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Author Topic: Squirrel design incidence: wing LE shim and stab washout  (Read 1688 times)
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Ashu
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« on: February 06, 2018, 03:02:01 AM »

I am working on my first rubber powered model Squirrel by Darcy White.

The original design has a shim under the wing LE and the stab is horizontal/level with ground (zero washout..if that is the correct term to use for the inclined stab). Gary on Endlesslift has made foam version with camber but without the shim...he says thay his prop downthrust eliminates the need for it. I have also seen that in Gary's other foam glider designs, he doesnt give incidence to wings but has a taper on the end of the stick for a stab washout of 3 degrees.

So:
1. What aerodynamic purpose does each of these features serve? My understanding is that shim causes wing underside to be exposed to oncoming wing for lift; and stab washout causes an upward moment by pushing the tail down.

2. If only a flat wing without camber is to be considered, then 'putting a shim under wing LE' is equivalent to 'stab washout', or do both serve different purposes? In my view 'shim under wing LE with horizontal stab' = 'horizontal wing with stab washout'. Is that correct?

3. If point 2 is true, then I dont get how they are equivalent as the commercial props used in both design has a default downthrust? So shimmed wing with prop downthrust would be a different case, right? Does wing camber have anything to do with it? (I do not quite understand zero lift line and thrust line)

4. If I make a camber wing Squirrel and a flat wing Squirrel, how should I decide which feature to use (shim or stab washout)? What happens if I use both?
I want to know the effect of these permutations in designs so that I can pick one as well as do trimming, especially with kids when I take workshops later.

Since Balsa is not available to me here and I use alternative fuselage sticks which are not always perfectly straight across their lengths to ensure that the stick edges are parallel, the imperfection might automatically cause a stab washout or some angluar difference between wing and stab (decalage?). What is the difference between incidence and decalage, and what is their impact?
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OZPAF
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« Reply #1 on: February 06, 2018, 05:49:07 AM »

You have made a good choice for an easy to build and fly small rubber model.

I have used a version of this model to teach young beginners and it has been quite successful.

There is a bit of confusion in some of your points so I will just try to explain the basics.

First of all the physical angle that the wing and tail are mounted on the fuselage is called the angle of incidence. This angle is measured from the chord line of the airfoil to the fuselage datum.

The difference in the angle of incidence between the wing and tail was originally correctly known as longitudinal dihedral  and now is often described as longitudinal decalage. However it is the difference that is necessary for stability and the tail or aft surface should always be at a lower angle of incidence than the front surface or wing in this case.

The Squirrel uses a packer under the front wing for the original flat surface wing with tail flat on the motor stick. In this case the line of the motor stick is taken as the datum and the thickness of the packer determines the incidence of the wing and also the longitudinal decalage as both the wing and tail have flat airfoils - ie no camber.

However the sample made by Gary uses a cambered airfoil and this changes the situation. The actual angle of attack of any airfoil with respect to the air is determined by it's angle above it's zero lift angle. For a flat airfoil or symmetrical this will be at 0deg. However a cambered airfoil will have a zero lift angle below 0 deg - depending on the actual amount of camber - and for a typical Clark Y section could be around -3 deg. Thus mounting the airfoil flat on the motor stick with this airfoil would give it a longitudinal decalage of 3 deg relative to a flat airfoiled tail mounted flat on the motor stick.

This difference is what enables the model to fly at a stable speed with the corresponding centre of gravity. If there is no difference in the angles of incidence of wing and tail - then regardless of the position of the balance point the model will not be stable.

Down thrust for the propeller is another story and depends on the propeller size and the amount of rubber used and is generally needed as the model will fly faster under power than it's glide speed and thus the climb needs to be controlled to avoid a loop.

This is very much simplified and I would advise you to Google some of these terms for more information and also check further here on HPA.

In any case if you do not have access to balsa I would suggest that you duplicate the Squirrel in foam( meat tray or pizza tray) with a flat airfoil and not worry about the cambered airfoil to start with. as far as the motor stick is concerned you could make say the top side dead flat and mount both the wing and tail on top if it is too hard to find/make perfectly parallel sticks.

John
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Hepcat
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« Reply #2 on: February 06, 2018, 07:08:11 AM »

As I read Ashu's opening post I was wondering how I could help without writing a book.  Fortunately in #1 I find that Ozpaf has done a great job in explaining things Well done! Ozpaf.  It should be bottled and passed out to all newcomers so that they can take a few sips every day and they will soon be clear of 'Terminology Rash'.
John.

PS. I have just noticed Gary's remark about not needing wing incidence beacause of downthrust.  It may have happened on his particular model but should not be said without qualification.  There may be occasions when a strong propeller slipstream affects the wing lift but I think it is rare.  It is more likely that downthrust will give a force about the CG that reduced the wing incidence and a loss of lift.
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Ashu
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« Reply #3 on: February 06, 2018, 08:29:31 AM »

Thank you OZPAF!

So shim will give incidence. If I have stab incidence then that will add to the total incidence, right? I mean, original design has flat stab and only wing LE shim. If I add taper to create stab incidence, that will increase the incidence further?

I couldnt get the downthrust part totally. But from your comment maybe my misunderstanding lies in the perception that "downthrust further ADDS to the incidence (or angle of attack?), and so it is either wing shim OR downthrust". But you say that downthrust is different and is for countering pitching up during powered climb. So it shouldnt be seen in relation to incidence, right? As Hepcat also says that "There may be occasions when a strong propeller slipstream affects the wing lift but I think it is rare." Have I understood correctly? I couldnt understand the second part: "It is more likely that downthrust will give a force about the CG that reduced the wing incidence and a loss of lift."

Gary's post: http://www.endlesslift.com/foam-squirrels/
"With 4.1 degrees downthrust on the prop hanger and 3.6 degrees to the wing zero lift line, the attack angle in flight will be 7.7 degrees,  right where I want it without requiring a shim under the wing."
Didnt understand this at all. The zero lift line is also BELOW the fuselage stick, the -3 degree OZPAF mentioned above? So if prop hanger is 4.1 deg with stick and zero lift line is at 3.6 deg, then prop hanger makes 0.5 deg with the zero lift line?...how do the angles ADD together to give 7.7 deg? Or is 'attack angle' discussion is different from the 'incidence' one above? Any diagram to clarify this?
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« Reply #4 on: February 07, 2018, 08:13:00 PM »

Thanks John(Hepcat) - I found it difficult to explain in simple concise terms.
I would again recommend Ashu that you do a search through HPA for more information and pay particular attention to the many informative posts from Hepcat.

now to your questions.

Quote
So shim will give incidence. If I have stab incidence then that will add to the total incidence, right? I mean, original design has flat stab and only wing LE shim. If I add taper to create stab incidence, that will increase the incidence further?

If you raise the leading edge(front) of the tail or stab - then that will increase the incidence of the tail(ie positive incidence) but reduce the decalage or difference, as for example 3deg positive incidence on the wing less 1 deg positive on the tail equals 2 deg decalage. It's important to realise that it is the difference that provides the stability in conjunction with the balance point.

I would suggest that you leave the tail flat on the motor stick and just use the packer on the leading edge of the wing as per plan. You should only require a 1.5mm packer under the leading edge of the wing. The exact amount you will need will depend on where the model is balanced (centre of gravity) and how heavy it is.

Quote
I couldnt get the downthrust part totally. But from your comment maybe my misunderstanding lies in the perception that "downthrust further ADDS to the incidence (or angle of attack?), and so it is either wing shim OR downthrust". But you say that downthrust is different and is for countering pitching up during powered climb. So it shouldnt be seen in relation to incidence, right? As Hepcat also says that "There may be occasions when a strong propeller slipstream affects the wing lift but I think it is rare." Have I understood correctly? I couldnt understand the second part: "It is more likely that downthrust will give a force about the CG that reduced the wing incidence and a loss of lift."

When the thrust line is altered - in this case downwards to provide down thrust it alters the force balance around the CG. In simple terms angling the thrust line down will provide a nose down pitching moment( by introducing a vertical downwards component in front of the CG) to control the increased nose up pitching moment produced by the wing flying faster than it's gliding speed. The incidence angles of the wing and tail remain constant (they are fixed) however the developed angles of attack depend on the force balance (trim). As the model flies faster at the original angle of attack it will tend to climb and if not stopped could loop or stall, thus the nose down pitching moment reduces the wing angle of attack - thus reducing the wing's lift and controlling the climb. The down thrust needs to be carefully adjusted to give the climb angle required.

[quoteGary's post: http://www.endlesslift.com/foam-squirrels/
"With 4.1 degrees downthrust on the prop hanger and 3.6 degrees to the wing zero lift line, the attack angle in flight will be 7.7 degrees,  right where I want it without requiring a shim under the wing."
Didnt understand this at all. The zero lift line is also BELOW the fuselage stick, the -3 degree OZPAF mentioned above? So if prop hanger is 4.1 deg with stick and zero lift line is at 3.6 deg, then prop hanger makes 0.5 deg with the zero lift line?...how do the angles ADD together to give 7.7 deg? Or is 'attack angle' discussion is different from the 'incidence' one above? Any diagram to clarify this?][/quote]

As mentioned above I don't agree with Gary's assessment of the wing angle of attack. However his reasoning for the 7.7 deg is as follows
The cambered wing has a zero lift angle of -3.6 deg. As it is mounted with chord line parallel to the motor stick then it is in effect at 3.6 deg positive to the motor stick - it has been rotated positively 3.6 deg above it's zero lift position on the motor stick.

The thrust line is -4.1 deg(down) to the motor stick and thus relative to the thrustline the wing is at 4.1 + 3.6 = 7.7deg.

The theory of flight can be quite involved even as you have seen, in what appears to be simple scenarios. I hope I have helped but the best way to get answers is to study the theory basics and then use the knowledge in areas such as HPA as a reference.

John
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Ashu
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« Reply #5 on: February 18, 2018, 11:26:16 PM »

Thanks for the help. I wanted to reply only after making some model Smiley

I made foam and tissue Squirrels and could do just a few short test flights. I used a round stick made from some stem I suppose for the foam version as balsa stick is not available...that had some problems in attaching parts and needed sanding. For tissue version I made a 6" prop out of 2" plastic bottle designed like Poonker prop blades.

The foam one flies straight. No stab tilt and no prop sidethrust. How should I design other models to make them turn? I see stab tilt as one option but since stick is round and thin (3mm dia), the sanding is often inaccurate and cumbersome. Should I stick paper aileron to one wing?
I have used 3mm thick foam plate for it as that was the one I had available then. I could mybe sand it thinner. The prop was 6" commercial prop by Kelvin sent generously by Slater of Sciencetoymaker...he generously also sent me his walkalong gliders, dragonfly, TSS rubber and props.

For the tissue model I have put stab tilt that gives a right turn. I didnt put any prop sidethrust..just 7 degree downthrust. I used another stick for it that was flat and sanded it to size. It is heavier than balsa though, obviously Smiley. I had made wing with the same round stick I used for fuselage by splitting it...that broke after some crashes when I was still trimming. I had a friend from Delhi give me some balsa sticks that I used for making the new wing. However I realised happily that the round stick I am using is not too different in weight compared to balsa, althought that stick is not wood and is probably some plant stem used commonly in a kind of ice cream here and firework rockets.
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OZPAF
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« Reply #6 on: February 19, 2018, 01:01:01 AM »

Quote
The foam one flies straight. No stab tilt and no prop sidethrust. How should I design other models to make them turn? I see stab tilt as one option but since stick is round and thin (3mm dia), the sanding is often inaccurate and cumbersome. Should I stick paper aileron to one wing?

Although you could use paper ailerons on the back of the wing to help the model turn it is not the best way to do it. The aileron deflection say to give a glide circle will be too powerful when you wind the motor up more and it flies faster.

Ok now say to trim the Squirrel for a right spiral in the climb and right glide I use right rudder (just enough to give a 15metre or so glide circle). Now under power this will be too powerful and roll the model hard right - so what you do is add some down aileron on the right wing. Start with say a 10 deg bend down and try a flight under power - you are looking for an almost straight climb with just a slight turn to the right under full winds, keep adding more down bend in small steps until you see this. How much down aileron you will need will depend on the speed and thus the number of winds. This system of trimming is known as "crossed trim" - ie the adjustments are fighting each other and can be used to find a balance.

The down aileron should be added to the wing about half way out along the TE and should only be around 6-8mm wide by say 15mm long. If you need to bend them down more than 30 deg you either have too much right rudder or they are not large enough (make them longer for more effect.)

Side thrust helps to control the initial high power of the rubber motor and as you add more winds to the rubber you will need some. On your tissue mode you could probably bend in a couple of degrees right thrust into the wire bearings(carefully). I notice that you have a plastic prop hanger and propeller on the foam version. This has a bit of right side thrust built in and you can actually carefully bend in more if needed.

Rudder tabs can be added to the rudder on the foam version 4-6mm wide by say 10mm long. However with tissue rudder versions - I move the rear rubber hook about 20mm forward of the rudder post and slightly and carefully bend the fuselage stick to the right to give right rudder. You will have to be careful doing this - although it won't need to move much.

You can still use tail tilt to help the glide turn but you will need the rudder - aileron cross for power as tail tilt has no effect under power( when the model is moving fast.) However using tail tilt as well helps because you do not need as much rudder for the glide which means you also don't need as much down aileron(on the climb) and thus you will have a better flying model(less drag).

The down aileron(or washin actually) also helps a bit on the glide - it tends to keep the inside wing of the turning model up and the slight bit of extra drag helps the model to keep turning to the right.

Your models look pretty good and it's a pity you can't get access to balsa. What about online suppliers?

I think sanding the foam could make it too weak. cut some of the middle out and cover with tissue? Incidentally I only use un shrunk wrapping tissue for these models.

Good luck

John


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Ashu
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« Reply #7 on: February 19, 2018, 02:37:21 AM »

Thanks!
I dont know if I have understood all the things properly Tongue
Flight should be straight climb with some turn, then right turn under remaining power and finally right turn glide. I add right rudder and right wing washin without sidethrust.
For right rudder, one way is to bend the stick to right at the tail (I have to join 2 sticks anyway to make 30cm length as they are only 25cm long; this could be done, but then it might be problematic to join the sticks and kids might have more problem doing this..I will try that).
I need help in figuring out how to add rudder.

Foam squirrel
How do I make rudder tab?..and I dont know what that is. Any pics? Since I have made a triangular rudder, should I make it trapezium shaped (as in tissue version pic) and add rudder to the TE by squeezing the foam and bending it? That is how I would add the washin too...Otherwise I am unable to visualise how to add rudder to the hypotenuse of the triangular fin.
I am not yet sure if adding stab tilt would be easy...I will have to try a few more models to see how difficult it is on this round stick...but cross trim is needed anyway, as you said, so I might skip stab tilt.

Sidethrust
Now another confusion is the sidethrust that you explained. You said that it will be needed for more winds. When and how do I decide that I need a sidethrust and cross trim is not sufficient? In the tissue version with DIY prop, there is no sidethrust and there is stab tilt for right turn...it flies in right circles with 150 turns, and I did not get time to check it indoors with more turns.

Tissue squirrel
How to add washin here? And right rudder, if I dont want to bend the stick?

---
Balsa
Online stores are few and relatively expensive..they probably import it. My primary audience would be low income community kids in government schools and I dont want to be catering to the well off schools only, IF they are interested in the first place in the pervasive culture of rote learning. The govt schools are very resource deprived...I mean often these schools lack basic infra, electricity, water, washrooms, etc and the kids are often first generation learners stuck in badly functioning schools.
So the schools probably wont have funds to organise any workshop, unless the teachers pool in personal resources or some sponsor is sought... Slater has lovingly helped initially with materials and guidance for my learning and local adaptation; I have found an individual donor here who has promised to initially help purchase TSS for the kids to get things rolling. Let's see how it pans out in future Smiley. So I am imposing the same material and cost constraints on myself as kids will have, be it glue or stick or paper or anything else.
The other option for balsa is to import from China but it takes 30-40 days and is not that cheap relative to other material expenses.
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« Reply #8 on: February 19, 2018, 07:35:15 PM »

Quote
For right rudder, one way is to bend the stick to right at the tail (I have to join 2 sticks anyway to make 30cm length as they are only 25cm long; this could be done, but then it might be problematic to join the sticks and kids might have more problem doing this..I will try that).

If you need to join the sticks then add the tail section - the last 5 cm or so at a slight right angle - approx. 2 deg or so perhaps. What age group are you talking about? It may be worthwhile making the sticks and providing them your self. A simple jig could make this fairly easy.

Quote
How do I make rudder tab?..and I dont know what that is. Any pics? Since I have made a triangular rudder, should I make it trapezium shaped (as in tissue version pic) and add rudder to the TE by squeezing the foam and bending it? That is how I would add the washin too...Otherwise I am unable to visualise how to add rudder to the hypotenuse of the triangular fin.

Your rudder tab could be just a small piece of bond paper of the approx. size I mentioned with first couple of mm lightly stuck to the trailing edge of the wing with paper glue.(glue stick works well). I feel you would probably find it hard to get small adjustments warped into the foam and it would lead to the problem of damaging the rudder and the wings. paper tabs are a better idea.

Quote
Now another confusion is the sidethrust that you explained. You said that it will be needed for more winds. When and how do I decide that I need a sidethrust and cross trim is not sufficient? In the tissue version with DIY prop, there is no sidethrust and there is stab tilt for right turn...it flies in right circles with 150 turns, and I did not get time to check it indoors with more turns.

The Trim I mentioned was for a outdoor model with a steeper more powerful climb. As you are flying indoors you will need less power but you may still need to add some right thrust as you increase the turns.
I didn't realise you were flying indoors so you need to maintain a good turn (to miss the walls) and if it is turning right now - then just keep adding turns. If the right turn starts opening up then add a bit of right thrust.

Quote
Tissue squirrel
How to add washin here? And right rudder, if I don't want to bend the stick?

Paper tabs for the wash in. With the lower power needed for indoor - you may not need as much wash in because you are not trying to achieve a steep climb but a constant right circling flight. Only use enough to stop the right wing dropping.

Full marks to you for trying to do in the circumstances you have explained. I was wondering if you were a teacher.

You may be able to use reeds or stripped sections of bamboo or similar. it's a pity you could grow balsa locally Smiley I believe Indonesia has timber yards that supply balsa  http://www.karyadasarutama.com/balsa%20wood.html

Anyway here are a couple of pics - a different model but it shows what I mean by paper tabs - I actually cut these off envelopes using the sticky section of the tab to glue onto the wing.

Good luck.

John
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« Reply #9 on: February 20, 2018, 11:13:09 PM »

Thanks! I am working on it. And the model in the pics looks very beautiful!!

I had contacted this particular Indonesian supplier about a month ago and he had said that his min order is around $500 (or $1000...I am forgetting), shipping extra. I had contacted another Indonesian supplier through some online listing, Balsa Hobby, whose prices were super cheap but he said that the min shipping price was $40...while his website was showing an online store of shoes, he said that it had been hacked and he sent official documents of registration for his company to prove authenticity...anyway the shipping price is high.
---

You mentioned aileron and rudder tabs lengths as 15mm and 10mm. I havent tried yet, but that length seems small to me. Although you had sent different model's pics, I guess the tabs you have used are at least an inch long. Should I stick to your suggested lengths initially?
---

After your suggestion, I noticed that the round thin sticks are hard to stick perfectly straight and there was already a bend in the tail sections of the sticks I had made. I have marked the direction on the sticks now..I guess it makes sense to make the turn with tabs according to this bend right..right turn rudder-aileron for stick bending to the right and vice versa? The stick and tissue model with stab tilt for right turn also had a bend in the tail section towards right; so by accident it was in sync with stab tilt.
---

Since the material constraints involve quite a lot more work than the balsa wood and commercial prop version, I guess kids in grade 7 or 8 would be appropriate. Initial workshops would tell. I realise that just having balsa fuse stick and a commercial prop would have reduced tons of work and hassle, but that's how it is for now Smiley Anyway the added work is a great learning and skill building experience.
---

I tested indoors to begin with but kids wont have access to a stadium and all flights will have to be outdoors. I will now be testing outdoors.
You mentioned different power for indoor and outdoor. If I put same number of turns on the 1/8" rubber, than power would be same? What's the difference in flying in these two conditions and how should my design and motor winds change accordingly?
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« Reply #10 on: February 21, 2018, 04:36:10 AM »

That particular model of mine lodged high up in a tree ( 20m + and I wasn't able to get it down). It was the prototype for a class I taught (also 10-12 yr olds). I produced kits - ribs, motor sticks and supplied stick material, and even then they made hard work of it!

The model is 500mm Wing Span and thus the tabs are bigger. However the size is not too critical - particularly the aileron tab on the wing. Just visually scale them to suit your wing and rudder.

Another way I hadn't mentioned, to achieve some aileron affect is to swivel the right wing forward a few degrees - no more than 10. it's not as good as using tabs and it needs to be checked before each flight and to a lesser extent so will the paper tabs as they can be knocked to a different setting.

Quote
I have marked the direction on the sticks now. I guess it makes sense to make the turn with tabs according to this bend right..right turn rudder-aileron for stick bending to the right and vice versa?
Yes definitely. Try to ensure that the kids add tail tilt to match the bend in the sticks. You may be able to prebend straight sticks before they are used?

Quote
You mentioned different power for indoor and outdoor. If I put same number of turns on the 1/8" rubber, than power would be same? What's the difference in flying in these two conditions and how should my design and motor winds change accordingly?

The 1/8 rubber you are using is basically what I found works well for outdoor Squirrels. Flying indoor would probably require a drop in rubber size to 3/32 or so - depending on the weight of the model. The idea - outdoor or indoor is to fly for as long as possible. However with outdoor models it is usually achieved by a steep(ish) climb to a fair height followed by a glide descent. Indoor models on the other hand fly under a restricted height(stadium ceiling) and thus need to have a slow climb but a long motor run. Thus indoor models generally have a much longer rubber motor (sometimes 2-3 times the distance between the propeller hook and the rear hook) of lighter rubber. They do not generally have a glide section of the flight, but descend under power and land with the motor running.

Incidentally my outdoor Squirrels flew with 1/8 rubber around 1.2 x the distance between the prop hook and the rear hook. That was a loop of rubber around 250mm when stretched out straight.

Unless you have an indoor area available - I wouldn't worry too much about the indoor set up as it is a whole area of it's own.

If you do some time in the future wish to try indoor flying - I would suggest the Hanger Rat. Try googling Hanger RAt on U tube and it will show quite a few of these models flying.

The name of my model in the pics is Sky Bunny and it also has some good outdoor flying videos on U tube, as does the Squirrel which I guess you have probably seen a few of.

These videos are worth watching - providing you have time and it sounds like you are a busy person.

Here's a couple of pics of Hanger Rats made by our kids in 2011 for a postal competition and my prototype. I again provided kits for these models - particularly ribs and the prop hangers/bearings. These incidentally were made out of 0.8mm thick aluminium and if you have access to a drill press and a few tools are not all that hard to make.

One other thing - you will need a reasonable amount of room to fly the models outdoor, even in fairly calm conditions - particularly once you add a few winds. I would suggest an area no smaller than a soccer or cricket field and don't fly in anything over 5mph winds. The best times to fly can be early morning or late evening just before sundown.

Good luck.

John


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« Reply #11 on: February 21, 2018, 05:13:34 AM »

Thanks!
"Try to ensure that the kids add tail tilt to match the bend in the sticks."
I didnt get this part. I had in mind that: horizontal stab/tail for ease of sanding, and rudder and aileron tabs for turn...no stb tilt. Stick bend (by design imperfection) would be in sync with these. But you mention tail tilt by which I understand that one side of stab would be up and another side down, right? I was thinking that I would NOT put stab tilt and would do turns through tabs. Have I misunderstood something?
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« Reply #12 on: February 21, 2018, 10:38:31 PM »

Quote
Try to ensure that the kids add tail tilt to match the bend in the sticks."
I didnt get this part. I had in mind that: horizontal stab/tail for ease of sanding, and rudder and aileron tabs for turn...no stb tilt.

I guess I was a bit vague. Actually what I meant if you notice the kids having trouble getting the stabs on square, then try to get them to err towards stab tilt to the right. It will help the right turn where as if they have left stab tilt and right rudder the glide could be inconsistent.

Quote
I was thinking that I would NOT put stab tilt and would do turns through tabs. Have I misunderstood something?

Your understanding of tail tilt is correct and you do not necessarily need both tail tilt and rudder even though it can be better than rudder alone. However for the kids and these early efforts just use rudder and wing tabs.

John
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« Reply #13 on: February 22, 2018, 07:04:28 AM »

So I tested my foam Squirrel with commercial prop and tabs. Stick had a left bend at the tail so I put in tabs for left turn. I made 2×1cm rudder and 2.5×1cm aileron. (Pic)

I wanted to try 3/32" rubber. With 500 turns by hand, it flew for about 15 sec. There were still some turns left in the rubber when I picked it up. I guess it doesnt give that much power and maybe the weight of the plain is also substantial...? I also reversed the fin to have straight edge at the back to stick rudder tab.

https://youtu.be/PQXK3RvW82w
Thanks John! Have put your name there Cheesy

I will try the other models too. The DIY prop version is important as I wont be using commercial prop with kids. It does give some problems, especially with keeping things rigid and stuck. Hope that works too.
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Re: Squirrel design incidence: wing LE shim and stab washout
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« Reply #14 on: February 22, 2018, 07:29:28 AM »

Not bad!. Yes it needs more power. Try a loop of 1/8 rubber if you have it. If you only have 3/32 then use 3 strands. You will need to have a small loop in both ends of the rubber - one for the front propeller hook and one for the rear hook.

Good luck.

John 
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« Reply #15 on: February 24, 2018, 09:52:38 AM »

I am trying to alter the design for ease and incorporating other features. I read a very nice document by Chuck Markos on Science Olympiad construction tips.

New things I am trying:
1. Have split the second stick in half for tail section and then stuck it on right side of the motor stick as seen from tail to front. I have added about 2deg right bend for rudder and 3deg up angle for stab incidence. This splitting has reduced the tail weight, exposed a flat side on the left for easily sticking rudder (which I was previously sticking to the side of the round stick), and gives a round surface on the right for sticking the stab with a tilt for right turn. As in Olympiad doc, I stick the fin first and then the stab for tilt. Simply splitting the tail stick has solved quite a lot of prpblems...now I am hoping it wont be too weak. After glue dries, I put sewing thread and white glue to secure it more and fix the tail hook at the same time to the motor stick.

2. Am using 1mm thin foam plate instead of 3mm. While 3mm is easy to work with, especially for kids, I felt that 1mm should be much better for lightness. I was concerned about sticking the vertical winglets due to thin surfaces; so instead of cutting and sticking the winglets separately on the wing sections, I did not cut away the winglets and made a single pattern of one wing section and the triangular winglet. I then scorched the winglet-wing line with a blunt pencil, stretched the winglet down a little to expose some more foam, put white glue, and then bent it up, supported by a book at the bottom and a vertical wooden block to have it at 90deg. It came out well. Since the tissue version's winglets were getting the LE of winglets torn, I thought that that is getting more impacts. So I have put small sticks near the base of the winglet LE to see if it will help.

3. I had to put thin sticks at the bottom side of wing for wing rigidity. There is a broom available here made of these thin sticks, but they are dense, non uniform and sometimes twisted (next time I will use the same lighter stick as that in fuselage). I couldnt get a length of 30cm for the entire LE, so I used 3 sticks initially. However I felt that it has become heavier, so I removed the sticks near the tips to test if it works and wont break there. I put a small length of the stick at the TE too as foam would press and dent under rubber band pressure on the wing hold-down stick. I used popsicle stick for shim and wong hold-down stick...for the former I used the original 2mm thickness and for the latter I sanded it to 1mm to reduce weight. I was thinking of offsetting the wing to the right by 1.5cm for increased lift on right wing, but then didnt do it...I will probably try it on another wing I make next.

4. I hadnt given much thought to the paper clip bearing, probably because it was tough for me. But that would have made it almost undoable for kids. I saw this doc agin, sent by my friend Jan, and made a line figure on paper for the angle (https://translate.googleusercontent.com/translate_c?act=url&depth=1&hl=en&ie=UTF8&nv=1&prev=_t&rurl=translate.google.nl&sl=nl&sp=nmt4&tl=en&u=http://www.zininmodelvliegen.nl/indoor/full-proof.html&usg=ALkJrhj-ZG52AJ9RehD51OYFjF8TKNWw0A)
... Earlier I was simply making bearing without measurements and teying for around 7deg downthrust like the commercial plastic prop. Making the figure on paper and comparing the bearing step by step as I bent the wire made it more organised and easier. In earlier test flights I hadnt kept much track of downthrust but it was needed more than what I had initially put in. Then I also saw Bill Kuhl's artcle on Endlesslift where he too said that he had needed more downthrust on the plastic prop. So now I am trying 10deg downthrust. I also made the looped sections of the bearing almost vertical...earlier it was getting bent on nose impact and the downthrust angle was changing that I was correcting from memory...now having vertical sections would be easy to reset if the bearing bends.

5. I had made the earlier prop from plastic bottle and had stuck 2 blades to make a single blade to increase thickness. I am considering not sticking 2 blades this time. I am also thinking of making even lighter yoghurt cup blades but I am not sure if they will rigid and strong for outdoors. Any idea/suggestions? (6" prop, P/D=1.3)

------
Queries about turning:
Some questions might be those that I asked earlier, but I am still not too clear on some things.
1. Out of stab tilt, rudder through sidebent tailboom, rudder tab, wing offset to one side, and sidethrust, which ones work at high speed during motor run and which one work at lower speed? Some places, including the Olympiad doc say that sidethrust works when the torque is lower under reduced turns...thatvdidnt make sense to me. i thought it will give turn under high torque...?
2. Can sidethrust, sidebent tailboom and stab tilt (maybe wing offset too) combined, eliminate the need for rudder tab at any point? Or will rudder tab have to be always used under outdoor high power conditions?
3. Related to ques 2, how tight can I keep the circles? You said 15m circles which I felt might be too big...Will having smaller circles, say 7-8m dia, give faster climb and more height? Any downsides to that?
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Re: Squirrel design incidence: wing LE shim and stab washout
Re: Squirrel design incidence: wing LE shim and stab washout
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« Reply #16 on: February 24, 2018, 06:55:33 PM »

I agree with your improvements - they will certainly help. Splitting the sticks was a good idea.

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I have added about 2deg right bend for rudder and 3deg up angle for stab incidence.
The 2 deg bend for the rudder is about right but if you have the wing raised at the leading edge I doubt whether you will need 3 deg "up" angle for the stab. I find with my Squirrels that I only have about 1.5mm under the LE of the wing which is 60mm wide - somewhere around 1.5 deg difference between the wing and tail. I would suggest a total of 3 deg difference between the wing and tail and preferably with the wing raised and the tail at 0 deg.

The tail angle is used as the reference line and the wing is raised relative to it and the down thrust also relative to this datum/reference line.

If you have too much difference then you will find that more nose weight will be required to get it to fly without stalling.

The point where the model balances(with the rubber motor) along the wing chord should be around the halfway point. This will give a better climb and glide.

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I was thinking of offsetting the wing to the right by 1.5cm for increased lift on right wing, but then didnt do it...I will probably try it on another wing I make next.

I wouldn't worry about doing this yet at least for the kids, as if the model has a natural tendency to turn to the left due to building errors - then it could be very difficult to trim. It's far better to stick with tabs at first.

The paper clip bearing idea is a good one - bend the paper clip wire around a similar size nail to get tight bearings. Make the legs of the bearing slightly off the vertical - almost vertical at the back. You can adjust the down thrust easily by bending the front leg towards the vertical for more down thrust.

When you bind/glue them to the stick fuselage make sure you have the side thrust of around 3 deg.

I have only used commercial (Sig) nylon nose bearings and have not needed more down thrust but have sometimes bent in more side thrust.
 
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5. I had made the earlier prop from plastic bottle and had stuck 2 blades to make a single blade to increase thickness. I am considering not sticking 2 blades this time. I am also thinking of making even lighter yoghurt cup blades but I am not sure if they will rigid and strong for outdoors. Any idea/suggestions? (6" prop, P/D=1.3)

The single blade will be enough as actually the thinner blade will work better and it should be strong enough. Yes the Yoghurt Cup blades will work well. They are a little thicker but strong enough and very light. Your P/D of 1.3 and 6"dia sounds ok. They will be a bit more fragile however a lighter model will not collide with as much energy. The standard method is to draw a 15 deg line to the vertical axis of the cup and place the centreline of the prop blade shape along this. When looking down on the cup the top of the line should be to the left and this is whee the tip of the blade should be.


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Some questions might be those that I asked earlier, but I am still not too clear on some things.
1. Out of stab tilt, rudder through sidebent tailboom, rudder tab, wing offset to one side, and sidethrust, which ones work at high speed during motor run and which one work at lower speed?

Basically the aerodynamic trim adjustments (those affecting the generation of lift) increase with speed and these include Rudder Tab(bent stick is the same in effect),  Aileron tab and the incidence difference between the wing and the tail.

Stab tilt has very little effect until the model slows to a glide and this is why it is so helpful as it can reduce the need to have too much rudder which can lead to problems when the model flies fast(It can roll into a dive).

Offsetting the wing has a more or less constant effect but is not desirable in my opinion as it will be difficult to have the small amount of offset required. I don't think it's worth the trouble for something like a Squirrel and the aileron tabs are easier.

Quote
Some places, including the Olympiad doc say that sidethrust works when the torque is lower under reduced turns...thatvdidnt make sense to me. i thought it will give turn under high torque...?

This is basically correct. Under high torque the model will try to roll to the left(looking from the rear) and the side thrust is usually just enough to stop this happening by trying to turn the model to the right and thus generating a bit more lift on the left wing.
As the Torque reduces and the model is now in the cruise mode, then the side thrust helps to turn it to the right. this however is really only noticeable under high power. On low power flights the model may only climb in slow right spiral turns as your model did in the video. As the power increases - the model is trimmed for a straighter climb and then the adjustment of side thrust for the cruise becomes more necessary.

Quote
. Can sidethrust, sidebent tailboom and stab tilt (maybe wing offset too) combined, eliminate the need for rudder tab at any point? Or will rudder tab have to be always used under outdoor high power conditions?

The side bent tail boom is the same as having rudder tabs in effect and if you have the right amount then it is more reliable(can be knocked out of place) as it is built in. tilt is good for the reasons I have mentioned but cannot replace the rudder under power. Thus yes you will always need some rudder - either built in(bent tail boom) or rudder tab, perhaps both when you don't know how much rudder you will need until you test fly. Again if  you change the motor for a different more powerful motor then you will probably need to change the rudder setting as well.

Quote
3. Related to ques 2, how tight can I keep the circles? You said 15m circles which I felt might be too big...Will having smaller circles, say 7-8m dia, give faster climb and more height? Any downsides to that?

Yes you can trim for smaller circles - however the best flights will generally come with larger circles as there will be extra drag caused by the increased rudder and necessary aileron tab to keep up the inside wing. However they can still put in good flights with smaller circles.

The fastest steepest climbs are straighter. Just as an example my Squirrels when trimmed will climb around 35m with only approx. 1/2 to 1 circle turn.

John
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« Reply #17 on: February 24, 2018, 11:28:10 PM »

Thanks! That cleared up lots of things! Smiley

So I will try one more model with the same 3deg stab incidence and this time I will not put shim under the wing. I will put 3deg tailboom right rudder. No wing offset in any model. Sidethrust, downthrust and aileron tab as necessary.

For the final design I am hoping to settle on:
- 0deg stab incidence
- substantial stab tilt
- 1.5-2mm wing LE shim
- 0deg or more tailboom rudder as per results of these flights...if 0deg then rudder tab, or maybe it will still be needed even with tailboom rudder
- aileron tab as needed (in my tissue version flights with 3/32" TSS, alight right rudder, no tabs and slight stab tilt, I found that the sidethrust was giving right turns about 7-8m wide without the need for aileron tab...but I hadnt put about 300-350 turns only)
- initial sidethrust about 3 deg (while making paper clip bearing, I turn the loops on opposite sides of the shaft...that gives the initial right angle without needing to bend the bearing afterwards, unless it has to be increased later)
- downthrust depending on the these flight results
- 1/8" TSS for large fields, and 3/32" for smaller fields and indoors

---

Can you give me an idea about max rubber turns? I know that 80% of max should be put and 3/32 has about 129/inch and 1/8 has about 97/inch. If I have 2 strands of about 10" length, then the total turns would be according to EACH strand length and NOT the total motor length of 21", right?  So for 10" 3/32, I should limit to 129*10*0.8=1032 and for 1/8 I should limit to 97*10*0.8=776 turns?

I havent used lube till now. I will be using glycerine + liquid soap or castor oil. The standard ArmorAll is not available here. I asked at a lot of places, but the automotive parts shopkeepers didnt have any idea what I could use safely on rubber. I saw a car polish spray that said "Silicone Spray Body Polish", to be used for painted surfaces. But I am not sure if it is the same as ArmorAll and can be used on rubber. The leftmost bottle: https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcRR9_1wrigYDsXPWSTuxLx7paSzqMlkwueHxmpsuUVLMRisRlpc6GJnGEQC

Also any care to be taken for rubber storage? I currently wash the rubber after use, blot it with towel, let it dry and then store in my refrigerator in a cardboard box put inside a polybag with tight knot. How long does the TSS last? For these resource poor schools/kids, it will be super helpful if they can use the motor for a long time and not have to bother with repeated purchases which would also not be feasible at all.
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« Reply #18 on: February 25, 2018, 06:14:03 AM »

Your final design sounds good. Yes always allow for rudder tabs - you will generally not have quite enough bend in the boom. It also has a secondary benefit when teaching kids as they learn the effect the adjustment has on the mode. That's half the fun as you are finding.

Your rubber selection sounds about right - depending on the actual model weights. However I think you will be very close with new model with the lighter 1mm foam wing model after seeing how your heavier model flew.

Now for the harder question.
Quote
Can you give me an idea about max rubber turns? I know that 80% of max should be put and 3/32 has about 129/inch and 1/8 has about 97/inch. If I have 2 strands of about 10" length, then the total turns would be according to EACH strand length and NOT the total motor length of 21", right?  So for 10" 3/32, I should limit to 129*10*0.8=1032 and for 1/8 I should limit to 97*10*0.8=776 turns?

Yes 80% is a good guide and your turns numbers I think would be Ok however I'm not an expert and they usually test a batch to destruction to be sure! However I will give the very good advice of John Barker (Hepcat on this forum) who has flown rubber FF for years and developed this formula

The formula is: Turns = 44 x square root(length ^3/weight).  In words: take the length of the motor, cube it, divide by the weight, take the square root, multiply by 44.  This gives near breaking point turns, you can expect to break strands quite often!

It is wise not to think that any formulae or tables will give exact figures for turns.  Consider that rubber varies from batch to batch, from end to end of a batch, from the age and storage conditions, from the weather on the day you use it and from your winding technique and probably from a few other things I have forgotten.



Now checking say your 3/32 x 1/24 thick rubber - weight will be density (gms/ins3)* vol (ins3) = 15.4 * 20*3/32*1*24=1.203 gms.

Then breaking turns = 44 x sq root( length3/weight) =- 44*sq root(( 10) cubed/1.2) = 1268

And 80% safe winds = 1268*80 = 1014.

Thus the figure you gave seems quite OK> You can ratio the 1/8 off this by SQ root (3/32)/(1/8) = 878.

No 2 people will always give the same figures but I trust John as he is very thorough and has the experience as well.

In short - your figures seem quite OK - possibly conservative for the 1/8.

You should also bear in mind that you will need a winder to achieve this number of turns and you will need to stretch wind. That is someone holds the model - very securely and well away from their face, and you should then stretch the motor to around 4- 5 times its resting length(as a minimum) and then wind on half the turns or until you feel the rubber "stacking" (losing it's stretch). You then move in slowly while winding slowly the last 50% of the turns and holding as constant a tension as you can. The motor must be lubed to do this and you need to rest the motor after winding to 80% like this and flying.

Thus I would suggest that you limit the winds to much lower levels - say 400 or so for the 1/8, which will take a few minutes if winding by hand and is also easier on the motor - ie it will last longer. Another point to consider is that the model will go a fair way on the 80% turns when trimmed and if it is not trimmed may crash very hard.

I have had 30 sec flights on 400- 500 turns of very old 1/8 rubber with Squirrels - as a guide.

Quote
Also any care to be taken for rubber storage? I currently wash the rubber after use, blot it with towel, let it dry and then store in my refrigerator in a cardboard box put inside a polybag with tight knot. How long does the TSS last? For these resource poor schools/kids, it will be super helpful if they can use the motor for a long time and not have to bother with repeated purchases which would also not be feasible at all.

With the lower turns I have recommended - you could get 3- 4 winds in a session before resting the rubber. I have had my very old rubber last for months when used like this but only flying 1 -2 times a week. The rubber is more likely to break so as you mentioned in an earlier post - it should be kept as clean as possible.

Your storage approach is very good. It's better than what I have done with my Squirrel motors. I cleaned them - lightly brushed any loose dirt etc off them and kept them in snap lock bags - but they were never refrigerated. They were kept in a cool place but that's it.

I would recommend that you keep to your method as the TANSS rubber is worth treating well.

One last thing I have an article that you may be interested in and if you PM me your email address I can send it to you. It's too big and the wrong format for this forum.

John



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« Reply #19 on: February 25, 2018, 10:41:44 PM »

Thanks!
"Rest the motor" means you dont use it again in that session right? I will limit to 400-500 on 1/8" and maybe 700-800 on 3/32. But how long before I can use it again after resting? And supposing the motor doesnt break after many months of use, is ut usable for long or becomes unusable after some time?

Also any comments about that bottle of car polish spray I mentioned?
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« Reply #20 on: February 26, 2018, 12:48:24 AM »

Quote
"Rest the motor" means you dont use it again in that session right?
That's correct. Leave the motor till the next day's session. It will probably mean that you will need around 2 motors minimum per kid.

As long as the motor has been stored well, not over wound and lubed every second flight at least - it will last possibly for years. It all depends on how often it is used, but you will get good life if you teach the kids to take care of the motor as we discussed.

I was using old rubber that had not been used (it was black FAI) and was over 20yrs old. In my case some samples broke while I was tying the knot - but those that didn't lasted for months while being used. I imagine your good Tan2 will last a lot longer.

UV from the sun id the big killer other than dirt and overwinding.

Competition flyers go for broke(literally) and their motors sometimes only last one flight!

I'm not sure about the car polish spray - if it is silicone based and not too expensive it may be worth trying on a sample motor. Castor oil is the old favourite but some believe that it eventually destroys the rubber. If you do use a silicone spray - you will need to replace it every second flight. The castor is messier but lasts longer.

By the way don't lube before you tie the knot with silicone or castor. I keep the rubber wet while tying the knot.(actually I wet it with saliva which is more slippery than straight water - it may not be too hygienic though!)

John


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