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Author Topic: Seeking advice on towhook position  (Read 609 times)
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johnok
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« on: December 30, 2018, 10:31:57 AM »

Good day all,

For a couple of years I have been trying to get into RC. gliding on a shoestring budget. My first thoughts were to scratch build a Keil Kraft Caprice and fit it with 2 channel (rudder and elevator) control. About half way through the project, as the difficulties were mounting up, I had to make the decision to shelve it as it was getting beyond me. Looking for something a little more realistic I settled on the "Super Turkey" (RCM plan #716 - Outerzone ID #7858). This was duly built and flown by hand launches and every thing was good. In fact I was surprised at how well it went from a "javelin" type throw. Next step was to try the hi-start. Well the model immediately turned hard left, hit the ground at (probably about) 30 degrees left bank, smashing the left wing and destroying the horizontal stabiliser. Having completed repairs recently I was then ready to try again. But first the C of G was carefully checked: seemed to be spot to the plan. Then the tow hook position.  Working on a suggestion that the tow hook should be between 25 and 35 degrees ahead of the balance point I drew some lines on the side of the fuselage and lo and behold, it was pretty much in the middle of the band.

So why then did the model not tow up straight?

Not wishing to demolish the machine again, I then fitted a temporary tow hook approximately 40mm. ahead of the proper one. Using this I made my maiden flight (of probably about 15 seconds). A clumsy landing on my second effort damaged the stab and that was it for the time being. To try to avoid further stabiliser breakages I am now converting to vee tail and will be ready for another launch very soon.

I would really like to understand why the first launch with the hi-start was such a disaster and what if anything one can do to get a better result. All advice and suggestions will be most gratefully received.

Also any information as to suitable ruddervator? throws for the vee tail setup will be much appreciated.

Thanking you in advance,

cheers John K.
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VictorY
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« Reply #1 on: December 30, 2018, 05:24:47 PM »

Sometimes hi-starts are a gamble, especially on planes without aileron control. Launch straight into the wind and start with less stretch until you determine how much control authority your rudder provides. Then work your way up once you are comfortable with how the plane is responding to tow hook position. I've never heard of anyone using degrees to determine placement but placing the hook 10-20% of the chord in front of the CG should be a good place to start. Moving closer to the CG will generally give higher launches but comes at the cost of stability.
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johnok
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« Reply #2 on: December 31, 2018, 12:46:56 AM »

Thanks for the response. The information for the towhook angle evidently came from RC Soaring Digest (see attached) and it does agree 100% with the towhook position on the plan. However that's by the bye. Main thing is I can get the thing in the air and start to learn how to control it.

cheers John K.
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Yak 52
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« Reply #3 on: December 31, 2018, 04:35:06 AM »

Hi John,

There are a few possible reasons I can think of for your swerve on launch:

1. Unexpected wind direction. If you were caught out by a stray gust or if there was significant wind shear that you couldn't feel on the ground.

2. Warps or asymmetry in the wing. Unlikely if your glide tests showed good trim.

3. Stall of one wing on the launch. If the bungee is too powerful for the model and it's launched with a high angle of attack ie fairly vertically then the wing is very highly loaded and can stall. If this stall is more on one side from a slight yaw the model will 'drop a wing' and do a flick roll towards the stalled wing. It's unlikely you would have the rudder authority to counter this.

It's also possible that if the bungee is too soft or if the wind is very light you can get into to the same attitude, especially if you are pulling back on the stick to try and keep the model flying.

Can you tell us more about the bungee set up and weight of the model? It's key to get these adjusted to suit each other.

The hook position rule of thumb you mention is a good starting point but it's not set in stone, in fact some models can launch well even with the how behind the CG  Shocked (not suggesting this!) Moving the hook forward won't help if you do have a stall issue, in fact it will just load the tail more.

This video shows my Skidoo (also a very cheap model Smiley) on launch - nicely balanced bungee for the model weight, hook at about 15 degrees forward of the CG and a fairly vertical launch:

https://www.youtube.com/watch?v=S8GhpdDMMZ4&feature=youtu.be

Jon
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Konrad
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« Reply #4 on: December 31, 2018, 08:38:37 AM »

Hi John,

There are a few possible reasons I can think of for your swerve on launch:

1. Unexpected wind direction. If you were caught out by a stray gust or if there was significant wind shear that you couldn't feel on the ground.

2. Warps or asymmetry in the wing. Unlikely if your glide tests showed good trim.

3. Stall of one wing on the launch. If the bungee is too powerful for the model and it's launched with a high angle of attack ie fairly vertically then the wing is very highly loaded and can stall. If this stall is more on one side from a slight yaw the model will 'drop a wing' and do a flick roll towards the stalled wing. It's unlikely you would have the rudder authority to counter this.

It's also possible that if the bungee is too soft or if the wind is very light you can get into to the same attitude, especially if you are pulling back on the stick to try and keep the model flying.

Can you tell us more about the bungee set up and weight of the model? It's key to get these adjusted to suit each other.

The hook position rule of thumb you mention is a good starting point but it's not set in stone, in fact some models can launch well even with the how behind the CG  Shocked (not suggesting this!) Moving the hook forward won't help if you do have a stall issue, in fact it will just load the tail more.

This video shows my Skidoo (also a very cheap model Smiley) on launch - nicely balanced bungee for the model weight, hook at about 15 degrees forward of the CG and a fairly vertical launch:

https://www.youtube.com/watch?v=S8GhpdDMMZ4&feature=youtu.be

Jon
+1

I hope your hand launching from 2 meters to get the trim set showed no warps. If  you have a lot of rudder offset just to fly straight it is an indication of a twisted wing. Get the model to fly straight and level with the rudder close to center, before putting her on the high start.

I set set my hooks at about 7° forward of the Center of Mass. Center of Mass is a bit different than Center of Gravity. But what this means is that with polyhedral wing the hook will be a bit forward of the location for the same model with straight wings. The only real down side I've seen to having too far an aft hook is early pop off. This could be a problem for a beginner as it could surprise you. I don't like a real forward hook as it make it difficult for me to pop off the line when I want, should I need to, like seeing flutter or other control issues.

Did you actually throw the model into the air? Too many folks actually just release the model, resulting in a stall at the beginning of the launch.

V-tails are a variable I'd avoid right now. I like V-tails a lot but far too many are not properly sized. Make sure you have as much or more surface area on your V-tails as that found on the original cruciform tail. Most polyhedral wings ( 2 channel ships) fly best with the V-tail angle between 90° and 100°. For most gliders, I hate the ubiquitous Beechcraft 110°.

All the best,
Konrad

P.S.
Is thi the plan?
https://outerzone.co.uk/plan_files_07/7858/Super_Turkey_RCM-716_oz7858.pdf
« Last Edit: December 31, 2018, 08:55:45 AM by Konrad » Logged

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dephela
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« Reply #5 on: December 31, 2018, 09:00:55 AM »

The design that Konrad just related to in the url looks like it has about 4 degrees of incidence built in. The plane might need some down trim for launch to keep it from behaving poorly on launch.

Dennis
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Konrad
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« Reply #6 on: December 31, 2018, 09:19:23 AM »

I’ve never had any luck with the Jedelsky wing. Structurally mine have twisted far too easily at speed (launch). And they act like gross under cambered airfoils.

Some folks can get them to work, I just can’t.

All the best,
Konrad
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Yak 52
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« Reply #7 on: December 31, 2018, 10:06:57 AM »

I’ve never had any luck with the Jedelsky wing. Structurally mine have twisted far too easily at speed (launch). And they act like gross under cambered airfoils.

This would be possibility #4: Structural deformation of the wing under high load. If the wing twists when launched that could cause the roll.


I set set my hooks at about 7° forward of the Center of Mass. Center of Mass is a bit different than Center of Gravity. But what this means is that with polyhedral wing the hook will be a bit forward of the location for the same model with straight wings.

Just a pedantic point here for clarity. Centre of Mass and Centre of Gravity are the same thing but people often mean different things by these terms. When people talk about the CG being at a % of the chord, they are talking about the centre of mass in the plan view. The actual CG in three dimensions will be somewhere above or below this point. To get the centre of mass in the 'side view', tip the model on its side and see where it balances on a finger tip. It's this CG in the profile view that the towhook rule of thumb refers to. As Konrad says a Polyhedral wing will have a higher vertical centre of mass and so the hook may end up further forward.

In flight the model rotates about it's 3D centre of mass and the aerodynamic forces and moments act there. However while the model is on the line the centre of rotation becomes the tow hook (more or less) and the wing and tail forces pivot around it instead.
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johnok
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« Reply #8 on: December 31, 2018, 11:42:46 AM »

Good day,

Again,thanks for your interest. I will try to address points raised above (although in no particular order).

First, the equipment: model is indeed  the one  identified by Konrad. I made one notable change in constructing the wing. I used 1m. long balsa sheets to give a span of 2m. instead of the 72" specified by the plan and I did not build the wing polyhedral - I left the centre section as a solid piece (thinking it might better withstand the inevitable abuse) with the outer panels at 500mm. long, raised 175mm. at the tips. Notes on the machine suggest a range of weight between 26 and 38oz. Mine came out more to the heavier end at 35oz. all up.

The wing is not warped - this would be very apparent if it is placed on a flat surface as the bottoms of the wing ribs are all in the same plane. Also the wing balances side to side - it should do as the parts of each side came from the same piece of wood.

Hand launched tests were initially carried out by some very experienced RC glider pilots who were assisting me. Generally the control surface had been set in line with rudder / stab. In fact we adjusted one of the pushrods to get the elevator in line when the servo was centred.

The hi-start consisted of 15m. of surgical latex tubing - OD. 7mm. x ID. 4mm. (sourced from a local manufacturer) together with approx. 25m. of monofilament fishing line - no parachute, it had a cloth streamer attached to the business end.

As far as conditions went it was a calm day. If anything I would have maybe been too tentative - first launch I certainly wasn't going for any altitude records. I'm sure I gave it  boost, but again - maybe not enough - but I think it was flying when it hit the ground.

I definitely was not pulling back on the stick - I never even had time to get to the stick.

I hear your comments about the C of G being below the level of the underside of the wing, however in this case it could only result in the towhook position being a bit further forward.

Wing twisting? I have  no idea - it seeme quite stiff though.

As far as the vee tail goes I am just using the alternative on the original plan hoping for a reduction in maintenance.

Anything I've missed, please come back.

cheers John K.
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« Reply #9 on: December 31, 2018, 12:01:41 PM »

Higher AR, more wing area, and an essentially under cambered airfoil, but tails not increased in size...I'm going to guess, like Dephela, that's going to want more down trim at higher speeds (ie launches).

And like Yak 52, twist is always an increased possibility when extending wings.
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Yak 52
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« Reply #10 on: December 31, 2018, 02:20:53 PM »

The hi-start consisted of 15m. of surgical latex tubing - OD. 7mm. x ID. 4mm. (sourced from a local manufacturer) together with approx. 25m. of monofilament fishing line - no parachute, it had a cloth streamer attached to the business end.

That sounds about right for a model of the size and weight except for the fact that the mono part is rather on the short side. You could easily double or even treble that to 50 or 75m (provided you have a long enough field.) This should give you a more gentle kiting effect and considerably higher launch.

Can I ask what attitude you are launching the model? Is it a horizontal 'javelin' throw or a more nose up launch as per the video I linked to?

It's possible that if you are using a short powerful bungee, launching flat towards the horizon and relying on forward speed to rotate the model upwards, that the speed will build up very quickly, magnifying the rolling effect of any twist or wind offset and possibly even causing flutter. This 'overspeed' is even worse if you have a forward hook and forward CG. If you get the trim and throw right your model should get straight into kiting speed and attitude from the beginning.


The other possibility is that you have inadvertently launched the model with a fair bit of yaw - it's very easy to do, especially in a 'straight up' throw. This would give instant roll effect and/or stall one wing if it was already likely to.


One more possibility: If it felt calm on the ground you may still have had some wind above you at 10-20m from thermal infill and potentially a crosswind (roll effect) or downwind launch (more likely to stall.)

Not in any way casting doubt on your skills  Smiley just hopefully helping you to diagnose what happened.


Jon

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« Reply #11 on: December 31, 2018, 03:20:51 PM »

...
Hand launched tests were initially carried out by some very experienced RC glider pilots who were assisting me. Generally the control surface had been set in line with rudder / stab. In fact we adjusted one of the pushrods to get the elevator in line when the servo was centred.
..
Have these guys tried to fly her up the high start. Just about any experienced sailplane pilot should be able to identify many of the basic beginner issues you might be having. Hands on help is worth the wait, if it is a matter of scheduling.

All the ebst,
Konrad
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« Reply #12 on: January 02, 2019, 03:25:23 AM »

Hi,

Going over the latest comments: vtdiv - am I missing something? What is AR?

As far as extending the wingspan, designer has suggested alternative of 96" without mentioning increasing the size of the tail surfaces.

Yak 52: no offence taken - I don't HAVE any skills yet - still trying to develop some. I've got a much better idea of how I must throw, and I will certainly lengthen the fishing line section.

Konrad: unfortunately the guys that helped me initially are quite far away, and I just haven't been able to get back that way since. And the clubs around here all seem to be flying powered models. But with all the help coming in I'm sure it's going to work.

Progress on the vee tail has been good. I had already started making a new fuselage -  hoping to lose some of the weight, so this in now being finished with the vee tail. Mechanical mixer made and installed, tail surfaces ready to be fitted, will just need to finish sheeting the rear part of the fuse and put on a nose block. The original fuselage with the conventional tail I will make a new (this will be the 4th.) stab but I think I'm going to make it in 2 halves to "plug in" so that future repairs will be a little less drastic.

again thanks to you all John K
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Konrad
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« Reply #13 on: January 02, 2019, 03:39:19 AM »

AR= Aspect Ratio

This is found by dividing the wing area by the wing span to get the Mean Aerodynamic Chord (MAC). Then divide the wing span by the MAC to get Aspect Ratio.

Generally the higher the AR the more efficient the wing, Lift to Drag ratio (L/D).
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« Reply #14 on: January 02, 2019, 05:34:01 AM »

John, I understand that you are trying to avoid damage but the V-tail will add some complexity in handling and tuning which may make things more awkward at the learning stage. The main thing to be aware of is the likely need for differential movement of the ruddervators. When you have a V-tail, any control movement now acts in both pitch and yaw so you can find that rudder movements cause the model to pitch nose up or down at the same time as turning. This takes some experimenting with to tune properly unless the designer has done plenty of testing and given appropriate throws and differential on the plan.

I would recommend sticking with the conventional tail for a less experienced pilot as it's easier to perceive and understand how the model is reacting to your control inputs.

Increasing the wing span by 6" will have increased the aspect ratio slightly but I doubt the effects would be noticeable. Of course the tail surfaces are now relatively smaller (and tail moment arm shorter) so an increase of a few percent in tail area might be worth considering. The change in dihedral may have some effect on handling but from the details given the wing should have a similar EDA (Equivalent Dihedral Angle.)


Jon
« Last Edit: January 02, 2019, 05:44:17 AM by Yak 52 » Logged
johnok
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« Reply #15 on: January 02, 2019, 07:28:57 AM »

Hi,

Aspect Ratio. Now I understand.

Concurrent with the vee tail I'm now making a new stab to replace the broken one. On your advice I'll push ahead with that first.

Can you get training wheels for beginners?

cheers John K.
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« Reply #16 on: January 02, 2019, 11:25:43 AM »

Yes, go with the cross tail, leave the V-tail for the next experiment once you've got familiar with your glider.
I dont know why your model veers' off  immediately after launch, but there are two points to explain better.

Position of tow-hook. The difference between a very forward hook position and a aft one stays in the attitude the models takes after release. To much forward and the model will not rotate and climb as it should. Take an extreme hook position on the nose like a Catapult Glider. The model will follow a straight line till hitting the stake. To much back, just in line with the CG, the model will rotate into the vertical and most probably stall or veer off (not your case, as the hook position on the drawing explains!). But a slightly forward position can be safer for initial trimming!

There is also a relationship between pull and the inclination you hold and leave the model. At the beginning of the trim session you certainly do not pull a lot on the bungee and this pull has to be associated with a moderate angle of release, maybe 20, sure not more than 30°. If released to steep the wing may stall because there is not sufficient speed and pull. You have also to consider wind speed wich eventually helps to "give" speed to the wing. Wind from the front means the wing is already accelerated even holding it in your hands. Later, during the trim process, the more you pull or the more front wind you have, the steeper will be the release. If possible get a second person to release the model, so you can have your fingers on the sticks and be ready to feed-in corrections immediately!

Just for fun I draw the front view of your wing and the original dihedral. You have less than the original Super Turkey. But it certainly is not the cause of your disastrous first launch. There may be a slightly different roll behaving in flight, a different rudder feeling, but not a disaster.

Urs

Last minute add-in:
to my humble opinion the wing airfoil is to extreme even if it would be a free flight model without RC. The top curvature is 10% at 30% chord. Being a Jedelsky wing means the mean camber is between 7-8% which is simply to much.
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johnok
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« Reply #17 on: January 03, 2019, 01:09:18 AM »

Good day,

Urs, re: your comments on the aerofoil, do you think that the excessive camber is contributing to the directional instability? Would it be worth "converting" to a flat bottom? It wouldn't be too hard to fill in the spaces between the ribs. For a test even closing the gaps with parcel tape?

John K.
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« Reply #18 on: January 03, 2019, 03:54:24 PM »

Hi John, for heaven sake, dont fill up the undercamber. You would just aggravate the situation, creating a thick, fat, draggy foil with still to much camber! BTW, the section does not influence the directional stability.

Just for not shooting into the dark I digitalised today the Super Turkey's section to confront it to the original Jedelsky E 85. Surprise, they coincide quite well, both have about 6% thickness and over 8% mean line camber. That confirms that this foil is thought (if there was any doubt) for FF models and in no way for RC. FF models are build to fly at the lowest sinking speed, RC models should fly a bit faster at the best glide/sink ratio. So they can be flown against the wind and not been blown away, fly away from downdrafts and in general being guided by the pilot. So the requirements for the airfoil section is different.

The Jedelsky section could be adapted to a beginners RC model flattening the roof shape of the 2 balsa sheets to obtain about 4% mean camber. That would mean to rebuild the complet wing. Keep it as it is for the moment.

Urs
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« Reply #19 on: January 04, 2019, 03:52:52 AM »

Hi,

Sounds like the best thing I can do with the Super Turkey is to hurry up and finish the Gentle Lady I've been working on between times !

cheers John K.
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« Reply #20 on: September 18, 2019, 07:05:26 AM »

Hi,

Well, work on the GL has been at a standstill for some time while some hard to get materials are being sourced so, since the second fuselage for the Super Turkey was well started, this has gotten finished in the meantime. I borrowed a a few ideas from Carl Goldberg for the construction with the result that the weight is more than 200g. less, so that at least should be an improvement and the mechanism for the vee tail works well. (at least, on the ground).

Now the only bit left is where to position the towhook . . .

cheers John K.
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