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Author Topic: Help with indoor duration helicopter  (Read 10774 times)
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PiperCub49
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« on: November 19, 2010, 10:39:21 PM »

Hello all,

I have dabbled a little in indoor micro before, but now I want to take it up a notch--or should I say down a notch? I built Walt Mooney's DH-6 for rubber band power and later converted it to 3ch R/C. It was a great success. I've posted a few pictures purely to show what my skill level is (...be that good or bad. My intentions are not to show off here...I have a lot to learn!). I think that there could be some similarities made between peanut builds and indoor duration builds such as glues, laminating, wood density, and specialty tools. Maybe the pictures will help generate some relations to the challenge at hand.

Alright. So I am a young modeler who found Science Olympiad freshman year thanks to an engineering mindset and a modeling background. (I think that you guys are well-aware of what Science Olympiad is.) I'm now in my junior year. This year, the challenge in the "flight department" is to build a helicopter that is rubber powered for an indoor duration competition. The rules are basically as follows:

1) Max. of three rotors
2) No limit on rotor blades
3) Max. rotor span of 40cm
4) Min. weight of 4.0g sans rubber
5) Max. rubber weight of 2.0g

My plan, as of now, is to roll a MS. There will be one fixed blade at the bottom of the MS, while the top rotor will be powered. Both "props" will be two-bladed. My problem is that I don't know where to start with props. Should I build a proven prop design and then get rubber to suite it? Or maybe I should build a prop to suite the rubber...or could it possibly be a hybrid of the two? I can't make up my mind as to if I want flat or airfoiled blades even (for lack of an actual term). The ideal dimensions of the blades will take some extensive testing. After all, that is really the whole point of the event. But again, it would be nice to have a starting point.

Any help would be great, seriously.

Thank you,
Kody Smiley
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« Last Edit: November 20, 2010, 07:48:36 PM by Ratz » Logged
Greg Langelius
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« Reply #1 on: November 20, 2010, 08:00:46 AM »

Just offhand, read up on helical pitch. The fixed blade(s) can just as easily be mounted at the top of the tube, and might be better for stability. Blades can be made as stretched ribbons, anchored at the axis in a nearly vertical plane, and to a hoop at the tip, in a nearly horizontal plane. This can allow blade arrays that might even resemble a turbofan. Imagine a pair of contra-rotating turbofans. The motor tube might be several/two concentric paper tubes of different diameters, to provide the most strength for the least weight. Glue at a minimum to conserve weight, paint not at all. Start with the maximum rubber allowed, Lube the rubber (I use Armorall), and build successively larger models until you find the one that appears underpowered. The one with the best duration (of course) will be your final design. Unless you want to get beaten by your own design, use some discretion when you're testing designs.

Greg
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« Reply #2 on: November 20, 2010, 03:30:57 PM »

Greg,

Thank you for the help. I never really considered putting the fixed rotor on top. It sounds like you are saying that one rotor would be powered and one would rotate freely, each attached to separate motor sticks on a concentric axis (one motor stick inside the other). Let's say that there is only one motor stick. Assuming that the unpowered rotor is attached to the motor stick, would it not rotate opposite the direction of the powered rotor? Or maybe I'm looking at this wrong...

I did do some research on helical pitch. Surprising, I couldn't find that much info. I did learn that when I talked about "flat or airfoiled" blades in my first post, what I was actually trying to get at was "progressive and helical/constant" pitch. I also learned that constant pitch blades are "extremely efficient... but only at one rpm range", as one website put it. I would like to try a constant pitch prop first and then go to progressive pitch if necessary for reasons of simplicity. I don't know what the change in RPM will be yet, so it's hard to tell what kind of pitch will be ideal.

I still can't make up my mind as to if I want to start with an F1D (or similar) prop or go with a design of my own. I suppose that it will be the latter if I want a constant pitch.

-Kody
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Greg Langelius
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« Reply #3 on: November 22, 2010, 08:24:46 AM »

By fixed, I mean attached to the motor stick, so the motor stick and fixed rotor rotate as one. The concentric tubes are joined as part of a single motor stick.

Greg
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« Reply #4 on: November 22, 2010, 09:20:33 AM »

Kody,
I would suggest that you make the blades adjustable in pitch by having them plug into rolled paper tubes on the central propeller hub to receive the round ends of the blade spars.

The diameter is fixed by the rules leaving you to experiment with blade area, and constant or helical pitch.

I would suggest starting with say the fixed pitch prop find a motor length/size that sustains flight without excessive climbing and then try say a helical pitch(same pitch setting) prop on the same motor and record your results. Once you have found what looks like the best prop combination try reducing the motor size and increasing the pitch to get a longer run.

You will most likely need different pitch settings on the lower blade(more most likely).

Hope this gives you some ideas.

I also agree with Greg re having the fixed blade towards the top of your motor stick as this should help to place the Cg below the blades, although you may need to add weight on the bottom of the stick to achieve this.

John
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Hepcat
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« Reply #5 on: November 22, 2010, 04:58:32 PM »

Hello Kody
I must start by saying that your DH6 is a little gem and demonstrates talent that will have no problem knocking out a helicopter.

I have no experience of helicopters except for building the infamous Ron Warring design in what I think would be the late 1940s. There was a fuselage made to look like a full size helicopter (if you had a good imagination) that did nothing. There was a round motor tube about an inch diameter that was fixed on a pivot in the bottom of the fuselage. The motor tube rattled round in a square hole in the top of the fuselage and protruded a few inches above of the fuselage. The protruding part of the motor tube carried a pair of rotor blades and a nose block on the top of the tube had a normal noseblock carrying another rotor. Having established that I have no real experience with helicopters I can only offer some suggestions based on a general interest in flying objects.

First of all, forget about treating the rotors as propellers and think of them as wings. A propeller is designed to combine rotational speed with a high forward speed and variation of blade angle along the blade is essential to suit the direction of the apparent wind. An indoor helicopter should spend most of its time hovering or climbing slowly so the forward (upward) speed will be very small compared to the rotational speed and so blade angle variation is not required. (The 25% or so of blade near the hub, where there might be a case for angular change is almost always omitted anyway.) I think the blades should have a thin cambered section set at about 6 degrees angle of attack.

I think the rules determine a lot of the design. The rotors will be made to the maximum of 400mm, and I guess the chord will be about 40mm. The maximum 2g of rubber will be used, most likely as two strands of 1/8 and this will need a motor stick about 350mm long. It is probably better to mount the fixed rotor near the bottom of the stick to reduce interference between the two rotors and a lot of flyers favour some dihedral on the bottom rotor. Cut some representative pieces of wood early on to see how easy the 4g limit can be met and don’t forget that a 2g motor well wound will require a substantial motor stick.

I think the development work will probably be in determining how many rotor blades the 2g motor will drive. Start with two top and bottom for simplicity and then try three or four if you can stay within the 4g weight limit.

Good luck with the project
John
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« Reply #6 on: November 22, 2010, 07:51:58 PM »

Greg,

I see. So we are on the same page... Smiley


OZPAF (John),

I thought about the idea of using paper tubes to adjust pitch when I saw some F1D prop setups. They would really only be for changing the pitch during testing though because the blades must keep the same pitch during flight. In other words, no variable pitch rotors are allowed by the rules. Being able to change the pitch that quickly and easily could be a real asset to my brother and I.


Hepcat (John),

First off, thank you for the compliments on my DH-6. It's the prize of my fleet right now. Smiley Secondly, I don't think any of us have had much experience with helicopters, and that's why this is so tough.

I really like your thoughts on avoiding the phrase or idea of "prop" for the rotors. Taking the words literally, we are creating rotating wings, not props.

I'm sorry for having to ask this, but could you clarify what you mean by having a ~6* cambered section? Does this describe the airfoil of the rotors? And my other question is whether or not you (or anyone else, for that matter) would have any idea as to what thickness wood would be a suitable for the suggested two strands of 1/8" rubber. (It looks like the wood will be 5-6# B-grain.)

So here is the plan: I will start with a 350mm long rolled tube. The top and bottom rotors will both be 400mm. The bottom rotor will have a small amount of upward dihedral. I will start with a powered top rotor, but will eventually test a powered bottom rotor. I am not sure if it will be more efficient to reduce resistance by keeping the first 25%-33% of the blades as prop spar or save weight by making them built up, but that is the least of my worries right now.

I think that I have a good start here. The next step is to build something and see how it flies, frankly. Then I will go from there.

Thanks again everyone,
Kody
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« Reply #7 on: November 22, 2010, 10:17:23 PM »

Kody
Go here:
http://groups.yahoo.com/group/Indoor_Construction/photos/album/1388916912/pic/2005929829/view?picmode=&mode=tn&order=ordinal&start=1&count=20&dir=asc
and here:
http://groups.yahoo.com/group/Indoor_Construction/photos/album/1296994561/pic/1117803381/view?picmode=&mode=tn&order=ordinal&start=1&count=20&dir=asc

You'll need to join the group but you should do that anyway.
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PiperCub49
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« Reply #8 on: November 22, 2010, 11:25:17 PM »

Bill,

I've heard a lot of good things about Yahoo! Groups and the modeling community and you're right; it's about time I joined. I am a member of RCGroups, SFA, and HPA. Why not another? Wink

I was able to join the first group, but not the second. Huh

Also, do you know who the moderator is?

Thanks,
Kody
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« Reply #9 on: November 23, 2010, 01:50:04 AM »

Both links are from yahoo Indoor Construction. If you joined that group then you should be able to access both pictures. Look in the SO album for the second one and roy glider for the first one.

BTW - I'm in almost the same boat as you. I'm supposed to coach helicopter and have never built one.
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« Reply #10 on: November 23, 2010, 07:17:26 AM »

Most of my helicopter blades have been for model rocket recovery--if you do everything right you actually thermal away a rocket spinning on helicopter blades--the blades will generate lift. My best flight was 141 seconds. I've built the Penni Helicopter sold by A2Z Peck Polymers--works OK, but not much duration, so I started another one, but it is in the huge list of unfinished projects...

I've cambered helicopter blades by wrapping them against an aluminum or PVC tube with an ACE bandage. I've also wet tissued the blades first with Japanese tissue and Elmer's school glue gel, and them wrapped them around the tube. It is pretty easy to just tissue the inside of the blades, though I do have some examples of tissuing both sides.
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« Reply #11 on: November 23, 2010, 10:35:50 AM »

First of all, forget about treating the rotors as propellers and think of them as wings. A propeller is designed to combine rotational speed with a high forward speed and variation of blade angle along the blade is essential to suit the direction of the apparent wind. An indoor helicopter should spend most of its time hovering or climbing slowly so the forward (upward) speed will be very small compared to the rotational speed and so blade angle variation is not required. (The 25% or so of blade near the hub, where there might be a case for angular change is almost always omitted anyway.) I think the blades should have a thin cambered section set at about 6 degrees angle of attack.

John
You completely lost me on this. I can't see the difference what a prop does and what a rotor does. Does this mean that a prop running on a stationary normal airplane model would have more thrust if it was not helical?
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« Reply #12 on: November 23, 2010, 04:49:07 PM »

Kody
A couple of quick points.

In reply #5 I said:”... a thin cambered section set at about 6 degrees angle of attack.” In your reply #6 it came through on my computer as:”...what did I mean by having a ~6* cambered section?” Can you confirm your question?

I was wondering about solid sheet blades as mentioned by Zack and used on one of the photographs to which you have been referred. [In the following I have worked in mm and given some near inch equivalents in brackets]. I assumed such blades rigged out 50mm (2”) from, the hub leaving a blade length of 150mm (6”) and with 40mm(1.5”) chord and thickness 0.8mm(1/32”). The volume is 150 x 40 x 0.8 = 4800 cu.mm and assuming a minimum of 4 blades = 19200 cu.mm. Balsa weight is usually quoted as lb/cu.ft and multiplying this figure by 0.000016 will give the weight in g/cu.mm. So assuming you have some light stuff, say 6 lb/cu.ft, then the 4 blades will weigh 19200 x 6 x 0.000016 = 1.84g. That is nearly half of the 4g target weight so my advice would be to avoid solid balsa blades and go for conventional indoor construction of LE, TE and ribs with ‘Mylar’ covering.

Oldbill
Yes Bill, a propeller prevented from going forward would certainly give better thrust without any helical twist. It would of course need an angle of attack which would be the same all the way along the blade – assuming the blade section were all the same.

It is getting a bit late tonight but I will try and give a fuller explanation with some sketches tomorrow.

John
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« Reply #13 on: November 23, 2010, 05:58:24 PM »

I'm surprised that after four days and twelve replies I haven't seen the name "Penaud". Greg's reply up front described his machine to a "T", and we all seem to have been reinventing it.

And why shouldn't we? It was good, but after 139 years, and the invention of carbon fiber, epoxy, and Tan2, I bet there's room for lots of improvement.

Art.
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« Reply #14 on: November 23, 2010, 06:54:39 PM »

Hello,

Homework calls, so I will try to keep this short tonight.

First, I just wanted to say that during my small amount of spare time at school, I check up on this thread on my phone and I love all of the responses. Time is a rare commodity these days and it's only making me want to get started more.

Bill,

So I was able to join and figure out how everything works. I like the information there. So many tips and tricks are in the "files" section. They will definitely be of use to me. Also, I see that responses are quick and on-topic. Concise information like that is most useful, in my opinion.

Ah, I was hoping I would find a coach here! We can really learn together as far as the "helicopter thing" goes, but I have some work to do if I want to catch up to you in construction and aerodynamics! Cheesy

Hello Zack,

Thanks for sharing your experience. My brother and I were first in the state in the "Egg-O-Naut" (bottle rocket) event in our freshman year. Last year, we had two complete rocket failures and still received the silver medal. Our best time during testing was 143 seconds (without time bonuses). One egg, pod, and parachute still lies in a 70-80ft. tree! Shocked

John,

Yes, you are correct. I said, "I'm sorry for having to ask this, but could you clarify what you mean by having a ~6* cambered section? Does this describe the airfoil of the rotors?" I think that you could look back and see this, but I figured I'd post it again just in case.

My brain is tired today. I will have to tackle the "prop vs. rotor" discussion a little later. It is clearly one of the most important problems to work out here.

Hello Art,

The first thing that I looked up when I started research on designs and ideas was indeed Penaud's helicopter. More thoughts on that below...


To all,

Last night, my brother and I were debating on the "RPM" of the blades. If one rotor is fixed, the other rotor is powered, and the increased airflow on the bottom rotor is neglected, would the two rotors not spin at the same RPM? In other words, is the top rotor's airflow the only cause of the bottom rotor's increased RPM? Furthermore; if this theory holds water, then it shouldn't matter if the powered rotor is on the top or bottom, so long as the CG remains the same, correct?

Thank you,
Kody
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« Reply #15 on: November 23, 2010, 08:39:59 PM »

A possibly important point on Roy White's copter is that he used a bearing on both ends. I think the advantage of this would be that the motor stick was not necessarily spinning. I know that once the machine was on its own (in the air) there would probably be some equilibrium point where the two rotors would be going in opposite directions at close to the same speed and the motor stick would be rotating at some intermediate speed.

The fact that it did 2 minutes with 1g of rubber is pretty impressive.
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« Reply #16 on: November 23, 2010, 09:03:53 PM »

Is there a significant advantage to making the blades wide? I've used 1/32" thick 1/2" chord blades that were 8 inches long--they survived the launch and recovery just fine. Such blades would have 1/3 the weight of 1-1/2" chord blades.
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« Reply #17 on: November 23, 2010, 09:14:28 PM »

I've always explained the rubber motor as a long thin cylinder of torque, attached to a hook on each end. All it wants to do is untwist, and it doesn't matter to it whether the untwisting force acts on the fwd or aft hook, or is split between them. We see it at work on indoor airplanes with insufficient washin on the left wing. And we'll see it on a Penaud type copter, with just a tad more RPM out of the rotor that isn't carrying the motor stick/tube with it. I don't understand why one set of rotors needs to be set at a different pitch from the other unless you need their speed to match. Why would that be necessary?

Art.
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« Reply #18 on: November 24, 2010, 01:43:19 AM »

To all,

Last night, my brother and I were debating on the "RPM" of the blades. If one rotor is fixed, the other rotor is powered, and the increased airflow on the bottom rotor is neglected, would the two rotors not spin at the same RPM? In other words, is the top rotor's airflow the only cause of the bottom rotor's increased RPM? Furthermore; if this theory holds water, then it shouldn't matter if the powered rotor is on the top or bottom, so long as the CG remains the same, correct?

Kody,
The short answer is yes providing the blades are of course identical. If they exert equal resistance to rotation then the equal torque will generate equal RPM. However the pitch on the bottom blades will need to be different to the top, to provide equal resistance, due to the increased inflow velocity and swirl of the air flowing onto the bottom blade.

I am also puzzled by John’s (Hepcat) comment re constant pitch. The velocity changes radically along the blade for a constant RPM and this alone will alter the pitch AOA for a given CL. I imagine using a constant pitch blade will bias the span wise lift distribution towards the outer section of the blade and also increase the torque required.

I would think thinner blades would be better as long as they are strong enough, while the blade width/pitch would need to match the power available.

One other point that may help would be to lightly weigh the tips of all the blades. This should help with stability (gyroscopic reaction) and also reduce the coning angle of the blades under load.

However I must warn that I’m not a helicopter expert.

Penaud must be laughing.

John
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« Reply #19 on: November 24, 2010, 07:45:56 AM »

There is some thought that multiple blade props work better with short chord (narrow) blades, and there is also the lesson from full scale copters which have, indeed, narrow blades.

Blade tip shapes are probably a significant issue where efficiency is concerned. Efficiency and performance are probably closely related. I'd suggest researching Westland helicopters, they have done some advanced work on this subject. Bear in mind that at such small scales, such matters need to be simplified, but then again, you need to give the tips some shape or other and some shapes are probably better than others.

Greg
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« Reply #20 on: November 24, 2010, 08:43:34 AM »

What I was trying to get at in talking about having a prop bearing at both ends instead of having a fixed rotor on one end is that you could theoretically eliminate the rotating offset mass of the motorstick. Actually eliminating it would probably be very difficult but minimizing it could have a very beneficial effect (I think).
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« Reply #21 on: November 24, 2010, 01:24:50 PM »

I am also puzzled by John’s (Hepcat) comment re constant pitch. The velocity changes radically along the blade for a constant RPM and this alone will alter the pitch AOA for a given CL. I imagine using a constant pitch blade will bias the span wise lift distribution towards the outer section of the blade and also increase the torque required.

John is correct... a prop that is optimised for hovering will have the same blade angle all the way along the blade.. This is because for efficiency the AoA of the blade needs to be similar at all points and if there is no forward motion through the air then that is achieved by having blade angle. If the AoA is the same then the Cl will be the same. The actual lift will be greater at the tips because the tips have greater airspeed, but that's true for any prop.

For proof of this take a look at the blades on a full size or RC model helicopter.. the blade angle is more or less constant. I have a RC helicopter blade in front of me right now.. The blade it totally 'flat' no twist at all.

Steve
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« Reply #22 on: November 24, 2010, 02:23:52 PM »

Steve...

You need to define "Pitch" and "Pitch angle", or else I become confused.
A rotor with the same pitch thru the full span (like a helical propeller) will have a twist across the span; if it has the same pitch angle it'll be flat.

Clear me up on this.

Art.
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« Reply #23 on: November 24, 2010, 02:33:26 PM »

Art,
Quite right. I should have said constant 'blade angle' rather than pitch. If i can edit my post i will do so to avoid confusion
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« Reply #24 on: November 24, 2010, 10:11:04 PM »

Art, Steve,
I think I introduced the confusion by referring to pitch AOA, which I feel should be blade angle, as the actual blade AOA to the airflow I believe will not be constant even for a constant geometric blade angle. This would be due to the varying inflow to the blade caused by the variation in lift associated with the variation in speed along the blade.

Steve I agree with your comments re the blade angle on RC helis - I fly them as well, however this constant angle will not produce a Eliptical distribution(max effeciency) along the blade even for hovering flight. While for a heli that is required to have a speed range a twisted blde would not perhaps be suitable as the ideal twist would vary with flight speed/flying speed ratio, however for the hovering only case of this thread,the better effeciency of a twisted blade may be worth the effort.

Anyway I hope this is giving Kody more ideas.

John
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