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Old Nov 17, 2012, 03:06 PM
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Turbine Airwolf with forward EDF thrusters?

Hello,

I'm in the process of building an Airwolf based on a Trex 800 powered by the Wren 44 turbine.

I was thinking, how practical would it be to have two EDFs where the thrusters would be? I'd set the rotor pitch to 0 degrees whilst in forward flight (or on a rolling take off) flip a switch on the transmitter powering up the EDFs and using the cyclic and tail to steer. Would there be any chance of it being able to gain altitude like an airplane?
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Old Nov 17, 2012, 06:07 PM
Love my scale Whirlybirds
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I have seen one picture of a Vario Airwolf with two mini thrust motors installed. If you were to gain enough thrust to act on the heli to increase its flying speed, it would eventually fall out of the sky because of blade procession/recession. Basically, you would fly faster than the surface area of the rotor blades could generate lift and it would only generate it on the side that rotates forward. They will look cool but don't set your hopes up too high. Nice to have the room to work on such a large heli. Take care.

Don
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Old Nov 17, 2012, 06:35 PM
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Don,

Thank you for your reply. Yeah I've heard that can happen, hence why a real airwolf would be impossible. Im not so interested in a much higher speed, more the fact that I could accelerate from a hover in a straight line and pull back the cyclic, pitching the nose up whilst still accelerating forward and climbing like an airplane rather than slowing down and eventually go backwards like a heli. Im just wondering if this would be the result i'd get.

I'd love to get two small turbines instead of the EDFs but with the wren 44, my budget wont stretch that far!

My original idea was to make an exhaust for the wren with 4 tubes, 2 for the top, 2 for the bottom turbos and fitting an afterburner system to the bottom exhausts.. I decided this may be a tad too complicated

Thanks for your help

Jamie
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Old Nov 17, 2012, 07:42 PM
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I'll search for a video I saw - a guy put an edf on a little falcon 40 / walkera 4. When he kicked it in, that little puppy really went to town.
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Old Nov 17, 2012, 07:46 PM
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Thank you, that would be great to see.

Jamie
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Old Nov 17, 2012, 08:25 PM
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It was actually a honey bee FP

Honey Bee V2XT hehehe (5 min 58 sec)
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Old Nov 17, 2012, 08:35 PM
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That's insane! I suppose I should just try it. I'll have Align's APS system on it just incase it gets into trouble!
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Old Nov 17, 2012, 09:15 PM
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Yeah - when it's a disposable FP it's easy to get a little fearless.
I think, if it were me, I'd try to experiment with something cheaper than an 800 with a turbine, but that's just me. I don't have any idea what problems might spring up on an 800 that weren't there on an FP.
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Old Nov 17, 2012, 10:11 PM
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That is the funniest thing I have seen in weeks, ROFLMAO. More power to him, but like karlik said, I would not trust a multi-thousand dollar heli to an experiment. Take care.

Don
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Old Nov 17, 2012, 11:43 PM
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Quote:
Originally Posted by Keyrigger View Post
I have seen one picture of a Vario Airwolf with two mini thrust motors installed. If you were to gain enough thrust to act on the heli to increase its flying speed, it would eventually fall out of the sky because of blade procession/recession. Basically, you would fly faster than the surface area of the rotor blades could generate lift and it would only generate it on the side that rotates forward. They will look cool but don't set your hopes up too high. Nice to have the room to work on such a large heli. Take care.

Don
While this is a real world hazard called "Dissymmetry of lift", I doubt scale RCs are likely to suffer from this. RC Helis are so overpowered underweight that many hazards that full size helicopters have to be cautious of don't apply to RC. The rotor disc load just isn't anything close to full scale helicopters.

I'll spare the calculation, but at 1500 rpm, an 800 size heli as a blade tip speed of over 300 mph. Let's say you have the heli traveling at 100 mph, using the EDF thrusters, with only a small amount of blade pitch (1 degree), the advancing blade side would be traveling at 400 (300+100) mph, and the retreating side at 200 (300-100) mph (relative to the wind). So yes, the advancing side is producing much more lift. BUT the heli still has another 12 degrees or more of pitch it can use to properly compensate for blade speed, and what is even better, a FBL controller will automatically correct for this when it senses the heli want to to roll to the retreating blade side. Actually because of the 90 degree precession inherent in spinning masses (the rotor), the heli will want to roll to the retreating blade side and also pitch up.

As you lower the headspeed, and put on weight (scale fuselage), you will increase the risk of retreating blade stall, but I am completely confident that 100 mph is a safe speed for 800 size scale birds. In fact, without doing the calculations, I would even estimate that 150 mph would be just fine.
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Old Nov 18, 2012, 12:26 AM
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Most large helicopters, when in pod and boom form, will easily exceed 100 mph in full forward flight. The thrust is not known and if enough, would still create a speed that could destabilize the heli. A flybarless controller may be able to compensate for the dissymmetry of lift, but at some point, you will not have the control you need for basic flight, as all pitch/aileron movement will be taken up by the FBL controller. Even a full size heli has a "Do Not Exceed" speed so that control will not be lost. As I said, I would hate to see someone put thousands of dollars into a model heli only to loose it on an untried experiment. Seeing those thrusters installed is one thing, finding successful flights using them is another. Someone may have done it, but so far, it has not shown up in the ususal places.

Don

PS. High performance F3C style helis (and others) are already well over 150 mph so the speed would need to be much higher to loose lift. I am sure that if the stock Airwolf would reach 90 mph, a boost to 150 would not be destablizing but certainly would be shocking to the pilot the first time he/she tried it, lol.
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Old Nov 18, 2012, 01:24 AM
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Quote:
Originally Posted by Keyrigger View Post
Most large helicopters, when in pod and boom form, will easily exceed 100 mph in full forward flight. The thrust is not known and if enough, would still create a speed that could destabilize the heli. A flybarless controller may be able to compensate for the dissymmetry of lift, but at some point, you will not have the control you need for basic flight, as all pitch/aileron movement will be taken up by the FBL controller. Even a full size heli has a "Do Not Exceed" speed so that control will not be lost. As I said, I would hate to see someone put thousands of dollars into a model heli only to loose it on an untried experiment. Seeing those thrusters installed is one thing, finding successful flights using them is another. Someone may have done it, but so far, it has not shown up in the ususal places.

Don

PS. High performance F3C style helis (and others) are already well over 150 mph so the speed would need to be much higher to loose lift. I am sure that if the stock Airwolf would reach 90 mph, a boost to 150 would not be destablizing but certainly would be shocking to the pilot the first time he/she tried it, lol.
Agreed, anything zippy around the flying field at 100 - 150 mph is hard to keep in your eyesight for very long. Plus, helicopters are a little harder to keep orientation of at distance than a fixed wing.

I do think the idea of a scale Airwolf would be pretty cool to see realized. Even if the bird was just cruising around at 30 mph, having its forward thrust come only from EDFs would be a pretty cool thing.
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Old Nov 18, 2012, 03:52 AM
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Quote:
Originally Posted by Libtekguy View Post
While this is a real world hazard called "Dissymmetry of lift", I doubt scale RCs are likely to suffer from this. RC Helis are so overpowered underweight that many hazards that full size helicopters have to be cautious of don't apply to RC. The rotor disc load just isn't anything close to full scale helicopters.

I'll spare the calculation, but at 1500 rpm, an 800 size heli as a blade tip speed of over 300 mph. Let's say you have the heli traveling at 100 mph, using the EDF thrusters, with only a small amount of blade pitch (1 degree), the advancing blade side would be traveling at 400 (300+100) mph, and the retreating side at 200 (300-100) mph (relative to the wind). So yes, the advancing side is producing much more lift. BUT the heli still has another 12 degrees or more of pitch it can use to properly compensate for blade speed, and what is even better, a FBL controller will automatically correct for this when it senses the heli want to to roll to the retreating blade side. Actually because of the 90 degree precession inherent in spinning masses (the rotor), the heli will want to roll to the retreating blade side and also pitch up.

As you lower the headspeed, and put on weight (scale fuselage), you will increase the risk of retreating blade stall, but I am completely confident that 100 mph is a safe speed for 800 size scale birds. In fact, without doing the calculations, I would even estimate that 150 mph would be just fine.
I'd say (no offense intended) that you are void of hands-on experience with unstabilized rigids of that size, otherwise you would not be making above statements.

If you were to use 1500 RPM headspeed on an 800 size, I'd say you are slightly out of sync.... Not that it is impossible, but 1000 RPM is more like it.
You are never going to get it up to 150 mph, because at the tip of the advancing blade you are getting close enough to the speed of sound, to start some serious problems, let alone the power required to drive the rotor at such speeds will seriousely increase due to that fact.

Any tilting tendency caused by advancing/retreating blades, is purely in the direction nose up, there is NO longitudinal roll tendency due to this.
In fact, the roll tendency that actually exists, is invariably towards the advancing blade. I have observed this from 450 size to 800 size to be true, regardless of number of blades or headspeed. This fact is caused by the air above the rotordisc being accelerated downward as the helicopter passes underneath it. Because of this, the air at the front of the disc has zero downward velocity, but the air at the back of the disc, has a noticeable downward velocity. This decreases the AoA of the blades in the rear half of the disc, thus reducing lift in the rear half, and due to the 90 degrees shift in precession, the advancing blades will sink, causing a roll towards that side. Since I was at first also convinced it would roll towards the retreating blade, I was not only very surprised it rolled exactly opposite, it also took me a long time to accept that and figure out why.

I do fly unstabilized 800 size multiblades, and I would say, that depending on blade profile and take off weight, at around 75 to 100 mph, roll and "nose-up" tendency become that strong, that there will be not enough steering deflection left in the stick (you cannot really go over 5 degree cyclic to each side, or you can provoke a high speed blade stall) to be able to control the helicopter in a safe manner.

An FBL system will not get you any further, because it corrects for you, but does not alter the aerodynamic properties of the rotor. In fact, it completely robs you of any warning of the oncoming bladestall, and worse, because of its autocorrecting function, once it is in a blad stall, the corrective action will make sure, it will STAY in that condition, so it robs you of the possibility to recover....

So despite nice theories and calculations, in reality, with a large scaler of 800 size you can consider yourself lucky if you manage 100 mph without problems, actually regardless or headspeed.

And that is based on actually flying such helicopters unstabilized, knowing first-hand what rotorsystems of such size will do in the real world.

The fact that a 3D heli will easily do 150 mph, is not a real good guide: the disc load and power to weight ratio are so much better compared to a scaler, that all direct comparisions for safe speeds are off from the start.

Brgds, Bert
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Old Nov 18, 2012, 01:04 PM
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You are correct, I have practically no experience with very large, unstabilized RC helicopters. My experience comes from 3D helis and small scale (450) sized projects.

I will concede the pure pitching up tendency, I've "learned" this before (read in book and manuals), it just always seems to me that the helo would want to roll as well, but I do concede to your experience in this matter.

With multi-blade setup, obviously RPM will decrease. I had a 5 blade head on a 450 scale project that was unstabilized, so I am just a little familiar with their operation. I don't remember what RPM I ran it at, but it was far, far less than a typical 2 blade 3D RPM speed. I could fly it just fine, but the particular MD500 fuselage that I wanted has been sold out practically all year so I abandoned that project and I'm now doing a 450 size Bell 212 build (flybared).

I guess the only thing I can't agree with your post is the blade tips approaching the speed of sound. I fail to see how 1500 rpm with a 70" rotor diameter can produce tip speeds close to the speed of sound.

(rotor radius [in])*(2)*(pi)*(RPM)*(1[ft]/12[in])*(60[min]/1[hr])*(1[mi]/5280[ft])= 35*2*pi*1500/12*60/5280=312mph... not even half the speed of sound. Even with the RPMs a 3D Trex 800 will produce, the tip speed is still under 400mph.

How close to the speed of sound do the rotor tips have to be going to to cause trouble? (I'm not trying to sound snarky, I'm genuinely asking to being schooled on this)
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Old Nov 18, 2012, 02:30 PM
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I'm not really trying to gain extra speed with this idea, more just acceleration from a hover or rolling takeoff. I also just want to see whether putting the heli into a nose up attitude with EDFs running and 0 degrees (or small amounts) of pitch on the rotor would allow the heli to continue accelerating forward and gaining altitude rather than slowing down back to a hover.

I have a couple of Esky Belt CP V2s that I can try the idea on I guess. I may mount a second reciever to allow a second 'pilot' to control the power of the EDFs and auxillery things such as weapon pods of the airwolf.
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