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Old May 29, 2004, 07:21 PM
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Lynxman's Avatar
Troms, Norway
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The blade doesn't stall. The blade lag just shifts the rotordisc and balance of the rotor.
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Old May 29, 2004, 08:09 PM
Blade Springs "Rock"
Central Coast, CA
Joined Apr 2004
2,141 Posts
Helicopter Rotor Blades that fall behind, as in RC Heli's, and do not realighn due to centrifical force are considered to be in retreating blade stall. A real Helicopter Rotor Blade that experience's this condition, always ends up with death. The terms cannot be crossed over from a free blade system to a real rotor system, only the systoms that occur. Once a RC Rotor starts to lag, it is over. Vibration and Wobble...crash. A Real Helicopter equipped with fully articuated rotor system, can lead and lag as a design, but to a degree that could never cause the blades to stall. In stalling, a rotor blade only has to not produce the same lift as the other. Hover, Forward Flight makes no difference. True, that retreating blade stall is described in Real Rotor Craft as a condition of the retreating blade not producing lift.....the terms, and conditions outlined here remain the same.
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Old May 29, 2004, 10:07 PM
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United States, TX, North Richland Hills
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So a retreating blade lags, then becomes an advancing blade and still lags. Of course the other blade not is now retreating and should lag also. How come it does not also lag and the system go back into equilibrium?

If your hypothesis were true, then tightening the blades would eliminate the problem because it would prevent either blade from lagging. It might cause other problems, but the wobble would change.

The other thing that makes me think we don't have the cause right is I can move my blades off line and within a few low RPM revolutions they are lined up just fine. If they generate enough centrifigal force to do that at low RPM, then certainly they can restore their position at high RPM. If I find my scale so I can weigh my blades I'll calculate the restoring force just for grins but since it's porportional to the square of the rotational speed, there's probably a lot of force keeping the blade stretched straight out.

It doesn't change the fact that your fix apparently works, but one of the things I find fascinating about helis is their dynamic complexity. It's a lot of fun to debate, test, hypothesize etc.
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Old May 29, 2004, 10:12 PM
Blade Springs "Rock"
Central Coast, CA
Joined Apr 2004
2,141 Posts
This I knew was coming....so I took another picture to show just how much advanced the spring keeps the blades, the tension then never allows the blades to lag, this setup somehow keeps the blades in perfect alignment. I cannot explain it any further, and it is fun, exploring all the different ideas of retreating blade stall, and how it effects our flight in micro helicopters. Thanks.
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Old May 30, 2004, 12:08 AM
Blade Springs "Rock"
Central Coast, CA
Joined Apr 2004
2,141 Posts
I've read the threads....I've done my homework, and feel I had achieve the ultimate setup with the like90 blade package until just now....

Basically it's a stock Aerohawk, with the Like90 combo package, and a GWS Tail Rotor Blade...It has pretty good power, but in my search for more power, I ordered the best of the best from helihobby, which turned out to be a bunk, every one of the upgrades they advertise, for the Aerohawk. So Reading more and more about gear ratio's, and the ultimate Aerohawk, with it's 96 tooth main gear, I decided tonight to add the 11 tooth pinion gear to it......figuring that at about a buck a piece the Johnson 250 can be changed out quite frequently, and give me the ultimate performance. So I installed the 11T pininon gear, and found a new level of performance. If that Johnson burns up, I think I can get two for a buck at allelectronics and I can get several flights like this, who cares....It's hard to beat the performance.
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Old May 30, 2004, 05:38 AM
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Old May 30, 2004, 05:46 AM
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Troms, Norway
Joined Jan 2004
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Quote:
Being a heli mechanic your self, Lynxman, you know the weight of your word.
What's that supposed to mean?

The headstiffener won't increase the load on the head, it will reduce the load on the head greatly. The added weight of the spring is small, and the spring is close to the center of the rotor disc so the force it applies to the rotor head is insignificant if you ask me.

However, should the head fail for any reason the spring might shoot off, as will probably the blades. But the spring is not going to cause a head-failure in my opinion.
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Old May 30, 2004, 07:41 AM
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Old May 30, 2004, 08:00 AM
Helicopter Aerodynamicist
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Cranfield, UK
Joined May 2004
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Bubblehead's quote is completely correct.

The terms retreating and advancing blade only have meaning in forward flight where the terms are used to distinguish between blades moving forward relative to the forward flight velocity (advancing blade) or moving backwards relative to the forward flight velocity (retreating blade).

The terms are meaningless in the context of a rotor system in hover or axial (ascending/descending) flight.

In hovering flight the blades should not show any tendency to oscillate in their lead/lag degree of freedom although they may exhibit a steady lead/lag angle (depending on the balance of aerodynamic and inertial forces and moments). There may be an adjustment as you vary the throttle (as you have varied the aerodynamic and inertial loading) on variable speed rotors, but it should not oscillate.

Lead/Lag oscillations only happen in forward flight where they occur due to the radial shift of centre of gravity caused by blade flapping. You can experience this force yourself next time you stand on a merry-go-round. Try walking from the centre to the outside (or vice-versa) you will experience a force in the circumfrential direction, this is the force responsible for the lead-lag tendency of blades.

Scott
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Old May 30, 2004, 10:39 AM
Blade Springs "Rock"
Central Coast, CA
Joined Apr 2004
2,141 Posts
Summing it all up......

For me, the difference was tremendous, it went from a ten second hover, then wobble forcing me to land, to being able to hover a entire battery pack! In my findings and flight tests that I've done, this has fixed the wobble for me. I believe it to be 1000% safe, cheap and easy to make, and for my Aerohawk/Like90 package the answer to my helicopter's hover wobble/crash land problems. For the Record: The Like90 blades are lighter, 7.4 grams. Stock plastic blades, 9.2 grams. The stiffener plate is composite. The weight of the spring....unknown. But who cares, it has fixed my wobble.

The Pro's:
1. Eliminated the Wobble.
2. Increased Rotor Smoothness, no vibrations!
3. 100% Increase in Control, almost stick off hover's!
4. Increased hover flight time to the entire battery, easily 8 to 10 minutes.
5. Cheap, at $1.69 for five pieces 3 feet long!
6. Easy to make, took me about a half hour to make the first ones, now in about five minutes!
7. Crash Dampening Springs. They absorb the blade impact, and quite possibly saving the blades from breaking, now that is safer. And the cost's of replacement blades, composites, wooden, or stock, is one of the biggest expenses on these micro helicopters.

Con's:
1. It might fly off in flight. (In my opinion: impossible.)

So we've all learned more about rotor dynamics, and Retreating blade stall, and how it pretains to our RC Helicopters. Rotor Blade Dynamics are very interesting, and complacted as well, the forces acting upon our RC Rotor Blades, the the entire spectrum of how to eliminate the dreaded flight wobble.

This worked for me, My Aerohawk/Like90 package. I hope this helped, and many thanks to the forums, and the members who contirbuted to this thread.

Post your helicopter/blade package findings if this fix's your wobble.....
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Old May 30, 2004, 11:12 AM
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SShaw's Avatar
Cranfield, UK
Joined May 2004
19 Posts
Although Geoffrey seems to want to wind up the thread I would just like to make a few corrections so that we are all clear on the temrinology / fluid mechanics. Geoffreys device seems to work - just not for the reasons he has put forth.

Helicopter Rotor Blades that fall behind, as in RC Heli's, and do not realighn due to centrifical force are considered to be in retreating blade stall.

This isn't true. Retreating blade stall has a very clear definition which has absolutely nothing to do with whether a blade leads or lags. Bubbleheads definition earlier in the thread is a useful working definition for the purposes of the present discussion.

A real Helicopter Rotor Blade that experience's this condition, always ends up with death.

Not true. Retreating blade stall limits the forward flight speed of the helicopter. In a full size helicopter it is experienced as severe vibration / large control loads. It is unusual for the stall to be so severe that the pilot loses control (although this can happen).

The terms cannot be crossed over from a free blade system to a real rotor system, only the systoms that occur. Once a RC Rotor starts to lag, it is over. Vibration and Wobble...crash. A Real Helicopter equipped with fully articuated rotor system, can lead and lag as a design, but to a degree that could never cause the blades to stall.

Again the lead-lag dynamcs have neglible effect on stall (of any kind) on a rotor blade. Larger helicopters (multi-bladed) generally have lead-lag hinge. This hinge is used to relieve the hub from the large forces that may be generated by lead-lag leading to reduced hub-weight. A number of small (full-size) helicopters have no lead-lag degree of freedom, the forces are sufficiently small that the weight cost can off-set by the simpler mechanics (maintenance benefit).

In stalling, a rotor blade only has to not produce the same lift as the other.

Not sure what you mean here, but this is an incorrect definition of stall. In actual fact we use cyclic control as a deliberate means of reducing/increasing lift on blades at particular azimuth angles (position in the rotor disc) as our primar means of generating control forces.

Hover, Forward Flight makes no difference. True, that retreating blade stall is described in Real Rotor Craft as a condition of the retreating blade not producing lift.....the terms, and conditions outlined here remain the same.

This statement is just not true! For the reasons I and others have stated in the thread.

All of this should not detract from the fact Geoffreys device appears to work. But its important that bad / poorly understood science and engineering should not be used to provide an explanation no matter how excited we may get.

Scott
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Old May 30, 2004, 12:03 PM
Blade Springs "Rock"
Central Coast, CA
Joined Apr 2004
2,141 Posts
Thanks for your clarifications. Exactly what term then would you call this condition this wobble, and pitch up loss of control? I know that the wobble was due to the blade lagging, the spring has proven this. If the blade lags, it is then out of balance causing vibrations and a wobbling effect. Tightening the blade bolts helps some RC Rotor systems, Rubber bands for other's, and of course springs. They all help keep the blades from lagging. I used the term retreating blade stall. I was incorrect in it's use. Blade lag, or the cause of it, would have been more clear. Despite my poor science, it's discovery and use are working, sorry for any confusion I may have caused.

Tell that to the Air crew who lost there lifes due to retreating blade stall. Personal friends of mine, and the investagation results and eye witness reports of the accident determined it was retreating blade stall. The UH-1 fell out of the sky from 10,000 feet in a ball of flames.

Your technical expertise on Rotor Dynamics is much appreaciated and welcomed here. Although simular, there are major differences in the way a collective pitch rotor system, and a fixed pitch rotor system work. I was only trying to find terms that are simular to collective pitch to a fixed pitch rotor system. The fact remains: RC Fixed Pitch blades start lagging, causing vibrations and the wobble. How this relate's to collective pitch real world rotor dynamics, and the terms of it's workings still is of some question....
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Old May 30, 2004, 12:38 PM
Helicopter Aerodynamicist
SShaw's Avatar
Cranfield, UK
Joined May 2004
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Not sure how I would describe it Geoffrey, but I guess its probably just a blade balance problem related to too much lead/lag freedom (as Lynxman suggested much earlier in the thread).

I'm not disputing that the source of the control problem is the leading / lagging of the blades (at least in a steady sense) as this will (*must*) happen to any system having a lead-lag degree of freedom.

All of the methods you describe essentially do the same thing - they limit the degree to which leading/lagging can happen. In the case of your device I'm guessing from the way you have arranged the spring that it actually acts as a blade stop (full size helicopters sometimes have blade stops on the flapping hinge to prevent mast strikes) which limits how far the blades can lead or lag.

One of the problems I have with the instructions for my Hummingbird is the rather loose (forgive the pun ) way in which they describe how tight the blades should be. This factor appears to have an enormous influence on vibration. My (limited) experience so far with the HB is that the blades should be as tight as possible (ie you should try to eliminate the lead/lag degree of freedom completely) although this has implications if you hit anything while flying.

Re: your final paragraph. You will notice that I very carefully used the word unusual. My main objection was your use of the statement 'always leads to death' which is far from the truth.

Scott
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Old May 30, 2004, 12:55 PM
Blade Springs "Rock"
Central Coast, CA
Joined Apr 2004
2,141 Posts
The spring device I fashioned to the blades, I don't know if it could be described as a blade stop. I believe what is happening now is the blades start slightly forward automatically as it just rests on the spring. As the rotor spins up, centrifical force is now acting on the blade/spring, the tension of the spring, as a wild guess on my part, must be keeping the blades in perfect alignment and balance. Sudden power changes on the blades seem to have no effect on the blade balance, the spring tension increases, thus the blades are realigning themselves. I liked the idea from the beginning of flying with the blades loose, the springs if fashioned simular, should work on all fixed pitch micro rotor systems, experiencing the dreaded wobble.

You are correct, Retreating Blade Stall doesn't always end up in death. But it is a severe flight condition of the rotor, and should be avoided at all times.

Note: I purposely flew my Aerohawk into the couch, although not a solid object, the base of the arms are pretty stout, and nothing. No damage, the blades just stopped. Those Like90's are tough. The springs did dampen the impact, then I inspected the rotor, I did misalign the flybar paddles on impact, I readjusted them, set it down and it hovered perfectly again! Crash testing, although the springs provide so much more control, it's actually hard to crash now, is done. In my opinion: Flawless Performance, and Flight Safe.

I too was a helicopter mechanic for 12 years. I was the Functional Check Flight Engineer. Rotor smoothing and balancing was my speciality, and went to many schools to learn rotor balancing techniques.
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Old May 30, 2004, 01:03 PM
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Troms, Norway
Joined Jan 2004
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Our Lynx helicopters have a drag dampers on each hub arm to dampen the lead/lag action of the blades. You could call your solution a drag damper even though strictly speaking it's not a damper, but a spring.

The Lynx damper is an oil damper and actually just dampens the flexing of the hub arms which are rather long. The blades is mounted with two blade bolts so any lead/lag (hunting) movement happens as flexing of the head arms (sleeves/dogbones). The Lynx head is very rigid, the only hinge point is the feathering hinge (blade pitch) on each hub arm.
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