Homebuilt Coaxial Helicopter (RELOADED)

Rotor Rooter

Dave Jackson
To my understanding, Birdy's questions in post #149 are ;
1/ "The system of yaw control in a differential collective coaxial machine”, and
2/ “yaw control is reversed dureing an auto”​

The subject is a difficult one for even for the experts. Referring to the note in post #155.
~ Kamov is, arguably, the most knowledgeable company on coaxial helicopters.
~ Gareth Padfield is the head of Helicopter Aerodynamics Section at Defense Research Agency in England. His book Helicopter Flight Dynamics: The theory and Application of Flying Qualities and Simulation Modeling is over 500 pages and extremely complex.​

This sketch shows the Yaw control for different configurations.
For ‘conventional’ coaxial and intermeshing helicopters it is by differential torque between rotors. These ‘conventional’ helicopters cannot vary the rotation speed between their 2 rotors because they must maintain constant blade crossing azimuths; both coaxial and intermeshing. But, the blades on these helicopters can change their pitch; by swashplate, etc.

Rotors that do not have the ability to change the pitch of their blades must; vary their RPMs differentially, or put drag tabs on the blades, or place a rudder in the rotors downwash, or have separate small propellers etc.

Chuck mentions in his latest post that “Bensen's yaw control was via a small engine/propeller blowing on a conventional rudder.” In other words, the yaw control on Bensen’s coaxial helicopter has nothing to do with the rotor system. This is basically the same as the two small propellers in Pegg’s PAM-110B, which is linked to in post #157.

It is interesting (for me at least) to see that Pegg's newer 1/5-scale helicopter does not appear to have these small propellers.

For the ‘conventional’ coaxial and intermeshing helicopters, autorotation, is a meaningful concern since they do not use the so-called ‘band-aid’ type of solutions.

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Rotor Rooter

Dave Jackson
Birdy, I think that this will answer your specific question.

Coaxial and Intermeshing helicopter use differential collective (differential torque) for yaw control.
The overrunning clutch is located between the engine and the gearbox.
The gearbox consumes 4% of the engine’s power.
The gearbox is hard coupled to the rotors.

During powered flight;
~ 96% of the power gets to the rotors and the yaw authority is strong.
~ Higher pitch on the CCW rotor will rotated the fuselage CW.

During autorotative flight:
~ Higher pitch (drag) of the CCW rotor will react back into the gearbox.
~ It will want to yaw the fuselage with it due to the friction of the gearbox (gears, bearings, seals and oil etc.)
~ Higher pitch on the CCW rotor will rotated the fuselage CCW, BUT it will be very weak.


C. Beaty

Gold Supporter
Torque differential between rotors yaws the airframe whether the rotor is driven by the engine or by the airstream.

In autorotation with engine disengaged, the rotor with greater pitch has greater drag than the rotor with lower collective, the torque difference yawing the airframe, exactly the same as engine drive except torques are in reverse direction.

Bear in mind, the rotors are geared together and can’t act independently as would be the case if each had its own freewheel clutch.
Or like Bensen’s torqueless drive, a differential gear between rotors.

C. Beaty

Gold Supporter
The Gyrodyne QH-50 had yaw control via rotorblade tip brakes, small vanes at blade tips that would pop out and increase the drag of its rotor.

Yaw control was identical whether the rotor was driven or autorotating. There was no reversal.


This article doesn’t mention tip brakes, a Google search might be in order.

Rotor Rooter

Dave Jackson

The Gyrodyne patent describes 3 methods of tip-brake yaw control; radially moving flap, radially extending probe and trialing edge clamshell.

The tip-brake (drag) is their way of getting around the inability to use differential collective for yaw during autorotation.
(Reference paragraph #2 in the link on post #167 above.)

The tip-brake is very complex but like you I assume (not having seen an exploded gearbox) that it eliminates the need for left and right pedal reversal during autorotation.



Gearbox drag has nothing to do with it. It is torque differential between rotors.
Back from my boreing drive, and cept for the rum boiln off this morn'n, its sunk in.
I savvy why the reversed peddle response now. :)

Rotor Rooter

Dave Jackson

You asked a very interesting question in your post #149.

I have participated in perhaps a hundred postings, plus research, related to yaw control in auto-rotation on twin-rotor helicopters.

First the easier of the two. :)

Flettner used only the rudder for yaw control during autorotation.
Kaman used pedal reversal in autorotation.
DeGraw used pedal reversal in autorotation.
This is a VERY discriptive presentation of the K-MAX yaw control

Autorotation, Kamov, Yaw & moveable tail-planes
Aerodynamics ~ Yaw, Autorotation & the Coaxial configuration
Gearbox drag has nothing to do with it.
You may be correct about my speculation on the gearbox drag. - It may depend on the design of the gearing in the box(es)

It is torque differential between rotors.
It is definitely not torque differential between rotors.

I savvy why the reversed peddle response now.
Your thinking would be of interest. Sincerely.​

Apparently, pedal reversal on the coaxial is of little to no value in autorotation.

My speculation:
Perhaps when the collective is fully down (i.e. autorotation) the application of pedal will increases the angle of attack on one rotor, BUT it may MAY decrease the angle of attack on the other rotor down into Windmill Brake State. :noidea:

No one, to my knowledge, has given a definitive answer to your question, That is why it is such an interesting one.



A logging pilot up north had to auto in an old husky, He added a little yaw & found it acted in reverse, spun in & rolled down a hill. Walked away, He said the damn peddles worked backwards WTF? Rudders don't do well unless you have forward airspeed.


Wots a Husky??

Dave, ina coaxial, under autorotation, with fixed geared rotors, if you slow one,[ add pitch] it slows the other.[ coz they are hard linked]
The difference between the two is the one with extra pitch is be'n slowed by the extra drag. The other is be'n "torqued" slower, [ reversed torque]. This reversed torque is transferred through the pinion, which is hard rigged to the frame.

But, only half of the yawing power of the slowing rotor is transferred to the airfame. The other half it used up to slow the other rotor.
And it don't matter wether you take on degree of pitch off one and add one to the other, its still a 2* differential.

Don't wurry, it still don't sound rite to me either. :(
I need anatha brain numbn drive to ponder it sum more.

Rotor Rooter

Dave Jackson

Intermeshing Husky:

From the link to the sketch in posting #161;
The Kaman Huskie uses both differential collective pitch and opposed longitudinal cyclic together. But what about during autorotation?
The Flettner used differential torques (differential collective pitch change) and rudder and only the rudder could give steering during autorotation

You, and Chuck, are correct if the gearing is the simple arraignment of 1 pinion driving 2 crown gears. My speculative posting #162 posting was poorly thought out and presented. Sorry.

However, when the cyclic is down, as it is in autorotation, there is a problem with yaw control and differential collective pitch will not solve it.

The problem with the Kamov coaxials has already been discussed.

Regarding the Sikorsky ~ S-69 ABC ~ From; Department of the Army Historical Summary: FY 1976. Weak directional control power in partial power descents and autorotation was observed. In future testing, improvements in autorotation directional control at low collective settings and high flare angles will be emphasized.

Here is another interesting report on yaw;
Regarding the Advancing Blade Concept (ABC) High Speed Development ~ May 1980 ~ (#3):
"However, with the rotor flying at close to autorotation at high speed, differential collective pitch has little impact on yawing moment ....."

I will stick with my speculation on #171 for now. :)

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There seems to be a lot of positive in these systems and there are numerous models but still way in the minority, while they seem to be gaining in popularity I wonder why they really never seem to have been a serious contender over a conventional helicopter.

Stan V

There seems to be a lot of positive in these systems and there are numerous models but still way in the minority, while they seem to be gaining in popularity I wonder why they really never seem to have been a serious contender over a conventional helicopter.
FAA certification would be a biggie!


For the extra set of blades,swashplate etc you could outfit another airframe or build a few batches of tailroter systems, Production costs. another compramize in the grand plan of flight.