Further Hybrid/ Firebird testing.....

[Aussie_Paul]

....One of the problems that we continue to have without a tall tail is the prop torque, particularly in rough air. With CLT and an effective h/stab we still have the problem with torque roll when the rotor is unloaded in rough air.

A tall tail is not effective enough with a cabin, particularly if the engine fails and you have to land in a cross wind. Rare, yes, but possible.

Today I conducted some tests by having each side of the stab set at a different AoA, in favour of helping the torque roll. I was surprised how well this worked.

In am looking forward to conducting further tests in 3 weeks when I come home from Western Oz, where I get to fly a Magni, and a modified Raf.

Geez!!! it's a tough life but someone has to do it!!!!!!!!

Aussie Paul.

 

[StanFoster]

Paul: That is very innovative what you are trying.

Quick question...and I am not trying to promote HTL..ok? But...I have always wondered that having the CG hanging below the thrustline ......that this in itself produces a moment arm to counteract some torque. It would just seem to me that it does. I would think a CLT machine would roll easier on the thrust axis.

I posted this same question earlier this year...but never got a response.

Again....we all know the disadvantages that HTL have when compared to CLT. Is this one area where it helps?

Stan

 

[Aussie_Paul]

That is a question that is beyond me Stan!!!!

Hey, the rest of you clever guys, what say ye?????

Aussie Paul.

 

[Doug Riley]

Paul: You've re-cycled Cierva's method of dealing with pork roll... er, I mean torque roll. He used differential HS settings, too. It would be a lot more effective, however, if that HS were more deeply immersed in the prop blast. In my experience (with the Gyrobee), raising it even 6-8 inches makes a considerable difference. The propwash's airspeed is in the neighborhood of 100 mph. The force created by an airfoil varies as the SQUARE of airspeed. You get lots more kick for your money when the fin is in the slipstream.

A HS that spanned the whole prop dia. right in the middle of the propwash would do the same thing as a tall tail, with respect to countering torque roll.

(It wouldn't help torque yaw one bit, of course. The absence of torque yaw when using a tall vertical tail is a wondrous thing, IMHO.)

 

[Aussie_Paul]

Aaah, it is an interesting life I lead.

Aaah, it is an interesting life I lead.

Thanks Doug for your info. I have a new set of tail feathers to try shortly.

Four, yes 4 stabs on a full flying tail. Two in the prop wash and two that are a 1/4 of the way up the tail. We can adjust each stab individually, and can compare stabs in the prop wash, to stabs that are some in and, some out of, the prop wash!!!!

Aaah, it is an interesting life I lead!!!!! Luckily my wife keeps me!!!!!

Aussie Paul.

 

[chuter]

Stan, your idea makes sense to me. I seems that having the CG farther from the thrustline, either higher or lower, would slow down the torque roll.

More opinions?

 

[gyromike]

If having the CG below the thrustline were actually effective at reducing torque-roll ( I don't know if it does or not ), would you be willing to trade less torque-roll for a greater thrustline offset?

Maybe Doug or Chuck or someone could run the numbers, but I don't think having a low CG will really affect torque-roll. Even though the thrustline is higher than the CG, the aircraft should still want to rotate about it's CG, and not about the prop thrustline.

At least that's how it appears to me.

 

[Victor Duarte]

Paul, it is a good experience, if you can, with Doug's ewplanations, write a little report, i would quote that on another thread.
Thanks

 

[Al_Hammer]

I have always wondered that having the CG hanging below the thrustline ......that this in itself produces a moment arm to counteract some torque. It would just seem to me that it does. I would think a CLT machine would roll easier on the thrust axis.

I posted this same question earlier this year...but never got a response.
Stan

I think the answer is no, Stan. I quote...

"Torque" in the sense we refer to it here is actually what in engineering term would be more properly called a "couple". In engineering, a "torque" is a force on the end of a moment arm, such as the thrust of a helicopter's tail rotor acting on the moment arm of the tail boom. A couple is purely a twisting force, such as the twisting effect in the helicopter's main rotor shaft, or the twisting in a prop shaft.

The key here is that a couple applies the same pure twisting effect to the structure regardless of where on the structure it is located You could mount a prop + motor on the tail, on the nose, all the way out at one wingtip, or down on the landing gear, and the plane would still experience the same twisting force from it.
http://www.djaerotech.com/dj_askjd/dj_questions/pager_torque.html

 

[chuter]

Concerning torque roll and CG/thrustline, consider this:

Say you have a screwdriver held horizontally, this represents the thrustline. You also have a 1lb weight, this represents the CG of the gyro.

Now, put the 1lb weight on a 1 foot long rod and attach it to the screwdriver shaft at a 90 degree angle. How hard would it be to turn the screw driver? (discounting gravity of course). The actual rotation may not be around the thrustline, the thrustline may try to climb around the CG, or a combination too complex for my limited visualization abilities.

Now, put the shaft of the screwdriver through the center of the 1lb weight, how hard would it be to turn the screw driver? I would think it would be much easier.

Assuming this is true, it certainly isn't reason enough to accept big CG/thrustline offsets. A CLT gyro or one with the thrustline below CG will ELLIMINATE PPO, where as a large offset will only slow torque roll, not illiminate it.

Disclaimer: I could be quite confused.

 

[Al_Hammer]

Michael, I understand your argument, but, as we know, the gyro rotates about it's cg no matter what the thrust line is. It will rotate the same, no matter where the torque is applied. It sounds counter-intuitive, but it's true.
In the case of the screwdiver, you are holding the screwdriver in a fixed location, so the weight will always rotate around it, and the distance the weight hangs down will affect the torque required to turn it. If you tried it while floating in space, you would rotate about the cg, just like the gyro. That's precisely why the astronauts have to be clipped to the structure of the spacecraft in order to use tools.

 

[Doug Riley]

Al's right (no surprise). The technojargon for the type of twisting tendency that an engine produces is a "couple." The reason for this name is that this type of twist is the exact equivalent of two straight-line forces, of equal size and opposite direction, producing a pure torque around a central axis.

Basic vector mechanics teaches us that a force may be considered to act anywhere along its line of force, with no change in the results.

If engine torque is applied somewhere other than right on the CG, the two forces that can be considered to make up the couple can be "slid" down to the CG with no change in the results. Bottom line: the torque is the same.

I'll post a crude MS Paint if I can.

 

[Doug Riley]

A scribble about couples. In the left image, we see the "original" couple. On the right, the pair of forces that makes up the couple has been "slid" down, along the lines of the forces. The torque, or moment, remains the same, because sliding forces along their lines does not change their effects.

 

[chuter]

OK, I think I'm getting it............

Say you have 200ft lbs of torque and a 1ft CG/thrustline offset, this means you have 200lbs pushing on the thrustline in the roll direction to roll it around the CG?

And if you only had a 6" offset it would be 100lbs trying to roll the craft?


(sorry to be hijacking your thread Paul)

 

[Al_Hammer]

Almost... If you have 200 ft-lbs of torque, it is the same as 200 lbs of force acting on a 1 ft arm, or a 400 lb force acting on a 6" arm.
Torque can be thought of as acting anywhere on the airframe. Think of a prop driven by belts. The belts can be any length and the torque remains the same, as long as the pulley sizes do not change. This torque acts to turn the the aircraft about the cg, again, no matter what the prop offset.
It s easy to be confused by the fact that that prop offset does matter when thinking about pitch stability. Thrust acting at a distance creates a torque.
Here we have a torque acting at a distance, and the distance does not matter.

 

[chuter]

OK, I think I got. Thanks Al and Doug.

Stan, you shoulda known that.