Gyroplane Thrustlines vs. Center of Gravity

[C. Beaty]

Sport Pilot TV, Mark, is a commercial enterprise owned by Dan Leslie.

That film clip would have been paid for by RAF and most likely scripted by Don LeFleur.

[bones]

I got no problem with that but look at the way it is nosing up and down just after it takes off

[wisemo]

yes bones i noticed that. was that guy doing a pitch control - but instead of the hub moving ...the whole gyroplane moved instead...?

[C. Beaty]

Originally Posted by bones

I got no problem with that but look at the way it is nosing up and down just after it takes off

Your eyes must deceive you Bones. The pilot, Dofin Fritz, absolutely denies that RAFs bobble.

[bones]

Originally Posted by C. Beaty

Your eyes must deceive you Bones. The pilot, Dofin Fritz, absolutely denies that RAFs bobble.

Yeah well even my misses who is not even ready to solo yet went "holy crap" why is it doing that.
A very good gyro promotional video apart from the fact the machine clearly does do what "The pilot, Dofin Fritz, absolutely denies that RAFs bobble"

[mceagle]

That seems to happen with all RAF's, even with 'on type' very experienced pilots. Even after-market horizontal stabilizers do not seem to stop it - only reduce the number of cycles. They look terrible taking off at dusk with the headlights pointing straight down the runway. Most RAF pilots deny that it happens.
Perhaps it has something to do with that 'magic' stability bush.

[Doug Riley]

Tim, there are a couple reasons why the after-market stabs on an otherwise stock RAF aren't sufficient to stabilize it.

First, they typically lack the power. Tail power is a function of tail volume (HS area x lever arm) and of airspeed squared. A 12" high thrustline with 600-700 or more pounds of thrust produces a nose-over torque of 600-700 foot-pounds. That's more than a reasonably-sized H-stab that is outside the prop slipstream can counteract. A couple of probable PPO accidents here in the States, involving H-stab equipped stock RAF's, confirm the math.

Second, even if they have the power, they are not set up correctly. To do the whole, job, the H-stab must have enough negative angle of attack to generate a 600-700 ft.-lb. nose-up moment. Even a very large H-stab will need several degrees of negative incidence to do this. However, if you add this much tail down-load, you'll also need to re-hang the gyro and move the rotorhead back enough to eliminate the built-in tail-heavy hang spec. (A RAF set up to factory specs has a hang angle of -7 degrees; the standard for small autogyros is -11.5).

This is too much work for most people to bother with, I guess. So they bob and tolerate the residual PPO risk.

A properly-set-up gyro will fly either hands-off or stick-locked indefinitely, until it runs out of gas. Pilots of unstable gyros warily remove their hands from the stick for a few seconds, as if they were riding a unicycle.

[GrantR]

I am a little confused about the how Rotor thrust vector and CG align.

When it is stated that a stable gyroplane has the RTV behind the CG. This CG is the horizontal not vertical cg correct? With an airplane you do a weight and balance to find the H CG how do you find the H CG of a gyroplane?

So can a CLT gyroplane be tail heavy with the CG behind the RTV?

I have 3 drawings of gyros 2 HTL designs and 1 CLT. One of the HTL has a angled mast and the other 2 have 90o angle mast to keel. All appear to be tail heavy except the angled mast.

If the rotor is tilted back about 10o in flight and the keel is level in flight that would put the RTV out ahead of the mast near the pilot.

So what am I missing?

[Doug Riley]

Grant, the CG is normally located some inches ahead of the mast. Both vertically and horizontally, it's located roughly at the pilot's belly button in a one-person pusher gyro.

Bensen angled his mast to coincide closely with the rotor thrust line, though he couldn't put it right ON that line without running it right through the pilot's skull. The rotor head attachment plates of Bensen and other gyros put the head a little forward of the centerline of the mast for just this reason.

The Bumblebee and its derivatives have vertical or even forward-leaning masts. This puts the mast quite out of line with the rotor thrust and increases the bending stress on the mast as a result. One advantage of it, though, is that it allows more room for the front fan housing of an upright Rotax engine.

The horizontal location of the CG on a gyro has somewhat less significance than it does in a FW plane. After all, the rotor swings through a wide arc and will naturally end up where it needs to be to balance the aircraft. We adjust horizontal CG location simply so that the machine cruises level.

The VERTICAL location of the CG is more important on a gyro, since we can't swing the prop thrustline around in flight as we can the rotor thrustline. The relative positions of the CG and prop thrustline are fixed, and need to be fixed so that there are not large moments trying to pitch the nose one way or the other -- especially not down.

[GrantR]

Ok so what the hang angle does is set the horizontal CG and get the joystick aligned? So the rotor seeks too align the RTV with the cg to balance out lift and drag from the rotor? I guess a draggy nose pod would work against this like a HTL. Thats where the H stab comes in to balance out any drag working below the CG and to damp pitch.

[GrantR]

In the rotorcraft flying handbook is states that

if you have a gyroplane where the CG is below the propeller thrust line, the propeller thrust gives your aircraft a nose down pitching moment when power is applied. To compensate for this pitching moment, the CG on this type of gyroplane is usually located behind the rotor force line. This location produces a nose up pitching moment.

Then it goes on to say fuselage drag, pitch inertia and the addition of a Horizontal Stab can alter when the CG is placed.

A high TL cause a nose down pitching so the rotor has to be tilted back more to overcome this nose down pitch or the gyro will dive. So when the rotor is tilted back that automatically puts the RTV further forward and ahead of the CG thus making it tail heavy. This is how it compensates for this nose down pitching moment. With no horizontal stabilizer tilting the rotor back is the only way to over come this.

So in a high thrust line Gyro the engine thrust is fighting with the rotor thrust. The rotor wants to keep the nose up and the engine wants to put the nose down. When one variable changes whether rotor thrust or engine thrust it causes pitching, nose up or down in the airframe since the change causes an imbalance in the opposing forces. This plus being tail heavy makes the aircraft unstable.


Now I am going out on a limb and say it seems to me a HTL is safe with a large HS that is trimmed and capable of creating enough down force on the tail of the gyro to offset the nose down pitching. It seems if its trimmed just right it could even cause a nose up pitching with increased engine thrust. Now if the gyro sees 0 or negative Gs this is where it gets confusing. I know from reading the H stab mods for one type of HTL enclosed Gyro is not big enough to offset the massive amount of nose down pitching the HTL causes.


Using an effective H stab at full power, is there a bunt possibility since the H stab is trimmed to create enough down thrust to offset the nose down pitching? Or At full power is there a bunt possibility since the H stab is trimmed to create enough down thrust to offset the nose down pitching in normal flight with a loaded rotor?

[Terry]

I am really struggling with that article Chuck.

I can find no reason to believe there is a forward component to the rotor on an autogyro as 4b. seems to indicate. It looks like Voodoo.

Also, while the article seems to want to debunk the idea there is a sum drag component to the rotor, it continues, textually, to affirm that it exists.

I understand that the rotor blades have a forward component of lift in relation to their plane of rotation, but it is still negative in relation to the angle of incidence of the rotorplane and aircraft, and working contrary to propeller thrust.

So, it seems for the entire aircraft in straight and level flight there does exist rotor drag <b>and</b> structural drag which are aerodynamically balanced against propeller thrust or drag. When the rotor is then unloaded, the drag component of the rotor diminishes rapidly as the rotor plane rotates toward parallel with the wind.

It seems to me that both are correct. Stability issues are both that of balancing thrust near the center of mass, and about balancing thrust near the center of drag. Or, at least, about balancing these two balances against each other. The only problem being that the center of drag moves dramatically, vertically dependent upon angle of incidences, and is not as stability seeking as conventional aircraft.

Adding to this confusion it seems powered gyroplane design not only needs to consider propeller thrust but propeller drag when powered down; more so than conventional aircraft because of the misalignment of all these offset forces.

As a newbie, it sounds like this idea of "rotor thrust line" is a carryover from helicoptereze and, because it is so esoteric, causes confusion in the discussion of autogyros.

Terry Graham


Moved by willed purpose.

[Mike Schallmann]

Oh Terry -you have so much to learn --all of this has been hashed out and discussed over many years both on this forum and the previous one- ( Norms)- start by doing a lot of research on the subject -- you cant change the laws of Physic no matter how hard you try--The statements you made display your lack of knowledge -- study hard young grasshopper and you to can become a master--live long and prosper--

[All_In]

Hi Terry
I'm still learning it myself.

Your confusing a few things that's all (Or more probably I’m confuse as to what you really meant!). But you’re on the right track!!!
This thread is a good one to read from start to finish and there are several others.
There is no quick answer, except the math, so for most the only way to really learn it is to read all of the threads and then the light begins to shine.

[Terry]

You're right Mike, I do have much to learn.

Yet what I said seems to also coincide with the ASTMs posted on the PRA site. Is it my terminology or what?