Gyroplane Thrustlines vs. Center of Gravity

C. Beaty

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There is no difference between tractors and pushers; all else being equal.

A propeller develops a line of thrust, not a point of thrust. Where the propeller happens to be located along that line is irrelevant.

There are of course practical differences. A tractor may have a longer moment arm to the tail surfaces and the airflow the propeller receives is most likely cleaner.
 

Doug Riley

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A pusher also allows you to get away with no streamlining of the fusleage if you don't mind a slow cruise speed.

A gyro with a bundle-of-sticks airframe like a Bensen but with tractor propulsion has a horrendous amount of drag. The sticks (and the pilot) are stuck in the 100 mph air exiting the prop. The pilot-and-sticks have 4 times the drag in 100 mph air than they have in 50 mph air.
 

MichaelBurton

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Pusher vs tractor for stability

Pusher vs tractor for stability

Chuck mentioned the moment arm to the tail as one area that will usually give a tractor additional stability. Another is the moment of inertia (MOI). As you move the weight away from the center of Gravity(CG) the MOI increases. You can try this with a yard stick and a movable weight. As you move the weight out the stick gets easier to balance. This is the reason that the man on the high wire holds a long stick with weights on the end. It makes him more stable.

All of this contributes to the stability of the tractor design. The biggest reason I see for pusher designs is simplicity.
 

Doug Riley

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A high MOI is something of a mixed bag. High MOI means that the gyro's airframe behaves like a heavy flywheel: it resist starting to rotate, but also resists stopping its rotation once it's started. It requires more control power, IOW, to stop a tandem or tractor airframe from rotating once it has started. This can lead to lag and overshoot.

The most persuasive argument for a tractor, IMHO, is crashworthiness. Well-known small-plane designer Ladiszlao Pazmany wrote that pusher power is lethal in a crash and ought to be banned by law. That may be a bit extreme, but the fact is that a rear-mounted engine is a cannonball riding behind the pilot, waiting to squish him or her in the event of a nose-down crash. The pilot acts an airbag for the engine in such a crash.

In a tractor, OTOH, the engine gets to interact directly with Mother Earth upon a nose-down rearrival, without the pilot being stuck in between.
 

LGoodhind

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>a rear-mounted engine is a cannonball riding behind the pilot, waiting to squish him or her in the event of a nose-down crash

Based on what people have seen over the years is a stable lawn dart drive into the ground a typical failure mode? Most of the pusher fatality incidents read like a PPO with the exploding tail, rotor separation, and followed by a distinctly non-aerodynamic tumble to the ground ... in a non-PPO control system failure it would seem that an uncontrolled vertical decent is a more likely scenerio.

What's a classic (80/20 rule) gyroplane crash look like if you ignore the PPO "rotor meets airframe" variety?
 

Doug Riley

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Larry, the argument to counter my argument has been that a free-fall isn't survivable anyway except by dumb luck -- so don't worry about it.

Instead, the argument goes, survivable accidents are those in which the rotor and control system continue working. In those cases, the pilot's best strategy is to pull back on the stick and use a vertical descent to cause a frame-level "soft splat." We know these are survivable in Bensen-style gyros because many of us have survived them. The gyro's vertical speed with an intact rotor and zero forward airspeed is on the order of 20 mph.

In that case, it doesn't matter whether the engine is in front of, or behind, the pilot, since it and the rest of the mass of the gyro have zero horizontal speed. What matters instead is the ability of the frame to collapse progressively in the vertical axis, within the space between the pilot's bottom and the aircraft's bottom.

This argument takes into account the difference between a typical FW crash -- involving a stall, loss of pitch control and a nose-down re-entry -- and a gyro that won't stall in the FW sense, and therefore retains pitch control.

Of course, a tractor gyro can be designed to have vertical crash-worthiness, too -- to go along with its naturally better frontal crash-worthiness.
 

wisemo

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Oh - now what about the reaction torque off the engine? how is that taken care of? In an aeroplane the engine's offset so one wing gets more lift than the other to counter this - what to gyrobabies (gyroplanes haha) do?
 

C. Beaty

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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

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

wisemo

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

bones

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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

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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

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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

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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?:wacko:
 

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Doug Riley

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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

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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.
 
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