The High Turn Over Angle

scandtours

scandtours
Joined
Apr 15, 2004
Messages
2,325
Location
Cyprus
Aircraft
Bensen,Brock, Parsons Tandem
I should like to hear from other gyro pilots how stable is their gyros on the ground?
To be more specific, how can we achieve a (high turnover angle) to prevent a gyro from ground looping or rolling over during landing.
The turnover angle is the angle (in geometry in Greek we call it θ=thita) and I know there is a formular that ( θ ) can be calculated.
Why I am asking this is because watching some videos I noticed that some gyros seemed to have difficulties even steering them on the ground.
Ok it depends on the mechanism of the nose wheel too.
 
A bigger triangle may be better.

A bigger triangle may be better.

Hello Giorgos,

I feel I don’t understand the question but I will give it a shot.

To me it seems simple enough to calculate turn over angle but in my opinion there is more to good ground handling than just the turn over angle.

In my opinion the larger the triangle the more angle it takes to turn over and the higher the vertical center of gravity the further it moves as the aircraft tips.

I feel ground handling is a place that Mark Givan has done particularly well with The Predator.

I often fly in strong gusting winds that change directions and land with more forward speed than most gyroplanes I have watched. I have well over 2,000 landings and particularly in the beginning I was sometimes not lined up well with the direction of travel on touch down.

I fly at busy towered airports so I want to scoot off the runway as quickly as I feel is safe. I feel that coming to a near stop landing ties up the runway longer than I would like. I have already slowed things down with my slow flight speed; I don’t want to exacerbate the delay with a slow exit. In order to not disrupt the traffic flow I don’t usually wait for the rotors to stop before leaving the taxiway.

The Predator has a very wide track, close to seven feet, and a long wheelbase so in spite of her high stance she does not feel in the least tippy like some other gyroplanes I have flown. It is hard to know how close to the edge I am at times so this is very subjective.

Her suspension is fairly stiff so the roll doesn’t begin in the suspension even though there is close to 5 inches of travel.

The suspension arms are long so there is not much camber change as the suspension goes through its travel. The geometry seems good so there does not seem to be much bump steer.

The steering by differential braking works much better than I expected in part because the brakes are very progressive and smooth in their action. When the nose wheel touches down in strong crosswinds it is not quarreling with the pedals like linked steering does.

She seems to have adequate dampening of the nose wheel and an appropriate about of trail, just over three inches, so I don’t get a shimmy unless she needs maintenance.

He wide stance and long wheelbase make her more difficult to put on a trailer but she is still under the maximum width for road trailers.

Thank you, Vance
 
IN a triangular suspension, the force tranfer to the mast also helps on turn over.
The rotor still flying, the major force, a bump on the wheels is transfered to the mast, depending how high it arm is conected.
If there is a conjunction of forces, pilot has to be extra quick to compensate. Over it goes . . .
Heron
 
Giorgios,

Martin Hollman discusses turnover angle in his book "Modern Gyroplane Design" (I added the words in square brackets)
Hiller specifies that [a turnover angle of] 35-40 degrees be used for helicopter designs. However, these angles should be larger for gyroplanes since the gyroplane is often in a crab attitude when touching down in cross wind landings. I recommend a minimum aft turnover angle of 45 degrees and a minimum forward turnover angle of 35 degrees for all gyroplane designs.
 
Gyro main gear width is many times determined not by proper calculated design but by the need to accommodate the gyro on a trailer for transportation.

.
 
Why I am asking this is because watching some videos I noticed that some gyros seemed to have difficulties even steering them on the ground.

Are you talking about stability or maneuverability issues?

If maneuverability you are not going to get better than castering nosewheel and differential brakes.
.
 
Hello Giorgos,

I feel I don’t understand the question but I will give it a shot.
Thank you, Vance


No wonder, Vance and Allan, that you were confused... It was my mistake since I’ve mixed two
different things in the same question. It is clear anyhow from your answers that you guessed correct what I meant.
Thanks
 
IMO, stability (i.e., roll axis while on ground) will be influenced by several factors, some more readily apparent than others:
- Center of Gravity
- Track
- Main gear camber and toe
- Suspension compliance, both compression and rebound
- Roll Center (the point about which the gyro suspension wants to rotate irrespective of CG)

Everyone talks about the first 4 items but very little is said about the last item. If we use an automobile analogy, the roll center (RC) is typically designed to be near the CG. The RC can be approximated by the intersection of the A-arms. The lower the RC relative to CG, the more the car wants to roll in a turn which adds 'bite' to the outside wheel. This roll tendency is reduced by the use of anti-sway (aka anti-roll) bars. In a performance automobile, the RC may be designed to be near the road surface. If too low, the car tends to want to "go-kart" with ineffective handling.

If we look at a gyro application, we see two basic extremes. Rigid suspension ala a go-kart with both the advantages and disadvantages of driving a kart plus the impact of landing mitigated only by rather extreme low tire pressures.

At the other end of the spectrum is systems such as the Dominator. Relatively compliant suspension, minimal damping in the air shocks, and relatively long travel. Locating the RC of the Dom is very simple - the point of intersection between the A-arms. Put a piece of tape there. Then compare it with the CG which can be approximated by the thrust line just for the sake of discussion as the Dom is a CLT machine. On my Dom, the RC is some 10" inches ABOVE the CG. This is a good design in that the lower CG with respect to RC tends to add stability to ground handling. This can be readily verified by taxiing the Dom with the rotor tied in place. With the differential braking and castering nose wheel, the Dom is surprisingly stable.

So why do the Doms have a reputation as being "tipsy"? The problem, IMO, is the forces exerted by the disc on the rolling chassis. It is, in effect, a big "hand" pressing on the end of a long moment arm (i.e., the mast). The Dom is unusually tall with respect of rotor to ground so it has a long moment arm. If pilot landing technique is improper, that force will unsettle the gyro regardless of suspension.

To mitigate this phenomena, most pilots choose to improve the damping of the stock air shocks by using springs and dampers. The net effect is not unlike the use of an anti-roll bar on a car with the improved compression and rebound characteristics slowing down the action and giving the pilot an opportunity to get the disk into a more proper attitude.

Proper technique requires that the landing be as square as possible - tail wheel touching first to insure proper yaw attitude by pulling the craft straight, but the pilot needs to work roll so that both mains touch together. IMO, a slight amount of wheel camber (top in) and toe-out will help settle the aircraft as it lands. And never, never, touch the nose gear first.

My thoughts reflect suspension engineering basics, not practical experience. Your mileage may vary.
 
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