Tractors and PPO

[bfwe]

Very new here. I am a fixed wing pilot but want to get into gyros for a variety of reasons. Mostly they are just very interesting. I will be getting a rating before building but I have a question.

As a pilot I understand the PIO/PPO situation invovled in these aircraft but PPO concern me most. It seems to be the single biggest problem in gyros. I fully realize that any departure from controlled flight can happen give the right circumstances in any design/configuration.

Is PPO less a problem with tractor designs?

Any advice from the people of this board would be greatly appreciated.

[KevinKing]

Welcome to the forums.

PPO is IMPOSSIBLE in correctly designed gyros, whether they be tractor or pusher. Search for "centerline thrust" or "CLT." You'll come up with tons of information.

[Udi]

The biggest advantage of most tractor gyros is that their tail group is typically located further aft from the CG, giving the gyro a superior stability in pitch and yaw. Regardless of the location and size of the tail group, as Kevin said above, a gyro would not PPO when the balance of pitch moments about the CG is kept neutral or slightly nose up. Both tractor and pusher configuration will have the potential for a PPO when the engine thrust line is producing a net nose-down pitching moment, and this moment is not neutralized with an adequate stab.

Udi

[GraemeMonro]

I thought that PPO was Power Push Over.
How can you have a power push over in a tractor. Maybe a power pull over? I don't know but I didn't think it was possible for one of these in a tractor configuration. But I'm sure some will enlighten us here please.

[rtfm]

Hi,
Theoretically, if you had a tractor aircraft (fixed wing or gyro) with the prop thrust line above the aircraft's CoG, then the tendency would be for the aircraft to want to nose-over. Add a long tail, with its large moment arm, and you dampen this tendency out. But the tendency would still be there.

Practically speaking however, it is extremely difficult (probably not even possible) for a tractor aircraft to fall prey to PPO (Pull Over).

I'd disregard it as a possibility, myself.

Regards,
Duncan

[Doug Riley]

PPO is very unlikely in a tractor gyro, unless you perversely set out to design one that could manage it. You'd have to make the tail boom short, the H-stab way too small (or skip it altogether) and point the engine thrustline way down toward the ground.

Many FW ultralights, and some larger FW's, do have some degree of pitch instability caused by high thrustlines. Ultralights like the Kolb Firestar have their prop thrustlines a foot or more above the CG. They nose down and speed up noticeably when you add throttle, and you have to increase back pressure to hold your airspeed. They have so much horizontal tail power, however, that neither PIO nor PPO is an issue.

It's simple to design a gyro that has none of these tendencies.

[bfwe]

Great input. This introduction has been a real pleasure.

[Udi]

I think the point we should make here is that, although it is harder to end up with a bad design (prone to PPO) in the tractor configuration than in the pusher configuration, the only reason to end-up with a PPO-prone pusher gyro is ignorance. We know how to design PPO-proof pusher gyros and there are enough of them already available. If someone is very concerned about PPOs (and they should be) they don't have to build a tractor gyro in order to have a safe gyro. Tractor designs may have other benefits over pusher designs but nowadays there is no reason to build a PPO-prone pusher gyro.

The main benefit of a tractor design, in my opinion, is its crash worthiness. Having the engine in front of the pilot instead of behind, and having a steel cage around the pilot could make for a big difference in certain types of accidents. On the other hand, sitting in a steel cage with a big engine in front of you makes for a totally different flying experience than flying in an open frame pusher gyro.

I believe that if there was a good quality 200-hr build time tractor gyro kit available, it would have been very popular. The only tractor gyro kit I know of is a poor quality and unrefined product. Building a good tractor gyro from plans (e.g. LW) is too difficult and time consuming for the average gyro nut.

Udi

[WHY]

bfwe

Consider a properly modified fixed wing fuselage as a good candidate for a tractor gyro, that's what I am working with, it started life as a "Texas Parasol"

Tony Stiles

[C. Beaty]

From an engineering perspective, a tractor is the more logical approach.

A large diameter tailcone provides a strength/weight ratio unmatched by any other configuration, whether it be a sheet metal monocoque typified by a Cessna or a welded steel tube and fabric structure pioneered by Aeronca.

A tractor will also be more aerodynamically efficient in general than a pusher. Drag is determined by the size of the turbulent hole (wake) left in the air by the passage of a body through it. The tapered tailcone of a tractor allows the hole to close up with less turbulence.

However, the bolted 2 x 2 aluminum tube structure originated by Dr. Bensen has its own merits; namely, ease and speed of construction by virtually anyone handy with hacksaw and drill.

The primary deficiency of bolted 2 x 2 pushers is that tail surface loads must be engineered around the propeller, resulting in structural offsets (doglegs) and the attendant weight penalty.

All other things being equal, there is no difference of stability between tractor and pusher. With propulsion unit bolted to the airframe, propeller thrust is applied along a line rather than to a point. It is irrelevant just where along that line the propeller is located.

The simplicity of Bensen style construction is directly responsible for the “killer” reputation of gyros.

Almost anyone can proclaim himself a designer, scale up a Bensen, print some fancy brochures and sell his “Saturday night specials” to an unsuspecting public that knows even less about the mechanics of flight than he does.

[WHY]

Hi Chuck

Did my e-mails come through???