aerobatic plane incident: was this a close call, or not?

Factual Questions
1

Video here:

Fast forward to 5:52 if it doesn’t start there.

Summary: aerobatic plane is performing a flat-spin maneuver when the prop suddenly separates from the aircraft. Pilot quickly performs a nice deadstick landing.

Questions:
How much does a prop weigh? The specs for a CAP 21 list it as a 2-blade constant-speed prop.

Being an aerobatic plane, I assume the center of mass was more aft (i.e. closer to the wing’s center of lift) than you would expect for a general aviation plane. Losing the prop would push the CoM even farther to the rear. Was there a chance this could have put the CoM to the rear of the wing’s CoL? Apparently the plane was still flyable - he landed safely - So I guess not. But I’m wondering if that was a really close call, or if there’s just no way that shedding a prop could have significantly moved the CoM.

2

IANAP but I think more than the lack of the prop’s weight, the lack of it’s counter-torque against the engine was an issue. You can see when he’s touching down he’s still rolled way too much to the left. Similar to a helicopter losing its tail rotor (though not quite as bad).

3

Wow. Pilots: did (s)he shit his pants?

4

I would have thought that the biggest concern was lack of uhh… propulsion.

5

Metal props weigh about 20lbs more than wooden ones. The constant speed props will be a bit more too, but some of that weight is from the governor which is at another place on the plane.

Haven’t seen the video, but especially with metal props, if you have prop failure, it certainly has the potential to rip the engine from the motor mounts, which would seriously disrupt the W&B, and almost certainly result in a fatality if you don’t get it shut down fast enough.

Haven’t seen the video, but will check it out now.

6

Seen it, and it’s good that it was probably a clean break with all of the prop going. Hell, I can’t land that good with a prop!

7

The entire hub separates from the engine. There’s nothing visible coming out of the cowling.

To the op, I’d guess between 125 and 150 lbs total.

8

I think he would have been coming in a lot hotter than what he did if it weighed that much. Went to wiki to learn more of the Cap 20 which OP supplied. Wiki says typical engines are Lycoming and up to 200 HP. These are the same engines Van’s aircraft use along with Hartzel C/S props. I have the same set-up, but a little bit less HP. Don’t recall exactly what the weights were, but I think for the metal prop and hub, weight was probably more in line with forties and fifties (lbs).

9

Just an observation on my part but the aircraft’s control surfaces don’t appear to have been damaged and air was still moving across the control surfaces. Except for level acceleration and climb, I assume the pilot could perform most basic maneuvers.

A stalled propeller or windmilling propeller, that was still attached to the engine, would increase aircraft drag and reduce gliding range. Discarding the prop would make it more likely that the pilot could maintain a rate of speed higher than stall.

Any change in front-of-center (FOC) by the loss of the prop would be more than compensated for by the decreased drag and increased speed.

It seems to me that a one-shot-only, dead-stick landing without the prop would be safer than a dead-stick landing with the prop.

10

If a heli loses its tail rotor, the rotation of the main rotor is resisted by the air. If the engine is still pushing that main rotor against the air, then the air pushes back on the rotor, the rotor pushes back on the engine - and the engine pushes back on the airframe, causing it to spin.

In the video I presented, the prop is gone. the engine has nothing to push on, so nothing can push back against the engine, and therefore the engine can’t push back against the airframe and make it rotate. Any roll angle seen in the video at touchdown is due entirely to the configuration of the control surfaces and/or the basic trim of the aircraft.

My concern was abou the shift in location of the center of mass of the aircraft due to the loss of the propeller’s mass way out in front. If the CoM shifts too far to the rear, the aircraft becomes fundamentally unstable. This makes for awe-inspiring maneuverability in fly-by-wire fighter jets, but it makes conventional aircraft with direct mechanical controls extremely difficult to control. So I was wondering if this pilot just got lucky, or if there’s no way the loss of the prop could shift the CoM anywhere near enough to result in that sort of instability.

11

The engine is still bolted to the airframe and the rotational torque of its crankshaft & flywheel are still going to cause the plane to twist in the opposite direction. Without the prop pushing against the air in the opposite direction the torque force will be significantly increased. Note that props don’t (usually) need to actually spin in the opposite direction as the engine (via a gear drive) but the propeller’s blades’ pitch has to be such that it counters the torque.

Watch a NASCAR or dragster crash where the car goes airborne at high speed. The reason they always start spinning along the long axis of the vehicle isn’t because of the physics of the crash, its because the tremendous rotational torque of the extremely powerful engine now has almost nothing resisting against it.

12

Loss of the engine is really rare. A long time ago, an Aeronca champ or something had the engine fall off. Yeah, OFF. He lost the engine until they found it on the ground, then it was not lost anymore. He instantly put the nose down to near vertical decent and then brought it up just before impact because that was such a massive change in CG, the loss of control for the elevators was going to be at a very high speed compared to normal. He was not hurt, so he did well.

Loss of part of propellers is more common, causes bad vibration & can get serious quickly, like in 1 rotation of the crankshaft.

Loss of the entire propeller is less common but is also less dangerous as the CG is not shifted so much that the control surfaces, elevators, become over powered.

After the prop came off, he must have quickly shut the engine down or it would have self destructed. The propeller is the flywheel on aircraft engines. You can get a small aircraft with a big engine to torque roll a bit on the ground but not really that much because the engines do not rev up that fast compared to a vehicle engine. 2500 RPM is high. Plus the propeller is a much bigger flywheel than most any land engine of similar displacement. It does resist turning more because it being heavier it has more inertia to overcome. The air resistance is not much until a higher RPM than you would expect.

Single piston engine general aviation aircraft do not have much torque. What little they have is off set with very small wing changes by the manufacture.

The turning force on take off, “P” factor is due to an engine propeller combination that is angled up for the most part. ( tail wheeled aircraft mostly ) The descending blade is grabbing the most air & pulling the hardest compared to the ascending blade.

Engine power gone for any reason, fixed pitch propeller windmilling = most drag.

Engine power gone for any reason, fixed pitch propeller stopped = much less drag.

Engine power gone for any reason, one that can feather = least drag.

Emergency, pilot has no gear, won’t come down, they foam the first 2000’ of the runway. pilot stops engines with the propellers stopped where they hopefully will get the least damage. Pilot & plane float way past the foam because they have no idea of how much they have turned their aircraft into gliders. Big change in how well they glide.

The guy in the video did a good job and I hope it was goofing off on purpose the things he did on landing. If it wasn’t on purpose, them he did a LOT less well.

Not that unusual to crack engine mounts on acrobatic airplanes, breaking a crank shaft out of the blue, I would suspect some other sudden stoppage of the engine in its past. Those engines seem to get overhauled more frequently and looked at closer on prefilight than most general aviation aircraft. except by pipeline patrol engines. IMO

Fun video anyway…

13

The aeroplane in the op banks at touch down because the pilot wants to, not because of torque. I doubt the engine was even running with the big prop flywheel missing, if it was running it would’ve been at idle.

14

In order for the engine to apply a torque to the airframe, it has to have another object to push off of. When the prop was there, that other object was the air. Without the prop, there is nothing for the engine to brace itself against in order to apply torque to the airframe.

Imagine an astronaut on the space station, holding a propeller-on-a-stick. He spins it; the prop torques the air, the air appies an equal/opposite torque to the prop, which gets transmitted through the stick to the astronaut, and he ends up rotating in a direction opposite to the propeller. Now take the propeller off of the stick, and just have him spin the stick. No prop means the stick can’t apply any torque to the air, and so the air can’t apply a resistive torque; the astronaut will not rotate.

Having said all that, by changing the engine RPM (and keeping it at that new RPM) it’s possible to impart a small amount of angular momentum to the airframe, but only for the period of time during which the crankshaft is actually changing speed. Moreover, it won’t result in much roll rate; the airframe has far more rotational inertia than the engine’s crankshaft (especially with the prop physically removed). So the pilot may have experienced a brief moment of slight roll torque at the moment he shut his engine down, but this torque would have ended the instant his engine reached zero RPM; With no propeller, there can be no torque on the airframe if the engine is at constant speed (regardless of whether that speed is 0 or 2500 RPM).

This is analogous to motocross racers, who are able to make mid-air adjustments to the pitch attitude of their motorcycles by tapping the brakes (wheels stop, bike pitches nose-down) or blipping the throttle (rear wheel accelerates forward, bike pitches backward). But once their wheels are completely stopped (or the engine hits max RPM), the chassis stops pitching.

15

Unlikely. When I had an emergency situation I was completely calm until I landed. Your training kicks in and you are too busy analyzing and planning to panic.

As soon as my plane rolled to a stop and all danger was over my legs started shaking uncontrollably. I assume that was from the adrenaline.

16

Hmmm, yeah you’re correct. The only (tiny) nitpick I can say about the astronaut analogy is that technically the astronaut’s body *would *counter-rotate against turning just the stick, but only at a rate proportionate to the (huge) difference between the mass of his body vs. the mass of the stick (which would be so small that the air resistance alone inside the space station would overpower it).

17

When starting from zero RPM, the first application of power to turn anything is fighting against inertia, not the air or some such. Torque rolling can be just as much using a heavy flywheel as is using a paddle wheel that has a lot of drag but the important part is that starting from zero, it can only twist anything because of ‘equal but opposite direction’ or some such.

You can get all geeky by adding ‘rate of change’ or / and ‘mass difference’ or anything else but pushing against the air by the propeller is just an artificial way of increasing mass of the rotating part. Just as adding lead weights to the engine until the mass of the engine is so great that the changing speed of rotation has a very small effect as far as observable rotation of the engine.

See LeRhone rotary aircraft engines…