The prop departed LN-KAP today. Media got the R/T recording from an online source.
Student and CFI walked away, and a/c mostly intact. Only some damage from the prop falling off.
This leaves to the old question: what is the trajectory of a prop that falls off? Forward? Backward? Sidewards? Random?
There have been cases, from the 1950s days of “airliners” which used the big P&W radial engines with loads of cylinders and a few thousand HP, when an engine would seize up (which they often did) and the prop shaft would break off (due to the inertia of the prop) and then the prop flew along or ahead of the plane, and once I recall reading about the prop did a “barrel roll” around the plane before disappearing. Sometimes the prop didn’t miss the plane…
So I think that in the simplest case if a prop comes off cleanly and under power, it will fly forward.
This chap was lucky the whole prop came off and not just one blade. The latter scenario is likely to rip the engine off its mountings in seconds (or less) and give you a massive W&B issue which will probably be outside the trim range of the pitch trim (i.e. you die). One has to shut the mixture really quick, but I have seen cases where it happened in about 1 second and the engine came right off. It happened on a TB10 at the runway holding point, so he was “lucky”.
This leaves to the old question: what is the trajectory of a prop that falls off? Forward? Backward? Sidewards? Random?
According to the instructor (?) interviewed at 0:50 he says that the propeller parted and disappeared backwards on either side of the aircraft. (a Swedes translation of norwegian)
So I think that in the simplest case if a prop comes off cleanly and under power, it will fly forward.
It can fly forward for a brief moment only. When it’s inertia/angular momentum is consumed by drag, the remaining part of the aeroplane will catch up on the propeller again and eventually overtake of hit it. If it is a constant speed propeller it may move to full fine instantly when the oil supply is lost (depending on the mechanism) and produce more drag than the aircraft and move away depending on which blade touches what part of the engin cowling first.
The only such instance that I’m aware of was with a C421 that I flew occasionally. They came back from a checkride and upon shutdown the engine backfired, which is not unusual, and the propeller shaft sheared off. The propeller simply fell to the ground in front of the wing onto the top of the spinner which remained flat instead of pointy for as long as the aircraft remained in my “field of view”. The examiner on that flight was known for treating the aeroplanes rather roughly, for example always shutting down an engine for real which one is not supposed to do with these complex geared engines. But neither him nor the guy who took the checkride talked about what happended on that flight, so if was filed under “fatigue cracking” of the propeller shaft.
QuoteWhen it’s inertia/angular momentum is consumed by drag, the remaining part of the aeroplane will catch up on the propeller again and eventually overtake of hit it.
That’s assuming you have multiple engines and the others are still pulling the aircraft forward. But in a single engine the airframe doesn’t have trust anymore, so it will slow down because of its own drag. So it’s a toss-up between the momentum and drag of the propeller vs. the momentum and drag of the airframe.
In any case, I don’t think a prop rotating on its own will be in a stable state. My gut feeling is that it will start to tumble immediately, instead of continuing to rotate and provide a measure of thrust. Especially since the plane of rotation might not be exactly perpendicular to the direction of movement – something that’s called p-factor while the propeller is still attached to the airframe, and is one of the reasons you need right rudder on takeoff. That tumbling will just cause more drag, way more than the relatively stable airflow around the airframe. So my money would be on the propeller moving backwards virtually immediately after detaching.
I saw a video of an RC helicopter where the owner hadn’t secured the jesus bolt. When the heli got about 3 inches off the ground the entire main rotor departed the mast and judging by watching the owner’s eyes, it must have climbed at least a couple of hundred feet (you could see his eyes watch it go in a big, curving arc, before he walks off to collect it!) which suggests that it remained reasonably stable for long enough to impart quite a bit of upward momentum.
Just a couple of thoughts:
If the aircraft propeller detaches and does move forwards cleanly, then the initial thrust force is several thousand Newtons, but the propeller mass is only a few tens of kilograms, so the initial acceleration is hundreds of metres per second squared.
But as the propeller disc gains forward speed, its angle of attack (and so its thrust) decays.
I think the steady state speed is where thrust equals drag, but now if all forces are of the same order as drag forces, we have a ballistic tumbling stick.
Because the initial acceleration is so huge I imagine this happens in a few tenths of a second.
(Order of magnitude, it should be an exponential decay).
The helicopter rotor case is different, because it moves upwards, not forward.
We start arguing like we did with the propeller, but now the steady state speed is where thrust equals weight plus drag.
So long as weight comfortably exceeds drag, we could have a reasonably aerodynamic rotor.
As the rotor rotation speed decays, more angle of attack is required to match weight, and the ‘steady state’ vertical speed changes accordingly.
Presumably this goes on until a stalling angle of attack is required.
Presumably this goes on until a stalling angle of attack is required.
The propeller will start windmilling once the aoa goes through zero. This will keep the angular momentum up and stabilise it.
I know the FI. Kind of funny to see him again in these circumstances. Shaken, not stirred.
For those who have not google translated the article, he says the propeller appeared to split in half so each of the blades went off to either side of the aircraft. They tried to reach ENKJ twenty something nautical miles away but ended up doing a power-off landing in a field.
