Quote:
Originally Posted by Graham Boak
Propeller tips are often bent forward when the engine(s) turning at impact. Remember that it is the rear of the blade that meets the air first (if this seems odd remember that the prop is seeing the sum of the forward and rotational velocities).
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Hi Graham. Yes, the bent forward tips from contact with turf, soil or water occur where the prop is turning under power and the blade tip chord line has a positive angle of attack to the material it is passing through. In this state, the length of blade passing through the turf, soil or water under power is usually quite short and so the deflection of the blade tip is relatively small. Usually, this is the result of a slight prop strike under power on a T/O or run-up that does not involve full or further contact. In the case of blade tips contacting when turning not under power, such as landing with reduced throttle, if the angle of attack is negative relative to the turf, soil or water they pass through, the tips will be deflected back. In both cases, if the contact develops into gross contact the blades are likely to be grossly deformed, usually ending-up bent backwards if the aircraft continues traveling forwards as the engine slows and stops.
In the case of the aircraft coded K, here fully on its belly, you can see that the left engine propeller sticking uppermost has a near 90 degree sharp bend of the last 30cm or so. Additionally, the lower blade partly visible on that engine in the first photo, looks distinctly bent forward over its length. The right engine on K, that has had a heavy impact and lost a blade, has a slightly forward bent blade. I would judge that the left engine blades were bent forwards as the aircraft finished its movement backwards with the U/C collapsing and the engine stopping in a single turn. The right engine has undergone such a heavy and complicated impact that, it is impossible to be certain from these photos exactly how that upper blade bend occurred. Cheers
SM