Props Driving Piston Engines

[SB4200]

In prop aircraft obviously the engine is designed to drive the propeller as opposed to the other way around. As I understand it, windmilling props (props driving engines) are a bad thing. Could someone please explain why with respect to piston engines.

For a flight with proper descent and approach planning I believe this should only occur during the landing phase and for a short time. What are the implications of an aircraft operating with a windmilling prop for extended periods of time?

Examples: - flight training simulated engine failures
- fixed pitch prop aerobatics

Obviously the engine is not going to fail the second the order is reversed, however does it affect engine life and TBO?

[EDML]

This rule comes from the good old days of radial engines.
It was caused by lubrication problems with the special
crankshaft concept used in a radial when it is driven the
"wrong way round".
That is no problem with the current engines in GA aircraft. -
With current engines there are only the fears of "shock-cooling"
the engine during long decents with very low / no power.

Marcus

[k12479]

This article describes it pretty well:
http://www.warmkessel.com/jr/flying/td/jd/78.jsp

[turnarounds]

Any geared engine the engine should always drive the propeller eg GTSIO-520 not so critical in ungeared engines.

[411A]

With large geared piston engines, the propellor should not 'drive' the engine for two very distinct reasons...

1. Accelerated master rod bearing wear.
2. Piston ring float (chatter).

Number one clearly applies to radial engines, number two to both, radial and opposed.

Keep in mind that these two conditions are present during landing, and this is not a problem.
However, prolonged flight at low manifold pressures (and/or BMEP) with higher RPM's should be avoided.

For further information on large piston engines consult www.enginehistory.org

Pay particular attention to the subject of bifler pendulum dampers, and the bushings these ride on, as mishandling the throttle on these large engines is NOT good.

[A37575]

Keep in mind that these two conditions are present during landing, and this is not a problem

True. I recall landing a DC3 and the captain lambasting me for reducing power below 20" MP on final approach as he said it would damage the engine. He was one of the old school who had thousands of hours on DC3's but from his lack of knowledge of engine handling it was clear he had never bothered to read a book on the subject. There are mechanics around of the same old school.

Even with full flap down it was well nigh impossible to reduce to correct threshold speed because the old man in the left seat held his hand firmly behind the throttles. With excess speed we floated for ages with power still on and finally thumped down in three-points which DC3's don't take too kindly too.

I wrote to Pratt & Whitney to get the good gen on engine handling of the 1830 and the reply said that at the low indicated airspeeds in the circuit and on final, there is no chance of engine damage with low manifold pressure and rpm at 2050 or 2300 rpm. I showed this to the old bloke and after squinting at the document through his look-over glasses stuck on the end of his nose, he snorted with derision and threw the letter into the waste paper basket saying he had flown DC3's for decades and wasn't going to change his opinion on engine handling.

[AHRS]

stubborn ol timers!

Boy! Dont these ol geysers bring amusement with their eccentricity in the cockpits.I am glad it was a DC3.the same ol chap would be quite bewildered with al his machismo moustache and a pipe clenched tightly between his lips...and bi focal semi lunar look overs on his nose to match.with all the loaded digital technology; he would seretly look up to the felow on the right hand seat for guidance as a blind ol man being led by a competent and strong youth!

You might even benefit from him by authoring a satirical text on flying.i would be only too willing to co author it buddy!

[Feather #3]

In the case of one major operator of big round engines, negative torque has caused the overhaul of more than one of their VERY expensive toys!??

AHRS' comment about eccentricity is fine up the point that it makes the operation dysfunctional!

G'day

[100BMEP]

Hence my "nick"

100 BMEP...guess it gives my age away!
DC-4, 6,7, L1049, C46..lets see..R2000, R2800, R3350...
I often wondered about the different engine shutdowns, failed cylinders, etc..how many were caused by someone earlier..pulling the power off!

....and then I learned about P7s...EPRS...N1s...N2s..N3s...EGTS..almost forgot to the Dart's TGTs.

[BigEndBob]

Always thought it was bad for any piston engine to run off load, as the conrod is no longer push the crankshaft, but the shaft is banging against the conrod.

I always wince when my car goes in for MOT and the tester thrashes the engine for the emission test.

[Brian Abraham]

During ground school on the R1820 seem to recall (fading memory) one of the exhibits was a piston which had the top removed as a result of underboosting. Memory being what it is, would this be a result of high power and snapping throttle shut. Or am I up a tree.....again?