FP Prop that has stopped in-flight

[Bad Sheila]

I you are at a high altitude and the engine has quit and the prop has stopped. Is there a speed you can acheive that would start the prop windmilling for an engine re-start if the starter motor won't do the job? Just curious? (I have an RV6)

[Kahuna]

yes and it all depends on everything. Your prop, your engine, your compression. Go try it and report back your results. Other results are in the archives under the search function.

[David-aviator]

Originally Posted by Bad Sheila

I you are at a high altitude and the engine has quit and the prop has stopped. Is there a speed you can acheive that would start the prop windmilling for an engine re-start if the starter motor won't do the job? Just curious? (I have an RV6)

If the engine quits and the FP prop stops, look for a place to land, the engine has seized and no amount of trickery will get it fixed in flight.

If the engine quits and the prop is deliberately stopped by slowing considerably, it can be restarted at some high speed. A couple guys here have done it may report what they saw.

[airguy]

Not an RV, but in my C-172 I've stopped it inflight, and I need to get up to about 110 kts to get it turning again without the starter.

[terrykohler]

IIRC, in my 160HP RV9A, If I pull the mixture in flight, I need to get below 80 kts to stop the prop. To restart, I push the mixture in and just let the nose drop slightly- never had to use the starter.
Terry, CFI
RV9A N323TP

[Kyle Boatright]

I've shut off the engine in flight to do glide testing. At 70 or 80 knots the prop stops. With my wood prop (low inertia which means it stops easily on the compression stroke), I needed 140 knots to get it to windmill sufficiently for a restart. That's a pretty steep power-off dive. ;-)

A FP metal prop would probably continue to windmill at a lower airspeed, and on restart would probably start windmilling at a lower airspeed too, just because of inertia.

[Snowflake]

Originally Posted by Kyle Boatright

A FP metal prop would probably continue to windmill at a lower airspeed, and on restart would probably start windmilling at a lower airspeed too, just because of inertia.

A stopped prop has *no* inertia. Wood or metal, the same diameter and pitch will start moving again at the same speed.

[simpkinsona]

Originally Posted by Snowflake

A stopped prop has *no* inertia. Wood or metal, the same diameter and pitch will start moving again at the same speed.

A stopped prop still has inertia. Inertia just means it resists change in velocity. A wood prop resists change less than a metal prop. I think what the previous post was pointing out is that the prop with the higher inertia will tend to use it's momentum to overcome the compression of the engine. As you dive and start the prop moving it will slowly overcome the compression of the first cylinder. When it goes "over center" and swings into the next compression stroke it will have more inertia to overcome the next compression stroke if it's a heavier prop. I suspect this would be a small difference but real nontheless.

-Andy

[n5lp]

I copied this post from another thread regarding my testing. Upon further reflection I don't know if it was cooler oil or just a cooler engine that caused different windmilling start speeds. My propeller is a Sensenich 72FM8-83

"I did rudimentary glide tests in my fixed pitch RV-6 with the engine off and a windmilling propeller and with the engine off and the propeller stopped. It did glide better with the propeller stopped but the difference was not great. I attribute that to the very course pitch of an RV fixed pitch propeller.

As you point out, you do lose glide while slowing down enough to stop the propeller so there is a crossover point that I didn't try to figure out. As for me I wouldn't stop the propeller unless the engine failure happened quite high, like maybe 4,000 AGL or so. With a warm engine I had to get down to about 60 knots to stop the propeller. It was more like 70 knots after the oil was cooler.

Another factor, depending on the nature of the failure, is that it would probably be much simpler to get the engine running again if it is already windmilling."

[Kyle Boatright]

Originally Posted by simpkinsona

A stopped prop still has inertia. Inertia just means it resists change in velocity. A wood prop resists change less than a metal prop. I think what the previous post was pointing out is that the prop with the higher inertia will tend to use it's momentum to overcome the compression of the engine. As you dive and start the prop moving it will slowly overcome the compression of the first cylinder. When it goes "over center" and swings into the next compression stroke it will have more inertia to overcome the next compression stroke if it's a heavier prop. I suspect this would be a small difference but real nontheless.

-Andy

What you posted is spot-on, except for the last sentence. The difference between an air restart of wood vs metal props is pretty substantial. It is similar to starting an engine with a wood prop vs a metal prop when you have a weak starter or battery - the metal prop's inertia makes a big difference.