Windmilling prop/alternator?

[201er]

I'm curious if in the event of an engine failure the windmilling prop provides enough power to keep the alternator charging? On the other hand, if electronics are no longer needed, does pulling the alternator field reduce drag and extend glide any?

[carusoam]

Alternator output is directly proportional to prop rpm, in a not so linear fashion.

In my O, the alternator will provide so small of an output while taxiing (700rpm?) the red light comes on indicating low voltage condition.

To get an actual answer for your alternator...  The alternator company provides a chart for output vs rpm.  You may have to include a pulley ratio in your calculation.

somebody had posted this chart for their secondary alternator for their IO550 a few months ago.  

See:

http://www.bandc.aero/pdfs/quickfacts_bc410-h.pdf

From fuzzy memory, over 1000 rpm will get you some electricity, 1700 rpm will get you a lot more, 2000rpm get so much the battery will recharge....

So, yes, with the prop windmilling, you are generating electricity.  Can you feel the braking effect when you downshift into 2700rpm?

There is a lot of friction when compressing air molecules in the cylinders.  They aren't just rotating pieces of metal...

Also keep in mind that the vacuum pump will be working as well.  Keep an eye on the vac gauge to be sure...

Disabling the alternator should also show a 'technically measurable' effect of saving mechanical energy.  This would require a torque vs output chart from the alternator company.

We need to set up some Mooney glider races, don't invite Seth!     (Seth has a Missile with a full feathering glider advantage)

An engineer's PP thoughts, not a mechanic or CFI....

Best regards,

-a-

[cnoe]

On 7/16/2016 at 8:23 AM, 201er said:

I'm curious if in the event of an engine failure the windmilling prop provides enough power to keep the alternator charging? On the other hand, if electronics are no longer needed, does pulling the alternator field reduce drag and extend glide any?

As a J driver who's done a fair amount of glide testing in my bird I'd say it's wasted effort attempting to extend a glide by minimizing the alternator load. Theoretically yes, but practically no.

Instead pull out the prop control, keep the airframe clean, and nail your best-glide speed the whole way down. My J glides MUCH further than what it says in the POH.

I truly believe that from a high altitude you would greatly extend your glide if you slowed to an airspeed low enough to stop the prop and THEN trimmed for best glide. But to this day I've been too chicken to test it out in person.

[mpg]

We did some engine out glides in my CFIs hand propped Airkknocker,,,

then slowed it way down to stop the windmill,,  even better glide.

Dive a little, a little more, faster,, then the windmill started up, feed it some mixture and its running again!

[Guest]

Pulling the prop to full coarse sure extends the glide in my RV4.

Clarence

[Raptor05121]

4 hours ago, mpg said:

We did some engine out glides in my CFIs hand propped Airkknocker,,,

then slowed it way down to stop the windmill,,  even better glide.

Dive a little, a little more, faster,, then the windmill started up, feed it some mixture and its running again!

This is another point- if the engine stops. Do you attempt to stop the prop from windmilling? I've heard a spinning prop has increased drag over a stopped one.

[carusoam]

Yes, when you have lots of altitude, and you want the extra glide.  

There are challenges....

- you have to slow the plane a lot to get the prop to stop.  Slow enough that you are bellow the best glide speed for a while.

- At best glide, the prop may begin spinning again.

- To be knowledgable about how this effects your plane, you would need to practice stopping the prop and know what speed it starts back up at.

I believe CNOE (or somebody recently) may have done this type of work (?) outlining his glider experiments...  You may review his writing on the subject.

I have not determined these numbers for my plane myself.  I would make note of them if anyone has IO550 and or TopProp data.

Best regards,

-a-

[cnoe]

Actually as I understand it you can stop a windmilling prop most easily by keeping the control full forward so that it's against the stops. Keeping it there lessens its tendency to resume windmilling (which results in less drag and enhanced glide). But if you're windmilling then pull the control back to coarse pitch by all means. John Deakin discusses this considerably.

Pulling the prop control out in an engine-out sets up bad harmonics in the IO360 but it beats coming up short. I may gather some data on stopping the prop entirely and how it affects glide in the coming months. I will share my results.

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