RV8/8A Best Glide

[scsmith]

Originally Posted by wawrzynskivp

I did my testing by pulling the mixture cut off and allowing the engine to windmill no-power.

It seemed that 12" MAP or higher improved the glide. Throttle at idle stop was the worst position. Again, no power just tinkered with windmilling MAP.

Interesting. OK. I was wrong.

[wawrzynskivp]

I assume each engine/prop/airframe combination will be different.

With the prop at full coarse pitch I observed that the prop spun faster as I began to open the throttle up to about 12"

There is the conundrum that a spinning prop is extracting work from the air and it might be reasonable to assume that if it spins faster then it is taking more work. This was certainly true of decreasing pitch, but seemed not so much the case when simply tinkering with MAP.

I really got the impression that allowing a little more flow of air into the engine reduced the work being taken by the prop.

[scsmith]

For a given pitch setting, the prop will spin fastest when no work is being extracted from it. Any braking torque from turning the dead engine will slow the prop down. So your observation of the rpm is consistent with your observation of the effect on glide.

[rvbuilder2002]

The engine off glide testing we did this past summer indicated that prop control position had zero influence on glide performance. That at normal best glide speed’s, relative wind just doesn’t spin the engine fast enough for the governor to have control of the prop. At least that was our experience.

[skylor]

Originally Posted by rvbuilder2002

The engine off glide testing we did this past summer indicated that prop control position had zero influence on glide performance. That at normal best glide speed’s, relative wind just doesn’t spin the engine fast enough for the governor to have control of the prop. At least that was our experience.

This seems to be dependent on prop governor. On my IO-360A1B6 with MT (Avia) governor, I have full prop control down to very low engine speed. In my own "true power-off" glide testing (mixture pulled), at 100 kias with the prop lever forward, the engine windmills at ~ 1500 - 1600 RPM. with the prop lever pulled back it will windmill at 650-700 RPM. This makes ~ 20% difference in altitude loss through a 360 degree gliding turn at 45 degree bank. At lower air speeds, the engine will spin even slower with the prop lever back - down to 200 RPM. Also of note, if my engine is shut down with the blades at full coarse pitch, they will stay there for some period of time until I push the prop lever forward. I found that out when the engine was new and I cranked it with the plugs out to build oil pressure before first start. I inadvertently had the prop lever aft. A little while later, after I was done with the pre-oiling I noticed that the prop looked "different". I realized that the blades were in the full coarse pitch position, which I wasn't used to seeing. When I pushed the prop lever forward, they returned to fine pitch.

Skylor
RV-8

[scsmith]

That's interesting Skylor. This suggests there is a check-valve feature in the governor that prevents or restricts bleed back of the oil pressure.

My old MT governor gave control to very low RPM, just as Skylor describes. My Hartzell governor loses control at 1600 RPM or so, so has no effect on engine-out.

Would be interesting to know how other mfg's governors behave.

[skylor]

Originally Posted by scsmith

That's interesting Skylor. This suggests there is a check-valve feature in the governor that prevents or restricts bleed back of the oil pressure.

My old MT governor gave control to very low RPM, just as Skylor describes. My Hartzell governor loses control at 1600 RPM or so, so has no effect on engine-out.

Would be interesting to know how other mfg's governors behave.

Steve,

I don’t know if there is a check valve in the governor or if the internal oil pump is a constant displacement pump that just didn’t have any significant internal leak-by when new and therefore just didn’t allow backflow when the governor control valve was in the increase-pitch position and the engine wasn’t turning.

Also, my engine will increase RPM if I experience brief oil starvation during aerobatics thus in flight the governor doesn’t seem to lock up pressure to the prop hub although I can speculate a number of reasons why this might be the case.

Skylor

[Saville]

Originally Posted by Ironflight

I’ve been doing some prop testing recently which involves a lot of sawtooth climbs. The tricks are to do it first thing in the morning, when the air is very smooth, and to start well below your target altitude band, to give you time to get on speed.

I dive to 1,000’ feet below the planned band, throttle at idle for the dive, then slow to the target airspeed, then add throttle to climb power while concentrating on airspeed. There is a little bit of airspeed wobble going through the power transition, but by a couple hundred feet below the target band it is sorted out, and if the air is smooth, the speed can be held - you just have to divide your attention between the ASI and the pitch picture outside, and stay ahead of the oscillations.

Paul

Hi Paul,

I went out again after reading your post, here, and used your technique.

It made getting ON speed and staying on speed much easier. So thanks.

I also found that getting a simple to use digital stopwatch made things a WHOLE lot easier. I was using a timer that had buttons and it was hard to do everything I needed to do and get the fingers on the buttons.

During multiple practice runs, I detected another technique error I was making:

Once on speed I was correcting any slight deviation by using the same technique I used on the throttle when formation flying:

1. Add a little power (if sucked)

2. Just before position removed added power and then some - you have to stop the acceleration

3. Go back to the "nominal" power setting.

Well I was doing that with the pitch. At the slightest speed deviation I would adjust pitch by adding more (if I had to slow down)

then taking the addition out and then some

Then going back to the "nominal" stick position.

That, as you might expect, failed.

So I learned to just make the tiniest of stick adjustments in the proper direction and hold that. That worked much better.

I still have to practice - I'm looking at the airspeed indicator way too much - but that's for the next ride.

Thanks everyone.