steve murray

steve murray

Well Known Member
I went up this past weekend to establish best glide speeds for engine out condition. Climbed up to 12.5K and shut the engine down. I have a 3 blade MT electric constant speed prop. It took me a while to figure out how to stop the blades, feathering the prop and slowing down to a stall speed would not stop the prop. I needed to drop the nose and pull up to an agressive stall to stop the blades.

After the blade rotation was stopped, I transitioned from 90 mph glides and then 80 mph glides. At 75 mph I began to get stall buffeting? this is about 20 mph higher than my normal power on stall speed.

I climbed back to 12.5K and repeated the test with same results. The 75 mph stall speed really suprised me. The stalls were much more gentle than normal, and it appeared more of a porpoising up\down vs. clean stall break that I am used to with the engine running.

Any thoughts on the much higher stall speed of 75mph? :confused:
 
What aircraft?
Your 20 mph lower stall of 55 mph/ 48 knots seems too low. I'd suggest your static port is being affected by the prop.
 
Stall speed will be much higher at 12.5K!! and if its an 8 they often buffet early due to the leg intersections.

Dave
 
Stall IAS doesn't change with altitude. Stall TAS does, but that's not what we fly by.

If you stall at 45 KIAS at sea level, you'll stall at 45 KIAS at FL180! But your TAS could well be 105....
 
A couple of thoughts:
I see nothing wrong with testing stall speed with the prop not turning.
Realistically, your stall speed and best glide speed should be determined with the prop windmilling, especially in light of the fact that as you stated, it was quite hard to get the prop to stop. In other words, it took some time to make that happen and looking for a place to put down would be a better use of your time during an emergency rather than trying to get the prop to stop windmilling.
All production airplanes list these speeds with the prop windmilling.
I can't prove this and I haven't tried it but it seems to me that even an idling prop produces some power at low speeds and may in fact help you lower the stall speed. Clearly a power on stall occurs at much lower IAS than a power off stall.
 
I loose 20 kts on a stopped prop when my prop is horizontal. Interrupts air over the wings. Would be my best guess here on your 3-blade.
 
Steve,
On standard Hartzell C/S Props which are consdered to be Constant Speed Non Counterweighted Props they have the capability of low positive pitch an high positive pitch and do not have an air charge and do not feather... so lets looks at how they work... and perhap shed some light on your electric.

Centrifugal twisting action on the blades move the blades to low pitch (less bite)

A spring in the dome assist in moving the blades to low pitch when RPM decays and takes the prop to the low pitch stops whe the prop stops.

Oil pressure opposes the spring and the centrifugal twisting to move he prop to high pitch setting ( Cruise setting ) (bigger bite)

If on this type of Hartzell prop you lose oil pressure to the governor, the prop will automatically go to low pitch ( Take off setting) or engine off setting. because the spring is not opposed by the oil pressure.

Therefore in flight if you kill your engine, the prop will go to low pitch regardless of where the prop control is set at because you have no oil pressure.. this is like putting a piece of plywood out in front of the aircraft an will slow you down. Increase stalling speed) because drag is increased)

Your electric prop might react in a similar way with a spring in the dome when it detects no rotation... It could? bypass any electric governor's action?????? taking the prop to low pitch...?

Just food for thougt.

Smilin' Jack
 
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steve murray said:
I went up this past weekend to establish best glide speeds for engine out condition. Climbed up to 12.5K and shut the engine down. I have a 3 blade MT electric constant speed prop. It took me a while to figure out how to stop the blades, feathering the prop and slowing down to a stall speed would not stop the prop. I needed to drop the nose and pull up to an agressive stall to stop the blades.

After the blade rotation was stopped, I transitioned from 90 mph glides and then 80 mph glides. At 75 mph I began to get stall buffeting? this is about 20 mph higher than my normal power on stall speed.

I climbed back to 12.5K and repeated the test with same results. The 75 mph stall speed really suprised me. The stalls were much more gentle than normal, and it appeared more of a porpoising up\down vs. clean stall break that I am used to with the engine running.

Any thoughts on the much higher stall speed of 75mph? :confused:
Click to expand...

I do not understand how a stopped prop can change stall speed by 20 mph. The prop just sits there, the blades creating some drag depending on pitch. The entire wing can not have its air flow disrupted by the dead prop, it does not make sense. Something else is going on here with regard to IAS.

But if it is so, clearly you are better off with a wind milling prop due to a lower stall speed on landing. The crash foot print will be quite different contacting earth at 75 mph verses 55 mph.

I've had one engine out landing with the prop stopped due to a prop failure and do not practice the maneuver. The airplane flies just fine with no power and there's plenty of time to find a spot to land if it happens at altitude.
 
David-aviator said:
I do not understand how a stopped prop can change stall speed by 20 mph. The prop just sits there, the blades creating some drag depending on pitch. The entire wing can not have its air flow disrupted by the dead prop, it does not make sense. Something else is going on here with regard to IAS.

But if it is so, clearly you are better off with a wind milling prop due to a lower stall speed on landing. The crash foot print will be quite different contacting earth at 75 mph verses 55 mph.

I've had one engine out landing with the prop stopped due to a prop failure and do not practice the maneuver. The airplane flies just fine with no power and there's plenty of time to find a spot to land if it happens at altitude.
Click to expand...

When you try it you will see and feel the action. 4 bangers dont get a prop stopped at horizontal due to clocking and compression location.
On my RV-8, with the 6cyl, and an 80" prop, I have to pull to vert hammer to get the prop to stop. It has a lot of momentum and weight. After stop, flying at best glide, the prop will move to each compression slowly. Stopping on each compression stroke for 10 seconds or so till the compression bleeds down and it moves onto the next one. When it stops on the horizontal, you had better hang on, the whisps over the wing are immediate, and stall happens at ~70kts. The disruption of air flow over the first couple of inboard feet make a very big difference in stall speed.
Why bother? Well its good to know what the plane will do with a stopped engine. Sudden stoppages do happen of course for a number of reasons. And in my case, i have concluded that a high risk item is engine stoppages in a hammer 1'k agl. Our worst stoppage location risk is tops of hammers. If mine quits, I want to know exactly what to do and I practice it.
Further, even airborne zinging along from A to B, sudden engine stoppage is possible.
 
So, the long and short is: why not try your stalls w/ the prop windmilling and see what happens. Then you'll actually know what your stall Vs is.
 
whiskeypapa said:
So, the long and short is: why not try your stalls w/ the prop windmilling and see what happens. Then you'll actually know what your stall Vs is.
Click to expand...

The stall speed with prop (throttle fully retarted) is about 55mph, this is why I was quiet suprised regarding the 75 mph stall speed with a stopped prop. The stall chararterics with the prop stopped was a greater amount of buffet but the break was more gentle and mushy vs. a clean quick break that occurs with the engine at idle.

I think I will go back up again and measure the descent rate with windmilling prop (engine off) and stopped prop. One of the earlier replies regarding the time and lost altitude of stopping the prop may not be worthwhile and I definitelty would not want to make contact something at 75 mph if 55 mph is possible.

I think the descent rate in my RV8 with windmilling prop is going to be -1000 fpm. Any thoughts from folks on what is maximum vertical descent rate you can land ("controlled crash") and expect to walk\limp away from in an RV8?


Steve
 
steve murray said:
I think the descent rate in my RV8 with windmilling prop is going to be -1000 fpm. Any thoughts from folks on what is maximum vertical descent rate you can land ("controlled crash") and expect to walk\limp away from in an RV8?


Steve
Click to expand...

I don't know what kidn of vertical woudl lead to no injuries, but I'd hope that yu plan to flare before impact. :D
 
steve murray said:
The stall speed with prop (throttle fully retarted) is about 55mph, this is why I was quiet suprised regarding the 75 mph stall speed with a stopped prop. The stall chararterics with the prop stopped was a greater amount of buffet but the break was more gentle and mushy vs. a clean quick break that occurs with the engine at idle.

I think I will go back up again and measure the descent rate with windmilling prop (engine off) and stopped prop. One of the earlier replies regarding the time and lost altitude of stopping the prop may not be worthwhile and I definitelty would not want to make contact something at 75 mph if 55 mph is possible.

I think the descent rate in my RV8 with windmilling prop is going to be -1000 fpm. Any thoughts from folks on what is maximum vertical descent rate you can land ("controlled crash") and expect to walk\limp away from in an RV8?


Steve
Click to expand...

Anything above "O" fpm at touchdown will add to the outcome of an emergency landing. Save some for the flair.

The closer to 55 mph at roll out without hitting something, will help you survive the crash landing.

Throttle to idle abeam the numbers in my 6A. At 85mph, I see 800/850 fpm descent rate.... Very easy to control to touchdown. Turn base when the touchdown spot appears from behind the wing.
 
buffet with stopped prop

I have noticed the same thing and came to the conclusion that it is not pre-stall buffet but turbulent air passing over the tail surfaces. If you continue to increase the angle of attack the stall occurs at the normal indicated air speed. It is much worse in some aircraft than others.
Chuck Ross RV4
 
Steve, my -10 has the two-blade Blended airfoil prop and it makes as much as 400 FPM less sink (for the same speed) when I pull the prop knob all the way out to coarse pitch, and I've demoed that to quite a few guys.

Your electric prop SHOULD be able to go to coarse pitch as long as it has power, I believe.

I've stretched my otherwise short-of-the-runway, 180 deg power off approach by doing this...it'll really surprise you.

Best,
 
Steve,

8's are prone to tail buffet at certain AOAs. There is a strake mod added to the fuselage just above the inboard leading edge that can help mitigate. A search on the site will turn up likely some discussion on this topic and, perhaps, a photo.

Cheers,

Vac
 
pierre smith said:
Steve, my -10 has the two-blade Blended airfoil prop and it makes as much as 400 FPM less sink (for the same speed) when I pull the prop knob all the way out to coarse pitch, and I've demoed that to quite a few guys.

Your electric prop SHOULD be able to go to coarse pitch as long as it has power, I believe.

I've stretched my otherwise short-of-the-runway, 180 deg power off approach by doing this...it'll really surprise you.

Best,
Click to expand...

I had the MT7 (electric) prop with -7A and changing to flat pitch for a long range descent made quite a difference also. The course pitch was set manually or by resetting rpm and would not change with power loss, a good feature with a dead engine but not so good for a go around.

The observed buffet reported here being attributed to tail buffet makes good sense and is most useful information should engine rotation stop in flight. It is information most of us do not get from usual phase one testing. Thanks for contributing it.
 
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