Stalled!!!

[birdy]

Just wunder'n,how many others have stalled their props???[wot prop sizes and engines].

[Doug Riley]

Birdy, I'm not sure what you mean. Totally stalled? (Haven't seen that.) Too much pitch so that too much of the prop is stalled? Sure have seen that. In that case, the gyro won't climb well and will under-rev at climb airspeed, but as you speed up, it comes alive as if you'd just lit an afterburner.

I once had an over-propped Air Command 447 that only woke up over 70 mph. Scary with a high thrustline and no HS.

[birdy]

I had just opened the throttle fully,0 ground speed and as the machine started to move ,the old 462 w/c rotax screamed up to god knows wot rpm,but I wasn't moving.It felt as though the prop had cum off.As the engine wound down to a more sain rpm,the prop "grabbed" the air again and performed as though noth'n happened.

Only thing I could think of was it stalled.

[Chopper Reid]

My guess is that the prop was cavitating....just like a pump impeller when it cant pump any more water , you can rev the motor harder but the pump just cant push anymore water out.
Would love to see what the prop is doing when thats happening ...via a video camera :D .

[mceagle]

Blade "stall" is common on some of the higher horsepower engines with a slow propeller speed and running high pitch settings. Some of the Subarus running a 2.62:1 ratio have typically from 17 to 19 degrees pitch at the tip and they stall at high power settings at anything below 25 to 30 mph. The thrust from the stalled prop is minimal and the pilot caught out below these speeds has to immediately throttle back partially to regain thrust.
As well as lack of thrust, the "stall" is identified by a roaring noise and an abnormal vibration.

[Doug Riley]

Tim, Birdy, it's the sudden jump in revs that puzzles me. An airfoil doesn't experience a sudden drop in drag just because it stalls. A stalled wing, for example, is plenty draggy -- it's only the lift that drops off drastically when you go past the stalling AOA. The drag keeps growing as the AOA increases. A blade at 90 deg. AOA has lots of drag.

I wonder if the prop on the machine Birdy was in was torsionally limber. A cambered airfoil without any trailing-edge reflex has a tendency to twist leading-edge down, i.e. to de-pitch itself. We put reflex in rotor blades to keep them from doing this.

Props are usually stiff enough to resist the de-pitching tendency without a need for reflex, but a limber one might act like a rotor blade and tend to de-pitch. As you get the aircraft moving, the prop blade's AOA goes down (because of airflow into the prop) so the effect might not show up so readily once you're flying.

(Last weekend, I got to fly a new Dominator in which the prop blade grips were apparently not tight enough. The prop de-pitched and would barely fly with the Rotax screaming at about 7500 RPM. Once the prop was re-adjusted and tightened down, everything was fine.)

Other than that possibility, you got me.

[Chuck Irby]

I never heard of an air prop cavitating. I don't see how that would be possible. A water prop, or impeller, cavitates when it grabs air instead of water. Doug's implantation sounds reasonable. What type and make of prop is it Birdy?

[birdy]

Ya got a point there Doug. :confused: The increased rpm[no load]don't add up.
Maybe stall isn't the right word.
Cavitation only happens in water[or non compressable fluid].fluid behind the impella blade is under such a vacuum that it vaperises,causing bubbles.[well thats my SCG understanding.]
It appers to be a combination of both. :confused:

I know that when you shut off the nozzle on a fire fighting unit [centrifical pump] the engine has less load,open it up and the engine works harder.
This is wot seemed to happen when it "stalled".

It appears,a vacuum formed around the disc,causeing the prop to loose "lift" ,and have no [or very little] drag.
Or is my SCG status realy showing now. :D

BTW Chuck, it was an Ivo.Very flexable blades,but this would only have ment it would have flexed negative,decreasing the pitch and preventing it from "stalling".Maybe. :)

[darrellwittke]

On a simple cow growers note it seems to me the physics of the situation would say if no thrust being generated then would not newton's conservation of energy say that it has to be dissipated in rising rpm, ie conversion to kinetic energy? I like McEagle's post although I do not know if cavitation is the right word for it. Any truth to any of this Doug?

Ahh. Never mind. I just reread where you said a stalled blade doesn't lose its resistance therefore the energy must be converted into non useful vortices around the stalled blades. Why does this decrease blade friction however.

(I may be simple but I'm slow, bear with me I'll catch up!)

[mceagle]

Doug is correct according to my experience. The stalled blade creates more drag and in fact rpm's do not reach maximum.
If Birdy's 462 had a "C" or "E" box, then it is possible that it had a large gear slip on the propeller shaft. I have had experience with this and it dosen't happen for long. the shaft galls and it grabs again or the large gear retaining nut tightens itself again.

[birdy]

?????????????????
Aren't the gears KEYED to the shaft?
Wot longterm damage would that cause?
If this could happen,without damage,it'd make sence to me.

[Doug Riley]

Darrell, yes the energy of that burning gasoline has to keep going somewhere. However, drag and lift (or thrust) aren't two different things, they're the same thing.

When we shove air around, it reacts by pushing back against the object doing the shoving. To the extent that this reaction is in a helpful direction, we call it lift or thrust. To the extent it's not, we call it drag. With an airfoil, the reaction is usually in a direction that's somewhat "liftish" and somewhat "draggish." Therefore we speak as if there are two separate forces, lift and drag. This is often a handy way of looking at airfoils, but it's not strictly true and can be misleading.

A stall comes from too much angle of attack. In a stall, the air's total reaction force doesn't suddenly get smaller, it just changes direction. Right at stall, a larger chunk of this reaction force starts pulling in a direction that's "drag-wise" instead of "lift-wise." The reaction is still there just as in pre-stall, but it's not useful anymore.

As a result, I wouldn't expect a prop that stalled to surge. The thrust would drop off, but the drag would actually increase, so the engine would still be loaded.

A limber prop that de-pitched itself, though, would by definition experience a decrease in AOA. Less AOA means the air isn't being whacked so squarely... hence the reaction force is less. This means both lift and drag are less, less energy is being absorbed per unit of time from the engine, and the excess energy WILL then being used to accelerate the prop.

[mceagle]

Birdy, no the large propshaft gear is not keyed to the shaft. It is tensioned onto a taper.