Determine stall of prop at Static?
 

How do you determine when a prop will stall at Static (plane pulling out of a hover)? I thick that for practical purposes a 15 x 4 prop will not lose much efficiency at 12,000Rpm due to the prop stalling at static, while a 15 x 15 will lose static efficiency at 12,000 Rpm because much of the blade is stalled. My guess is that it has something to do with not only the pitch, but also the speed at which the air is moving over it. For instance, I would guess that the 15 x 15 would not stall at 50 rpms and the 15 x 4 would have stall at 50,000rpms. Please someone shed some light on this for me. Thanks
How do you determine how much thrust you losing at static?

RE: Determine stall of prop at Static?
 

You've picked a couple of extremes here, that's for sure.

RPM's don't enter into the issue. It's all strictly about angle of attack. For a prop that's not traveling forward the angle of attack is given by the local angle of the prop blade. In reality it's a trifle more complex than that because the prop tends to create it's own airflow with the air being drawn into the disc on the front and pushed away to the rear so the angles are somewhat mixed up. But basically a low pitched prop will not be stalled and a high pitched one will if there is no forward speed of your model.

Basically if the 15x4 isn't stalled at 1500rpm it won't be stalled at 15,000 either. At 50 rpms the 15x15 SHOULD be stalled but low speeds of that nature can do some funny things sometimes. But ignore that example. Certainly I think it's safe to say that by 1500 rpm it would be stalled and stay that way regardless of the rpms.

But all this is for static operation at zero or very low airspeeds. As the airspeed rises at some point the 15x15 will become unstalled because the air is moving into the prop disc faster and the angle of attack seen by the blades will become less. Assuming the drag of the airplane and the engine power will let the prop reach that speed. If it can wind up and the prop is operating well the final speed of the model will be very high for a given rpms from the engine. Meanwhile the 15x4 will accelerate very fast thanks to it's low pitch but it'll top out it's speed at a much lower value assuming the engine is turning the same rpms. But all this is ignoring the fact that to turn these props at the same rpms will take probably someting like 6 to 8 times the power for the 15x15.

But a stalled prop still produces thrust. It just can't spin as fast as the engine would like because the extra drag of the stalled blades prevents the engine from revving up to it's peak power rpms. This is why fixed pitch racing full sized and model aircraft often take a lap or a bit of time and distance to come up to full speed.

RE: Determine stall of prop at Static?
 

I understand that the dynamics change as the plane starts to move forward-this is why I specified static. Yes I also know about H.P. and the relation of of prop pitch and diameter. What I don't understand is at what pitch do I lose efficfency do to the blade being stalled. You say that the speed at which the air is flowing over the blade(RPM) has no baring on stall, just angle of attack, there seems to be something wrong with that-but for this aplication what you say is propably true.

I know that I would take a lot more h.p. to turn the 15 x15 than the 15 x4 12,000 RPM But if the Static thrust measured with the 15 x 4 was 16LBS.

What would you guess to be the static thrust of a:

15 X 1 at 12,000 rpms
15 X 2 at 12,000 rpms
15 X 6 at 12,000 rpms
15 X 8 at 12,000 rpms
15 X 10 at 12,000 rpms
15 X 12 at 12,000 rpms
15 X 14 at 12,000 rpms
15 X 20 at 12,000 rpms

At what pitch does the stall make the prop inefficient at static?

RE: Determine stall of prop at Static?
 

According to my Thrust/HP calculator, thrust stays constant with same diameter and RPM. So, every one of those different pitched 15" props turning at 12000 would produce the same thrust. It would require different amounts of HP though.
basmntdweller

RE: Determine stall of prop at Static?
 

First of all you have one thing limiting your efficiency before you start checking different prop pitches. This limit is your rpms, a prop produce its maximum efficiency at tip speeds between .80 - .90 of the speed of sound. Anything over that range and you start dealing with compressibility issues. Once you have calculated your max rpm for your blade then you pitch is the determining factor to maximize thrust. As Bruce stated, a propeller will stall at a certain AOA, just like a wing, when in static conditions. In order to determine this we need to know the Cp factor for the propeller and normally thats not readily availble at a modeler level unless you request specs from the prop manufacturer. A prop will reach a maximum amount of thrust at a certain angle and certain rpm and any increase in pitch angle afterwards will just increase the amount of drag and lower the amount of thrust. Every propeller manufacturer uses a different airfoil combination so the Cp factor will change from each manufacturer. You could perform static tests using a fish scale and your model at different prop pitches. You will noticed that the best static thrust are usually in the 6 to 8 pitch region for the same diameter props.

RE: Determine stall of prop at Static?
 

BMatthews,

Are you saying that the 15 x 1/2 would develop the same static thrust as a 15 x 6 turning at 12,000 rpm. I know the thrust calculator indicates this, but it seems like there is something missing here. I would like to see someone do some Empirical tests here.


6 - 8 pitch sounds good for maximum, although I thought it would be closer to 4 to 6 because I have herd people talking about 8 pitch props having stall at static.

Thanks for all the help everyone.

Steve

RE: Determine stall of prop at Static?
 

Here's a short video (307 Kb, Divx) of my electric pylonracer during launch. It has a 5x5 prop turning about 22000 RPM. You can hear the "woosh". After about 2 seconds the prop unstalls and the "woosh" dissapears quite suddenly.
[link=http://213.84.42.244:8087/paulraaf/stalled_prop.avi]http://213.84.42.244:8087/paulraaf/stalled_prop.avi[/link]

Paul

RE: Determine stall of prop at Static?
 

That's just WRONG! THere's something missing there.

RE: Determine stall of prop at Static?
 

Steve, if you're looking for 3D performance with hovering and other prop hanging stunts being a priority then you are best to stay with props in the 3 to 6 inch pitch range and experiment with options until you find the one that works best for you and your model. But I'm pretty sure you won't find a 15x3 and even a 15x4 is going to be scarce. For example I see that APC only makes a 15x6 as the finest in that diameter but they do have a 14x4W that is spec'ed for Funfly.

Paul, nice sound track. You can definetly hear the difference.

RE: Determine stall of prop at Static?
 

Steve,

The the equation that ThrustHP use has a fixed Ct which does not take into account pitch....this is not a very good reflextion of whats going on.(Thrust=Ct*q*rps^2*diam^4). Ct is set to .0888 no matter what the pitch. Actually, for most props, Ct range from .05 to around .12.
(see [link]http://naca.larc.nasa.gov/reports/1932/naca-report-378/[/link])

In reality, if you take the 75% blade angle of a prop it will roughly compare to the normal stall pattern of a wing. At around 10 degree absolute AOA the blade will start to stall at around 19 degrees the prop is pretty well stalled (but still generating thrust). Around 6 degree AOA is the most effeceint so I would recommend a prop with 7 to 10 degrees AOA at 75% of blade if you want the greatest static thrust and practical use for 3d.

If you want to find the Blade Angle at 75%:

Angle = acr tan(pitch/(2*pi * (.75*r))) or for pitch

Pitch= tan(angle)* 2 * pi * (.75*r)


steve

RE: Determine stall of prop at Static?
 

Thanks for all the help guys I need all I can get.

Very impressive DipStick! Please keep me posted on your progress and if there is any info I can read on the subject on line please make me aware of it.

Paulas, I can hear the video but not see it. The wosh is very pronounced.

O.k. this is going to rock my world if a 5 pitch prop that is only 5" and tuning at 22,000 is in a stalled state. Either the prop is not holding 5 pitch or it does matter what the speed of the air is that is passing over the prop or both.

RE: Determine stall of prop at Static?
 

I believe the "whoosh" we hear at first is cavitation just before the airflow reaches steady state. Othewise the prop would continue in the stalled position. A wing or prop would not "unstall" unless there is a decrease in AOA.

RE: Determine stall of prop at Static?
 

Cappio777

I was just doing some calculating and according to my #'s in order to even get to .8 the speed of sound a

13" prop would need to turn 16,000rpm
14" -15,000
15"-14,000
16"-13,000
17"-12,000

Do I have these numbers correct? If so, a sport motor would not achieve these numbers at static with a 6 -8 pitch prop. I know I was the one who started with the extremes. I do want to learn about this stuff.

RE: Determine stall of prop at Static?
 

Does this cavitation disappear if you don't let the plane move forward?
I though of "cavitation" as a result of the blades being stalled. What is cavitation?

RE: Determine stall of prop at Static?
 

Technically I don't know if we can call this cavitation since I always understood the term to refer to air coming out of suspension in water or the water "boiling" in the precense of very low pressure. In the case of boat propellors close to the water's surface the air can be sucked down to sit on the low pressure side of the prop and they call that cavitation. In the case of submarines or deeper surface props the prop driving too fast creates such a low pressure zone on the forward face that the water actually boils to a gas state even though cold. The problem goes away as the depth and pressure rises. I'll bow to anyone that can show me a different definition though.

But there is no doubt that props can suffer from bad separation bubbles ( which is a sort of cavitation I suppose) on their forward faces due to the low pressure and that shows up as that woosh sound.

I was involved in a bunch of static thrust testing of electric motors and props a few years back. We noticed that wooshing sound on many props. The current always took a big jump for the amount of thrust we got when that sound was present. It didn't take long to figure out that the blades were stalled and that was what caused the extra current demand. The stalling happened on various props and pitches but it was usually in the 8 to 10 inch pitch range. Bear in mind though that we were using airfoils on these props that had some undercamber to them and carefully shaped blades. The stock Zingers we used as a comparison showed stalling figures consistently in the 8 inch pitches and some in the 6 inch pitchs IIRC.

Anyway, the separation bubbles will dissappear as the Angle of Attack reduces. This reduction takes place automatically as the model gains speed. And that's why Paul's model audio is getting a bit quieter just before and after the woosh stops. He's launched it and the model is accelerating away from him and the microphone.

RE: Determine stall of prop at Static?
 

Yes, there is a critical point where the separation will occur regarless but before that its a matter of the motion of the ambient fluid (air). Given that a propeller is first started when the ambient fluid is at zero velocity the intial acceleration of the fluid mass could take it near compressible conditions, but as soon as the fluid passes the transient stage into the steady state condition the velocity becomes non-zero and the compressibility effect is reduced. Thats when the sound stops and the thrust increases. This effect is more noticeable when there is a duct around the propeller (ducted fans and wind tunnels).

RE: Determine stall of prop at Static?
 

If I am understanding correctly, you are saying that you think that the blades are stalled on the 5 x 5 prop that is turning 22,000rpm. My guess is that they would not be stalled at 10,000 rpm. Did I miss something here. Was it that the blades were accelerated so fast that the blades became stalled? Would that mean that it does matter what the speed of the air is traveling over the blade? The tip speed of this prop is only 480FT/SEC. Is it true that the blades will come out of stall if the plane stays static?

RE: Determine stall of prop at Static?
 

OK, I am no aerodynamic engineer, but I will try to explain this as best as I can....

Why a 5x5 prop is stalled?? Simple, the pitch we see in these props is the theoretical forward advance the prop would make in one revolution, so in one revolultion, theoretically, this prop would advance 5 inches. Given it's small diameter, the angle needed to acheive this is higher, so the prop stalls. This pitch is measured at the 75% radius so this would give:

Radius at 75% -> (5/2) * .75 = 1.875 inches
Angle for 5in pitch = atan(pitch / (2 * pi * radius at 75%)) thus, with pi = 3.1416:

atan(5 / (2 * 3.1416 * 1.875)) = aprox. 23 degrees

Standard airfoils (without fancy mods to stall at higher AoAs) stall at 10 to 15 AoA....So at 23 degrees that prop is certainly stalled.

But as the model speed increases, the angle of attack of the prop decreaeses, due to simple vector physics. The AoA is:

Pitch angle - atan(RadialVelocity / ForwardSpeed)

Radial velocity (in m/s) is given by:

2 * pi * (RPM / 60) * Radius at 75% in meters.

RE: Determine stall of prop at Static?
 

Now let's use as an example a 15 x 4 prop. The angle at 75% radius would be:

Radius at 75% = 5.625in
Angle = approx 6.5 degrees.

A 15 x 5 prop (same pitch as the 5 x 5) would have a:

Radius at 75% = 5.625in
Angle = 8 degrees

So, these props aren't stalled.

From the other equations, further conclusions can be drawn.

The higher the pitch, the better the high speed performance because the relative AoA (the angle at which the air strikes the blade) is higher with higher speeds, so larger pitch props have a higher blade angle, so they take longer to achieve 0 or negative lift.
The lower the pitch, the better the static thrust because the prop is near it's most efficient stage, so it develops more thrust at this stage. But on the other hand, as speed buildups and the relative AoA takes a shorter time to equal the blade angle, thus the blade get's negative thrust at lower speeds.

I hope this clarifies what you're asking for....

RE: Determine stall of prop at Static?
 

That's also why the prop "unloads" in flight, the real AoA (the angle which the blade is striking the air....) get's smaller with higher flight speeds, thus the blade drag decreases and so does the load on the engine, then it increases it's revs.

RE: Determine stall of prop at Static?
 

Wow, interesting discussion. Your explanation makes sense Eagleone!

Thanks,
Paul

RE: Determine stall of prop at Static?
 

Thanks for the info. It would help if I knew more about props-what pitch was and stupid me, but I never put together that 5 pitch on a 5" prop had a different AOA than a 15" prop ect. Thanks for working the equations out. I guess thats how you learn... by asking stupid questions and exposing your ignorance.