RE: Prop effective speed
 

The problem is that the thrust vs power curve of a prop is complicated. At full throttle, level flight the thrust and drag will become equal. Depending on the drag of the model this may not be a very efficient regime to operate the prop. Worse is that top speed behavior may not have anything to do with how a model performs generally. For a glider, climb performance may be more important than top speed. For best climb you would want to use the largest prop possible, so 8 inches.

If I were designing, I’d look for a reasonable power to weight ratio and then see if my maximum size prop and available pitches gave something reasonable. I’d think using 6 W/oz is overkill for a glider, but you seem to want max-it-out. So that’s 6*54=324 Watt. At 7.4V, it requires a 44 Amp motor. The question then is whether an 8x4 is better than an 8x8 for this application and does it require gearing to get good efficiency. I don’t know. I’m sure the guys here with more experience can answer.

I can make a comparison to the 25 size biplane in my avatar. It has a lot of wing area, wing loading of about 10oz/ft^2, about 1/2hp (about the same as 370W) and is 3.25 lb. It flies like a sports plane with a 9x4, 9x5 or 9x6. It is very fast with a 9x6, much faster than a WW1 plane should fly. I usually fly with a 9x4.

It sounds to me like flying a souped-up 54 oz glider with an 8 inch prop is possible. And I don’t think a 40-50 Amp motor is that expensive. Maybe someone else can comment on a better configuration.


Hope that helps,

Carl

RE: Prop effective speed
 

The answer is Yes, a propeller with a certain diameter and pitch has a maximum forward speed where it stalls, being unable to further increase the speed even if the RPM is increased.
That's because the prop is then unable to further increase the thrust in order to exceed the drag.

Similar situation may occur during the take-off where the RPM is high and forward speed is low, a coarse pitch propeller blade is then at a higher angle of attack than optimum and may easily stall. Some racing aeroplanes may be sometimes unable to take-off for this reason.
At the other end of the scale, a propeller designed for greatest efficiency at take-off and climb (highest thrust) will be working at lower angles of attack than optimum at high speeds. This prop will accelerate the model very quick from standstill but will lose efficiency rapidly as the forward speed increases.

RE: Prop effective speed
 

The reality is not sinking in - -everyone has tried to say the same thing - that drag MUST be considered.
Siefring - that glider (? these are not gliders they are powered speed planes ) -unless I missed the point about being a pusher
If it is a pusher on a pylon then that's another thing
Based on the size mentioned - Iwould bet 200 watts is a minimum
and a folding prop on a pusher ? sumpin odd here ----

RE: Prop effective speed
 

Dick,

I'm not sure what he is looking for. Maybe he is trying to do glider racing?

I'm just trying to change the discussioin from prop efficiency near Mach 1 to something that will be more useful.

Anyway, I'm a big beleiver in looking at models that fly something like how you want to fly. Start the design there. Then you can extract and calculate your way to a new design with a fair amount of confidence.

Carl

RE: Prop effective speed
 

Thanks big time!:D I don't necessarily want souped up wild speed. I have been told, by others who built this model, that the recommended power setup is barely flyable. After much searching I found that we were stuck with a small, fast prop. I believe this glider will fly quite well with 32 oz's of thrust. P-Calc tells me I need close to 15,000 rpm's to achieve this. What I wanted to know was, is this prop really doing any good at this high of rpm's. If not, we would have to settle for fewer amps/thrust.

I really wanted to know if there is a point where the prop stops achieving any further thrust.


Thanks for everyone's help.

RE: Prop effective speed
 

The power is needed only for good response getting to altitude. All wanted to know was "does a prop STOP increasing thrust, power, whatever......when it reaches certain rpm.

The fuse is special design where the motor is mounted inside the fuse just behind the cockpit. Can't be changed without major structure mods and with degraded appearence.

RE: Prop effective speed
 

I don't see any problem with turning an 8" prop at 15,000 RPM, but I've never looked into this before.

Consider the SSC combat planes. The requirements are >2.5 lbs, >400in^2 wing area, 8x3 prop, maximum RPM on the Ground <17,500 RPM . Basically a souped-up glider :D. In my opinion, they have a good climb rate, for a glider.

The 8x4 may be best, as the 8x8 will try to get you >100mph.
It's unfortunate there is not more selection in prop pitch for your application.

Carl

RE: Prop effective speed
 

I am no expert in aerodynamics but I think that the point is being missed at some level on both sides. In any situation, there are a number of isues that have to be considered. At least until you have a prop tip speed in excess of Mach 1, you will get some increase in thrust. I don't know exactly how quickly it drops off beyond that but effeciency drops rapidly and noise increases a lot. Even if you don't hit the magical "no improvement" line, you will certainly hit a point of diminishing return where the "cost" for greater thrust/speed is too great in terms of weight penalty (bigger motor/batteries) or in terms of reduced flight time. Are you willing to double the motor size and double the battery weight for a reduced flight time if you only get a 10% increase in effective thrust. Will that extra weight more than negate the added thrust by giving you poorer overall performance and possibly less speed and lesser climb rate? Added weight is rarely a good thing. As others have mentioned, the ultimate efficiency and the added benefit of more prop speed may depend upon the drag of the model. Another thing that may be a consideration is how you are going to fly. Is this a "climb and glide" scenario where the ultimate aim is to produce as much altitude as possible in a few seconds or are you interested in doing some flying under power? Is duration of any interest?

So, I STILL haven't given you an answer about the question of whether added thrust drops to zero at some magic RPM or tip speed point. I doubt that it does but I can't say for sure. I can tell you, however, that the added benefit of more RPM will drop to less than zero at some point as the model weight is increased to accommodate the equipment needed to gain the extra thrust. Factors that are almost impossible to determine emperically include the aerodynamic characteristics of the model, the "mission" of the model and what are you willing to consider in terms of cost($$) and reduced flight performance in a "power off" mode.

In answer to your most recent comments, an 8" prop is still doing its thing quite well at 15,000 RPM.

Ross

RE: Prop effective speed
 

In short, NO. As far as I know thrust always increases with rpm. Problem is that the power needed increases a lot quicker than the thrust increases as compressibility effects become important, i.e. when tip speed gets close to the speed of sound. In other words, the propeller becomes very inefficient in converting power to useful thrust.

For an 8" diameter prop, tip speed approaches speed of sound at approx 32000 rpm if the plane is stationary and a little less if moving at typical model aircraft speeds. In order to achieve this RPM with an 8*8 prop you will need one hell of a motor in your glider.
To the Formula One and FAI pylon guys 15000-17000 rpm with an 8" prop is nothing, they are used to RPMs in the 25-30k range.

To summarize: The problems you may encounter will not be associated with the propeller stopping to produce thrust or going supersonic, but rather how to get enough power from your motor in order to turn the prop at RPMs that are even close to where compressibility effect start to appear.

BTW, APC also has 8*9 and 8*10 props in their range if your plan on going very fast and have enough power available.

/Red B.

RE: Prop effective speed
 

Whether or not the plane's forward speed reaches the prop's pitch speed (pitch*rpm) depends on how big/small the plane is related to the prop's size.
For instance, a prop with too little diameter in relation to the plane's size will hardly make the plane's forward speed reach the prop's pitch speed regardless of how much rpm increases.
Also, a constant pitch prop is designed to give best results at one airspeed and rpm.

RE: Prop effective speed
 

Adamone wrote:
Of course. My statment was about thrust vs. rpm and NOT about whether a certain aircraft can reach "pitch speed" or not.

BTW, I'm not a fan of the term "pitch speed", simply because it tells very little about the propeller itself. Most propellers develop significant thrust at "pitch speed". Whether or not an aircraft will be able to reach speeds higher than "pitch speed" or not is mainly dependent on the drag of the aircraft.

/Red B.

RE: Prop effective speed
 

I brought up the pitch speed in attempt to illustrate that there is a max rpm above which a prop will stall, no longer increasing the thrust, unless the rpm and the flight speed keep changing in step with one another, which has a natural limit.

RE: Prop effective speed
 

I’ve been hesitating to contribute to this, because I don’t think I understand it completely. So take my comments with a grain-of-salt.

The propeller is not stalling when it is approaching Vel=RPM*Pitch inches/minute. It is reaching the case where the Radial Velocity and the Forward Velocity combine to give a zero-lift angle-of-attack at the prop (i.e., if the prop where a symmetric airfoil the angle-of-attack would be zero). Then all of the energy from the engine is being consumed by "prop drag" and efficiency goes to zero. In reality the engine/motor is unloading at the same time so the RPM goes up. Soon you reach a balance for a given throttle setting where drag=thrust. So three variables are important, the prop thrust curve, the airframe drag, and the motor torque curve.

I would think the prop behavior at cruise speed is not the determining factor in the performance. Maybe it’s important for getting the last mph out of a racing plane or getting the extra mpg in endurance flying.

Carl

RE: Prop effective speed
 

No, I don't think this is the case.
If a prop has a geometric pitch (i.e. airfoil angle of attack) that is less than the stalling angle of that airfoil, it will not stall at any rpm.
The worst case is at zero airspeed (forward speed of propeller through the air). As airspeed increases, the angle of attack of the propeller airfoil is reduced at a given rpm reducing the risk of stalling the propeller even more

If on the other hand the pitch of the propeller is such that the propeller is stalled at zero forward airspeed, increasing the airspeed will reduce the angle of attack of the propeller airfoil. As the airspeed becomes sufficiently high the propeller will be operating in an unstalled condition.
The Schneider racers of the 20s and 30s had fixed pitched propellers with very high pitch. These propellers were stalled at the beginning of the take-off run making the take off runs extremely long.

/Red B.

RE: Prop effective speed
 

Siefring, you are absolutely right.

/Red B.

RE: Prop effective speed
 

As I said to you guys/gals before, I am sure it's my question that is the problem. I'm asking the wrong thing, I guess.:) I was using thrust in the static form. I really just wanted to know if the 8 inch prop could work at 15,000 rpm. I think the notion that it quits working is not at rpm, but, drag where drag is enough to keep the prop from moving it any faster. The thrust I am measuring is from the bench and wanted to see if there was a way to know if the 8x4 prop would work fairly good at 15,000 rpm before buying the motor that would get it to that rpm.

I just noticed there were 3 post since I started writing this message.:) I don't know what they said.

RE: Prop effective speed
 

Thanks again, everyone. I did pick up some good information from the exchange anyway!:D

RE: Prop effective speed
 

Siefring,
I didn't say that the prop stalls when the plane reaches the pitch speed, but I meant it does when an increasing in rpm no longer gives an increasing in forward speed. At this point the prop is unable to increase the thrust despite increasing of rpm.
The graph below shows how the power thrust required changes along with different flight speeds.

RE: Prop effective speed
 

Adamone wrote: Nope, if the geometric angle of attack of the propeller blade is less than the stalling angle of the propeller airfoil it will NOT stall if the RPM is increased. This is true even if the airspeed remains constant.

/Red B.

RE: Prop effective speed
 

Then, give us an example when the prop is stalled regarding rpm and forward speed.
Enlighten me.

RE: Prop effective speed
 

Adamone wrote: O.K. it's time for some Liljeholmens :-).
As an example consider a typical 11"*8" prop at a constant 10000 rpm. At zero airspeed this prop will be operating in a stalled condition. If airspeed is increased to above 20 mph (approx. 32 km/h) the combined effect of the rotatational speed and forward airspeed will cause the propeller to operate below the stalling angle.

If the same prop is operated at 15000 rpm the airspeed needs to be increasd to approx. 28 mph. (approx 45 km/h) in order to allow the propeller to operate below the stalling angle of attack.

If a low pitch prop is chosen, say a 11"*4" this propeller will not be stalled at zero airspeed no matter what the rpm. This is true also at higher airspeeds.

/Red B.

RE: Prop effective speed
 

A good web site (in general) with some nice diagrams for visuallizing propeller workings


[link]http://www.auf.asn.au/groundschool/propeller.html[/link]



Carl

RE: Prop effective speed
 

här kommer en Stockholmare [8D]
So, in order to avoid stall, the forward speed has to increase as the pitch speed increases (rpm increases) right?
This because the prop's angle of attack starts increasing when the airspeed no longer increases at same rate as rpm.

Ledsen but I can't agree with that because as I previous stated a prop's AOA increases when the rpm (pitch speed) starts increasing without being followed by the airspeed.
So, even if the 11*4 prop might not be stalled at zero speed it is operating at very low efficiency then, and it will stall if you keep increasing rpm at static condition.
Jag vet, jag har sett det.:D

RE: Prop effective speed
 

At zero airspeed it doesn't matter what the propeller RPM is, the angle of attack of the propeller blade will always be the same.

At airspeeds higher then zero, the angle of attack will decrease with increasing airspeed (assuming constant RPM) and vice-versa, we agree on that.

N.B., as far as angle of attack is concerned, zero speed is the worst case scenario. If the propeller is not stalled at zero airspeed it will never be stalled at higher airspeeds no matter what the rpm. It's simply a matter of geometry.

Can you explain exactly why an 11*4 propeller at zero airspeed that is not stalled at low rpm should stall if rpm is increased (still at zero airspeed)? Other than cavitation I can see no reason for this. In general most airfoils used for model aircraft sized propellers usually perform better at higher Re-numbers not worse.

/Red B.

RE: Prop effective speed
 

So you mean that with such a prop that never stalls you may increase the thrust as much as you want provided you can get enough rpm? Where is the limit?:eek: