AgCat1982

Does anyone know the maximum useful speed of props? I figure it is different for different props. If it is, is there a chart or program to determine what that is?

What I have are two eight inch dia props. One with four inch pitch and one with eight inch pitch. They are the longest props that can be put on this eight foot wing span glider that I am building from a kit and I need to know how many rpm's I can effectively use to get my needed thrust. I am going to use brushless since the recommended motor is just barely useable according to others who have built this glider. It is a pusher, thus the limit on prop dia.

I need this information for folding props and Master Airscrew props.

Any help would be appreciated.

w8ye

My Feedback: (16)

There's no reason why you couldn't spin a prop at the maximum safe rpm limit of the armature in your motor? The actual factor would be to prop your motor where it will turn on the ground a speed closer to the torque peak of the motor?

Enjoy,

Jim

AgCat1982

Thanks, but, I understand that there is a maximum speed of effectiveness of a prop, where any faster does not give any additional benefit, it's just beating the air. That's what I was wondering about.

Rotaryphile

Propellers of around 24" diameter were wind tunnel tested many years ago, and it was found that efficiency started falling off at around Mach 0.65 to 0.70, while full-scale size props don't usually fall off until around Mach 0.85 tip speed. The props don't just beat the air at higher tip speeds; some of the pylon racers commonly run props at nearly Mach 1.0. I guess that they figure that the extra urge available from the engine at very high RPM more than offsets the loss in propeller efficiency.

If you are able to manipulate your propeller RPM by using a reduction gearbox, you should get best efficiency from a prop with its pitch roughly equal to its diameter. When pitch is around half the diameter, propeller tests showed that the efficiency loss was around 15% or so, compared to a prop with pitch equal to diameter. Control line speed guys used to get pretty good efficiency with pitches 1.4 times diameter and more, and I have used 20 inch diameter, 30 inch pitch props on a very clean, high powered UAV to generate maximum airspeed.

AgCat1982

Are you saying an 8 inch dia, 8 inch pitch prop at 15,000 rpms would still be working fairly efficiently? That's what I am trying to find out. On a prop this small is there a point where it would not gain anything to increase the rpm's? This is the largest prop that can be put on this glider.

Geistware

My Feedback: (16)

The folk lore that I have heard is that 0.8 mach is the fastest you want a propeller tip to travel.
Depending on the diameter of the propeller, this will have a different RPM.
For noise reasons, people keep it down to around 0.65 mach.
If you are looking for effeciency, then it may be a lower RPM depending on the cut of the propeller.

rmh

max torque on an electric motor is ZERO rpm.
These are not anything like IC engines
You look at amps being consumed -more amps -more torque- so for speed setups you can really load on the prop -as long as the plane is moving
This is tricky - as you can't calculate the prop efficiency at speed
you can guess at it but it is linked with the airframe it is pulling.

Rotaryphile

An 8x8 inch prop working at 15,000 RPM will be quite efficient, provided that it is matched reasonably well to the drag of the airplane, so that it will be advancing at least 7" per revolution in steady flight - a "slip" of about one inch per rev, or 12.5%. If the drag of the airplane is too high for it to advance that far, it might well be less efficient than, for example, a 9x5, due to the excessive slip. Its tip speed at zero airspeed would be 524 feet per second, around Mach 0.476. It would not start losing efficiency until tip speed approaches around Mach .7, which would occur at 22,060 RPM. Always bear in mind that the RPM will increase considerably in flight, and that the forward speed of the airplane also increases the tip speed in flight.

I have used 8x8" props turning at a maximum RPM of about 3500 with an indoor RC model, and the 8x8 was considerably more efficient than lower pitched props turning higher RPM.

AgCat1982

Thanks Rotaryphile. I think you answered my question. An online calc says the needed thrust would occur at around 15,000 rpm for this prop. And, as I said, this is the largest prop I can put on the 8 foot wingspan, 54oz glider without considerable structure mods which would make the plane look bad. I have been searching for different motor setups and the above is the only thing that looks usable. I had been told that there is a point where small props just start 'beating the air' and gaining nothing. I have been trying to find out if this prop turning at 15,000 rpm was above that point. If it was we were going to have to settle with a sluggish climbing glider.

Thanks all you guys for responding.

hopkimf

My Feedback: (56)

I remember something about full scale props needing to run at less than the speed of sound. If the speed of sound is about 1085 feet per second, that's the limit. Prop tip speed is calculated from the formula v = r x omega, where v equals tip velocity in ft/ second, r equals prop radius in feet. Omega equals rotational speed in radians per second. Assume a 24 inch prop turning at 7000 rpm. Radius r equals 1.00. Omega equals 7000 x 2 x pi/60, or about 732 rad /sec. Multiplying out v = r x omega, or v = 1 x 732 = 732 ft/sec. Well below the sonic velocity. It's about 70% of sonic velocity.

Maybe someone can check my numbers.

rmh

I thot the question was about electric powered model .
As a suggestion , also check with the electric powered , competition discussion groups.
As for the calcs--- don't bet the farm on these

AgCat1982

It is electric.

Thanks

HighPlains

My Feedback: (1)

Your calculation of tip speed has neglected the speed due to forward motion of the aircraft. Take your rotational speed (in feet/sec) and square the value. Take the forward velocity (in feet/sec) and square it as well. Add both squared values together and then calculate the square root of that sum. THAT is the actual tip speed.

PS. The speed of sound is not a constant, and goes down with altitude.

hopkimf

My Feedback: (56)

HighPlains is correct. If the calculation for tip velocity should include rotational speed and forward velocity. In the example offered above, I calculated the tip velocity and ignored the forward speed. Assume the plane is flying at 60 MPH. Then the tip forward velocity is 88 ft/second (60Mi/hr x 5280ft/mi / 3600sec/hr) .
Next find the resultant of the rotational and forward velocities. Using my rotational speed of 732 and forward speed of 88, the resultant tip speed is 738 ft per second. That's the square root of 732 squared plus 88 squared. Very little difference when the forward speed is considered because it's so small compared to rotational speed.

Same goes for speed of sound. HighPlains is correct here too. Speed of sound in air has a temperature depenence and also depends slightly on the presence of water vapor. Since these change with altitude, speed of sound changes as well. Not enough for me to be concerned with for models.

Dick Hanson. Sorry if this was supposed to be about electrics.

rmh

don't be apologetic- I just thot some hands on stuff from powered glider flyers might shed further info.
The the techno info on full scale props is interesting .
We are doing a variable prop setup for an electric .
On this setup -the prop will do positive as well as reverse thrust.
The prop blades we designed,are thin and flat, made from a composite of long carbon fibres and sheathed in thin, dense ply (1/64)
Others have already done these collective props with good -and poor results
the real issues were blades (non commercially available) and the precise control of the collective -which we now have these figured out.
the unit is simply a commerdial tail rotor assemblyfrom a model copter-fitted to an outrunner motor.
Thanks to the advances in radio design --the rpm and pitch control can be programmed to work in sync .
My new JR 9303 tx allows the throttle stick to add power / blade angle in various relationships.-in a seamless fashion
The blade efficiency simply is a matter of correct power /angle combos as the blade must be symmetrical in shape for pushing and pulling.
(where is the prop theory for that?)
Oh yeh -the model will fly forward and backward -tho the reverse flight will be very limited.
It is almost ready to go --
I love all the new techy bits which permit this kind of stuff.

Rotaryphile

Just a little addition on speed of sound - it does not depend on altitude, except to a very small degree, but on temperature. The speed of sound is roughly 49 times the square root of the absolute temperature in degrees Rankine. At zero deg. F., the absolute temperature is 459.6 deg., and the speed of sound is 1050 feet/second. At 100 deg. F, the absolute temperature is 559.6 deg., making the speed of sound 1159 ft./sec., or 790 MPH. The speed of sound normally decreases with altitude simply because the temperature decreases, and at -80 deg. F., the speed of sound is 955 ft./sec., or only about 651 MPH.

speedster 1919

My Feedback: (3)

Gee Wiz I don't work for NASA ,but logically your talking an 8" prop ,pusher at that. Stated max RPM for prop won't get you to speed of sound. Pusher probably limits your choices. They are fairly cheap so buy a few sizes and experiment. I would imagine to get 8' started or in stiff wind you need tourqe so smaller pitch would be better.

AgCat1982

We are talking about a little bit heavy electric and an 8 inch prop.

Question was: "Does anyone know the maximum useful speed of props?"

The operative words being "MAXIMUM USEFUL". Meaning anything faster is NOT USEFUL. I didn't ask for the MOST EFFICIENT prop. I keep saying that I understand there is a point where turning a prop FASTER gains NOTHING and just beats the air faster.

No one seems to want to respond to this "urban legend".

I thought the question was reasonable and simple. I do NOT want to know what is the BEST speed!

I think I am having trouble asking the question correctly.

"What prop?" is not the question. I repeat. I can ONLY use an 8 inch prop.

The question has to do with what MOTOR to use.

Example (extreme numbers on purpose):

A. Setup that will turn an 8x8 prop safely at 15,000 rpm

B. Setup that will turn an 8x8 prop safely at 20,000 rpm

C. Setup that will turn an 8x8 prop safely at 30,000 rpm

Pretend that ALL three can be done safely

Question: Will installing one of these setups be a total waste of effort and money over any of the others?

I hope this helps.

Rotaryphile

An 8x8 inch prop turning at 15,000 RPM in the air needs an airspeed of at least 100 MPH to get its slip down to about ten percent. If the model is too draggy to attain this speed, the 8x8 is the wrong prop, regardless of the RPM. The same prop that propels the model at 100 MPH at 15,000 RPM would propel the model at 200 MPH at 30,000 RPM. The 8" diameter prop, turning at 30,000 RPM would have a tip speed in the air of nearly Mach 1, and would lose quite a lot of efficiency and make a terribly noise at this RPM. The efficiency of a given prop on a given airplane should be relatively constant over quite a range of RPM, for airspeeds well above stall, where induced drag is not much of a factor. If the 8x8, for example, turning 15,000 RPM in the air produces an airspeed of only 75 MPH, you need a lower pitched prop, probably something closer to a 9-5. If the 8x8, however, produces airspeed of over 100 MPH at 15,000 RPM, you may want to try a higher pitch, smaller diameter prop. An 8x8 would probably not be a good prop to use with any model having more drag than a pylon racer, regardless of the power behind it.

AgCat1982

Thanks for trying, but, I have to repeat that an 8 inch prop is the largest dia prop that I can put on this glider. Unless a SMALLER prop would provide more thrust, I can't change the prop dia! The ELECTRIC motor the ONLY thing I have that can be changed. I need the a motor to provide enough power to spin this prop as fast as it can be spun, but, I don't need (for obvious reasons) to buy a motor what will spin it faster than its ability to add additional thrust.

This glider weighs approx 57 oz, is being built from a kit and has a wingspan of 8 feet. So, it's not going to go 100 mph. It's design limits the prop size. The suggested motor provides only marginal thrust, from accounts of other owners. As the cost of brushless motors and ESC's increase greatly as they get larger, I do not want to just keep buying motors and ESC's to match until I get the best one!

It does not matter what the maximum thrust number is that I can get from this prop,.... just that I GET WHATEVER THAT NUMBER IS!......Without buying an unnecessarily large motor, ESC and battery pack that can provide additional power that cannot be used as additional thrust!

rmh

You need to ask more questions
"An 8" prop" -is not enough input
The Master airscrews?
odd --
-you need to mention a potential power source qualification.
Are you going to use 100 watt setup ?
200 watts??
the 8" prop used will be strongly related to that .
the 4" pitch may be best with 100 wats -or -but the 8" pitch may be best with 200 watts
It is all related to the drag of the model .
calculate all you want -
unles you have all inputs
motor size / watts available /calculated airframe drag at given speeds --you can't begin to select the right prop.
So why not buy the most power you can afford - then get some props and try it to see which ones produce best speed within the watt limitations of the motor/esc/batt
remember that it takes roughly four times the power to double the speed -- - so don't be suprised to find a bigger motor doesn't add a lot more speed.

Red B.

agcatsbest wrote:
Excuse my ignorance but why can't you use a bigger prop? On a glider there are usually no limits to the maximum size since they are usually hand-launched and have no landing gear.

agcatsbest wrote:
As long as you can't be more specific about your question the best answer you will get is what has been stated previously:
Ordinary propellers doesn't operate efficiently at speeds where compressibility effects become inportant, i.e. close to speed of sound.

At the other end of the spectrum: Like wings, propellers become very inefficient if operated at angle of attacks which cause the airfoil to stall. For a propeller this of course depends on not only the pitch and prop RPM but also the speed of the aircraft.

Assuming 10000 RPM, the 8" pitch prop need a forward speed of more than 25 mph not to be in a close to stalled condition. For a 4" pitch propeller you won't reach a stalled condition at 10000 RPM no matter how low the speed of the aircraft is.
If the flight speed is around 35 mph a 4" pitch prop will absorb about as much power at 14000 RPM as a 8" prop would do at 10000 rpm.
With these assumptions the 4" prop will be operating at a slightly higher efficiency and also generate more thrust.
If the speed is significantly higher, say 50 mph, the 8" pitch prop will be more efficient than the 4" pitch prop.

/Red B.

AgCat1982

OK guys. I won't bother anyone any more. If this doesn't help, I will proceed to do it, on my own, the hard way. I don't need to supply anymore information than I have given for someone to answer this simple question. I have supplied way more information than is necessary. Apparently I just can't ask the question correctly. I don't need any technical stuff about the speed of the prop tip. I keep saying that I don't need to know the "most efficient" speed of the prop.

Once again, no one has even responded to what I jokingly labeled an "Urban Legend". I have asked this question from the beginning. And, the answer I need is directly related to that question! Here it is again------

IS THERE SUCH A THING AS A SPEED WHERE A PROP HAS REACHED ALL OF THE THRUST INCREASE POSSIBLE FOR THAT PROP! i.e. if a prop is turned faster than that speed, it will not cause an increase in thrust, thus wasted rpms, battery weight, and motor weight.

If that "urban legend" is not true, then, THAT IS ALL I NEED TO KNOW!!!!!!!!!!!!!!!!!!!!!!!!

Nothing else!!!!!

If it is true, is there a way to find that speed? That's would be the only other information I could use.


I thought I explained why I would not "just buy the most expensive motor I can afford".

As for the motor and power setup data........ THAT'S WHY I NEED THE ANSWER TO THE ABOVE QUESTION.......to determind what power setup to get!!!!!!. Like it or not it's going to be an eight inch prop. Period.

I explained in the thread why the prop cannot be any larger than eight inches.

I am sorry for having caused a problem. If I am still not getting across as to what information I need, I will won't bother anyone trying to ask again.

Thanks for trying

Siefring

Yes. Consider the case where there is no drag. The prop will speed up until it is moving through the air at RPM*Pitch. At that point there will be no thrust.

You are wrong, this is not the only information you need!

AgCat1982

This is getting sad. Really. The correct answer to my question, a useful answer to the question, is what I am looking for. How the heck do you know what I need? If you don't want to help me then why bother responding?. I said it was my fault that, for some reason, I am not able to ask my question correctly. I am not cutting anyone. I said thanks to everyone who was trying to help me. You still didn't even answer the question. There is no such thing, in the real world, where there would be NO drag after reaching a certain speed. If anything being moved thru the air, there is ALWAYS drag.

I'll figure this out on my own.