Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

prop spinning too fast?

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Old 11-22-2007, 09:56 PM
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Flying freak
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Default prop spinning too fast?

Do you believe that it is possible to actually get a prop to spin to fast i got to thinking about the fact that a stock jett turns 18k nelsons have been known to turn 3?k do you think it is possible that were simply spinning props too fast that maybe packing the rpm down would maybe increase speed

7.4(pie) = 23.235 (inches per revolution)
23.235 X 32 000 = 743552 (inches per minute)
743552/12= 61962.6667 (feet per minute)
61962.6667/(1087)60 = mach#
61 962.6667/65 220

So the tip of the prop is at a speed of mach .95 which is normally considered to fast because the airflow has a hard time staying with the contour of the prop.

“the speed of sound is 331.3 meters per second (1,087 feet per second) in dry air at 0 degrees Celsius (32 degrees Fahrenheit)”

so without looking at the engine side of this (ie tune pipes and such) could it be that speed could be gained by dropping the rpm a bit and going with a more aggressively pitched proper maybe shortening the props now and increasing there pitch..??

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Old 11-22-2007, 10:25 PM
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Default RE: prop spinning too fast?

The guys who are running the course care less about theory and more about results. Every possible combination has already been tried many times...often without any shared results.
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Old 11-22-2007, 10:33 PM
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Default RE: prop spinning too fast?

Hey FF,
Love your math. When I took flying lessons and planned on building my own plane. I read everything I could about it. (Bought plans too) If I remember correctlly, (fat chance), you want to keep prop speed about 85% of mach 1. You need to add the forward speed of the plane/PROP also.
Hope this helps.
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Old 11-22-2007, 10:43 PM
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Default RE: prop spinning too fast?

FF:

In my much younger days I recall the worlds best engineers claiming that a dragster could never exceed 170 mph, or a number around this one, due to the maximum friction available at the surface. Now, they are actually well over 300 mph, close or in excesss of twice the maximum design speeds. I recall seeing a claimed 34,000 rpm out of the new nelsons, so use that as a good number. The real question here IMHO is, does the prop quit pulling once the speed of sound starts to move down the prop as the rpm's increase. Sort of asking, does a airplane quit flying over the speed of sound? My best answer here is: t If the speed could be increased by increased pitch, they would be doing it in the real world. I'm not throwing rocks at you as I enjoy the thinking and questioning mind, even though mine quit several years ago. Oh yea, if you assume a "stock Jett" will only turn 18,000, wrong wrong, wrong. The Jett 35, 40, 50, and 60LX all turn over that and they are sport engines. Don't know for sure that the Jett raceing 40's are fully up to par with the Nelsons right now, but if not, they will be shortly. Years past, the Jetts turned "fastest times" many times, even though the pilot did not win the race. Must be hard to accept to have the fastest plane out there and get your clock cleaned by some old tretcherous man called Fred Burgdorf. Comment made with respect and affection for Fred. ENJOY
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Old 11-23-2007, 12:36 AM
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Default RE: prop spinning too fast?

NASA has tried the whole supersonic prop thing.... it doesn't work. Look into the data, it clearly shows that props are more suited for low-subsonic speeds. Make no mistake, the F3D guys really are looking at this stuff today, because it suits the kind of contest and engines they are flying today... but up to this day, they're all running subsonic.

I'll go out on a limb here: but the first person to perfect a supersonic prop will dominate F3D for years (if the secret doesn't get out)
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Old 11-23-2007, 07:20 AM
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Default RE: prop spinning too fast?


ORIGINAL: rednekk58

Hey FF,
Love your math. When I took flying lessons and planned on building my own plane. I read everything I could about it. (Bought plans too) If I remember correctlly, (fat chance), you want to keep prop speed about 85% of mach 1. You need to add the forward speed of the plane/PROP also.
Hope this helps.
85% is close i ahve it ranges from about 85 to 92% depending on the source, I read a full scale prop test where they noted that once the tips reacher mach one the actull trust did NOt increase but the noice did the more power they gave they got VERY little more trust out of it.

Of course once you calculate the speed of a q40 set up (that what i have been using 32krpm and 7.4 inch prop diameter).

A super sonic prop could be interesting... although it would put a ton of stress on the prop/engine as it breaks into the transonic barrier.

So i might try and calculate a more reloistic set up, how fast does a Q40 fly? What rpm while flying? and one last thing is it correct they use a 7.4 inch prop?

Q40 may already be breaking the sound barrier they really arnt that far from it and i havn't factored the speed of the aircraft.

Steven
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Old 11-23-2007, 10:34 AM
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Default RE: prop spinning too fast?


ORIGINAL: Flying freak

Q40 may already be breaking the sound barrier they really arnt that far from it and i havn't factored the speed of the aircraft.

Steven
When considering aircraft speed while calculating the actual tip speed use the formula below. The tip speed does go up with increasing aircraft speed but it is less then you might think.

The "helical" tip speed in flight Vtip is obtained by adding the airspeed V to the rotation tip speed Vr using the rule of vector addition.

Vtip = SQR( Vr ^ 2 + V ^ 2 )
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Old 11-23-2007, 11:32 AM
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Default RE: prop spinning too fast?

well you could take the above speed and add what ever speed the plane is traveling at if the aircraft is traveling at 100mph than the tip is going another 100mph faster (just now its in a "cork screw" pattern). You don't really have to take into consideration of the direction of the prop do you[&:]?
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Old 11-23-2007, 01:47 PM
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Default RE: prop spinning too fast?


ORIGINAL: Flying freak

well you could take the above speed and add what ever speed the plane is traveling at if the aircraft is traveling at 100mph than the tip is going another 100mph faster (just now its in a "cork screw" pattern). You don't really have to take into consideration of the direction of the prop do you[&:]?
Wrong! Use the formula to calculate actual tip speed. If the tip speed is 500 mph and the aircraft is moving 100 mph forward the actual tip speed is only about 510 mph.
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Old 11-23-2007, 02:14 PM
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Default RE: prop spinning too fast?

I'm surprised that the tip speed increases by that much! You're not talking about unloading here, but a geometric difference in tip speed? I can only picture tip speed controlled by rpm, with airspeed having nothing to do with it.
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Old 11-23-2007, 02:41 PM
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Default RE: prop spinning too fast?


ORIGINAL: combatpigg

I can only picture tip speed controlled by rpm, with airspeed having nothing to do with it.
What if the forward speed of the aircraft is faster then the tip speed?

Anyway, the above formula is listed on the "Joe Supercool" web site and I have never heard anyone say he didn't know what he is talking about. Also, unloading is a different issue that needs to be considered.
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Old 11-23-2007, 04:25 PM
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Default RE: prop spinning too fast?

I could see the speed of a moving prop as being additional to the speed of the arc it travels in if it was moving sideways. Too abstract for my little head. I do think it is very cool to know that I have a 7 inch prop that does about 650 mph [sideways at least].
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Old 11-23-2007, 05:12 PM
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Default RE: prop spinning too fast?


ORIGINAL: Mike Connor


ORIGINAL: Flying freak

well you could take the above speed and add what ever speed the plane is traveling at if the aircraft is traveling at 100mph than the tip is going another 100mph faster (just now its in a "cork screw" pattern). You don't really have to take into consideration of the direction of the prop do you[&:]?
Wrong! Use the formula to calculate actual tip speed. If the tip speed is 500 mph and the aircraft is moving 100 mph forward the actual tip speed is only about 510 mph.
but how can that be i mean lets say you ahve a deadstick at 200mph and manage to keep 200mph your entire aircraft is flying at 200mph wing tail battery everythign, right? how does this also not apply to the prop? i mean the engine is travelling 200mph so it should just be a simple addition ...
unless you can think of a reason it wouldnt be..
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Old 11-23-2007, 05:43 PM
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Default RE: prop spinning too fast?

FF, the individual speeds have to be in the same direction for them to be purely additive [on an equal basis]. In the example of a turning prop on a moving plane, the speeds are added, but trigonometry is involved and you need to do vector math to figure out how much the forward speed of the plane contributes to the turning speed of the prop blades. I would never guess that it ends up being as much as the example given by Mike C. Math is such a useful and exact science, I wish I could make better use of it.
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Old 11-23-2007, 06:43 PM
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Default RE: prop spinning too fast?

FF,
If your really want your head to hurt just go to this site a read for awhile.
http://www.supercoolprops.com/
There is a good article about prop tips breaking the speed of sound also.
I leave it to the guys in the rubber rooms eating raw meat to make the formulas then I just use them.
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Old 11-23-2007, 08:42 PM
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Default RE: prop spinning too fast?


ORIGINAL: Flying freak
but how can that be i mean lets say you ahve a deadstick at 200mph and manage to keep 200mph your entire aircraft is flying at 200mph wing tail battery everythign, right? how does this also not apply to the prop? i mean the engine is travelling 200mph so it should just be a simple addition ...
unless you can think of a reason it wouldnt be..
Well, run it through the formula to see if you get the right answer. In this case:

Vr = 0

V = 200mph

So

Vtip = (200^2)^1/2 = 200mph

So it is still applicable. The reason the numbers aren't anything like linear is the square root of the sum of the squares. The tip speed component is much larger than the aircraft component and the square root term minimizes the change caused by the addition.

Mark
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Old 11-23-2007, 09:02 PM
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Default RE: prop spinning too fast?

OK Mark, looking at things in reverse.....if you can add the vectored resultant of the models' speed to the props' rotational speed...then shouldn't you be able to do the same in reverse? Or only if the model is in a roll to the starboard side? [sm=bananahead.gif]
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Old 11-23-2007, 09:07 PM
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Default RE: prop spinning too fast?


ORIGINAL: combatpigg

OK Mark, looking at things in reverse.....if you can add the vectored resultant of the models' speed to the props' rotational speed...then shouldn't you be able to do the same in reverse? Or only if the model is in a roll to the starboard side? [sm=bananahead.gif]
I don't understand your question. Because vectors have both force and direction, you can work backwards to get any of the three terms if you have the other two. But you seldom have the overall tip speed and not one of the other numbers...

Mark
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Old 11-23-2007, 09:31 PM
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Default RE: prop spinning too fast?

I can barely fathom the idea of the planes' speed contributing to the overall speed of the prop. If this is so, the explanation that makes the most sense to me is that the prop tips have more air molecules blowing past them on a moving plane than on a plane that is parked.

I should be able to visualize this same addition problem in reverse where the rotating prop speed contributes to the planes' speed......not from a view point of prop thrust, but of the props' rotational speed. I guess what you are saying is that without knowing what the thrust is, there would be too many unknowns?
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Old 11-23-2007, 10:14 PM
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Default RE: prop spinning too fast?

I can barely fathom the idea of the planes' speed contributing to the overall speed of the prop. If this is so, the explanation that makes the most sense to me is that the prop tips have more air molecules blowing past them on a moving plane than on a plane that is parked.
This is a pretty good way to think of it.

I should be able to visualize this same addition problem in reverse where the rotating prop speed contributes to the planes' speed......not from a view point of prop thrust, but of the props' rotational speed. I guess what you are saying is that without knowing what the thrust is, there would be too many unknowns?
I think I get what you are after now. Thrust has nothing to do with the tip speed. What is happening is that the prop is traveling through the air like a screw moves through wood as you turn it. So the prop tips are rotating around the crankshaft and they are also moving forward along with the rest of the model. These two motions combine to make the prop tip path look like a spiral through the air. All that equation does is give the geometric addition of the two motions. It is the same equation as for finding the length of one side of a right triangle when given the other two sides. All I was saying is that, given 2 of the 3 variables, you can work out the third variable.

Did I get it this time?

Mark
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Old 11-23-2007, 10:54 PM
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Default RE: prop spinning too fast?

Mark, thanks for hanging in there with me, but I don't think we are on the same page yet.....If this scenario can be played in mathematical reverse, a plane traveling 200 mph with a spinning prop would be going faster than a 200 mph plane with a prop that is not moving. If the seperate motions are additive, what I propose must be true, but I surely know that this is non-sense!
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Old 11-24-2007, 12:44 AM
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Default RE: prop spinning too fast?

ORIGINAL: combatpigg

Mark, thanks for hanging in there with me, but I don't think we are on the same page yet.....If this scenario can be played in mathematical reverse, a plane traveling 200 mph with a spinning prop would be going faster than a 200 mph plane with a prop that is not moving. If the seperate motions are additive, what I propose must be true, but I surely know that this is non-sense!
Ah! Now I get it. The fact that the prop is spinning has no effect on the plane's speed. We were talking about prop TIP speed and that it is a composite of the rotational speed and the forward motion of the aircraft. How fast the spinning prop will pull the airframe has to do with RPM, pitch, drag, etc. The prop is rotating only because that is how it makes thrust to move the airframe through the air. The fact that the tip is moving faster than the airframe does not make the plane go faster.

What you need to do is shift your reference point to see this. If you are sitting on the nose of the plane, you see the air coming at you at whatever speed the plane is going. Now imagine you are sitting on the tip of the prop. You will see the air coming at you with the velocity from the rotation, plus the velocity from the forward motion. The only trick is that the two velocities are at right angles to one another (one forward, one in the prop plane), so they do not add directly together, but end up combining to make an angle. Think of a north wind being added to an equal velocity east wind. You end up with a wind direction of north-east and the total wind velocity will not be twice either, but actually 1.414 times one velocity, acting at 45 degrees (the north-east direction).

The question really was, can a prop turn TOO fast? And the answer is YES. When the prop tips start to close in on the speed of sound the efficiency falls off and it gets hard to make more thrust regardless of how much power you throw at it.

Mark

*edited because I can't type...*
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Old 11-24-2007, 01:21 AM
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Default RE: prop spinning too fast?

Q40 engines aren't turning 7.4" diameter props at 32k. The LS Nelsons are turning them at about 25k on the ground, and maybe 27-28k in the air. F3D's are turning about 32k in the air, but with substantially shorter props. My F3D props are anywhere from 6.5-6.75" diameter.
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Old 11-24-2007, 02:07 AM
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Default RE: prop spinning too fast?

The old nitro burning F1 engines used to turn around 27-28K in the air too, but with 8 to 8 1/4" diameter props. They unloaded more than the present piped engines.
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Old 11-24-2007, 06:55 PM
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Default RE: prop spinning too fast?

Yep, the tuned mufflers and pipes that are used in the AMA and FAI racing classes limit how much the engines unload.
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