FlooredCOBRA

A while back I found this program that calculated speed by prop size pitch and rpm's. I am not to sure how realistic it was but according to the program the diameter had no effect on the speed like the pitch does.

I know you get the speed from the pitch but it seems there has to be a spot where you start loosing speed with smaller diameter. What I mean is how far can you go on shaving until your past that sweet spot. What is the rule of thumb?

On my GRRT I use between 7.2 - 8 inch diameter props. I can get all of them to turn 24,000 to 25,000 rpm's. Now my questions is even with a decent rpm am I loosing speed even though the rpms say a certain number? Should I try a larger prop and shave down to a size that can work or just stay in smaller prop sizes?



There has to be some spot in there where one takes over the other. Just some props have larger meat area ( or pitch angle ) on different spots of prop. As for flying and seeing speed difference it is very hard to tell. But I can tell a difference on thrust on ground. But for speed in air thrust don't mean to much.

Just looking for a rule of thumb on this. I am not even sure if my question even made any sense. Just hard to word what I am trying to say by typing it.

ChuckAuger

My Feedback: (12)

When the thrust the prop can deliver at speed becomes less than the drag of the airframe, the return in speed will start to diminish.

I ain't a rocket scientist, so I can't give formulas and calculations.

Just use Dustflyer's analogy..you go so fast with a 7.4 X 8, turning X RPMs. Now cut the prop down to 2" diameter with 18" pitch...RPMs are still way up, but your plane ain't moving at all.

You have to have enough thrust to overcome airframe drag, and drag increases exponentially with speed. So the faster you go, the more thrust you need. Trouble is, to get that speed, you still need RPM and pitch. At higher speed you need a lot more thrust than the little prop can develop.

Which means you need a cleaner plane or more HP.

FlooredCOBRA

okay I see what you are saying. Makes sense.

I guess there is now way to know what speeds vary unless you have a way to record the actual speed. Like I was saying all those props I been using all seem to look about the same speed.

I know some props are faster than others, it is just hard to tell by just flying it. The only difference I can see is when launching. After that it all looks the same to me. I t is not putting around like a trainer thats for sure and everyone says its really hauling. So I guess that will do.

FlooredCOBRA

okay now I have another question I just thought of. I have a bunch of different APC props some are racing and others are pylon props or what have you.

I notice that the tips are different, some have a gradual taper to a point. Some of my other ones have more of a bolder tip to them.

Now my question is what seems to be the best way to finish after cutting or shaving down a prop. Some of mine I have cut off the tips and just round the hard corner into a taper kind of like a traditional prop tip look. Some of my other ones I made a gradual taper to the diameter I need. Those have more of a point to them.

The ones that I shaved down into a point kind of looks like I am loosing the meat of the prop by doing this.

I am just trying all sorts of different varieties to see what works better. I guess I get bored and it is a good way to kill time when it is raining outside.

ewstap

Just remember that as you shave the tip (wash-out area) you effectivly increase the average pitch of the prop....(ie. a 9 x7 could turn into an 8 x 8 by taking 1/2" off each tip..get my drift???????

FlooredCOBRA

Yeah I see that you are saying. Good point!

I was checking out a site earlier on carbon props and they have smaller sizes that I think may work just like they are. I am not sure yet so going to order a few and see how it goes.

I would feel much more comfortable with a prop designed for that size for optimal performance rather than shaving down some. I mean don't get me wrong it does still work but just seems like it would be much better with a prop made at that size that works best for your engine pipe set up.

Rudeboy

... and you get turbulence losses around the tips it you keep increasing tip pitch by cutting the prop...

Most props (the good one at least) have a number of different wing sections throughout the blade... if you cut the prop you could end up with a wing section at the tip which isn't suited for high (sub- or transsonic) speeds... result: major drag...

FlooredCOBRA

Ok say that you did cut past where it is best. How would you know if you are getting to much drag?

What is a indication that you are doing more harm than good? Just curious to how you can tell so I can eliminate the props that may be in this category. But then again I find it hard to tell a difference while it is in the air flying around.

But one thing I can tell a difference with is the fuel consumption. Some props use up more fuel more quickly than others. I may just stay with the prop I get better fuel consumption with since it is hard to tell if I am actually going any slower than the other props.

All in all it is still moving along at a good rate of speed. So I guess I would be more likely to stick with certain props that give me more run time.

To bad there is not a wind tunnel where you could test out different props sizes and sew what prop works better than others speed wise. It would be nice to see the differences on paper rather than just guessing.

The way it is now I have no clue on the actual speed it is going. lots of people ask how fast.. tell them I have no idea. over a 100 mph or so.

It would be cool to find other Dust fliers in Ga to compare with on the speed that may have a clue on how fast they going.

But it would be nice to know how fast really.

Rudeboy

Cutting and sanding (or even making) your own props comes down to this:

A LOT of trial and error...
Meticulous note taking...so you know what you did when you find that "magic" prop...
Patience...
Precision, getting the exact same shape mutiple times...
Knowing what you're doing, a solid base in aerodynamics wouldn't hurt...

Ask the F3D pylon folks... Dave Shadel and the likes... they'll tell ya...

For sport flying it might be more economical to stick to the available props... unless you really need a prop that no one makes...when you start using .90 size DF engines and stuff like that...

FlooredCOBRA

so when will you know if you past that sweet part of the prop. The part where it goes from good to drag? What is an indication that you have gone way to far?

I was wondering because was curious as if it was a small thing or a big thing. The way it is now all my props seem to be about the same. See I was wondering if maybe there is an obvious way to tell on if you have shaved down so far as to where you can noticeably tell a difference about if you shaved to far?

Is this only on paper the difference or is it noticeable? Well besides spinning a whole load or rpm's and not going anywhere?

banktoturn

FlooredCOBRA,

As you say, you need pitch for speed, and there is indeed a point where smaller diameter costs you so much efficiency that you don't some out ahead. I don't have a rule of thumb for you regarding where that crossover occurs. I do think that you are better off with a prop the way it is built than with one that has been cut off. A propeller blade is a little wing, and just like the main wings on your plane, there is an optimum distribution of lift along its span. If you deviate too far from that optimum distribution of lift, drag will go up. This is why main wings on planes should have an elliptical ( or nearly elliptical ) spanwise lift distribution. For a prop, the optimum spanwise lift distribution is parabolic, which means that if you made a graph of the amount of lift produced at each point along the span, it would be a parabolic shape. When you cut down a prop, you cause the lift distribution to deviate more and more from parabolic, and drag goes up.

Rather than thinking of choosing a prop for speed vs. thrust, it is helpful to think of choosing a prop for maximum thrust in your chosen speed range. A prop which makes a lot of thrust at high speed will have high pitch, making it less efficient at low airspeed, so that it produces low thrust at low speed. I have never actually gone through the process of trying different props to maximize speed, but I would probably start by choosing a pitch, and gradually reducing diameter until you can tell that your maximum speed goes down. Then you go to a higher pitch and try again. In a perfect world, you would end up with a prop that lets your engine turn pretty close to the speed at which it makes the most power, and has the right tradeoff between pitch and diameter. I assume that a number of people here can give you a more methodical procedure.

Good luck,

banktoturn

Rudeboy

You have answered your own question here mate...

Excessive drag losses on a prop can present themselves in a number of ways... the most obvious being the fact that you start loosing top end speed... but it can also be noise. Maybe you don't hear the noise because you piped, unmuffled Jett .50 is screaming so loud, but it will be there.

I usually determine things this way: if the engine is revving but the plane isn't moving, go to a larger prop... if the engine isn't revving, and the plane isn't moving either, reduce pitch until the engine revs and work from there... like I said: trial and error...

Try to keep prop shaving to a minimum. Taking a few millimeters off each tip because the prop seems to be just a tad too heavy is ok...
Making 8" props out of 10" props is tricky stuff.

FlooredCOBRA

The props I been using the most and like the best are 8 X 7. I do shave these down some to allow the pipe to kick in. After I measure the prop total diameter it is just at 6 1/4'' .

On the prop I just measured I ended up going a bit to small and more than I needed to. I just kept on shaving until I got it up on the pipe. Well after I got it done I found that the carb was only opening 3/4. After I seen this I kicked myself for not checking this out before hand. I did not pay attention to the travel on carb after I bolted the engine back on. So I am thinking like a 6-3/4 to 7'' even diameter may be just about right without having to shave anything.

I been looking at the http://www.bollyprops.com/index.html site and found a few I think I am going to try out. I don't like the idea of cutting down props to get them to run right on an engine. Like all that effort into the design of prop ends up on my floor in bits.