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

calculating speed

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Old 12-15-2002, 11:26 PM
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dannost
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Default calculating speed

I have seen some posts stating that the speed of an airplane with a certain prop spinning at a certain RPM should go a certain speed. Is there an easy formula for this.

I would like to know if I will get more speed from a given prop spinning at a higher RPM than a larger prop at a slower RPM. Or do you just keep experimenting.
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Old 12-16-2002, 01:49 AM
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roywiglesworth
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Default calculating speed

formula pitch x rpm x effency (1 would be a perfect airframe with no drag a .5 would be something
like a cub for jet style prop planes a .85 would be close to a real figure) x .00947 = mph

try this
roy
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Old 12-16-2002, 03:44 AM
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maverick
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Default calculating speed

Originally posted by roywiglesworth
formula pitch x rpm x effency (1 would be a perfect airframe with no drag a .5 would be something
like a cub for jet style prop planes a .85 would be close to a real figure) x .00947 = mph

try this
roy
Well this formula being:

7 (pitch) x 16,000 (rpm) x .8 (average efficiency) x .00947 = 848mph.....doesn't sound right!
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Old 12-16-2002, 03:49 AM
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Mike2002
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Default calculating speed

It seems like this formula is a little off since I am getting that a plane with an 8 inch prop turning 17,000 rpm's will fly at 1094.732. That would be a pretty cool speed but... or maybe I did something wrong??
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Old 12-16-2002, 07:49 AM
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Default calculating speed

Can you take the prop pitch in inches, let's say 8", then multiply by r.p.m. let's say 16,000... That should give 128,000" per minute...Divide by 12 =10,666.666...feet per minute. Devide by 60=177.777 feet per second (assuming no slippage, which is impossable of course) 177x.85% efficient(sp?)=150.45' per second...We all know, thank's to all the NASCAR guys on TV that 200 M.P.H.= a football field per second,(or 300')give or take a few ft. Now our prev. calc. of 150 ft. per sec.=100 M.P.H.(plus/minus a wee tad) What do you all think of my approximations??? Not very scientific, for sure, but kinda interesting and easy to estimate.(not counting a whole *#%@ load of variables not taken into account, of course) Just some food for thought...
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Old 12-16-2002, 12:34 PM
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MMallory
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Default calculating speed

It should be .000947. The decimal is in the wrong place above.

Mark M.
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Old 12-16-2002, 01:22 PM
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roywiglesworth
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Default calculating speed

yeah it needs one more zero .000947 i didn't look at it when i
down loaded it from a file sorry.
roy
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Old 12-16-2002, 03:36 PM
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Default calculating speed

Originally posted by roywiglesworth
yeah it needs one more zero .000947 i didn't look at it when i
down loaded it from a file sorry.
roy
That works!
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Old 12-16-2002, 04:05 PM
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Default web site

i posted a web site calculator on here before. it gives speed and static thrust. all you need is diameter and pitch and rpm. i'll look for it again.

http://freespace.virgin.net/barry.ho...e/goodies.html
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Old 12-18-2002, 02:13 AM
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Default calculating speed

All those formulas are not worth squat. Lets say a 46fx with a 10/7apc turns 14000rpm. It doesn not matter what answer you get. if the engine is mounte on a trainer, it is not going to pull as fast as it would on an Ultrasport.
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Old 12-18-2002, 02:23 AM
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Default value of formula

1)The static thrust is very useful for these hand launched planes.
2)Comparing prop loads is very useful for choosing props and effect on engine rpms.
3)Wing loading is very useful.
4)The calculated speed is a max, which a clean plane might reach. Everyone knows a cub will not go as fast as a whiplash. But you are not going to go faster than the calculated speed.

The formula is a place to start. It doesn't tell you the exact answer,but in combination with observed data I find it very useful in evaluating engine/prop combinations as well as comparing similar airframes.
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Old 12-18-2002, 02:33 AM
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roywiglesworth
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Default calculating speed

there is something i came across about a month or 2 ago
there was an article about the formular and max speed according
to it you can exced the max prop speed on the right airframe
the article was out of uk very interesting and lenghty it went in to details how this was possible .i didnt book mark it but i'll try to
find it again and if i do will post the site. if i rember right(like i said it was a long article) a delta wing was the most effecnt.

roy
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Old 12-18-2002, 04:24 PM
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Default calculating speed

It's not true that this formula is "not worth squat", but it is also true that you can't count on accurately predicting actual speed. Essentially, what this formula can tell you is whether a given amount of pitch is enough to reach a target speed. When you use the formula to answer this question, you are tacitly assuming that the engine can spin the prop at the target RPM, and that the plane has low enough drag. It's a reality check, and can give a ballpark estimate. If you were starting 'cold', with no information about how fast your engine can turn a similar prop, and no information about the drag of the airframe, then this formula would not be very useful.

These discussions about this formula remind me of the following quote ( or paraphrase ) from Albert Einstein:

"Things should be made as simple as possible, but no simpler"

Predicting the speed of a model airplane from "first principles" is complicated, and requires data that we generally don't have. Don't indict this formula, and don't try to use it for things it can do.

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Old 12-18-2002, 08:36 PM
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Default calculating speed

The formulas are not going to give you the ultimate answer your looking for which is how fast is my plane going? I have met piots and maybe you have to that are genuinely upset because theur calculations are not right. 'Well it should be doing 105mph but its only going 70mph tops. Well genius youonly have a 46fx on a 10 lb plane! Either get a radar gun or time and distance it.
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Old 12-19-2002, 02:58 PM
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Default calculating speed

My intent when asking the question was not really to find out how fast the plane was going. I wanted a formula to try to determine which prop will make the plane go faster, whatever the speed. I just wanted to measure the RPM's of each prop and know if the additional RPM's on a smaller pitch or diameter prop will translate into more speed. I think this will give me the answer (although I haven't put it to practical use yet).
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Old 12-29-2002, 02:44 AM
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Default speed

is this right for determining speed
pitch x rpm divide by 1 foot divided 1 mile x ?
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Old 12-29-2002, 06:43 PM
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Default calc

donzi,

The formula, stated another way, is:

MPH = RPM x Pitch / 1056

Pitch is in inches. Just remember the "1056".
FYI 1/1056 = .000947

.000947 = 60 (minutes/ 1 hour) / 12 (inches/ 1 foot ) / 5280 (feet / 1 mile)

Paul
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Old 12-29-2002, 10:21 PM
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Default calculating speed

Originally posted by dannost
My intent when asking the question was not really to find out how fast the plane was going. I wanted a formula to try to determine which prop will make the plane go faster, whatever the speed. I just wanted to measure the RPM's of each prop and know if the additional RPM's on a smaller pitch or diameter prop will translate into more speed. I think this will give me the answer (although I haven't put it to practical use yet).

The thing is once you find out what prop makes your plane go faster, you still have not gone anywhere. The formula won't tell you what kind of plane. I sleek propjet or a fat Cub
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Old 12-29-2002, 10:55 PM
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Default Yeah..we got that...

Cyclic..I think the original poster was looking for the theoretical speed a given pitch turning a given RPM would yield. Of course the airframe would play a very important part of any speed equation, as would the efficiency of the prop, etc. But thanks for pointing it out three different times in this thread, I'll bet they got the point.
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Old 12-30-2002, 02:38 AM
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Default calculating speed

3 times! I need to get a life. I can't resist. He does not want to know how fast the plane is going but what prop will make the plane go faster? :stupid: I think i can see thru this.

I don't want to be a smart &^% here but is there not more to it than just the formula.

Plane type
Weight
Engine
Altitude
Temp
Prop type

I am hardly the expert here and it just seems to me like a waste of time. I know some people like to now these things but results can be disappointing. Take that stuffed Mustang. I am putting the engine (G90) in a Kyosho Texan. Now The Stang ran its best with a 12/9 APC.
*That is what I am going to put on the Texan. It will move right along but that front end is as aerodynamic as a box so i am going to take a hit on speed and maybe i'll have to make a prop change?
I am lucky since we have several hundred flyers around here and i can see whats going on before i buy or build.
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Old 12-30-2002, 04:12 AM
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Default Well....

It might come in handy...

Say a guy has a trainer with a .40 and a 10X6.

His buddy might remark, "Man that thing must be doing 120!"

And the guy could tell him it was probably closer to 70.

But if he had no way of even guestimating via some sort of formula, he might just say, "Yeah, that's how fast it feels !"

Not everybody is after some magical, absolute formula..they just kinda wanta know 'bout how fast they might be goin'...
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Old 12-30-2002, 02:20 PM
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Default calculating speed

How about a simple "time and distance" thing?
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Old 12-30-2002, 05:34 PM
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Default calculating speed

That would work pretty nice if you do it the right way. Maybe not as accurate as some hi-tech gadget, but we're not going for the world speed record here, are we?

Just remember: the longer the distance, the smaller the error...
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Old 12-30-2002, 08:22 PM
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Default calculating speed

Originally posted by Cyclic Hardover
How about a simple "time and distance" thing?
We did this with good success some years back. I think the two gates were 200 meters apart and we wired up timers so that the first start button would start the other timer's clock and vise versa for the other direction. This was for speeds of 200+MPH. It's unsophisticated equipment but it takes some organizing to make it all work.
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Old 12-31-2002, 05:02 PM
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Default Right on Troy

Troy, what you described is the same system that we used in the memphis area for many years. It was difficult to get everything together, but once we did, it worked very well. To add to this, we had spotters that made sure we did not dive through the trap, and we each had to make runs through the trap both ways to eliminated the wind.


As for the calculator/formulas, they are not total crap. In fact, they are a very good tool to start making estimations with. The key is to know when to stop trusting them. As another person mentioned, they are definitely very good at debunking someone else's claims.
For figuring out what you need to go fast, they can also be useful. Drag and propeller inefficiencies aside, by using the calculator formula, you can find that you need an engine that will swing at least an 'X' pitch propeller at 'y' rpms to even begin to approach the speed you are shooting for.
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