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

Speed calculations


Old 07-03-2004, 03:35 PM
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Default Speed calculations

On here ive seen a few equations to calculate speed ,
What are they ? And how can they be very accurate ?
How can you add in drag , Airframe styles , weight and
other varibles? I understand they cannot be exact , But
can you really estimate aircraft speed based on
pitch and rpm ? Any info and/or numbers people ? [sm=confused.gif]
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Old 07-04-2004, 08:56 PM
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Default RE: Speed calculations

Heres one for Pitch/Rpm. I tested it on a couple of speed planes and i thought it was pretty close. I was having problems fnding the right prop . This equation told me what I already knew and helped me find one that worked.


14degree pitch divided by 12inch length = 1.1666 (ft per turn)

1.1666 divided by 5280 = 0.000220 (miles per turn)

0.000220 x 10,500rpm = 231 (miles per minute)

2.31 x 60 = 138 mph

Using 80% efficiency like this 138 x 0.8 = 110mph.

Now for speed is a time distance thing which works well and I'll use our runway and one of my planes.

Lets say my P-51 with a Saito 100 flys our 700ft runway in 5 seconds, It goes like this.

700ft runway divided by 5 seconds = 140ft per sec.

(140 x 60 = 8400) x 60 = 504000

504000 divided by 5280 = 95mph. (before corrections.)

Now on the first formula was with my new .40 size P51 with a Saito 100. My first few flights I was not getting the speed i wanted and was very slow and was also before i used the formula. Using this formula backed up my poor flights. I then picked out a prop using a specific rpm which gave me a nice looking number. So I went out to fly the plane and it went like a bullet.

Also on the first formula you see an "rpm" this is whatever rpm you choose to dial it in at.

I use a Saito 100/ 12/12 prop and 9600 rpm on one plane.
Have a Jett.50 with a 9/7 prop at 17500rpm

May want to print this to keep around. Later.
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Old 07-04-2004, 11:38 PM
iskandar taib
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Default RE: Speed calculations

A couple of reasons why this might lead to an underestimate:

1) In-flight RPM is more than ground RPM. Often a lot more.

2) Effective pitch may be higher than measured pitch (most people measure off the back surface). And besides, manufacturers' stated pitch numbers are often highly inaccurate.





Some exceedingly cool stuff here, including in-flight RPM and speed measurements using the Doppler effect.
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