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

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Old 07-03-2004, 03:35 PM
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mercula
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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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Cyclic Hardover
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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.

example:

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
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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.

See:

http://www.supercoolprops.com/ARTICLES/article_4.htm

also:

http://www.supercoolprops.com/articles.htm

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