larry11

My Feedback: (6)

i was wondering if there was a generic calculation to determine approx. how fast my plane would go using the prop size, engine rpm, and plane weight to calculate.

ie: plane is 10lbs, engine rpm is 9800 and prop size is 16x4

thanks in advance,
larry

cncswiss1

off the cuff, not very fast..

now the details: (prop pitch speed is a calculated ESTIMATE and reality will differ some)

9800 revs per minute * 4 inches per rev = 39200 inches traveled per minute (assuming no slip)\
39200 inches per minute is 2352000 inches per hour
divide by 12 to get feet per hour =196000
divide by 5280 feet per mile = 37.1 miles per hour.. not very fast..

to simplify:
revs * pitch /1056= speed in miles per hour


1056= 1 over ( 60 minutes/ 12 inches per foot / 5280 feet per mile))

Mike Connor

My Feedback: (5)

You can calculate the theoretical speed by using the pitch in inches and the rpm. A 16x4 prop at 9800 rpm would be 39200 inches or 3267 feet per minute. That would be 195999 feet per hour or 37 mph. That would be (((pitchxrpm)/12)x60)/5280=mph.
or
go to http://www.bmaps.net/ , click on goodies on the left and down load the Thrusthp calculator. It will do it for you plus much more.

Theoretical is just that, theoretical. At slower speeds like your situation the plane may be moving faster and if it bolted to a work bench it is going no where. It is often close to accurate though for average model aircraft.

Edit -- opps, someone beat me to it.

Rudeboy

In short, you can't calculate it... the best you can do that way is "guesstimate"...

The only way to get a reasonably accurate number is to measure it one way or another. You have basically 3 options: radar gun, doppler analysis or on board equipment (gps or air pressure based).
I prefer the second option for basic speed measurements: it can be pretty accurate (depends on your flying), and very easy to do. If you want to hold a speed competition, it has its drawbacks. But it's more than good enough to get a decent idea about what you are doing.

Cyclic Hardover

My Feedback: (3)

I use a simple time/distance thing. I ball park it with a stop watch. Lets say your runway is 700ft long and it takes your plane 5 seconds to fly by from one end to the other. The "60's) represent 60sec in a minute and 60 minutes in an hour.

700ft div by 5sec =140 feet per sec
140 x 60 = 8400
8400 x 60 = 50400
50400 div 5280 = 95mph

freakingfast

My Feedback: (3)

Here's another tool. A speed nomograph. It's close for "clean" models only, i.e quickies, diamond dusts, whiplashes and prop jets.
http://members.cox.net/mdconnor/speed.html

funflyerf4

My Feedback: (6)

Better yet strap the plane to the roof of your car ,go fast through your local radar trap and there you are!

larry11

My Feedback: (6)

thanks for the replies.

i maidened her this morning and she is pretty slow.

i think the calculations are fairly close

cobratraxxas69

My Feedback: (31)

Here's the link of the German Doppler Speed Program thread:

http://www.rcuniverse.com/forum/m_36...tm.htm#3616107

Fun little program, I've used it several times

johnc1go

My Feedback: (9)

Thanks for the chart. That means I have a 120 MPH Plane, not bad for my first speed plane but think I'm going to upgrade exhaust to macs header and pipe. RPM 17500 with 7x6 prop