Author Topic: Take off roll calculation  (Read 1355 times)

Don Ward

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Take off roll calculation
« on: July 17, 2010, 05:57:26 AM »
Does anyone know of a formula for taking the density altitude (or the individual values used to calculate density altitude), aircraft weight and winds to calculate expected take off roll?  I have never felt that using the chart in the POH is very precise, and that assumes I remember how to use it correctly.  I am  a flatland flyer considering a trip out west to higher elevations and would like to have a good way to make sure there is enough runway available.  I have an F33A/IO-550 and I recognize that the most accurate formula would need to be specific to the engine and model but would be grateful for a formula for any model that I could use as a starting point.

Tom Turner

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Take off roll calculation
« Reply #1 on: July 18, 2010, 09:01:11 AM »
Hi, Don.  The POH table is actually pretty accurate, assuming you use the prescribed technique as listed in the Associated Conditions and airspeed tables on the chart.  

The POH chart accounts for density altitude by comparing the raw information (pressure altitude and temperature) in the left-most block of the chart. It may help to copy it from the POH and blow it up for easier reading.

You might take AOPA's online Mountain Flying course before your trip west, and even consider a stop along the Front Range to fly with an instructor from the Colorado Pilots Association.  One item you'll see recommended frequently is to fly early, in the cool part of the day.  Another is to fly as light as possible--the Colorado Pilots Association, for instance, recommends no heavier than 200 pounds below maximum gross weight for your mountain takeoff.  If need be, fly out of the mountains, land and refuel to make it al the way home.  It's vital that you practice good mixture control for takeoff and climb, and fly the book airspeeds to get "book" performance--the attitude needed to get book speed will be lower at high elevations than what you're used to.

Have a good trip!  

Don Ward

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Take off roll calculation
« Reply #2 on: July 18, 2010, 11:05:27 AM »
Hi Tom,

I guess I have just never trusted my technique on the POH table. Blowing it up is a good idea, I'll try that... the POH version is too many small lines and numbers for my big hands.  I have built a spreadsheet that calculates W&B and was hoping to incorporate take off roll info into the same sheet if I could find a formula with a reasonable amount of precision.  I may try to build it myself from the POH table. My plane is turbonormalized so the leaning won't be so much an issue.  It also means the table in my POH should be wrong in a conservative way which is fine with me. On mountain flying, I took the AOPA course a while back but plan to take it again before departing.  I have also read Sparky Imeson's book, which I found VERY informative.  His discussions on surviving off airport landings are good info for all of us.... even the flatlanders.    Thanks for the feedback and info....

Don

Jeff Koonce

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Take off roll calculation
« Reply #3 on: July 19, 2010, 04:56:06 AM »
Don:

Have you looked at a "Koch Chart" for determining the effects of temperature and altitude on your 'normal' takeoff distance.  I live in Creede, Colorado (8680' msl) and fly a E-33 with the NA 225 hp IO470K.  I've found the Koch Chart to be very useful for high altitude airfields.

If you plan to use a grass or soft field you should increase your landing and takeoff distances by an additional 25%.

Jeff Koonce

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Don Ward

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Take off roll calculation
« Reply #4 on: July 19, 2010, 12:51:42 PM »
Jeff,

I have never heard of a Koch chart until you brought it up.  I Googled it and found an image that I could print.. It seems pretty straightforward.  I will play with it some.  I also found a formula for the chart that I may be able to play with to incorporate into my spreadsheet.  Thanks for the info!

Don