Without changing pitch, venturi dia and/or compression and presuming that I would aim for the same lap time and the same type of fast 4-cycle engine run, what is the relation between density altitude and nitro content? Does anybody have data on hand quantifying nitro change relative to density altitude?

My home base starting point is 10% nitro for 5.3 sec at a density altitude of 2500 ft. and I am looking for info on how to adjust nitro for 1000 ft. and/or 4000 ft. density altitude. Trivial Pursuit 62 oz  PA .75 2p  13 x4 3 bl. 64 ft x 0.015

Thanks for help

Peter
  I do not have any charts, but going up 3000 ft with my setup requires about 5% increase.
I normall use 5% in my setups in cooler months, going to 10% in hotter months, or when I change locations that have higher altitudes.
In the Summer I use a lot of 7.5%. When I was at the US NATs, for example, I take 5 and 10 % fuel with me, it is easy to go from one to the other, or I can mix them and have 7.5% fuel, work perfect as a tuning tool to keep the run the same
You could use 10 and 15% in the same manner, or even 10 and 20%

Regards
Randy

I'm interested in Randy's answer as well. I was always told that more nitro helped in hot weather.

Thanks Randy, 
I happen to have another special prop for Stalker that is 1/4" more pitch.  Have to give it a try in this hot weather we're haviing,
Alan

Thank you, Randy. I will do as suggested, and keep record and report.
Meanwhile on I have found on:
http://wahiduddin.net/calc/calc_hp_abs.htm  
a program calculating relative power of IC engines when entering:
- Absolute Pressure in in/Hg  (barometer at site)
- Air Temperature in °F
- Relative Humidity in %
I wonder if we could use this program to actually quantify the loss of power due to altitude and temperature? If so, all we would want to know then is how much nitro we need to restore power.

rgds

Hi Peter

You could probably get very close by using the programs "Dyno Correction Factor" and then multiply by 3 . In other words on a 100 degree day at 100% humidity and a standard pressure, you would get 88% hp, the dyno correction factor would be 1.126.
 If you are using say 10% nitro normally on a standard day, you would multiply 10% by 1.126 ,this would give you 1.12 %increase in nitro x 3 would total 3.36% increase to compensate..or you would run 13.36% nitro fuel.

A quick way to figure this,if you are adding nitro at the field would be 3.36x128=4.3 ounces of nitro added to a gallon of your fuel

Depending on your particular setup you may need to use a multiplying number of 3 (and I think 3 would be real close)  or it could be as much 4.5, or as low as 2.5, these numbers would get you very close.
 I don't think there would be one universal number that would work for everyones setup, and ever percentage of stock size nitro that we all use, but I don't think the range would be very far off from another persons,, but you could very easily use it and develop  your own multiplier number..ie find your factor..wether it be 2.0, 2.5,3.0,4.5 or what number works for your setup
You would need to buy one of the hand held weather stations that I have used, they would give you all the info you needed at the flying site.
They are available online

Regards
Randy

Without changing pitch, venturi dia and/or compression and presuming that I would aim for the same lap time and the same type of fast 4-cycle engine run, what is the relation between density altitude and nitro content? Does anybody have data on hand quantifying nitro change relative to density altitude?

My home base starting point is 10% nitro for 5.3 sec at a density altitude of 2500 ft. and I am looking for info on how to adjust nitro for 1000 ft. and/or 4000 ft. density altitude. Trivial Pursuit 62 oz  PA .75 2p  13 x4 3 bl. 64 ft x 0.015

   The density altitude correction should be pretty close. But I have had excellent luck just adjusting the nitro content until I get the same run time for the same volume of fuel.

    Brett

Thank you, Randy and Brett

Brett, when using the "same run time" method does this mean same T/O RPM or same needle setting?

Hi Peter

You can add nitro until you get the same run time as Brett suggested ,and also look for the same performance, the RPMs will NOT necessarily be the same.
Many times you will notice that you will have to change the needle until the engine "sounds" the same .
But if you get the amount of nitro by trial and error method, you could then back it up into a formula to use the density altitude method or the program you asked me about.
I use the method Brett suggested many times, For me, going up 3000ft in density altitude I have to about double my nitro..ie  7.5% I go to 15%, 5% I go to 10%. This is very easy to do the math for any density altitude.
This will get you very close but there are other factors that will affect the planes, even the surface you are flying off of will have an effect on the plane, example is black top vs. grass

I carry my fuel in a box that holds 4 gallon cans, I take normally 5% and 10%, if the contest is somewhere in between I use the 2 mixed to get 7.5%, If I am going to a really hot or high place I take along a gallon of 15 or 20%


Regards
Randy

I carry my fuel in a box that holds 4 gallon cans, I take normally 5% and 10%, if the contest is somewhere in between I use the 2 mixed to get 7.5%, If I am going to a really hot or high place I take along a gallon of 15 or 20%

   And of course, you can mix the fuel in the sryinge. Pull out 2.5 oz of 10%, 2.5 oz of 15 into the same syringe, and you have 12.5 percent with no pouring or mixing. I was doing that at the NATs this year at times. It's a lot safer in AMA than it is in FAI, since you have the extra minute at the end.

     Brett

Good Morning, Randy

its not so much a formula but an easy to carry chart showing the change of nitro content required to  compensate density altitude. I do remember how to find density altitude from std. temp deviation and QNH. From there, using the engine tuner website, rel. power can be defined. Is it too simple to assume that all that remains to be done then is adding enough nitro to compensate the loss? 
Trying to quantify „enough“ I was today running an older type OS .29 with three different fuels while measuring peak RPM. From  the rpm gain observed I have then calculated the relative power, assuming that a specific increase of prop RPM requires a power change by the cube. (i.e. to double RPM of a propeller it takes 8 times the power) Nitro was 99% and oil 15% KL-200 + 5% castor. Prop was 9 x 6 2bl.

10% Nitro, 11'600 rpm, 100% rel. power
12.5% Nitro, 11'750 rpm, 103% rel. power
15% Nitro, 11'850 rpm, 107% rel. power

Does this mean that if, when flying 10%,  I suffer a 3% power loss due to density altitude change, I can compensate the loss by going from 10 to 12.5% nitro, thus incerasuing power by 3%?

Humidity does affect air density which is what you are adjusting for with altitude changes.  Most of the savvy F2C teams use air density as an indicator.

Does all lthis take into consideration humidity?  When flying F2C we had to keep adjusting for humidity in some areas.  The one I remember most was Buder Park in St Louis.  Round by round there were humidity changes.  DOC Holliday

Hi Doc

The density altitude includes the dew point temp.
http://www.divemaker.com/trajectory/da.php

The above is a wing loading convertor for density altitude

And below is a simple density altitude chart form NOAA, I can include many other site that complex number calculations for this but I think this shows how we are affected
http://www.srh.noaa.gov/elp/wxcalc/densityaltitude.shtml

Regards
Randy

So reading between the lines, I take it that if you were to put up one flight using 10% fuel, and then another a short time later using 12.5% fuel, same prop, needle setting and fuel volume, your run time would go....up?  If so, is there simple math to estimate how much?

    The flight time would go down - it takes more 12.5% to get through a flight than 10%, because to get the same power you have to run the engine richer.

If you're flying at a contest where it's, say, 75 degrees in the morning and 90 degrees midafternoon, would you recommend starting the day with less nitro and adding more as the day grows hotter?  Does the increase in temperature tend to offset any change in run time due to increased nitro %?

     Depends on what the humidity does. Most of the time, the dew point stays the same throughout the day, so the morning might be 60% and 75 degrees, and later you have 30% and 95 degrees. Most of the time I figure out what nitro to run for a particular site, and figure I can adjust the needle to compensate. So, when I go from here to Muncie, I switch from 10 to 15 and get about the same. The variation from day to day once I am in Muncie is usually close enough that I don't need to change anything, and here, I just always run 10% and adjust the variations with the needle. If something drastic happens, then maybe it's a different story - David and I both had to go back to 10% at the 2007 Team Trials when we both started having the engine charge here and there - but mostly it stays the same.

     Brett