Friction coefficient of taxi/runways 0.023?

#1
Greetings

Could somebody critique my logic here please!

I have a Boeing 777-300 perf chart

The chart details the amount of breakaway thrust required at sea level/ISA day. The 777-300 was loaded at 770,000lbs (max taxi weight) and has a 1% uphill slope

Idle power thrust produced (per engine) 4013 lbs

Breakaway thrust required 9125 lbs x 2 (18250 lbs) to get moving

Feed this into AJPC roll power requirements and reverse engineer the friction coefficient (thanks Herve) until approx 18250 lbs is required to move v friction coefficent. The value obtained here is the friction required to be set within the sim = 0.024 to get the aircraft moving at 18250 lbs thrust

Interpolate a little for the uphill slope so make it a little less draggy

0.023 is the correct coefficient of friction for a hard smooth surface to modify within SIM1.DLL? (offsets shared in my other post)

As the 777 is at max taxi weight. 40% N1 is generally the limit for apron maneuvering so in table 1506 0 mach I set idle N1 at around 4013lbs becasue I know this value (4013/115000) 0.034895

and at 40% N1 I set thrust to equal 9125lbs (9125/115000) 0.079347 because I know the aircraft needed more thrust than at idle and imagine %40 is probably the most to be used

Smooth out the thrust curve so its flatter at low N1 and with thrust ramping up exponential from %50 N1 onward

Power ground handling complete!

Any comments?

Aware that all other airfiles will be screwed up and taxi like rockets!

thanks
 

Heretic

Resource contributor
#2
Well done(?).

If you have a registered version of FSUIPC, you can use a LUA script (included in the package) that will set the friction realistically at low speeds (for correct breakaway behavior, provided the .air file is properly set up) and then switch back to the default behavior at higher taxi speeds. Could be a better compromise across the board.
 
#3
I think Pete has removed that feature in FSUIPC 5 because the offsets keep changing every time the DLL is recompiled, I have just finished my editing and airfile work so about to test in P3d!
 

Roy Holmes

Resource contributor
#5
I'm not sure you can just
Interpolate a little for the uphill slope so make it a little less draggy
With a 1% uphill slope at a weight of 777,000 lbs there is a potential energy force equivalent to 770,000*sin(slope) backwards or 7,700 lbs reverse thrust equivalent.
Your manual says it requires 18250 lbs thrust to get moving under those conditions.
I think that since you are not testing on a 1% slope you need to consider subtracting the 7700 from 18250 for level conditions.
Roy
 
#6
Thanks Roy, that makes sense and I was hoping you would read this!

I have calculated a new coefficient of 0.014 (AJPC doesnt go this low so I have had to double the thrust and half the friction result)

The handling is very nice now, needs a bit of thrust to get moving but then rolls at idle at low MTW

Next part is to figure out the braking? is it now overbraked or underbraked?
 
#8
Hi Herve, the issue was that as I lowered the friction past 0.016, AJPC reported that it was impossible to roll at idle thrust? This was a bit odd as you would expect it to become easier
 
#9
Hi Herve, the issue was that as I lowered the friction past 0.016, AJPC reported that it was impossible to roll at idle thrust? This was a bit odd as you would expect it to become easier
Understand..In such a case, from AJPC calculation, it is indeed impossible to determine a proper thrust for taxiing since idle thrust is greater than what is required for an unaccelerated steady state move (that is you'll need braking from time to time)
 
#10
Just an update to this and close out, I have just been reading through some Boeing performance data for the 787-9, they quote that the tyre static friction used in their maths equated to 0.0135!
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#11
Hi guys. Sorry to dig an old post ...

I’m very newbie for these kind of things. I use PROSIM 737 with P3Dv4, and I really miss the old friction module from FSUIPC4.

Could anyone please help me modify the friction table on p3dv4 to help make taxiing more realistic ? I know that this requires work and time, so I’m willing to compensate for it.

I just can’t stand the default friction in P3dv4.

Thanks

Eduardo
 
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