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FSXA Reverse thrust

hotelfox

hotelfox

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germany
Is there a real life pilot or any other expert who can answer my question?

All my FSX aircraft (both, default and add on) which are reverser equipped simply show 'reverser_available= 1' in the aircraft.cfg. The SDK however clearly states that 'A value of 1.0 will cause the theoretical normal reverse thrust to be available. Other values will scale the normal calculated value accordingly.'

That would mean there is a way to fine tune the reverse thrust by setiing the value to for instance 0.73 or something.

My impression is that the value 1.0 means to overrate the reverse thrust. I can imagine that for example the reverse thrust of a RR Trent 772B only matches a certain share of the normal thrust.
Is there a rule of thumb how many percent of the full thrust is generated as reverse thrust?
I already tried to find corresponding information but didn' t succeed.

Thank you for your help.
 
Hi!

reverser_available is used for scaling the rev thrust, you're correct.

Via min_throttle_limit in [GeneralEngineData] you can adjust the max N1 for full reverser.

So I'd suggest first setting min_throttle_limit and then use reverser_available to get the most realistic reverser thrust.

Best regards,
Steven
 
Hallo SierraB!
Thanks for your help. Now I hope to get some input from an expert who knows about the power ratio of reverse thrust.
 
You need to know what N2% your engines run up to when in reverse.
Then fiddle with your aircraft.cfg to approximate the real value.
go to [GeneralEngineData]
and adjust min_throttle_limit=
to your liking.
It's a negative number. The larger the number the greater the thrust.
My VC10 is set to min_throttle_limit=-0.32
That equates to 93% n2 for the RR Conways.
vololiberista
 
Mille grazie!
That means you wouldn' t change the figure behind 'reverser_available' at all?
 
You would need a static thrust chart of your engine in reverse to know exactly. Every engine is slightly different, but FS does a good 'generic' job at full extension because they modeled it after the 737-400 which is fairly similar in thrust loss to most airliners. Clam shell reversers such as MD80s are a few percent more efficient than the bypass reversers used on wing mounted engines.

The best way to resolve this without that hard to come by chart is by setting breakaway power in reverse. The powerback %n1 required to produce movement should match the real plane. If this is set correctly, and assuming your forward thrust and ram drag are correct, the sim is producing the correct amount of braking power at full extension.

At partial extension, the sim will mess up some.
 
hotelfox said:
Mille grazie!
That means you wouldn' t change the figure behind 'reverser_available' at all?
Click to expand...

No, all you need to change is [GeneralEngineData]
min_throttle_limit=-0.32 <-- or your chosen value

If I change -0.32 to -0.42 for example that's enough for the a/c itself to go in reverse!!! Which in the case of the VC10 is not possible.
vololiberista
 
The min throttle limit does not affect the amount of reverse thrust produced. It only adjusts the throttle range as a percent of your CN2 vs throttle table.

The scalar you referred to does.

To answer your question directly:

My impression is that the value 1.0 means to overrate the reverse thrust. I can imagine that for example the reverse thrust of a RR Trent 772B only matches a certain share of the normal thrust.
Click to expand...

No, as I stated before FS used the 737-400. This will produce about a 35% loss in thrust. So setting the scalar to 1.0 = Forward thrust times 0.35.


Looking through some formulas here is what I've found:


forward thrust / bypass ratio = core thrust

forward thrust - core thrust = reversible thrust
reversible thrust * cos50(0.643) = useable reverse

So at static:

useable reverse - core thrust = net reverse

Cos50 is an approximation of the reverse airflow angle. The mechanical reverser will vary some.


Using the 737-400 for example:

22,000 / 5.9 = 3,728 core thrust

22,000 - 3,728 = 18272 reversible thrust
18272 * 0.643 = 11748 useable reverse

11748 - 3782 = 8020 net reverse at full power.

8020 / 22000 = 36% reversible



So if you know the airflow angle of the reverser you're in business.

*The C-17 Globemaster can reverse it's core thrust as well. Almost all other fans can't.
 
Many thanks for your help, gentlemen - especially for your detailed deliberations, jx! This is valuable info!
 
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