b737-800 reverse thrust

[kilo]

anybody know how much reverse thrust a 26k engine can produce when max reverse power is used?On a slippery runway,
probably it is the only effective stopping force.

[DJohnsen]

Kilo, I don't have the exact "net" reverse trust on a CFM56-7 but I know that an RB211-535 estimates 25-30% net reverse thrust. I doubt the CFM engines can produce substantially more than 30%... or in this case 7,800 lbs (static). Remember that you can only reverse the fan bypass air... and only "at best" 45 degrees in relation to your direction of travel... and then deduct your core thrust... and you are left with about 30%... The "total" reverse thrust also changes with your forward velocity... and is obviously greater at higher velocities... but diminishes as the aircraft is decelerating... Finally, your "window of opportunity" to apply full reverse thrust is relatively small... since you first have to get the "ground" signal for the TR to deploy... then wait for the TR sleeves to travel a certain distance to get the "throttle interlock release"... and then allow the engines to spool up from idle... and be back out of reverse thrust "at least" before reaching 60 kts...

The combination of ground spoilers, reverse thrust and "proper" use of brake application is your best bet at stopping the aircraft... even on contaminated... or as you said "slippery" runways...

:ok:

Dag

[mainwheel]

Do the math. Fan/core thrust of a CFM is about 5-1 ratio. What percent N1 is max in reverse. Lots!!

[DJohnsen]

mainwheel... I'm not certain I'm reading you here... or to whom you are addressing...?

Anyway... the "math"... a CFM56-7B26 is "flat" rated to 26,300 lbs static thrust at sea level and a bypass ratio of 5.1. Using that bypass ratio and "assuming" the N1/N2 thrust ratio is proportional to the bypass ratio... the fan should be producing around 80% (21,040 lbs) and the core should be producing 20% (5,260 lbs) of the indicated (N1) thrust. If we also assume and use 45° angle of the reversed air in relation to the direction of travel… and assuming no parasitic losses, the “net” reverse thrust from the fan will be cosine 45° (.7) X 21.040 = 14,728. Now the core will be producing forward thrust (5260 lbs) so subtracting this will leave us with 9,468 lbs… “net” reverse thrust… or around 36%

I have never ran a B737-800 to maximum N1 in reverse thrust… but since the VBV’s are partially open in reverse thrust… to improve your core stall margin… I can guarantee that you will not reach the same N1 as if you were in TO mode… ;)

Dag

[Flight Detent]

Hi all,
Good response mr johnsen.

I'd just like to add that:
1/ the reverse sleeves only need to travel 60% of travel before you can get passed the 'gate' and throttle up!
2/ approach minimum idle is set at around 76-78% i think with flaps or gear extended, and stays until 5 seconds after touchdown, to get better engine acceleration for reverse or go around.

otherwise i'm quite happy with your thought-out response,
Cheers