How much brakeing from spoilers
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How much brakeing from spoilers
Hi, I was just wondering how much braking comes from the spoilers on jet aircraft, spcifically I'm interested in the 737-NGs- but any figures are welcome. What I mean is proportionally how much of the brakeing force comes from the spoilers when compared to wheel breaks and thrust reversers?
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In a modern airliner, virtually all of the runway retardation force comes from wheel brakes. That braking force is enhanced by having more weight on the wheels, hence the need for lift dumpers/spoilers. Reverse thrust does have some braking effect, especially at high speeds. But even without it, your stopping should be assured by the use of wheel brakes only. The effect of each systems not working can be explained by the penalties they attract in terms of stopping distance. These are "ish" figures: Flaps 42 instead of 25: -10%; No reverse thrust: +5%; Emergency Reverse: -10%; Excess Speed: +1% per knot; No Lift dumpers: +30%; No Brakes = Arrange good life insurance and get the Fire brigade waiting.
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It is for 737!
From Boeing:
"Reference distance is for sea level, standard day, no wind or slope, VREF30 approach speed and two engine detent reverse thrust."
There is an addition for 1 or 2 revs inop.
Edited to add QRP Extract
From Boeing:
"Reference distance is for sea level, standard day, no wind or slope, VREF30 approach speed and two engine detent reverse thrust."
There is an addition for 1 or 2 revs inop.
Edited to add QRP Extract
Last edited by BOAC; 4th Dec 2008 at 15:04.
Re “Some performance figures for some aircraft require use of some reverse.”
Some scheduled performance figures do, e.g. JAR OPS 1 contaminated runway – and its probably max reverse, not just ‘some’; this is not necessarily so for wet / dry ops.
BOAC, could you confirm the exact 737 QRP reference; I understood that this data is unfactored, i.e. not for use as scheduled performance (pre departure planning).
Some scheduled performance figures do, e.g. JAR OPS 1 contaminated runway – and its probably max reverse, not just ‘some’; this is not necessarily so for wet / dry ops.
BOAC, could you confirm the exact 737 QRP reference; I understood that this data is unfactored, i.e. not for use as scheduled performance (pre departure planning).
BOAC, thanks. The only on-line reference that I have is from this 737 QRH. Page 396 and similar indicate that while reverse is considered in the data, it is unfactored, i.e. not scheduled performance.
Re how much braking comes from spoilers; see Takeoff / Landing on Wet, Contaminated and Slippery Runways, where page 119 indicates the proportion of actual stopping distance from reverse (unfactored distance). The effect of spoilers is not shown perhaps indicating that the ‘drag’ contribution is minor.
Pages 143/144 show the overall effect of spoilers on the ‘stopping’ force, where this force is the sum of reverse thrust + aero drag + wheel brake. This indicates that the main contribution of the spoilers is helping to reduce lift and place the weight on the main wheels so that the brakes can be used effectively.
IIRC in the BAe146 / Avro RJ (which have very large spoilers – lift dump), the difference in landing distance with the spoilers failed is 40%, but again this is probably due to reduced braking – the aircraft does not have reverse.
Re how much braking comes from spoilers; see Takeoff / Landing on Wet, Contaminated and Slippery Runways, where page 119 indicates the proportion of actual stopping distance from reverse (unfactored distance). The effect of spoilers is not shown perhaps indicating that the ‘drag’ contribution is minor.
Pages 143/144 show the overall effect of spoilers on the ‘stopping’ force, where this force is the sum of reverse thrust + aero drag + wheel brake. This indicates that the main contribution of the spoilers is helping to reduce lift and place the weight on the main wheels so that the brakes can be used effectively.
IIRC in the BAe146 / Avro RJ (which have very large spoilers – lift dump), the difference in landing distance with the spoilers failed is 40%, but again this is probably due to reduced braking – the aircraft does not have reverse.
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Going back to the original question, in descending order, the most effective braking is.....
(1) Wheel Brakes WITH Spoilers,
(2) Wheel Brakes WITHOUT Spoilers,
(3) Reverse Thrust (Not as much as people think),
(4) Spoilers.
As mentioned earlier, the drag of the spoilers on the ground and in flight is very small. On the ground, wheel brake efficiency is much improved by destroying lift and putting much more weight on the wheel brakes. In flight, Spoilers don't add much drag in their own right, they are extremely effective by destroying much lift, and causing the aircraft to be flown at a higher angle of attack, which DOES considerably increase drag.
Regards,
Old Smokey
(1) Wheel Brakes WITH Spoilers,
(2) Wheel Brakes WITHOUT Spoilers,
(3) Reverse Thrust (Not as much as people think),
(4) Spoilers.
As mentioned earlier, the drag of the spoilers on the ground and in flight is very small. On the ground, wheel brake efficiency is much improved by destroying lift and putting much more weight on the wheel brakes. In flight, Spoilers don't add much drag in their own right, they are extremely effective by destroying much lift, and causing the aircraft to be flown at a higher angle of attack, which DOES considerably increase drag.
Regards,
Old Smokey
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SP - copy. I was trying to simply answer a simple question from a 'racket stringer'
Answer "Virtually none"
What I mean is proportionally how much of the brakeing force comes from the spoilers when compared to wheel breaks and thrust reversers?
