Hit brakes after takeoff
Thread Starter
Joined: Jan 2009
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From: On a wing and a prayer
Hit brakes after takeoff
I am a student pilot on a single engine piston aircraft. After takeoff my instructor insists that I pump the brakes to stop the rotating wheels.
On questioning the logic he answers that 'It reduces drag'.
Asking how yields no answer.
Does the Magnus effect have anything to do with this ?
Could anyone enlighten me on this ?
Regards,
VT-ASM
On questioning the logic he answers that 'It reduces drag'.
Asking how yields no answer.
Does the Magnus effect have anything to do with this ?
Could anyone enlighten me on this ?
Regards,
VT-ASM
Joined: Sep 2012
Posts: 222
Likes: 73
From: UK
Hmm, I could be wrong but I always thought that after take-off the wheels would automaticly stop rotating anyway (due to some kind of WoW switch). This is so when you are about the land, the wheels are not spinning. If they did, they would act as a massive gyro and when you touch the ground would cause the aircraft to jump. This could just be on large a/c though, Im sure someone else will know!
The wheels would normally be contained within a fearing anyway, so I doubt the drag difference would be too much. Furthermore, once you have breaked to stop the wheels rotating, the airflow would soon make them spin again. I suspect your instructor is mis-informed, but maybe not, I'm half guessing too!
The wheels would normally be contained within a fearing anyway, so I doubt the drag difference would be too much. Furthermore, once you have breaked to stop the wheels rotating, the airflow would soon make them spin again. I suspect your instructor is mis-informed, but maybe not, I'm half guessing too!
Last edited by V_2; 26th October 2012 at 06:53.
Joined: Mar 2012
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From: Central London
Pretty poor of the instructor not to be able to back up his teaching with reasons!
I was taught the same. I was told it it was to get into the habit of dabbing the brakes after takeoff, ready for moving up to a complex SEP, where apparently this is done to prevent vibration caused by retracting spinning wheels.
I was taught the same. I was told it it was to get into the habit of dabbing the brakes after takeoff, ready for moving up to a complex SEP, where apparently this is done to prevent vibration caused by retracting spinning wheels.
Joined: May 2009
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From: HKG
Absolute rubbish about drag. In a fixed gear aircraft it is pointless stopping the rotation. In basic retractables that do not brake automatically you do it to stop the gyroscopic effect as the wheels go up, think of the way a main gear retracts and how a force acts on a gyroscope, perpendicular and 90deg in plane of rotaton, or something like that. So every time you put a rotating wheel up it produces a twisting force that the leg is not designed to take over and over.
On large aircraft, such as an A330, if you have an inop brake on one main wheel you have to leave the wheels down for two minutes after take off so as to allow the rotation to stop. There is a lot of rotating mass just after take off so the twisting on the leg would be quite large. If the brakes are working they stop rotaton on selecting gea up.
On large aircraft, such as an A330, if you have an inop brake on one main wheel you have to leave the wheels down for two minutes after take off so as to allow the rotation to stop. There is a lot of rotating mass just after take off so the twisting on the leg would be quite large. If the brakes are working they stop rotaton on selecting gea up.
Joined: Apr 2012
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From: planet earth
On questioning the logic he answers that 'It reduces drag'.
Sounds like one of those instructors who passes on things that he learned himself withouw knowing the reason why.
Maybe there are some, but I don't know any single engine, non-retractable a/c that requires the pilot to hit the brakes after T.O.
Taxistaxing first reply was spot on.
Topic can be closed.
Joined: Mar 1999
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From: Australia
Agree that the technique is for retractables for the gyroscopic reason mentioned. There are many, more complex aircraft where the brakes apply automatically on retraction to stop the wheels turning so the dab on the brakes is not required. They also have brake inhibit to stop the wheels being locked on touchdown.
There is absolutely no problem with the wheels turning when you touchdown.
As an aside I know that there are a few pilots who found out the hard way that an A340 will not let the brakes on in flight when they pulled a circuit breaker during an engine test, the aircraft thought it was in flight and disabled the park brake and they found themselves impaled on a concrete wall.
There is absolutely no problem with the wheels turning when you touchdown.
As an aside I know that there are a few pilots who found out the hard way that an A340 will not let the brakes on in flight when they pulled a circuit breaker during an engine test, the aircraft thought it was in flight and disabled the park brake and they found themselves impaled on a concrete wall.
Joined: Jan 1999
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From: north of barlu
To sum it up the brake after take off thing
Non retractable SEP waste of time.
Retractable SEP/ MEP I don't buy the Gyroscopic thing but might be worth doing to stop mud or snow & slush being flung into the wheel well.
Transport aircraft Normaly the brakes are applied from the up line in the landing gear hydraulic system.
Retractable SEP/ MEP I don't buy the Gyroscopic thing but might be worth doing to stop mud or snow & slush being flung into the wheel well.
Transport aircraft Normaly the brakes are applied from the up line in the landing gear hydraulic system.
Last edited by A and C; 27th October 2012 at 10:19.
Joined: Jan 2008
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From: Reading, UK
Transport aircraft Normally the brakes are applied from the up line in the landing gear hydraulic system.
Joined: Dec 2007
Posts: 175
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From: Cyprus
Let's take some hypothetical figures: an aircraft gets airborne at, say, 100 knots and accelerates to 150 knots with the wheels still spinning. The air velocity past the bottom of the wheel is 50 knots while the velocity over the top of the spinning wheel is now 250 knots. Given that drag increases as a square of the air velocity what is the total drag of the rotating wheel compared with a non-rotating wheel? Just nit-picking for the fun of it 
.
.
Joined: Jul 2000
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From: London
I was always taught to dab the brakes in light aircraft after take off just to stop the gyro forces. This was especially true in the light twins, upto a C404. As soon as you move onto more complex, faster aircraft, the systems will snub the brakes on retraction, for the same reason.
I once, just the once, fell into old habits and dabbed the brakes on the XL after rotation.. I wont ever do that again.. it was violent how much of a bang and nose down (momentary) force i got!..
In light aircraft, SEP, even fixed gear, often the wheels spinning freely after takeoff vibrate so badly its a relief to stop the spinning! Thats a good enough reason in itself!
I once, just the once, fell into old habits and dabbed the brakes on the XL after rotation.. I wont ever do that again.. it was violent how much of a bang and nose down (momentary) force i got!..
In light aircraft, SEP, even fixed gear, often the wheels spinning freely after takeoff vibrate so badly its a relief to stop the spinning! Thats a good enough reason in itself!
Joined: Mar 2006
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From: England
Most large commercial aircraft apply the brakes during retraction, but this is done very gently to avoid problems as in #13.
Harsh manual application in some aircraft might even damage the wheels / brake rotors. Check the ops manual / DDG for MEL dispatch with the automatic system inoperative.
Harsh manual application in some aircraft might even damage the wheels / brake rotors. Check the ops manual / DDG for MEL dispatch with the automatic system inoperative.
Joined: Aug 2008
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From: North America
Cardinal wins............
................ the best word picture I have read in a long time!
As a dog owner.......now that is funny!
In primary flight training, fixed gear piston singles, I was never taught to apply brakes after takeoff.
Seems to me I was told to when I checked out on senaca's, navajo's and titans.
-Northbeach
shook like a wet dog after takeoff
In primary flight training, fixed gear piston singles, I was never taught to apply brakes after takeoff.
Seems to me I was told to when I checked out on senaca's, navajo's and titans.
-Northbeach
Last edited by Northbeach; 26th October 2012 at 19:27.