Hit brakes after takeoff

[VT-ASM]

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

[V_2]

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!

[taxistaxing]

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.

[taxistaxing]

I always thought that after take-off the wheels would automaticly stop rotating anyway (due to some kind of WoW switch).


Surely not on an SSEP?

[SloppyJoe]

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.

[Cagedh]

On questioning the logic he answers that 'It reduces drag'.:rolleyes::rolleyes::rolleyes::rolleyes:

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. :ok:

[Roger Greendeck]

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.

[A and C]

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.

[DaveReidUK]

Transport aircraft Normally the brakes are applied from the up line in the landing gear hydraulic system.

And the nosewheel, not having integral brakes, is typically stopped by a snubber in the NLG bay once retracted.

[Lancman]

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 :).

[A and C]

You must be short of things to think about !!!!

[VT-ASM]

Thank you everyone. And besides, I forgot to add that I fly a fixed tricycle gear.


Regards,

VT-ASM

[smallfry]

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!

[zondaracer]

I find that on a lot of small fixed gear airplanes that the tires aren't balanced and the spinning of the wheels causes quite a bit of vibration so I like I dab the brakes like smallfry.

[Capn Bloggs]

Please forgive my ignorance, but @#$ is a SEP? I hope it's not a Single Engine Plane!

[Lancman]

Thankyou A & C for your closely reasoned and constructive contribution.

[DaveReidUK]

Please forgive my ignorance, but @#$ is a SEP? I hope it's not a Single Engine Plane!

Single-Engine Piston (as in licence categories).

[Cardinal]

Zonda - I'm with you. Flew an old C182 with spring steel gear legs an out-of-balance wheels that shook like a wet dog after takeoff, that's when I got into the habit.

[PEI_3721]

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.

[Northbeach]

................ the best word picture I have read in a long time!

shook like a wet dog after takeoff

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

[typhoid]

As has been said, retraction braking on a fixed gear aircraft just makes you feel more in control, but has no engineering reason behind it.

On transport category aircraft, most type have a feature called retraction braking, that applies the brakes to stop rotation before the wheels are fully up. Typically, the de-rotation happens over 2s or so whilst the gear is travelling. The principle reason for this feature is to prevent a burst tire retracting into the bay whilst spinning and allowing the tire to damage hydraulic lines, fuel tank walls and anything else in proximity etc. Gyro effect is also a consideration but given the wheels are still spinning at the start of retraction this feature does not help so much.

The reason the brake application is limited in torque (typically to a reduced psi for hydraulic brakes), is to reduce the loads on the gear during the braking itself. These cycles have to be taken into account when calculating the fatigue spectrum for the gear. High loads = lower total gear cycles or beefier gear.

For the nose, a snubber may be used to stop the wheels in the bay or they are left to spin down naturally - there are less critical systems in the NLG bay compared with the mains.

Alex

[boofhead]

I agree it is a waist (sic) of time.
Gyro effect? Negligible.
Increased drag? Mental drag maybe.
Fixed gear? Only if the rotating wheel is out of balance and that bothers you. Risk of creep if you hit the brakes while the wheels are spinning fast (tire will rotate on wheel and if tubed might pull the tube stem out, and the tire will go flat).
Retractable? Follow the POH. Most airplanes will not require it, because the tire bay is large enough to take the spinning wheel and many have automatic brakes/snubbers anyway. If not, the POH will specify the procedure.
Some airplanes take off so fast that the wheels are really really spinning and will have a bigger diameter while doing so. If you find yourself flying a jet fighter, check the book and follow the recommended procedure.
If you misjudge your height above the runway, you could touch down with the brakes on during the retract process, which will spoil your whole day I guarantee it.
Letting the wheels spin for a while before retraction will throw off water, mud, ice etc. Applying the brakes too soon could stop this crud from being removed and you could face extension problems after a long flight, or the prox/micro switches could be contaminated.

Sounds a lot like downwind turns, turning into the dead engine, and other urban legends many otherwise smart pilots believe without verification.

[captjns]

I can see itnow. “Pilot loses control of aircraft doto gyroscopic precession of retracting landing gear:eek:. One gear would not retract and the effect ofthe retracting gear caused the aircraft to spiral out of control. He should have tapped the brakes:{.” A good script for a lousy movie:}.

[Check Airman]

If your CFI insists that you tap the brakes, do it, but please don't pass on the misinformation he has given you.

The spinning wheels don't produce any drag that I've ever noticed. In addition, if you're flying a Cessna, look at the wheels and see how long they spin. Within a few seconds of leaving the ground, they are stationary.

[A and C]

Just think about the inertia load that the mass of a spinning tyre stopping will apply as torque between the tyre and the wheel when you stop it by braking in the air.

Now think about the torque loading that is applied when stopping the aircraft even at a low speed.

Who thinks up with this tyre creep rubbish !?

Lancman as well as being short of things to think about you also seem to be short of a SoH.

[Miserlou]

So what do you do in an aeroplane with no brakes?

If you can feel vibration due to an unbalanced wheel, go ahead.
Otherwise, don't sweat it. I've never seen any reference to doing so in a POH.

[ShyTorque]

Risk of creep if you hit the brakes while the wheels are spinning fast (tire will rotate on wheel and if tubed might pull the tube stem out, and the tire will go flat).

I agree with A&C's comment about braking on landing. Much more likely to get creep on landing, the brakes are trying to stop the wheel, the earth is trying to keep it spinning!

Now, on the other hand...if the aircraft were to be landed on a conveyor belt....

An ex-colleague of mine was told off in the air by his Jet Provost QFI for allegedly dabbing the brakes at the point of take-off. He swore blind he hadn't done so. And he was later found to be correct by the groundcrew, who were asked to check for tyre flat spotting.

What he'd actually done was to press the undercarriage up button too early during a final runway "bounce", which defeated the WOW switch and allowed the bay doors to touch the runway during retraction. There was half an inch of metal missing from the ends of each main gear door :D

[sudden Winds]

Hi, didn´t care to read the whole thing, and probably someone already mentioned this...but I 'll just say it.
if the POH, flight manual, FCOM, etc instructs you to apply brakes after takeoff, do it, otherwise it´s not required and might not be desired.
rgds.
sw.

[Chu Chu]

If you pick up a spinning bicycle wheel, you'll find that the gyroscopic forces seem greater than the weight of wheel itself. Not that gyroscopic forces would have an effect on aircraft handling, but they could matter during gear retraction, when the mechanism otherwise has to deal with mostly with the weight of the wheel itself. (OK -- I guess "mostly" is an exaggeration; the weight of the non-rotating components and the drag on everything are bound to be quite significant.)

Or maybe it's to stop running up the odometer. ;)

[supermoix]

I'm with zonda in this one, i was taught since day one of training, that on single engine airplanes to slightly brake after airborne to avoid the vibration on out of balance wheels, which you will normally find in used and abused trainers and small airplanes flown by too many people.

Never used this technique in anything else but Small SEL up to C-207.

[SURGEBLEEDVALVE]

Gyroscopic effect is the reason behind Automatic braking on large aircraft after takeoff.

Most Large aircraft apply braking through the Autobrake system once the gear handle is selected up. If the Autobrake system is inop or one brake is deactivated, then the landing gear has to be left down for 2 minutes after takeoff.

All this is due to the gyroscopic effects because they are too great and might cause a twist in the landing gear strut, hence damaging the strut and preventing the wheels from fitting in the landing gear bay.

The nose wheel will not experience those gyroscopic effects, thats why only brake pads are fitted in the nose wheel bay to stop the spinning once the nose gear is fully retracted.

Now with regards to light aircraft with retractable landing gear some don't have the Autobrake feature, so thats why Instructors get into the habit of teaching this technique.

But for non retractable landing gear ?? it is a useless procedure :ugh:

[FE Hoppy]

The Gyro effect on large aircraft is a red herring. The actual reason as already stated is to prevent possible damage from a burst tyre.

[itsresidualmate]

Just a 'for info'; many transport aircraft (737, BAe146 to name but a few) use the landing gear UP hyd pressure to operate an auxiliary set of pistons (146 anyway) on the brake units. Usually operation of the gear handle also disables the anti skid to allow the brakes to work.
After a 146 brake change, it's gear pins IN, hydraulics on, then gear UP to check for leaks. I always dread that 'clack clack' as the downlocks release and the gear actuators try to break the gear pins!

[Clandestino]

Tapping the brakes?

Type and circumstances specific.

Follow your POH/FCOM/AFM, not tales.

[DaveReidUK]

Just noticed that the video posted in this thread

http://www.pprune.org/passengers-slf-self-loading-freight/349752-nervous-flyer-thread-4.html#post7493102

has a good illustration of the brake snubbers in a 747 NLG bay (at around 11:00).