Applying brakes on takeoff - or not as the case might be.

[Onan the Clumsy]

There's a thread (http://www.pprune.com/forums/showthread.php?s=&threadid=167917&perpage=15&pagenumber=1) running in JetBlast at the moment regarding this picture
(sorry about the width btw)

http://img21.exs.cx/img21/4498/454thcrashwinserta5ds.jpg

Someone found an explanation here (http://b24bw.proboards33.com/index.cgi?board=B24Aircraft&action=display&num=1106355336) Accident involved take off for combat mission, in witch, airplane lifted off run way, plot applied brakes
to stop main gear rotation and the airplane settled back down, as a result, the nose wheel collapsed and
the aircraft slid off the runway about 150 yards until a dirt embankment arrested movement and the craft
came to rest in this unusual position. The report recommends “ All pilots should be instructed on the dangers
of retracting main gear too soon and of applying brakes too soon after take-off.”

My question is: If the only thing applying the brakes could do would be to transfer the angular momentum
of the wheel to the a/c causing a nose down pitching, even if there's a lot of mass in the wheels and they're
spinning quite fast, could that really be enough to pitch the entire aircraft nose down?

I figure you're all a lot smarter here than those zoo creatures in JetBlast :}

[airborne_artist]

Yes, I think it could, especiallly if the aircraft was in quite a nose high attitude
at the time the main gear hit the deck with the brakes applied.

The aircraft could/would have been heavy with fuel/munitions, so the momentum of
the nose falling could have been quite great.

Just my sixpenn'orth

[Rainboe]

That's really funny AA! This is a Liberator- 4 engine bomber. Applying the brakes would have zero pitch effect. The reason for applying the brakes after lift off is to stop the wheel rotating and throwing mud and debris into, and possibly damaging, the wheel well. I would suspect possibly it settled back with the brakes applied, but I think collapsing the nosewheel wouldn't necessarily result. I don't expect the pilot survived, but I would think there was more to this than conjecture about brakes. Maybe an overrun on takeoff and striking objects?

[Onan the Clumsy]

Thanks, but that's not really my question. I'm wondering why applying the brakes to an airborne aircraft
would make it return to earth.

As well as mud and dirt, rotating tyres are also wider and might not fit too well in the wheel well.

btw, 6 people died.

[pigboat]

Well, the n/w would not have had brakes. The mains on the B-24 were attached to the main spar beneath the numbers 2 and 3 engines and retracted outwards (spanwise) so I don't see how applying the brakes contributed to the accident. Unless the author means the aircraft was flown off the ground prematurely and then settled back on its belly after the nosewheel downlocks had disengaged. It appears to have slid along the ground, hit the embankment, went up on it's nose and by some chance in a million the tail didn't fall back down but remained vertical after the airplane came to a stop. :confused:

[Touch'n'oops]

The a rotating wheel does not cause the aircraft to sink or climb!!!

The reason why the brakes are applied after take-off is to stop the wheel rotation before retraction. A rotating wheel creates tremendous gyroscopic forces. Retracting the gear while the wheel is still in rotation places a lot of stress on the gear and hydraulic system.

Modern jets do not require the pilot of apply brakes after take-off because the anti-skid system does this automatically after lift-off.

[keithl]

What on earth are you all on about?! Your quote, Onan, clearly says, "... the aircraft settled back down, as a result the nosewheel collapsed..."etc.

So, what tipped it over was landing with the brakes applied. Seems very straightforward to me.

You'll notice the great effort I'm making not to descend into JB language!!!

[Onan the Clumsy]

Not quite so simple keithlpilot applied brakes to stop main gear rotation and the airplane settled back down I think you're explanation is probably correct, but the way it reads, one action caused the other.

I suppose you could also read it as two unconnected event, which it probably was.

Ok then: poorly worded sentence.

[keithl]

Yep. Hate poorly worded sentences. That's why I'm a pedant.

[Onan the Clumsy]

Where is the subject of your middle sentence then? :p

[Rainboe]

keithl! So, what tipped it over was landing with the brakes applied. Seems very straightforward to me.
Stuff and nonsense! I don't thinkthe brakes were probably applied at all! Everybody forward of the wing died, so who is to say the brakes were applied? Doing that may cause a gear collapse, but have no effect on tipping the aeroplane nose up like that. The aeroplane would not 'settle back because the brakes were applied after T/O'- that has no effect whatsoever! In fact I think the Liberator retracted main undercarrige backwards into the wheel well, so gyroscopic forces would not apply, but they would into a sideways retraction a la Spitfire. This is either a take-off or landing overrun hitting embankment/ditch and tipping up. The reasons for that may be overload, wind effects, wrong flap, lack of full power or engine failure.

[Onan the Clumsy]

B24 MLG retracted outwards ref: Revel 1/48 scale kit :ok:

So not "stuff and nonsense", maybe just "Stuff" or "nonsense".

[Piltdown Man]

IMHO, having brakes full on (or whatever is the correct term for "wheels locked") in an aircraft of this size would have little effect on the pitch of the aircraft but it would seriously affect tyre life.

[Fg Off Max Stout]

Sorry rainboe can't let you get away with that.

From your first post on this thread:
I would suspect possibly it settled back with the brakes applied
And from your second:
I don't thinkthe brakes were probably applied at all!
I detect some inconsistency in your assertions.

Applying the brakes would have zero pitch effect.
Sorry, you are quite simply wrong. Applying braking to spinning wheels will produce a nose down pitching moment*.
It may be small, it almost certainly will be smaller than the pitching moment created by the elevators but it
is definitely not zero.
* Assuming that the axis of rotation at the time of braking is parallel to the axis of rotation in the gear down
position. This will almost always be so, as the wheel will almost invariably braked before a change of axis if this
is incurred during retraction.

who is to say the brakes were applied? Doing that may cause a gear collapse, but have
no effect on tipping the aeroplane nose up like that.
So you're telling me that an aircraft coming into contact with the ground with brakes applied will not
experience a pitching moment. I think you need to look at Newton's Laws of Motion again, old chap.
Incidentally, someone who states he has an ATPL on 747s should recognise the fact that the nose
is down, not up, in the picture.

I am not familiar with the cause of this accident and it's quite clear that you aren't either, so do us a
favour and stick to what you know.

Cheerio.:ok:

[keithl]

It's a fare cop,Onan I conseed. You can be Chief Pedant I will send my pedants hat if you say where to. I will take a year of to happily do whatever.

[Slim20]

The B24 was notorious for its tricky handling characteristics on takeoff and landing (in fact in any low-speed regime) due to its very long Davis wing section (long span, thick cross-section but relatively short chord)

Due to the overloaded condition on any given takeoff, especially from hot airbases common to Italy, N Africa and the Pacific where most were based, the a/c would generally use every available inch of runway for its takeoff roll.

Pilots had to exercise great restraint in not rotating the nose too early or too quickly even though the end of the runway was approaching; to do so would cause excessive drag or worse, the (relatively shallow) wing stall angle to be reached, and the a/c would begin to settle back down on the runway.

In this case, it would appear that the pilot either over-rotated or rotated too early, checked the wheelspin on the main gear, and then as the a/c began to settle back down, forgot to release the toebrake pressure in his preoccupation with the loss of lift. The result at max TOW and flying speed of touching the ground with toebrakes on would very likely result in the mess in Onan's picture.

[Rainboe]

FO Max Stout- the first posting showed a quote which reads that someone thinks applying the brakes after lift off causes a pitch down effect which makes the aeroplane settle back on the runway. Touch n oops corrected this same assertion. I wish to make clear that hitting the brakes has no effect whatsoever on pitching an aeroplane nose down to make it settle back on the runway.

Now, if you applied the brakes and then for some reason touched down again, I would think it 50/50 you would either wipe away your undercarriage (if traction was good) or burn rubber and drastically overheat your brakes. It would not cause a fully loaded B24 to tip nose down in itself and go to that attitude. That final attitude was due to external factors.

The cockpit crew died. I think it is disrespectful to their memory and the job they had to do to start blaming them for brake operation! Nobody knows, and it's so glib and makes them look stupid. This was probably an overrun accident for any of those myriad reasons it used to happen in the war. That aeroplane left behind widows and orphans and parents without their sons. And now it has become a cartoon. But then I should stick to what I know, shouldn't I?

[hercboy]

it all depends on the aircraft strength in the undercarage.

oh yeah Fg Off Max Stout

I think you need to look at Newton's Laws of Motion again

im not saying your wrong but im just putin g foward a point of view. i know that if you hit the braks all the weight will be transfered to the frount of the aircraft and it will pitch down but the messerschmitt 262 swallow pilots had to apply brakes to drop the tail so they could take off. but hence im not saying you incorrect.

but everone knows that when you have to brake heavily that you pull back on your controll colum to take that weight off the nose wheel

[411A]

Tromping on the brakes just after liftoff is not always a good idea, as the tire might indeed spin on the wheel, leaving the tube in same (no tubeless then) just a bit ah....flat.

Keep off 'd friggin' binders, Clyde!:ugh:

[Onan the Clumsy]

I've heard it was discouraged also because it could spin the tyres around the, suddenly stationery, wheel.