Safe Speed Forums

Just don't get any driving or political advice from the 'dope, especially the message boards...

I dunno where you got the 360 mph figure from though

As your reference shows, the motive force of an aircraft comes not from contact between tyres and ground, but from thrust. Unless braking or steering, the undercarriage merely supports the aircraft while on the ground. In this respect you just can't compare aircraft with cars; and doing so to "show" that brakes of the runaway BMW could or could not have stopped the car is using a false analogy ... which was the whole point to posing that question.

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Will

Hi willcove.

I’m not one for hijacking threads, but I fear this is what’s happening to this one. I’ll happily continue on this sub-subject if someone starts a new thread in general chat.

I realised I made a booboo with an earlier reply; it’s been edited as appropriate.

My new answer follows:
747 thrust = 220000 pounds
747 takeoff speed = 180mph
Assuming equal thrust at 180mph, gives 80MW or 105k bhp
(don’t forget, a jet engine wont achieve full power until a lot of air is rammed into it)

Using your assumptions, dividing this power between 24 brakes would mean 4.4k bhp (3.2MW) per brake.
Its fair to assume that the far larger disks (compared to any car) would have much better ventilation (open design), the effectiveness of which would be multiplied at speeds of 180mph; so the brakes will be able to dissipate at least an order of magnitude more power than a car.
Hence I think the 747 analogy isn’t so erroneous.
(This will be my last post on this sub-subject, please start a new thread if you wish to continue)

The aircraft and car brakes analogy is flawed. The brakes on a car are acting directly to counter the motive force, through the drive train. The brakes on an aircaft can only serve to introduce a retarding force, without in anyway reducing engine thrust.

On the conveyor belt thing, the increased wheel speed wil tend to make the aircraft more reluctant to rotate at takeoff speed. This is because the axel/bearing friction will induce a nose down pitching moment.
It would still get airborne though.

OK, see here http://www.safespeed.org.uk/forum/viewtopic.php?t=6381

About this plane on a conveyor belt thing, I think it's a load of cobblers , surely the airflow over the wings is caused by the movement of the plane relative to the air? If the airflow was generated by the engines, why do aircraft carriers turn into the wind before planes are launched?

And if it really worked, surely vertical take off could be acheived by restraining the plane by a longish chain or somesuch, and powering up the engines... then when off the ground, releasing the chain. This would remove the need for a runway at major airports.

Or is this whole thing one big wind up?

A friend of mine did have his pedal weld itself to the rubber matting under his feet once and was forced to plough into a herd of emu at over 120 mph. But that was in 100+ degree heat in the middle of Australia's Outback, not the A1!

I wonder if this chap just had a burning desire to ride a roller-coaster but didn't have time to nip over to Alton Towers for the day.

Maybe the call he placed to police should have been more like this:

Wwhhheeeeeeee... yahoooo... um... yeah officer... um... I'm going to die... yyiippeeee... um... help me someone... wwhhheeeeeeeeeeee!

The point is that the conveyor belt can't prevent the aircraft from moving forwards. Unlike a car, the motive force comes not from the friction (and hence relative speed) between tyres and road surface; it comes from thrust. Except when braking or steering, the undercarriage merely supports the aircraft - so the aircraft will accelerate down the conveyor belt until its airspeed is sufficient for take-off. The only difference between the aircraft being on a normal runway and on the conveyor belt is that on the conveyor belt the wheels rotate more quickly!

HTH.

PS. Since I don't have posting privileges in Technical, I can't join in the aircraft power discussion. So I'll take the opportunity to mention two other reasons not to compare the brakes on a 747 with those on a BMW 318i:

• The majority of braking effort on a 747 comes not from the wheel brakes but from reverse thrust.
• If the same braking effort as available from reverse thrust were applied via the wheel brakes, there would be a large moment tending to collapse the nose gear. This doesn't happen with reverse thrust because that is applied on the thrust line, which is much closer to the aircraft longditudinal axis.

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Will

Yep, that does indeed help, Willcove, thanks. I understood that the conveyor belt would always compensate in such a way that the plane remained with a groundspeed of 0.

The last word on the topic, from James May in the Telegraph.

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Anyone having problems with the conveyor belt thing, think of a floatplane. If the floatplane is on a river can it takeoff against the prevailing current? Answer, yes.
Or an everglades fanboat or a hovercraft?. Can they move upstream against the current? Answer, yes.

Excellent, I'm off to the patent office to protect my vertical-take-off-Jumbo-on-a-chain idea (NOT!)

This is really driving me crazy, so I have to get this worked out.

I contend that there has to be airflow over the wings to enable takeoff.
Are the "yes it will takeoff on a conveyor belt even if groundspeed is zero" saying this because they believe that the jet engines cause the local airflow which passes over the wings?

What if the jet engines were mounted in such a way that they produced horizontal thrust, but no airflow over the wings??? (e.g. on a long mast).

As far as the float-plane is concerned, is the airflow over the wings produced by the propellor(s)?

Hovercrafts/fanboats are not comparable, as vertical lift is not produced by airflow over wings.

Will some smart-alec please put me out of my misery and give me the definitive answer????!!!!

There is airflow over the wings - and the aircraft is not still at take-off.

Consider what would happen if the conveyor belt did not move. The aircraft would accelerate down the conveyor belt (just like a normal runway) until its forward motion resulted in sufficient airspeed to take off. With this so far?

Now consider what would happen if the conveyor moves. In an aircraft the engine is not coupled to the wheels, which are thus free to rotate independtly of the engine. So, providing the wheel brakes aren't on, the conveyor belt will have no effect on the aircraft other than to turn the wheels. So, when the engines power up, the aircraft will do exactly what it would if the conveyor were still, except the wheels would turn a little faster. It will accelerate until its forward motion is sufficient for take-off.

HTH,

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Will

Thanks Will, but you are assuming that "This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction)" does not maintain a ground speed of zero.

What if the groundspeed is maintained at zero? e.g. by a conveyor belt system AND chain tethered to a fixed point.

If you tether the 'plane then clearly it can't take off.

But that's a completely different question to the one that was asked!

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Thanks, JT, I think we agree (but the question is badly worded then)...

...however, now I'm waiting at the door of the patent office, so I want to know something else:

can a regular (i.e. non-VTOL a-la Harrier jump jet) plane produce its own airflow over the wings to enable it to takeoff in the "tethered" example? e.g. by propellors or jet engines positioned in front of the wings.

I think a helicopter does that by 'spinning its wings'.

I also think that if you could devise the required airflow-inducing system then it would be possible to get a plane to lift off by blasting air over its own wings. I have very substantial doubts that any such thing could ever be practical, however. I think you would just need too much air moving, and of course the drag at the margins of the flowing air where it met still air would be massive power-sapping turbulence.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Since I'm guilty of posing the question, I assumed nothing. FWIW, I phrased it carefully to avoid the conundrum of the conveyor keeping the aircraft still and I posted the question just to illustrate the difference between cars and aircraft WRT their engines and brakes.

@Paul: A gas turbine exhausting over an aerofoil would not provide laminar flow, and so would produce no lift. However, if you could provide smooth air at the right speed and angle of attack, I suspect the thing would leave the ground!

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Will

I’m guessing some may have confused groundspeed with airspeed. Relative airspeed is the factor generating the lift; for this example I think it’s reasonable to assume airspeed = groundspeed (a windless day). Groundspeed is irrelevant – which is why the conveyor won’t make any difference. An example, the film Flight Of The Phoenix, the stationary plane (no groundspeed) almost took off during the sandstorm because it had a lot of airspeed over the wings.

A jetstream directed at an aerofoil/wing will produce some lift but nowhere near enough for lift-off; it will be acting on only a small portion of the wing (if the jetstream is diffused such that it covers the whole wing, the resulting airmass speed will be too slow to produce lift). Wings are usually so wide such that they can pass through a great cross section of airmass and produce the required lift. The exception being the Harrier which has enough static thrust even when its jetstream is directed vertically (via nozzles) – it does not direct the jetstream overt the wings to produce take-off lift.

IT's perfectly practical. The airflow-inducing system is called "the wind" and the aircraft "a kite"...

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CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!