Safe Speed Forums

Certainly there have been a number of aircraft that use the so-called 'blown-wing' or 'blown-flaps' principle. Bleed air ducted from the engine compressor is, as the name suggests, blown over the aerodynamic surface(s) to induce lift. Its seldom sufficient to produce enough lift to get the aircraft airborne on its own, but it does enable it to attain lower forward airspeeds and yet not fall out of the sky; useful on landing or maneouvring. The Blackburn Bucaneer was an aircraft that used this system and there are various STOL transports that position the engines forward of and above the level of the wings to help induce lift.

And as for the conveyor belt thing...the groundspeed won't be zero, it will be whatever it would be were there no conveyor belt. The wheels will just be spinning like crazy (twice their normal speed in fact) to accomodate the forward push from the engines and the rearwards motion of the conveyor.

Re: last few, especially JT, Paul, smeggy, Will & Rigpig.

I have thoroughly enjoyed this thread, and while I felt like Mr. Gumby ("my brain hurts!") for a while, thanks to you, I have now recovered confidence in my Physics A-level and you all get my prize for technical-knowledge services to the internet community
http://www.geekculture.com/geekcultures ... /caps.html

These sort of brain teasers remind me of the old joke about the two intellectually-challenged removals men. The customer sees one of them struggling with a heavy cupboard and enquires why his mate is not helping him. "oh he is, he's inside holding all the clothes".

As regards the notion of generating lift by passing the engine thrust over the aerofoil wing, all that is doing is vectoring some of the thrust downwards along the exact same principle as the Harrier jump jet.

In other words instead of the engine thrust all going out of the back and pushing the 'plane forwards, you are deflecting some of it downwards so that the net reaction is forwards and upwards. The Harrier goes a stage further by ducting and deflecting ALL of the engine thrust downwards so that the resulting reaction vector is vertially upwards, but the principle is that same.

This same effect can even be seen on primitive prop driven aircraft, which can maneouvre on the ground by applying rudder and then opening the engine up, so the propwash is deflected sidways causing the 'plane to turn.

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

I've heard two theories why helicopters fly...

(1) They don't actually fly, they simply thrash the air into submission... or

(2) They don't fly, they're so damned ugly that the earth repels them.

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"Politicians are the same the world over... We build bridges where there aren't any rivers." - Nikita Kruschev

I’m glad you enjoyed the thread supertramp.
Here’s a link to a photo showing a Harrier in action.

http://www.richard-seaman.com/Aircraft/ ... UpHigh.jpg

You can clearly see the right forward (cold) nozzle; if you look carefully you can also see the right hot nozzle toward the centre of the craft. All nozzles are pointed downwards for vertical vectoring – the jetstreams are never directed at the wings or control surfaces.

What I was meaning in my previous post was that the vectored thrust nozzles and their associated ducting are deflecting the jet thrust downwards, and that mounting the engine ahead of a wing or control surface has the same effect, albeit in a much cruder and less effective way.

Thus an aircraft that had an engine blowing thrust over it's wings could achieve a shorter take-off roll, and the Harrier jump-jet effectively illustrates what happens when you take this principle to its logical conclusion.

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

I agree.
If directed at the wings, I’m not sure how well they would stand against the hot exhaust stream from the inner jet – at least that part of the wing would never suffer ice buildup

Since energy can be neither created nor destroyed, the blown wing giving lift causes an equal loss of energy available for thrust. That's always been the down-side and why blown wing technology has never really caught on. The additional weight of a wing suitable for this is also a downside in the overall equation.
You know, flying is fun and you can go as fast as you like. Maybe I should give up cars and go back to it. I had my pilot's licence before my driving licence.

All you need is a landing strip in your garden and you'd be laughing!

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Science won over religion when they started installing lightning rods on churches.

Whoops - I have missed this thread, which is a shame because I would have been able to contribute on two counts:

a) I have had the throttle stick fully open, which while scary for about a second didn't mean crashing or much danger to anyone else, however it was in a Renault 4 which has manual gearbox, and no power anything. Use clutch to disengauge power and get to a safe place to stop (partially lift clutch if you need a little power) then turn off engine.

The cause was a frayed accellerator cable where the frayed cable snagged on it's housing in the throttle fully open position; trim off or bend back frayed bits, limp home and get a replacement cable. Oh it was so easy in the old days.

and

b) I worked at British Aerospace Kingston and later Dunsfold which made the Harrier and Sea Harrier (and Hawk). I sat in a portacabin a few hundred yards from the hover test pad where all first flight harriers were tested, and know quite a bit about the technicalities. Not that I can give any secrets away.

The Top Gear test track is around the runway and perimeter track of the Dunsfold Aerodrome and the studio is in one of the hangers. The Dunsfold site was closed years ago, and the design centre and manufacturing site where Harriers were built (Dunsfold did final assembly) in Kingston Upon Thames (actually Ham Common on the road to Richmond) became a housing estate as it was very nice land backing onto the river Thames - it closed a few years before Dunsfold.

The plane on the conveyor belt question is a paradox.

Rules state that belt speed automatically matches wheel speed. If that's the case then the plane can't make forward progress. To move forwards, wheel speed must exceed the belt speed and thus the rules are broken.

The belt is reactive and thus the wheels must move, and thus the plane must move before the belt can react and match the wheel speed. This differential is the paradox because the rules state the speeds must match.


In short, as soon as the pilot opens the throttle, the rules are broken and the "test" is over. Paradox.

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No paradox.

It is perfectly feasible for the plane to travel at 100mph, with the conveyor belt travelling backwards at 100mph, and the wheels rotating at the appropriate speed for the 200mph differential that now exists between plane and conveyor.

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

Uh huh........

The problem arises in the acceleration.

Mathematically there's no problem with a plane on the belt with a 200mph groundspeed but 100mph airspeed.

The problem is that for the plane to initially move off from rest the wheels must rotate as the plane introduces a rate of change to it's speed.

For the plane to move the wheels must turn faster than the belt beneath it. How the wheels are turned is totally irrelevant. It's the speed differential that is the paradox.

The problem is that :

Wheel Speed - Belt Speed = 0

That's what the rule is actually saying, yet for the plane to initially move, Wheel Speed - Belt Speed cannot equal zero.

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I think you are becoming embroiled in a theoretical,nay a philosophical point, no different to that which your car faces every time you drive off. Namely that for an object to go from being stationary to moving, no matter how low the speed, then for an instant it's acceleration is infinite.

In any case, the problem doesn't really arise with the 'plane. For anyone to conduct a practical experiment they would have to measure the speed of the aircraft and then accelerate the conveyor to match it. Thus during acceleration there would always be a slight lag in conveyor speed.

Either way, it doesn't affect the practical issue of "could the plane take off", to which the answer is a resounding but theoretical "yes".

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

Absolutely agree

Just being pedantic though - as a brain teaser it's a non-started

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That is most definitely what the rule is not saying. I was very careful not to mention wheel speed when I posed the question. As the aircraft moves forward, the belt moves at the same speed in the opposite direction. Thus, if the aircraft moves forward at 100 mph, the rule says that the belt moves backwards at 100 mph. Yes, the wheels rotate twice as quickly as they would if the aircraft were on a normal runway, but that doesn't affect how quickly the aircraft fuselage moves relative to the ground.

HTH,

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Will

The last few posts have, for me at least, completely cleared up my confusion regarding the brain teaser, thank you jamie & willcove.
So I can now put the finishing touches to my soon-to-be-patented Vertical take-off airport design...
1. Jumbos are fastened to a long chain some way back from the aircraft.
2. A big fan is placed in front of the aircraft.
3. Jets are powered up, and the fan is started.
4. When sufficient fan-generated airflow is moved over the wings, the jumbo will rise vertically a few metres.
5. Chain is released, and the pilot has to continue his ascent over the fan under jet-power.

Simple, and genius
Right, all you impatient investors, please form an orderly queue

A great idea; unfortunately, the aircraft will make quite a mess if it leaves the fan airstream before it reaches take-off speed.

Looks like it isn't.