OT: VTOL X-Plane

[Adrian Caspersz]

Anyone got any spare fans?

VTOL X-Plane Phase 2 Concept Video

"program seeks to provide innovative cross-pollination between fixed-wing and rotary-wing technologies"

Whh...

[Davey]

Didn't the V-22 Osprey do that, when it wasn't crashing?

[The Natural Philosopher]

I simply cannot believe that they are serious, after all we did the Harrier years ago and the basic problems of vertical take-off at the Pure physics level are well understood.

If you take two basic equations - F=ma which fives the thrust of an 'air acceleration' system, and P=1/2mV^2, which is the power required to drive it, you will discover that the least power to lift an object at very low speed occurs if you accelerate a huge amount of air a very very little.

Which is why planes have big wings and helicopters have big rotors.

The power to lift that thing is a function of the sum of the cross sectional area of all the fans, which is STILL a lot less than its wing area. So although its a neater way to package a helicopter as a series of small ducted fans, its still a power hungry brute of a thing, inherently. Not to mention the weight of the wing swivel mechanisms.

And we did it years ago with the incredibly noisy Fairey Rotodyne..

[harry]

Looks to be design with lots of drag. Hardly fast.

[ARW]

Faster than building a runway

[The Natural Philosopher]

Well of course as you might have guessed its not a design with lots of drag, and ducted fans are in fact what jet engines are, so they can be quite fast.

[Vir Campestris]

It's a little more complicated than that - as I suspect you know.

Dragging a wing through the air takes power. The faster, the more power

- speed cubed IIRC. But as you go faster the drag needed to generate lift goes down. Economical cruise is when the sum of the two is minimised. A bigger wing lets you fly slower - but also means you must fly slower.

Of course you can cheat, and go up where the air is thinner. Airliners at cruise are surprisingly close to stalling.

Wish I'd seen it. Amazing idea. Rocket on each rotor tip...(1)

Andy

[Brian Gaff]

Have not we been here before? Carrying about engines that are only used to do vertical take off and landing is going to be wasteful. Brian

[Bob Eager]

No, the wings (with engines) rotate, so the same ones are used for forward flight.

Really, it's an electric Harrier on a bigger scale.

For real waste, research the R101. That had four eng> Have not we been here before?

[harry]

There are two sorts of drag. That caused by the need to displace air. (Increases with speed) That caused by friction on the aircraft. (Reduces with speed.)

There is therefore a minimum drag or optimum cruise speed for every aircraft. Faster or slower increases drag.

[harry]

Why do you suppose we don't use helicopters for everything?

[Nightjar]

What is your answer to that question Harry?

[The Natural Philosopher]

"Because the wings aren't big enough to fit solar panels'

arf arf!

[The Natural Philosopher]

yes and no. In general the most economical speed is a little above stall speed, and few aircraft exceed three to four times the stall speed.

Indeed. I think the U2 was pretty much just over its high altitude stall speed all the time, and was quite dangerous to fly.

Yup. Would do it with another way these days.

[Vir Campestris]

Isn't that what I said?

Andy

[Vir Campestris]

I was surprised by that. A quick google leads me to believe otherwise, but it's difficult to find economy cruise figures at the same height as stalling speed.

Do you have a source?

Andy

[harry]

Bollix. The stall speed is constant, but as aircraft get higher they must fly faster to generate sufficient lift in the less dense air.

They are limited in speed by stall speed at one end and "Velocity never exceed" at the other. Exceeding Vne can cause flutter and the aircraft to break up.

At some point as they get higher they can't generate sufficient lift because they would exceed Vne and that's as a high as they can go.

[harry]

The U2 was flying at close to Vne. At operating altitudes it had a range of speeds of only ten knots. ie couldn't go hardly any faster or slower. Could not even safely maneouver to avoid the missile that got it. The missile missed, the aircraft broke up when it exceeded Vne. Hence the pilot escaped.

[The Natural Philosopher]

Another post has highlighted the issue, Ex of wind, you want to fly at the best L:D ratio and slow enough that skin friction doesn't start escalating.

In general that seems to be around 1.25 - 1.5 x stall for most wing sections.

Airliners don't necessarily fly at the most economical speeds either, because passenger miles per hour is what they earn.

And of course altitude lowers the pressure and the friction, and increases the stall speed.

http%3A%2F%2Fhome.anadolu.edu.tr%2F~mcavcar%2Fcommon%2FStall.pdf

has a graph showing a 747 stall speed increases from around 180 knots at ground level (ex of flaps one assumes. IIRC a 747 lands around

140-160mph) to a whopping 300-450 knots at cruising altitude (30-50,000 feet)

And that is where those babies fly, at around 1.3 x stall - i.e. 400+ mph in general.

The equations for the actual power and efficiency are fairly simple.

If you consider that you must generate enough lift to carry the weight and the total energy to transport the plane a given distance (in still air) is the drag times the distance, then you want to have the plane fly at the minimum drag to lift ratio for that amount of lift.

Where that optimum L/D lies is a design exercise. Delta wings that can operate an massive angles of attack and generate a lot of lift (and a lot of drag) can stall slower for similar top speeds.

Airliners are optimised in overall flight profile for the sorts of flights they will be undertaking. There sis not much point in using a

747 to waste fuel climbing to 55,000 feet and then glide it down again. Better to use a slower ones optimised for lower altitudes for short haul, which is why you see turboprops take over for short haul routes.

And its why in general airliners climb at low altitude, at a lower speed ...that's the most efficient speed at that altitude..

[harry]

Because they are complex, expensive to build and run, slow, relatively small and unreliable, noisy and use lots of fuel.