Why Rotors ?

[Vfrpilotpb]

Good Morning all You Rotorhead's,

Why in this modern era of computers that will balance or mix anything do we need to still rely on rotors for lift and directional stabillity, Look at the Harrier Gr1 it doe's not have a tail rotor but can do anything that a Heli can do, why are there no moves toward full vectored thrust , I am sure we must have the engineering and design ability, or do we, what do you folk out in the real world think?

[ 16 August 2001: Message edited by: Vfrpilotpb ]

[Grainger]

Question of energy efficiency, V.

The Harrier achieves lift by shooting out a small mass of air at high speed.

This uses a lot more energy than taking a large mass of air and changing its momentum by a small amount, as happens with a rotor.

So the Harrier can only hover for a few minutes before using all its fuel up !

More to the point - can all the computer technology be put to use working the rotorcraft. I believe a lot of the military helicopters do have autopilot, autohover etc.

Computers do go wrong though. Oo-er. Think I'll stick to the cranially mounted biocomputer system.

[Vfrpilotpb]

Hi Grainger,
I was using the Harrier as an example to indicate the vectored thrust idea, in a copter I am sure it wouldnt need something as big as a Pegasus! :D

[RW-1]

Well .. here is the reason.

While I'm sure you could do it, and keep it small, you would have a problem with ...

(our favorite killer ...)

Noise!

Imagine the hovering vectored thrust vehicle near the scene of an accident, etc. I'm sure that no matter what the configuration, that the thrusted vehicle will be much louder than the conventional rotorcraft ...

I'll allow for being wrong, possibly someone could come up with something to muffle it, but so far all designs are large, and noisy.

Figure out noise first, and the other issues brought up above (power, fuel usage) and it would sell quickly IMO.

[Vfrpilotpb]

Hi Marc,

That prove's that getting up early is not quiet right for me, I didn't consider the noise, just all that lurvely thrust, now the day (or my day) is nearly over it was a bloody silly post in the first place, Thank you for pointing this out to me.
My Regards :confused: :D

[ 16 August 2001: Message edited by: Vfrpilotpb ]

[RW-1]

Hey my friend,

It wasn't silly at all, all points considered we just don't have the technology to make it viable in the civilian sector, where noise would naturally be the #1 concern (Besides it falling out of the sky due to major power loss hehe ...)

I thought it an interesting post to consider.

While we might not see a vectored thrust aircraft replacing the heli for hovering ops, it still has a viable shot as a transport vehicle, where takeoff and landing areas may be at an airport, etc. Like the V-22, only different.

And I was thinking, I haven't seen data yet on the noise of the "Lift Fan" system of one of the JSF's, it might actually be quieter than the turbine powering it ...

[ 17 August 2001: Message edited by: RW-1 ]

[Lu Zuckerman]

If you pay a visit to the Fairchild-Dornier facility in Gilching, Germany you will see a vectored thrust aircraft about the size of a Canadair Regional Jet. The Germans tried it and it didn’t work. Maybe with the newer engine technology it could be made to work but Fairchild-Dornier is placing their money on the FD-728 which is a twin engine passenger jet.

[vorticey]

at banks town last year there was a vectored thrust idea which used a cylinder fan (for lifting thrust)in front of the turbine engine which was used for forward thrust (as well as driving the fan).
the only problem i could think of on the spot was wasted power driving the fan in forward flight.
i think the thrust was delivered to the wingtips and then down. :rolleyes:

[Nick Lappos]

The power needed to produce lift in a hover is determined almost purely by the disk loading (the weight of the aircraft divided by the area of the rotor or lift fan nozzel).

For a helicopter, about 5 to 10 pounds of weight are lifted by each square foot of rotor, for a tiltrotor it is 15 to 25 pounds per square foot, and for a vectored thrust, it is about 1000 pounds per square foot. This makes tilt rotors need about 45% more power than helicopters for the same total gross weight, and vectored thrust machines consume about 100 times more power that a helicopter!

This power efficiency for the helicopter means that they burn less fuel in a hover, carry more payload, make less noise and cost less (since it is the machinery that makes and transmits the power that drives the cost of the machine).

The advocates for non-helicopter configurations always mention the advantages (speed, especially) but often never quite get around to mentioning the significant cost of that speed in terms of reduced payload, purchase price and maintenance burden.

[heedm]

Nick,

It seems to me that the disk area you mention is really the cross sectional area of the downwash (for want of a better word). If the area of the downwash was somehow increased below the disk, would this reduce disk loading and thus reduce power required?

I know that this is very hypothetical, because the effective downwash narrows due to pressure gradients, vortices, etc. but it seems consistent with your explanation.

If that is the case, why are the thrust nozzles of the Harrier small? Wouldn't it be more efficient to increase their size to decrease "disk" loading?

Matthew.

[Nick Lappos]

heedm,
You are very correct, the actual efficiency is determined at the edge of the shaped plume. Rocket nozzels are carefully shaped to squeeze everything they can out of the flow. If the rotor has no shroud around it, the plume behaves about the same for all exhausts. If the shroud or nozzel is perfectly shaped. the equivilent disk loading might be halved, which turns a vectored thrust from a very very high power hovercraft into a very high power hovercraft.

The power is eye-watering. The Lockheed Joint Strike Fighter has a forward lift fan and an aft downward pointing nozzel. The lift fan can consume 27,000 Horsepower (that is the right number of zeros!)

For a 50,000 pound hover weight, this would make it need about 50,000 HP (including the aft nozzel). For comparison, a 50,000 pound helo needs about 5,000 HP, and a 50,000 pound tilt rotor needs about 7,000 HP.

[Vfrpilotpb]

Good morning Rotorheads,

Well that seems to put my career as a vertical lift designer well out of the window, but I have now arrived at the answer that I suppose I was asking in the first place, However perhaps Nick could help with this, Why is there such a huge difference in the required HP that you quote from the spinning rotor to the vectored thrust, is it to do with the aerodynamics of moving air,or in the case of the Heli is it that the MR can be so much larger than the fan on a vectored thrust system and will be therefore more efficient?

[Nick Lappos]

vfrpilotpb asked:
Why is there such a huge difference in the required HP that you quote from the spinning rotor to the vectored thrust, is it to do with aerodynamics of moving air,or in the case of the Heli is it that the MR can be so much larger than the fan on a vectored thrust system and will be therefore more efficient?

Nick sez:
Both, really. It is locked into the aerodynamics, where the momentum change that generates the lift can be imparted on a big package of air moving slowly (a large rotor) or a smaller package moving quickly. The greater the velocity, the much greater the power needs. Theoretically, if the rotor is infinately large, it could generate lift with virtually no power.

The configuration of the air mover dictates the range of disk loadings that can be achieved. Helos have the advantage, since they have a vast, overarching rotor. Tilt rotors must be packaged so that the rotors cna only have about 40% of the area of an equivilent helo, since the rotors must not pass over the cabin. If a Harrier had a big rotor, it would have an even bigger fuselage, and would weigh tons more than it does.