VTOL Aircraft

[sp00ln]

Originally Posted by JC

Same reason a rocket does, lots of vertical thrust.

After the vertical thrust, how does it fly foward? Without dropping altitude.

[Dean]

Originally Posted by sp00ln

After the vertical thrust, how does it fly foward? Without dropping altitude.

By spinning it's wheels....


Or tilting its jets/ducting. As long as total thrust > plane's weight, you can use any extra thrust to go forwrd until your wings start providing lift.

[Nick Koan]

Originally Posted by sp00ln

After the vertical thrust, how does it fly foward? Without dropping altitude.

er...

talking about the Harrier? The jets swivel to slowly transition the vertical thrust into horizontal thrust, then shut off.

[Dean]

Originally Posted by nKoan

The jets ... then shut off.

And then you drop out of the sky

[Nick Koan]

well, the VTOL jets turn off, and the main jets take over

[M3n2c3]

Hah, I just LOLed.

Yes, I stuck the -ed at the end of LOL again.

Some days I wish I had VTOL jets on my car.

[JC]

Originally Posted by sp00ln

After the vertical thrust, how does it fly foward? Without dropping altitude.

I'm no VTOL expert, but there are generally three ways they work. They have seperate engines that provide vertical lift, they rotate the engines, or they have nozzle that direct the vector of the thrust. Regardless you aren't going to instantaneously go horizontal and no vertical. They probably start to climb while they move forward until they have enough lift to only have horizontal thrust. That make sense?

[Dean]

Originally Posted by nKoan

well, the VTOL jets turn off, and the main jets take over

As far as I am aware the current "in use" VTOLs, Harrier, and V-22 both use their main engines for both vertical and horizontal flight by means of rotating the engines themselves(V-22), or the ducting(Harrier) behind the engine to redirect thrust.

I think one of the JSF designs had/has an additional lift fan, but the main engines do most of the heavy lifting.

[Nick Koan]

Oh really? I just figured it was a two engine design, but it makes sense to be a single engine because you need a massive amount of vertical thrust to take off vertically. A smaller support fan just wouldn't cut it, would it?

[sperry]

The AV8B Harrier II has 4 rotatable main engine exhaust nozzles that can rotate slightly past 90 degrees, providing thrust for VTOL takeoff, as well as rotating horizontal for conventional flight.





There is a single engine feeding all four nozzles, though IIRC hot exhaust exits only the aft nozzles, while the front are cool air from the turbofans.



In addition, for yaw/pitch/roll control while hovering, there are reaction thrusters in the nose, tail, and wingtips that can push the plane about while it's balancing on the main nozzle thrust.



In addition, the Harrier is an *extremely* difficult plane to fly in hover. The original Harrier had something like a 1/5 fatality rate for the pilot training program. Much has been improved in the Harrier II, though a full fly-by-wire system would probably make the plane much safer yet.

Finally, the Harrier rarely utilizes true vertical take-off. It's much cheaper on fuel to use a short roll-out w/ a 45deg thrust vector. Plus it allows the plane to take off with a much heavier payload. And for the really heavy launches, there's the ski ramp:



/me used to be a big military aircraft nerd.

[M3n2c3]

Originally Posted by M3n2c3

Some days I wish I had VTOL jets on my car.

Originally Posted by sperry

In addition, the Harrier is an *extremely* difficult plane to fly in hover. The original Harrier had something like a 1/5 fatality rate for the pilot training program.

/considers retracting his wish

That's nuts.

[sperry]

Oh, and Dean... the Joint Strike Fighter is supposed to have a VTOL variant:



There are competing engine designs. One is just like the Harrier, except with three nozzles, (fore and one aft). The other uses a drive shaft that turns a nose-mounted fan instead of the forward exhause nozzle.



There is a hatch on the back of the plane that opens so that air is pulled into the main engine from above (prevents injesting dust from the regular intakes while in hover).

[Dean]

Originally Posted by sperry

Oh, and Dean... the Joint Strike Fighter is supposed to have a VTOL variant:

Yeah, I know VTOL was a design requirment, that's why I mentioned iot. I just couldn't remermber if the seperate lift fan design was the one that won the contract or not. There is actually a really good Discovery channel show on the design competition.

[doubleurx]

The harrier also has the worst crash record and highest maintenance cost of any naval plane.

[sperry]

Originally Posted by Dean

Yeah, I know VTOL was a design requirment, that's why I mentioned iot. I just couldn't remermber if the seperate lift fan design was the one that won the contract or not. There is actually a really good Discovery channel show on the design competition.

Actually, I don't know which one they selected (if they've even got that far). But my money's on the Boeing design that's sans the ducted fan and associated driveshaft/clutch... that's just too many bits to break. Stick w/ what works... the design similar to the Harrier w/ the forward nozzles.

[sperry]

Originally Posted by doubleurx

The harrier also has the worst crash record and highest maintenance cost of any naval plane.

I believe it.

On the same note, I bet it saves the English Navy a buttload of money, since they don't have to have full-size aircraft carriers like we use in order to have a capable naval air-force.

[Dean]

Originally Posted by sperry

Actually, I don't know which one they selected (if they've even got that far). But my money's on the Boeing design that's sans the ducted fan and associated driveshaft/clutch... that's just too many bits to break. Stick w/ what works... the design similar to the Harrier w/ the forward nozzles.

Sorry, Complexity won out, and the Lockheed lift fan won.

One of the primary reasons was it has better resistance to heat soak. (The engine injesting its's own hot exhaust and losing thrust) This is one of the Harrier's biggest problems. Losing thrust in close proximity to the ground in VTOL configuration.

From http://www.jsf.mil

Shaft Driven Lift Fan (SDLF)
Lockheed Martin developed the idea for a Short Take-Off Vertical Landing (STOVL) lift system that uses a vertically oriented Shaft Driven Lift Fan (SDLF). A two-stage low-pressure turbine on the engine provides the horsepower necessary to power the Rolls-Royce designed Lift Fan. The Lift Fan generates a column of cool air that provides nearly 20,000 pounds of lifting power using variable inlet guide vanes to modulate the airflow, along with an equivalent amount of thrust from the downward vectored rear exhaust to lift the aircraft. The Lift Fan utilizes a clutch that engages the shaft drive system for STOVL operations. Because the lift fan extracts power from the engine, exhaust temperatures are reduced by about 200 degrees compared to traditional STOVL systems.

The SDLF concept was successfully demonstrated through a Large Scale Powered Model (LSPM) in 1995-96 and during the flight-testing of the X-35B during the summer of 2001. The Lift Fan, a patented Lockheed Martin concept, was developed and produced by Rolls-Royce Corp. in Indianapolis, Indiana and in Bristol, England.

[sperry]

Originally Posted by Dean

Sorry, Complexity won out, and the Lockheed lift fan won.

One of the primary reasons was it has better resistance to heat soak. (The engine injesting its's own hot exhaust and losing thrust) This is one of the Harrier's biggest problems. Losing thrust in close proximity to the ground in VTOL configuration.

From http://www.jsf.mil

Wow... I figured the dorsal intake would prevent the hot air injestion issue, and that the front nozzles would be cool air anyway.

'Course, I also thought the YF-23 would be picked over the YF-22.

[AtomicLabMonkey]

Merits of a design sometimes have little to do with which competitor actually wins in a military design competition.

[JC]

My lab did some of the background and safety stuff for the JSF. It's crazy the way they make decisions, let me tell you. Or rather not tell you. haha

[sperry]

Originally Posted by JC

My lab did some of the background and safety stuff for the JSF. It's crazy the way they make decisions, let me tell you. Or rather not tell you. haha

Lemme guess... you could tell us, but then you'd have to kill us, right?

[tysonK]

This thread is informative. Today when some of the other managers were talking about VOTL aircraft I totally laid the fucking smackdown on them.

[JC]

Originally Posted by sperry

Lemme guess... you could tell us, but then you'd have to kill us, right?

No, I'd probably just get fired. I dunno what's allowed to be public and what's not, so I wouldn't risk it.

[Dean]

Originally Posted by tysonK

This thread is informative. Today when some of the other managers were talking about VOTL aircraft I totally laid the fucking smackdown on them.

Tyson makes me laugh. I can totally picture 3 or 4 guys staning in the middle of the warehouse floor, or coffee room with Tyson spanking them with his wisdom...

[Dean]

The JSF Fighter competition discussed earlier is on Nova starting in 30 seconds... Channel 5.