Bell X-Hawk
 

What do you think about the Bell X-Hawk?

Is this real or yust a marketing gag?

I can't believe this is a competitor for a conventional Helicopter.

the only application I can think of is a military one, because the fans are not exponated and can be protected inside the hull.

What do you think?

http://www.urbanaero.com/whatsnew.htm

http://www.urbanaero.com/whatsnew_im...dsizexhawk.jpg

Separate lives?
 

Separate cockpits! Is that for pilot/co-pilot pairs who can't get on with each other?

It could work in a limited military environment or even perhaps EMS.

When they [UrbanAero] first suggested it they were marketing a rescue machine that was envisaged as bumpting up to the side of a high rise and taking people off a burning building... with all the problems that might meet - seats for 4 but what would the effect be if 14 got on board?. Yes straight down. Pie in the sky marketing that few gave a second thought to even though there had been similar [both concept and flying] fan machines before.

Now with Bell marketing the more sensible modified 'military jeep' concept it might just stand a chance and if the military go there and fund the project it might even make its mark as a high rise rescue vehicle by default!

The lift fan concept is certainly viable, but has not taken off (pardon the pun) mostly because the higher disk loading means that the engine power must be at least twice that of an equivilent helicopter, if not 3 times. This means more fuel burned, greater initial and operating cost, more expensive overhauls and much lower payloads. The tilt rotor barely works, and it requires only 50% more power than a helicopter, what does that say about a lift fan that needs 2 to 3 times the power?
Additionally, there is no autorotation capability, tolerance to engine failures is very poor and the control has (in the past) been much more marginal than a helicopter.

Let the market decide!

Think I'll stick with the traditional helo thanks very much. And the tilt rotor would seem to have some possible failure modes that dont bear thinking about.

Thanks for pointing this one out Hotzenplotz.

I'm curious how the X-hawk offers better gust stability than a conventional helicopter, unless just by virtue of the downwash velocity. It may have stabilisation, but then a conventional heli would benefit from stabilisation in the control system. I have no idea why this design would be better in icing conditions.

I can see the benefit of having enshrouded rotors in built up areas though, but the lack of autorotation capability in a machine hungry on fuel is not a good mix. Figure on it being the equivalent of a conventional with rotor diam 1.84 times rotor diam shown, since fairings add ~30% lift. I wonder if the pilots would be trained to fly sideways for better climb rates? :}

Nick, how much of a safeguard is a multiengine design against autorotation?

Mart

Not much, if you're unlucky enough to put contaminated fuel in. :(

The mock-up they had at Farnborough certainly had a gun or two attached. But was rather lacking in the stretcher stakes.

Considering that such a desing would have very poor autorotational capabilities, would il be possible to equip it with a parachute system similar to that of the Cirrus Desing aircraft?

Aw, come on guys. The idea can't be that bad.


The bidding on e-Bay for the Solo Trek got up to 6 million dollars. http://www.unicopter.com/RollLaugh_2.gif

The idea is that bad!!! Anything that is pretty much guaranteed to kill you in the event of a power failure is a bad idea in my book. There is no shortage of bad ideas in the helicopter world, whether they be X-Hawks, Solo Treks, or whatever.

-- IFMU

Robbo Jock, I asked the Bell rep at Farnborough about the chain gun on the starboard side and he pretty much dismissed it as a 'pretty' for the show. Not that it might not happen more that the mock-up was there to raise interest [like this thread!] in the project.

The reason I asked about the gun was that it was pretty obvious that it represented a significant weight and there was no counterbalancing 'lump' on the opposing corner of the craft.

I guess there might be a need for some pretty significant control inputs to counterbalance a single off centreline chain gun rattling away and of course spewing lots of little cartridge cases near the front fan!

Artistic licence by Bell!

That's a tough requirement!
 

Thanx Nick. How is this demonstrated in primary structure? I take it once the fatigue cycle and material characteristic is established, this is the main criterion for airframe life. What is the powertrain power loss probability requirement in a (relatively) low disk loading heli, and how is that demonstrated?

Mart

PANews,

Thanks for the info. Interesting.

Graviman,

For any aircraft, the primary structure must be designed to show that the probability of catastrophic failure is "extremely remote" or 10e-9 This is done by design analysis, and test, and is inherent in the certification of any aircraft. Additionally. for engine failures, helicopters must be safely autorotated to be certified (this is the underlying method to show that a power loss is not a catastrophic failure).

I speculate that for a flying machine that is held up and controlled by engines like the Xhawk, the safe completion of a flight must have this same integrity, so the engine power needed to maintain control and to make an emergency landing must have the same integrity (probability of failure) as the structure, or 1 to a billion. If not, then the FAA would be allowing a catastrophic failure (power loss) at a much higher probability - not at all likely.

Thanx again Nick,

I had long forgotten about 6 sigma for vibration fatigue and gust loads - i had better read up to refresh my memory! I remember now a conversation with a mathemetician friend of mine who explained that non-linearities in actual airframe structural deformation meant that 6 sigma was almost impossible to achieve in practice. However, this is not the forum or thread for that discussion.

One thought about rotorcraft powertrain: most engine failures can be attributed to either wear (eg cambelt, valve and seal) or material degredation (eg turbine creep, conrod failure, or head deformation). One interesting point is that brushless DC electric motors (like the ones in the french TGV train) and batteries do not suffer catastrophic failures, but lose performance with time as the electronics degrade. To achieve the speculated FAA 1E-9 failure requirement, an electric emergency drivetrain might be suitable.

This may well be better suited to a fly by wire approach to design, where triplex or even quadruplex systems achieve the failure requirements. While i am not a fan ;) of unecessary high disk loading in a design, it is interesting to speculate on what solutions might be offered for this problem.

Mart

Graviman,
You are right, 10e9 is really not achieved on airframe structures in practice, but it is analytically. 10e7 or 8 is attained in the field, and even that would be quite difficult for an aircraft whose control is based on the power train. FBW is of no hope, because it is the flight power system that falls short.

Graviman,

Your comments about the flying ducted platform and an electric drive suggest the possibility of a viable rescue platform. A flying platform where it's genset plus 'pilot' are ground based, and the two pieces of equipment are linked by a power/control umbilical cord.

Perhaps Sikorsky would be an ideal manufacturer of such a vehicle. They have experience with the Cypher and one of the three authors of the book 'Axial Flux Permanent Magnet Brushless Machines' works for United Technologies Research Center.

Dave

Where are the Cyphers?
 

Dave,

Cypher seems to have dissapeared off the face of the earth. Wouldn't it still be around if it was viable? It would seem to suffer from high disk loading, maybe not as bad as the X-hawk with a gun on the side!

-- IFMU

Cypher was never considered for carrying people, so the comparison to a dream that has seats for the wife and kiddies is not correct. Having no people on board is a very distinct advantage when you can't autorotate, and can't even maintain rightside-up if the power quits!