> > I don't trust tiltrotors anyhow. They are like skydiving -- even if
> > the
> > pilot does everything right, they can still kill you Just Because.
> >
> > If ANY flaw appears in the transmission or power control systems, the
> > aircraft will most likely crash.
> >
> > In helio mode, the same differential thrust problem that you have in a
> > 310 now becomes a differential LIFT issue, with a very long moment
> > arm.
> >
> > In airplane mode, the rotors are too large to allow it to land like a
> > plane, and even if you shut down the engines and dead-stick, you will
> > catch the blades long before your landing gear is anywhere near the
> > ground, and it will flip you over on your back before you have the
> > time
> > to swear.
> >
> > Autorotation is relatively ineffective, because there just isn't
> > enough
> > rotor area.
> >
> > Just because a technology is attainable doesn't mean that it's
practical.
> >
> > Keith W
> >
> > The props may or may not flip you over. Depends on how they stop. Also
carbon props tend
> > to shatter rather than bend. For our small PAV single tilt rotor we
advocate large airbags for
> > the pilot and pax instead of chutes as they inflate faster.
> >
> > http://www.rotaryeng.net/VTOL.html
> >
> > For high altitude tilt rotor PAV problems a ballistic chute will work
fine.
> >
> > I think the new variable twist blades
> > Boeing is working on will enhance auto rotation. It will certainly
mitigate the vortex ring
> > state. Variable twist blades will make the disc area more effective in
helicopter mode.
> >
> > Tilt rotors are the future as they solve a lot of current car traffic
and airport access problems.
> >
> >
> > Paul Lamar
>
> > ------------------------------------------------------------------------
> >
>
> Got an airbag big enough to englobe the whole aircraft?
>
> A carbon-fiber blade which is strong enough to be 1/2 of the support of
> a fully loaded aircraft is NOT going to politely break up into
> bite-sized pieces in a dead-stick landing -- they are going to be a big,
> nasty lever, especially if fully feathered (thus hitting the ground in
> the direction of their greatest cross-section). It doesn't matter what
> position they stop in, even with one blade straight up, there are
> several feet of blade below the landing gear. So check the video of that
> Navyator who snagged a cable with the hook while 3 feet off the deck.
> The plane hit the deck so hard that the fuel tanks ruptured and the
> plane exploded into flame like a Ford Pinto that's been rear-ended by a
> dump truck. From 3 feet up. Landing with the right side up. Now
> imagine that's your tiltrotor, digging the blade on each side into the
> ground from 5 feet up. Or worse, digging ONE blade in from 5 feet,
> while the other side is still clear of the ground.
>
> I just don't see any other likelihood -- those rotors are just too big.
> It would be much more chance of survival if they used 4 instead of 2.
> The Vought / Hiller XC142 Tiltwing was capable of operating like a
> plane, because the blades were shorter than the clearance to ground.
>
> "The _______ of the Future" is usually an evolutionary dead end, or the
> Zeppelin would still be queen of the skies, we'd all fly personal
> rotorcraft to work, and UPS would be delivering packages to Luna City.
>
> For that matter, weren't the Vega and Pinto touted as the "Future of
> American cars" . . ? That predicted future faded pretty fast after
> people started dying in them. Just imagine how eager people will be to
> climb aboard the tiltrotor and commute to work when they've just watched
> the news video of another just like it falling out of the sky, ballistic
> chute wrapped around as it tumbles . . .
>
> Nope, sorry, just don't see it, and if it is the future, I'll stick to
> the good ol' days.
>
> Keith W
>
> There is a big difference between a tail hook and a thin blade in
> bending
> it is not designed for. Not a good analogy. Blades are primarily
> designed
for axial
> loads and not bending loads. In fact they are usually hinged much like
helicopter blades.
> If a helicopter blades is not rotating it will collapse upward and not
support the
> weight of the helicopter. What keeps a helicopter up is the centrifugal
force
> on the blade while it is rotating. Helicopter blades are not rotating
wings
> despite the appearance.
>
> It is also possible to blow the blades with explosive bolts if the crash
mode
> you describe is typical.
>
> The Vega and Pinto are also irrelevant analogies as they both had minor
flaws
> easily corrected.
>
> My philosophy is solve the problems rather than give up on high
> potential
> technology. I am glad Mazda had the same philosophy when it came to the
rotary.
>
I'm not making myself clear.
Tilt rotor blades are NOT helicopter blades. They have to be
significantly stronger to handle the severe gyroscopic forces involved
with changing axes. What little axial change the helo rotor disk makes
is done BY the blades, through aerodynamics. The tiltrotor axis change
is done TO the blades, at the root.
I do have to make a correction -- if the rotor is stopped with one blade
vertical, there is a 20" space between the ends of the lower blades and
the ground, with the aircraft sitting on the gear. However, the same
Navyator who told me that also noted that there is no way to force the
rotor into this position in flight, in any failure mode involving the
engine, control system or transmission. In other words, anything which
would result in a dead-stick landing will result in a random position of
the rotors.
Look at pics of the Osprey at rest. Compare them to any helo you have
ever seen. The helo blades droop, the Osprey blades don't. That same
strong and stiff structure, necessary for flight, becomes a major hazard
in any dead-stick landing -- assuming, of course, that you can keep up
enough airspeed to actually have life from the wings.
Go look at the record of the Osprey. Out of 10 reported major
incidents, 8 were related to the technology (the other 2 could happen to
any turboprop), and 30 people died (7 of them in front of the
Congressmen and other officials who had been invited to a dog and pony
show demonstrating the new aircraft).
That said, I'm not looking at the "Osprey-only" issues (such as cracking
rotor heads, etc) except as symptoms. Any new aircraft can have those
sorts of problems. I'm condemning the whole technology, not the design
and manufacturing problems of a specific model. In other words, I am
not looking at the "Hindenberg," I'm looking at "airships."
Why would this be the "Aircraft of the Future"? It is significantly
less efficient than real fixed-wing aircraft. Using the V-22 as the
only available example, the payload/distance chart is incredible (ferry
range for a NO PAYLOAD flight is only 500 nm -- that 2100 nm range they
brag about is for a V-22 filled to gross weight with fuel and no
payload). Tiltrotors cost several times as much as comparable
fixed-wing or rotorcraft, and need far more maintenance. And this
ignores the very real and significant threats to life inherent in the
technology.
It's not manufacturing issues which have led the V-22 to fail BOTH of
the OPEVALs it has taken, it is those technological threats.
At best, the tiltrotor is a niche-filler -- it flies 100 MPH faster than
a helo -- but there are just too many places that the "Golden BB" can
bring it down . . .and it comes down with a big, expensive and
fatal-sounding noise.
Sorry, to me, the tiltrotor concept is even more dead-end that the
Reciprosaur, and for the same reason -- it only works when absolutely
everything is working properly, and too many of those things are too
easy to break. It is about as practical as a day-to-day workhorse as
TACIT BLUE.
Oh, and as far as Lockheed looking into tiltrotors, well, Lockheed has
always looked at EVERYTHING. Doesn't mean they'll ever build them as a
manned aircraft.
I'm going to be gone for a couple of days on another business trip, so I
guess this discussion will die here. We'll have to agree to disagree.
Keith W
The gyroscopic forces on the blades when changing changing axes is
minimal
as it is
done over a relative long period of time. In contrast a propeller
changes
axes
in an airplane at about 60 degrees per second during a stall or spin. The
tilt
rotor does 90 degrees change in 20 or 30 seconds.
Perhaps a small electric motor could be added to orient the blades in an
impending
crash.
It is not just that it flies 100 MPH faster. It flies much much higher
and
much much further
than any helicopter.
http://www.rotaryeng.net/VTOL.html
"No discussion on the controversial V22 tilt rotor would be complete
without a discussion
on Vortex Ring State or VRS for short. An issue has been made about the
V22
Osprey's 47
degree blade twist and how it affects the Vortex Ring State. Unlike a
helicopter high
blade twist is necessary in a tilt rotor to get good propeller efficiency
at high speed.
This high twist can cause blade stall in the vertical landing mode. I
also
suspect it
affects the ability of the V22 to auto rotate.
The V22 can roll over if one rotor stalls and the opposite side rotor
does
not stall.
Special landing techniques have been devised to overcome the problem
namely
by limiting
the decent rate to 1000 FPM.
What the V22 Osprey critics don't realize is an off the shelf variable
twist rotor
blade such as we are proposing above is already on the market and has
been
proved
to work. Namely the IVO. Thousands have been sold over the past ten
years.
Contact IVO and see if they will build a variable twist rotor for your
needs.
http://www.ivoprop.com/ if I were Bell/Boeing I would be looking long and
hard at
this option for the V22 rotors.
We are not affected to the same extent as the V22 is to VRS as we have
only
one rotor axis. However blade twist negatively affects the efficiency of
the
rotor in the vertical take off mode. Never the less Boeing is working on
their
own system of variable blade twist. Here is a graphic that gives some
estimated performance improvements."
I believe for every proplem there is an answer. Just like the Wright
brothers.
--
Paul Lamar ...No rotor no motor.
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Copyright 1998-2007 All world wide rights reserved.
Keith,
What is your favorite source of data on the V22?
Do you believe Wikipedia is materially in error?
http://en.wikipedia.org/wiki/V-22_Osprey
They say that the combat operating
_radius_ is 416mi - 830mi
out-back.. I would presume that would include at least some mission
load.
They show a max speed of 316mph for the V-22 and a cruise speed (at a
sub-optimum sea level alltitude) of 246mh. I'd guess that the long
range at-altitude cruise is going to be more like 270mph.
One of the craft that the V-22 is to replace is the CH-46:
http://en.wikipedia.org/wiki/CH-46_Sea_Knight
With a combat radius of 184mi - less than half. 165mph is the max
speed allowed.
100mph faster is really useful. Being faster can definitely save
lives for many kinds of missions.
There are all sorts of things you can do with a normal helicopter
which are completely fatal. For instance many helicopters can sever
the tail boom with the main rotor under certain maneuvering
situations. The CH-46 suffers from a problem (as do all twin rotor
designs) where the transmission can have a mechanical failure which
causes the two rotor systems to get out of sync - causing loss of
blades or at least loss of control. There are lots of other problems
with helicopters. Engineers come up with ways to mitigate the
issues..
The BA-609 is a much smaller vehicle than the V-22...
http://en.wikipedia.org/wiki/Bell/Agusta_BA609
It's a real flying airplane that is going through FAA certification -
supposed to be completed in 20011.
Matt-
To start with, the V-22 CAN autorotate, the problem is that it has a 5000
foot per minute sink rate in this configuration. The problem is not that
it's a tiltrotor, the problem is an extremely high disc loading; the Osprey
isn't as much a rotary wing aircraft as it is a powered lift vehicle.
Since the rotors aren't intermeshing, getting out of sync is not a problem
but any condition that would do this would probably mean a destroyed
transmission. In the Osprey, a total loss of power is dealt with by a
forced landing same as a fixed wing aircraft.
A smaller vehicle can do things in a much simpler manner, look at the
differences between the BA-609 and the Osprey. If the Osprey didn't have to
fold up to fit on a ship, the systems could be much simpler. The wing could
have been longer and it could have bigger rotors. Remember that weight goes
up as the cube of increase in linear dimension. A two or four seater may be
able to use a two blade rotor with a teetering hub. A ballistic chute
system would be usable. A 2000 lb vehicle with two twenty-foot rotors might
auto-rotate very well. A two-blade rotor with the blades feathered in
horizontal position might glide reasonably well.
I don't care is this thread may not be specific to rotary engines. The
rotary engine is an enabling technology and the more new vehicles that are
designed around them, the more this system will be developed. Discussion of
new applications enabled by rotary engines is not irrelevant.
Steve Brazil
The Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site.
http://www.rotaryeng.net
Copyright 1998-2007 All world wide rights reserved.