Nick Lappos was a sikorsky senior engineer for decades. Is now the boss of the US EH-101 program. These comments PREDATE his work in that capacity.(15 may)
His comments are pro and con, and obviously impartial.
http://yarchive.net/air/tiltrotor.html
The successful development of tilt rotor technology depended on two important
technologies that were of sufficient maturity to allow the XV-15 to be the
success that it was:
1) Engines light enough, yet powerful enough to lift the machine. Tilt rotors
must have small highly loaded disks, unlike multi rotor helicopters whose
disks stay above the aircraft, and don't have to swing past the fuselage. For
this reason tilt rotors need about half again more power than helicopters to
lift the same payload, so the engines must be light, and efficient. While
demo aircraft were possible in the past, only now can the modern turbines
allow a practical production tilt rotor.
2) Theoretical computation to control the dynamic interactions that exist
between the rotors and the wing. Without the excellent computers available
today, the search for acceptable aero-elastic solutions would take very
expensive and time consuming trial and error. The limiting factor in all
early tilt rotor vehicles was the interaction of the rotor frexquencies with
the wing, creating resonances in many flight modes. The XV-15 and now the
V-22 are reported to be quite free of such worries. An important other
technology is the composite structure which allows tuning the wing and rotor
structure to avoid these interactions with relative ease.
<b>I must comment that any given tilt rotor has LESS range than a helicopter with
the same power, cost and empty weight. This is because the tilt rotor starts
off with half the payload, and so even though it is quite efficient in cruise,
it carries so little payload relative to a helicopter that it never catches
up.</b> The tilt rotor has a significant speed advantage, of course, probably at
least 75 knots and perhaps as much as 125 knots (150 to 160 for a helicopter,
225 to 300 for a tilt rotor).
You have already seen responses as to yaw control, which is done with
differential cyclic control.
Part 2:
Auger,
Your question is a good one. <b>The V-22 and the H-53E have similar
systems and penalties for Marine shipboard operations, and the
generalization that the helo will beat the Tilt Rotor in range is
accurate for that comparison.</b>
The wing length and prop length issues for the tilt rotor are
really layed out by its basic geometry, not by any shipboard
requirements. For the rotors to be at the ends of the wings, and
then convert to props, the rotors must not come too close to the
cabin. <b>This sets the size of the rotors for all basic tilt
rotors, and makes them small and highly loaded disks, and
therefore less efficient by about 40% than an equivilent
helicopter</b>, which can have the rotors overlap the fuselage (like
some of the big Russian machines, like the V-12).
<b>The efficiency is directly tied to the size of the rotor disk(s)
for the weight of the machine.</b>
Also, if the wing on a tilt rotor is too long and thin, it
becomes a real elastic nightmare for avoiding the various
resonance modes that I touch on in the post above. These
modes are kind of like the pylon rock some helicopters
exhibit. The rotors can begin to pump themselves, twist the
wings and feedback in resonance. The shorter,
lower aspect tilt rotor wing is quite a bit stiffer, and is less
prone to resonances, that is why it is used on the V-22 and the
XV-15.
<b>Tilt rotors have very high drag on the wing in a hover, where the
downwash on the wing is "felt" as extra weight and therefore lost
payload. On a helicopter, we package the fuselage tightly to
keep the vertical drag down to about 5% of the total weight,
which means that a 50,000 lb helo has about 2,500 pounds of lost
payload due to downwash. A tilt rotor wing is flat and big and
completely exposed to the high speed downwash, making the penalty
of vertical drag at least 10% of the gross weight.</b>
<b>Therefore,
for a 50,000 lg tilt rotor, the lost payload is twice that of an
equivilent helicopter, a difference of perhaps 2,500 pounds or
so.</b> This fact is why some folks use a tilt wing instead of a
tilt rotor.
<b>All this fits into the sweeping statement I made, which is true
for helos vs tilt rotors, that with the same power, cost and
empty weight a given helicopter will have more range that a tilt
rotor, but of course much less speed.</b>
Nick
Part 3:
Nick sez:
On the H-53 series, the whole tail section folds automatically, after the
tail rotor is disconnected and motored into correct position. The main rotor
is indexed, the pitch of the blades are all locked with special mortorized
pins, and the head is rotated into position by a special motor. All
automatically, and all in winds up to 45 knots.
I think you are trying to explain the tilt rotor's shortfall in payload as
something to do with shipboard issues, and it won't wash. <b>Specifically, given
the same power, cost and empty weight, a helicopter will carry almost twice
as much useful load as a tilt rotor.</b>
Part 5:
Nick sez:
This is where I wasn't clear enough. <b>If you lay out a tilt rotor to fit any
given area of deck space, it will have two very small rotors as compared to an
equivilent helicopter. That is beacuse the two disks must not touch the
cabin, so they must be small. Generally, a tilt rotor must have only about
40% of the rotor disk area of an equivilent helicopter, which means that ot
will need about 50% more power for the same payload (or that it will have much
less payload for the same power). This is not salesmanship, zrassler, it is
physics.</b>
Part 6:
Nick sez:
You misunderstand the problem, zrassler! <b>If the tilt rotor has less useful
load in its hovering takeoff, it can't carry the fuel to get the range. The
awesome panalty of having only half the payload of a helicopter means that the
tilt rotor starts with one foot in a hole. It takes off with half the payload
for the same power and cost, so it can carry only half the fuel. Even tough
it is more efficient in cruise (and surprisingly, only a bit more efficient)
it never gets to go as far as the helicopter, because the helo can take off
with more fuel!</b>
best,
Nick
Google this guy, his SCIENCE is very hard to argue with.
<img src="http://i22.photobucket.com/albums/b343/m21sniper/OnTheJobEnhanced.jpg" border=0>
<b>"One post, One Kill".</b>
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