John Lynch wrote:
paul,
I got this letter back from Scientific American. This is my reply.
Regards,
John
Tim Beardsley wrote:
Dear Mr. Lynch,
I have investigated the points you make about the Moller skycar. At the
root of this is that Moller and Freedom Motors were publishing in different
places on their websites information about the motors measured under very
different conditions, yet they did not make clear the differing measurement
conditions. Moller and Freedom motors have told us they will revise their
websites to clear up this source of confusion.
The inconsistency in their published data affected both stated powers and
weights, and it led us into error in our published account. Specifically,
Moller says the Skycar's engines weigh 135 pounds for a pair, not each.
There are 4 pairs of engines. The power that Moller cites is maximum power,
for emergencies (5 minute rating). The power for cruising is less, and so
this invalidates the fuel consumption calculation you did.
I include some information from Jack Allison of Moller:
"The numbers quoted for the engine weight are incorrect in that both engines
together in each nacelle weigh 135 lbs for a total of 540 lbs. It should be
obvious that the total horsepower provided is intended to provide sufficient
power to deal with an emergency during takeoff, hovering flight and landing
and far exceeds the power needed for cruise.
In fact, at 20,000 ft and 300 MPH the required horsepower is 150 HP because
the lifting surfaces are sized for cruise rather than takeoff. At a
specific fuel consumption of .4LB/HP-HR, which we have demonstrated to
others, our fuel burn in cruise is 60 lb/hr = 180 lb for a 900 mile flight.
With four passengers this totals 1400 lbs, which leaves 800+ lbs for
airframe, fuel reserves, parachutes, etc.
Boeing did an extensive study of the Skycar which resulted in their
validation of the technical viability of the design. Two Boeing engineers
and six airline pilots have become stockholders in our company."
And from Paul Moller:
"The reference on the Freedom Motors website refers to a continuous long-life
industrial/marine application of Freedom Motors three rotor Rotapower
engine, including engine mounts, large flywheel, etc. In the Skycar we have
continuous
Horsepower rating of 80 HP for a two rotor Rotapower engine with a maximum
capability of 120 HP in an emergency role (5 min. rating). Because the
Skycar application uses the fan as a very light flywheel and emphasizes
weight reduction, the engine weighs only 67.5 lbs.
I am sorry for the confusion and will revise our websites to clear this up
for future readers."
We will put some sort of note in the magazine clarifying the matter of the
engine weights. Thank you for your interest in Scientific American.
Sincerely
Tim Beardsley
Associate Editor
Tim Beardsley
John Lynch wrote:
Subject: Re: re. Moller Skycar
Date: Wed, 01 Dec 1999 00:34:09 -0800
From: John Lynch <jr_lynch@earthlink.net>
To: Tim Beardsley <tbeardsley@sciam.com>
Dear Tim:
Thanks for your reply to my recent e-mail about the Moller Skycar. I understand
that their interpretation of fuel consumption and component weights is different
than mine, but I would point out a few things in response. First, my calculation
of fuel consumption was very crude, admittedly, but it works surprisingly well
for many types of vehicles. (By the way, there was a complete lack of any
technical information on their website, particularly under the heading “technical
information”). And you quote Jack Allison as saying that “It should be obvious
that the total horsepower is intended to provide sufficient power to deal with an
emergency during takeoff, etc.”- and I would say that it’s not so obvious.
Helicopter engines run at 100% rated power nearly all the time when hovering, and
at 75% power or so when cruising. Likewise, most aircraft takeoff and climb at
full rated power, and cruise at 75% power. Now Moller’s vehicle is apparently a
“mixed-mode” device, somewhat like a Harrier VTOL jet, and relies on both
vectored thrust and dynamic lift from the small surfaces. It seems that it will
have to spend a fair bit of time operating in hover and in transitional flight.
So unless he’s thought of something that everybody’s overlooked, the Skycar will
have to operate at full rated power a lot, just like helicopters, the Harrier
jet, and most airplanes.
That brings me to my second point: the engines. They are claiming that each
engine will weigh 67.5 lbs. and produce 120 horsepower. For comparison, a
Lycoming O-235 aircraft engine (115 hp) weighs 240 lbs. That means that they are
claiming to be 3.5 times better (power to weight) than equivalent HP air-cooled
aircraft engines. One good way to evaluate performance projections like this is
to look at the current state of the art, and then plot your projected design on
that chart in comparison. I’ve attached an image of an Excel plot showing how
Moller’s projected engine compares in power-to-weight ratio to the industry
standard engines. Notice that it’s even better than one of Garrett-Airesearch’s
gas turbine engines for helicopters. (My source for these weights is Jane’s All
The World’s Aircraft, 1975-76 Edition).
When viewing such a graph, you then have to ask yourself, how likely is this, or,
what have they done differently than everyone else? You may then conclude that
either they have made a technological breakthrough, in this case a huge one, or
they have deluded themselves and made a mistake.
They will probably say that they have demonstrated this engine. I would point out
the standard of comparison. Aircraft engines must run with nearly absolute
reliability for about 2000 hours between overhauls. Lycoming and Continental have
produced hundreds of thousands of these engines over the years, and there is now
an overwhelming statistical database that proves their reliability. And those
engine designs are the result of hundreds of thousands of hours of testing and
refinement that took place in the past 50 years. The Skycar’s engines will need a
lot of reliability testing to show that their engine can meet this standard.
Anything less will be very fatal in this vehicle, even more so than traditional
aircraft or helicopters.
I didn’t mention stability and control, noise, voting computers and the other
complexities of the Skycar yet, but Aviation Week noted recently that the Marine
Corps is still losing Harriers at a high rate, and it is powered by a reliable
jet engine and has a long development history. Suffice it to say that I am
willing to bet that no Skycar will fly a significant distance (say 300 miles) in
the next 3 years, and I certainly don’t expect to see it at Oshkosh, exceptt
perhaps on a trailer. I guess my point is this: I respect good engineering and
encourage innovation. What Moller is trying to do is possibly admirable. But, he
should show us an operating aircraft first rather than a website and sales
brochure. The real designers do.
Thanks for your time.
Regards,
John Lynch
Hang in there John. In three years there will be more Tracy Crooks
around :-)
I might even buy stock in Moller. :-)
Paul Lamar
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