X-Mozilla-Keys:
Fred wrote:
To further the discussion, the lift coefficient of a Lancair IV at 210 knots
IAS is about 0.2 to 0.25.
That sounds a bit high Fred. What was the race weight?
Correct me if I am wrong but a C sub L of .25 at
210 knots is 38 pounds per square foot of wing area. According to
Kitplanes
the Lancair IV has a gross weight of 3200 pounds and a wing area of 98
square
feet. That would be 32 pounds per square foot. Surely they don't race
at gross weight.
At Reno the planes are pulling over 3 g in the
unlimited class (I have heard as high as 5-6 g), and based on the estimated
bank angle I think the sportsmen were pulling over 3 g on their short course
as well. So let's assume 3.5 g. This means that the lift coefficient in the
turns on the Lancair IV would be about 0.7 (probably a bit less due to higher
IAS). At this g level, interesting things begin to happen. For example, on
short wings, winglets start to be effective in reducing induced drag. Also
the flow at the wing fuselage intersection starts to get complicated, and one
needs to exhibit more care at this location to reduce induced drag.
Based on careful measurement of many turboprop and late model bizjet aircraft
at OSH some years ago plus discussion with Bruce Carmichael (aerodynamicist)
I concluded the lowest induced drag occurs when the fuselage walls are
parallel to the free stream until the fuselage reaches the trailing edge of
the wing. The Lancair IV fuselage tapers inward 6 inches (3 inches each
side) prior to the trailing edge, so I thought I saw a drag reduction
opportunity. I convinced one of the factory people to build a set of
fairings based on my research and drawings, including keeping the fuselage at
constant width to the trailing edge. They were big fairings.
Result? No increase in cruise speed. Improved rate of climb, better
behavior near stall, but no more speed.
Why? I met Mark Kirchner, a retired Boeing aerodynamicist and Lancair IV
builder and posed the question. His answer: lift coefficient is too low to
provide any benefit. Got to be above 0.6 to be beneficial. For reference,
big intercontinental Boeings operate above this level when heavy, and so
carefully tested fuselage fairings and winglets become more attractive.
The inference is that these benefits can be attained but only in a high g
environment in our lightly loaded sport aircraft. I checked Nemesis, and low
and behold, it's fuselage is parallel to the free stream until passing the
trailing edge of the wing. And Nemesis operates at very low lift
coefficients in cruise due to the large wing required by its class. I don't
know what g loads are pulled in turns.
So if we go racing, keep the fuselage parallel to freestream over the wing,
and keep in mind the induced drag created by high g turns since one spends a
lot of time turning and scrubbing off speed when doing so.
Additional food for thought. Because propulsive efficiency of jets goes up
as aircraft speed goes up (small high speed columns of air are bad news at
low speeds, but necessary at high speeds, witness turbojets), why not
consider a highly loaded small diameter high RPM fan at the front of the
engine and dispense with the gearbox? Takeoff thrust will be BAD due to the
reduced static thrust, but with the power to weight ratio suggested and long
runway, this aspect should not be a problem. Keep in the mind the gearbox
weighs something, and consumes 1-2% of the transmitted horsepower and so
requires oil pumps, cooling, etc. Wonder if we can just get rid of it and go
to high speed cheese slicer (with a 3 foot prop?) suggested earlier. Might
be worth some research and calculation.
Fred
That is a very interesting idea Fred and I have always wondered about
that. Funny you should mention it as Vance and I were talking about that
on Sunday when he visited me at Santa Paula. There is a lot of
technological
risk in the idea and if I were doing a Reno unlimited racer I would take
a more conservative approach and use a gear box. On the other hand if
I were GE or Pratt & Whitney I might try it. If the direct drive
prop proves to be worse than a gear box and conventional prop that
could be a show stopper financially for anyone I know. Well maybe not
Bill
Gates :-)
Paul Lamar
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