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<< Lancair Builders' Mail List >>
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<< I understand the extended wing area is important for high altitude flight
so I will have to use some type of winglet. >>
Altitude does not matter. Some amateurs with impressive degrees will argue
with this; it really doesn't matter. Lift coefficient does. For a given
wing and aircraft, the only variables for lift coefficient are INDICATED
airspeed and weight. The only influence altitude has is that your power will
fall off with altitude, and with it, the IAS you can cruise at. High IAS
means low lift coefficient, which means less induced drag and more parasitic
drag; hence winglets (and wing extensions) can be a penalty instead of a
benefit. Why? Because the winglet adds wetted area and parasitic drag. If
the drag added is more than the reduction in induced drag, you go slower.
Looks cool though...
Low IAS (or VERY high wing loading like an airliner) means high lift
coefficient, which means more induced drag vs. parasitic drag; hence
winglets (or an aspect ratio increase) can be a benefit.
If you are going WAAAAAAAYYYYYYY over the aircraft's intended IAS, it is even
beneficial to clip the wings to shorter span and lower aspect ratio.
Mustangs have been modified for this and the crossover on the current Reno
Unlimited course is about 2,000 HP. Below that, they're slow enough that the
bigger stock wing's turning performance makes up for the extra drag of all
that surface area. At 3,000 to 3,400 HP, Dago, Strega, and Voodoo have
proven that reducing the wetted area with the wing clip was worth the induced
drag penalty.
Holding the same IAS and climbing will have an effect on stability; albeit a
minor one until you go transonic. For the Lancair IV, this starts around
M.52 to .56 on different parts of the airframe. It is likely that the
aircraft will hit Mdd at M.62 to 64 and no amount of power is going to make
you much faster. Mdd is the drag divergence Mach number, above which the
collapsing shock waves in the pressure recovery regions of the wing will
cause separation of the boundary layer and a large drag rise. M.52 at 29,000
ft. is 307 KTAS and 192 KIAS; M.64 at 29,000 ft. is 378 KIAS and 236 KIAS.
Before everyone gets their hopes up about flying at airliner speeds on the
power you use to get 236 KIAS at sea level, remember that thrust from a prop
varies with true, not indicated airspeed. Since many, many owners have shown
that 350 HP from their TSIO-550's can push them into the 240 KIAS range near
sea level, we can expect that it will take ~1.6X the power to go 236 KIAS at
29,000 ft than it does to go 236 KIAS at MSL (given the same np across all
that change in altitude and TAS; this isn't true, but let's set that aside
for now and get back to winglets). This is where everyone gets the drive for
more and more power.
If you are going to fly at over 180 KIAS and > 20,000 ft., the factory
winglets add a couple of percent of needed yaw stability. Below 170 IAS,
they start to make <some> efficiency improvement. If you want to run around
140 KIAS at any altitude, tehy're probably a very good idea. There are a
couple of owners who want to make some range records with their Lancair IV,
either winglets or a wing tip extension make sense for them. If you hot rod
the aircraft with an AE or modify a TSIO-550 with ceramics and FADEC, you
will likely want to cruise at over 180 IAS at high altitude. The added yaw
stability will be needed; though this is accomplished with far lower drag
impact by a simple ventral fin that increases your effective vertical
stabilizer area.
If you have a stock engine and plan on cruising from 10,000 to 18,000 ft (max
canula altitude) and 165 to 175 KIAS, forget the winglets. Anyone's
winglets. This is what the stock wing was designed for and it serves that
purpose admirably. If you power up and go to higher IAS, the winglets will
cost you some of the speed you just paid dearly for.
I hope this clears up some misconceptions about the mysterious subject of
winglets. I will close with one, real world, layman's test of whether
winglets are right for you or your aircraft: show me the before and after
test data. Flight test, not CFD. I make my living finding and fixing
aerodynamic flaws that don't show up on most CFD. It's a useful tool, as are
wind tunnels; neither is always right or even perfectly predictive. We fly
in air, not computers. So once the CFD has shown us what to try, we fly it.
If it doesn't work the way we thought it should, the good engineers go back
and fix the code. At this time, no commercially available CFD can find the
things that most design houses claim and match them to flight test. So fly
it yourself and make your own decision.
Eric Ahlstrom
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LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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