Thanks for the various addresses mentioning planform and design software.
Without wanting to do too much self advertisement, I will mention that a
program for designing a wing in plan, using actual wind tunnel test data
for the section concerned, was published by me in Herk Stokely's 'Soartech
Number 0ne' in 1983, or about then. In those days I used a hand held TI 51
programmable calculator. The system worked, with some examples given in
Soartech 2. I don't know if any of the currently available programs use the
same methods but it would be relatively easy for anyone to re-write my TI
51 program to make it usable in a modern spreadsheet. (No, I haven't done
it myself.)
The essential principles remain for all of us - it is most necessary to
avoid the outer parts of the wing stalling before the inner panels, but we
want also to taper the wing to achieve at least an approximately elliptical
lift distribution.
The traditional use of taper and washout to achieve this at low (soaring)
airspeeds, is not desirable since it produces a bad situation at high
airspeeds. The outer wing, washed out, begins to 'lift' downwards as the
airspeed rises. In old time full scale sailplanes, at speeds above about 70
knots, you can see the wing tips beginning to bend down, indicating
reversed loading and high drag, spoiling the fast glide. Exactly the same
happens with models if the wing is washed out, and even more so if the
wing, under load at high speeds, begins to twist appreciably.
The required control of the stall spanwise is achieved best through a
combination of careful planform design and WING SECTION CHANGE FROM ROOT TO
TIP. You need to choose the tip section with a later stall than the root
and, preferably, a higher available maximum lift coefficient. This is not
very easy when the tip Re number is always lower than the root, but it can
be done from the wind tunnel results. Increasing the camber slightly and
thinning the section towarwds the tips is usual, with attention to a gentle
stall character.
Then you can keep the desired elliptical load throughout the whole range of
airspeeds, and not have the downward bending tips. This is how full scale
sailplanes are designed now.
This is dealt with in general terms in Chapter 6 of my Model Aircraft
Aerodynamics book (and was there twenty years back the 1978 first edition).
The required design methods are given in the old NACA Report No 865,
"Method for Calculating Wing Characteristics by Lifting-Line Theory Ussing
Nonlinear Section Lift Data." This was published in 1946, all done by slide
rule in those days, and is still readily available through the USTIS. To
make it suitable for modern computing should not be too difficult. Any
volunteers?
The effects of small amounts of sweep bacxk or forward are usually not
enough to matter, but for large sweep angles in eihter direction, more
advanced design techniques, panle methods rather than lifting line theory,
are necessary and I can't commit myself on these.
Martin
>From Martin and Jean Simons,
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