Adding washout to a 4 panel wing - where?

[fprintf]

In Dr. Drela's 5 panel wings I see his recommendation is to add .5 degree of washout to the mid-panel, which lets the entire tip panel carry washout without twist.

Is this an appropriate thing to do for a 4 panel polyhedral wing? That is, to apply twist to the two center sections so the tip panels carry the washout without twist?

Another question, regardless of the answers to the above, is there a formula for calculating how much washout is .5 degree over a certain distance? My ruler measures in fractional inches or millimeters. I assume I need to know how long each panel is to determine how much to lift up the trailing edge.

Thanks!
Stuart

[Bernie Wolfard]

Stuart,

Is the plane you are setting up have the Drela series of airfoils? The reason Dr. Drela sets up his wings this way is because the airfoil sections change from root to tip and the tip airfoils actually fly at a different (lower) angle of attack than the root sections. His way of adding twist is unusual because of this and pretty hard to do accurately.

The first question is why do you want to add washout? Is your plane tip stalling enough to make it hard to fly? Washout, while increasing stability, decreases performance so without a good reason to do it don’t. If you are trying to stop a tip stalling issue try setting the training edge at the tip up about 1/8”, test then increase in small steps if more is needed or until the desired flying characteristics are reached. On most poly ships twisting the tips while heating with a heat gun is all that is needed to set washout.

Bernie

[nuevo]

Stuart,

The recommended twist is corresponds to the characteristics of the individual airfoil sections at each station along the wing, and the speeds the plane was designed to fly at. At least for Mark's published plans, it does. :rolleyes:

Not clear if this is what you're asking, but it is not appropriate to make a generalization that you should add XX degree washout to an arbitrary wing planform.

This gets into airfoil and planform design. A glider designer generally wants to avoid tip stall, so chooses planform and airfoils appropriate for the root, mid-section, and tips so that the root will stall first, or that the entire wing will stall at the same time. Various safety margins may be put in place to not allow tip stalls in tight turns, too. Other goals are minimum induced drag, pull on launch, turn handling, etc. It's all a comprimise, depending on the exact goals the designer is after. The designer can choose a combination of geometric washout (twist) and aerodynamic washout (differing airfoil zero lift angles) to achieve the effect he is after.

In "the very old days of full size gliders", where no wing taper was used, a lot of geometric washout was used to obtain the lift distribution desired at "thermalling speed". But at very high speeds, the tips would generate negative lift (due to excessive washout) and actually bend down.

Jon

[fprintf]

Ok guys, thanks very much for the education about washout and why it is used. The plane in question is my Marauder which has a dual taper (LE and TE) tip section. The plans do not call for any washout.

When trying to go into a turns from level flight, particularly into the wind (turning across the wind) I am finding that one wing drops rather rapidly causing me to lose quite a bit of altitude before I build up some airspeed and flatten out the turn. This could be because of a tip stall, or it could be because I am just flying it too slowly. Either way I think based on your responses above I have some work to do learning to fly the plane a bit better before messing around with washout. I'll check the wings again for warps and wash-in. Perhaps the plane needs a whole lot of up elevator earlier than I am used to after initiating the turn or perhaps I just need to fly this airfoil faster.

Thanks again!
Stuart

[nuevo]

If the wing is built true to plans, I believe your conclusions are valid. Try a click or two of down trim for general flying. Also get a local expert to watch you initiate your turns or fly the plane himself. You may be applying too much rudder to start your turns.

[Soar_dude]

Stuart
I do not think you have a tip stall problem. when making a downwind turn the wing that is on the outside of the turn is going to force the glider into to a steeper turn because the wind is working with the lift the wing generating. That is why making downwind turns are so much quicker then upwind turns. On the upwind turns the wind is having to work against the lift the wing on the inside of the turn is creating.
So my rule of thumb is less rudder on a turn to downwind and more rudder for a turn to upwind.

try it and see if that helps
Soar Dude

[nuevo]

downwind turn :eek:

[TLyttle]

I've been reading about the dreaded downwind turn for over 50 years now, in model mags as well as full-size: PLEASE lets not get into that one, or will take an hour to load the thread!

I have no experience with Drela, but on simpler models flat center sections with a little (just a little!) washout on the tips plays it safe, I like the idea of twist & try until you get what works for you.

As far as calculating washout is concerned, I have a table taped to my toolbox that has to do with natural tangents (I think), and I just measure the difference between the inner and outer ribs on the panel (I use dowelling gummybanded to the wing) and use the table to calculate. If you are measuring over a 10" section of the dowels, you will get a measurement, ie 1/8", between the dowels; you look up on the table to see what is closest to that number, and that is the # of degrees. This sounds difficult, but if you do it a couple of times you will see that it may be crude, but it is quick, and it sure works!

[Al M]

I just cured a similar problem by reducing control throws.

[scififlyer]

Another question, regardless of the answers to the above, is there a formula for calculating how much washout is .5 degree over a certain distance? My ruler measures in fractional inches or millimeters. I assume I need to know how long each panel is to determine how much to lift up the trailing edge.

Thanks!
Stuart

Geometry functions on your average pocket calculator will work to high accuracy.

Already did your homework for you: for .5 degrees multiply .0087 times the chord measurement to find your washout. Good luck, building to within 1/2 degree accuracy without precision machinery is very difficult.