How much Washout

[coosbaylumber]

Looks I may need to add in some washout on to a model from an old plan.

Now, does it matter the amount as to if the model A/C has round wing tips or near square ones?


If the aircraft is of an aerobatic type, with near symetrical airfoil, should there be ANY washout added in?



Wm.

[rofujiyama]

Hi,
Hard to say, a little more info on the plane, weight, wing area, etc. and the reason for the washout. Quite a few of the older Sig ,low wing sport planes have washout.
If tip stalling is the problem, more wing area , or coupled flap/elevator is fun, on a strip aileron setup ;) not to sure if washout will be whats needed on a low aspect ratio wing. If it is the reason for the washout. :confused:

[Ollie]

It depends on the lift distribution that you want. Use this program:
http://aero.stanford.edu/WingCalc.html

[Ollie]

"If the aircraft is of an aerobatic type, with near symetrical airfoil, should there be ANY washout added in?"

It depends on the wing plan form taper. If the taper is more than ~.6, then the tip stall is fine. If the taper is smaller than ~.6, then the tip stall for slow landing is a problem but fine for snap rolling. It is a trade-off.

Consider no wing twist (zero washout).

[coosbaylumber]

What have been having troubles with is an A/C which went only 80% towards production in 1960. It is a near duplicate to the (Berkeley or SIG) Astro Hog. I have original plans on a DMECO Stit's Playboy. It is of constant chord (13") except at the tip where it is 1/2 round. Per spec's on original plan: Wing area 820 SI, 66" wingspan, loading of 12/SF.

Has lottsa dihedral, thick sturdy wing, box fuselage, was originally designed in 1960 by Hal DeBolt for the then new K&B .45 to use in the First ever International Radio Control Competition in Switzerland. Wasn't referred to as F.A.I. at that time yet.

Oddly even with all the dihedral, the A/C flies inverted just fine. It settles down and flies rock straight, perhaps in small part due to the big dihedral, but with all the massive chord, it takes a hot landing, or it will stall out one wingtip (80% of the time to the left) and come in near sideways. Hal said he did not have the time to iron out this sudden feature, for he wanted to get it into producton ASAP. It may have been one of the mysterious reasons for NOT going into production also. For several more prototypes would then have to be made up, small changes added, etc. At his age then, he just could not remember the reason. My plans and blueprints here are dated March 1961, and looks to the final variant prior to production.

The much more popular Viscount was on the drafting board in October 1961 and it lived an awfull long time.

Wm.

[JetPlaneFlyer]

Wm,
If it has parellel chord wings and uses the same airfoil throughout then tipstalling should not be a problem (I'm assuming that there are no warps)... I would suspect that the unusual handling is due to some other factor... perhaps all that dihedral?

Ollie,
On the wing analysis program you gave the link to; I figure that objective would be to achieve a Cl distribution (the blue line) that is greatest near the root and decreases toward the tip. Hopefully then the root will stall first and there should not be tip stall.
So what's the red line signify?

Steve

[Ollie]

The red line is c/cavg.
c is the coefficient of lift.
cavg is the average coefficient of lift.

[JetPlaneFlyer]

The red line is c/cavg.
c is the coefficient of lift.
cavg is the average coefficient of lift.

Ollie,
I've been thinking about this. For the red line I believe 'c' is the chord and 'cavg' is the average chord.
The actual red line therefore shows Cl (coef of lift) x c/cavg, which is basically lift distribution over the wing. For a tapered wing this is a different thing from coefficient of lift distribution, as indicated by the blue line.

Steve

[rofujiyama]

Hi wm
I have been trying to find an example of your plane, but the closest thing I have is, an Sig Astro Hog build manual , and plans from 1 I built when sig came out with their version of the astro hog. The build manual show the original plans and story of the original astro hog by fred dunn,also. It had a elevator set on the bottom of the fuselage and barn door ailerons. Is yours similarly designed? I built mine after a friend here built 1 , and we both agreed it could use a little more rudder area , so i made mine a little longer , and it had very docile low speed handling characteristics.
I think adding washout , wouldnt be detrimental to your plane, ( if you can get it to twist ,lol) you got sooo much wing there. But if you have barn doors , you could just do a simple test to see if some washout would help the low speed handling , by lifting the aileron trailing edge an eigth of an inch or so up, to get some temporary washout effect. Wouldnt look to clean , but just a test , so ok .
Also , another example to consider. when the laser and extra mid wings became popular , as arf kits , my friend got the Diablo version, it had a shorter height rudder and more bulbuous canopy, as compared to the other 2 , and , that one had a vicious tip stalling if you came in to slow, as compared the the other 2 with taller rudder/fins, with very similar wing planforms.
It was such a transitional time for model airplanes, I think, using more current air craft design philosophy would certainly make it a more friendly model to fly. Even Sig moved the stab placement more in line with the engine as compared to the original plan design of the astro hog.
Hows about a picture. :)
Roland

[coosbaylumber]

Steve....

You are not first person to mention such. Two other "Senior citizens" mentioned that back when their short solution was to strip off all the silkspan, and recover the model, using a bit of washout twisted in the next time. In their experience they though with wide chord wings there is a tendensy to obtain uneven wing air friction. And thus the A/C wants to turn one direction all the time, but not noticable until the powere is cut off. Like at anding. I mention the Astro-Hog of 1957 for it is of similar dimensions, and was noted in Hal's letter, and people remember that design easily. The A-H has a 71 inch wingspan and a 12 inch chord, similar lines to fuselage, 'cept the Hoz-stab is smaller and engine displacement smaller. Fred and Kathleen Dunn used to live about 30 miles south of here.

Roland.....

The ailerons for this version are large, but certainly not approaching the BarnDoor variety as had been popular. (The Viscount as mentioned did have BarnDoor ailerons, plus assisting speed brakes, on the one year later design). There are two positions shown for Hoz-stab. A high and a low stance. One uses rubber bands from below, the high version ahs fin/rudder stuck on permanently. The Playboy is shown within the October 1960 edition of M.A.N. and the Sept. 2003 edition of R.C.M. Neither too good. I can easily strip off the covering, add in some twist, then re-cover. I may not notice the addition of only 1/8 inch, thus had been wondering in 1/4 or 3/8 inch would be more appropriate here due to large chord. But I still wonder if due to huge chord, the wing drop off is not inherent to some sort of air friction.

Wm.

[JetPlaneFlyer]

Wm,
Just thinking about your description of the symptoms; "it will stall out one wingtip (80% of the time to the left) and come in near sideways"

Can you expand on exactly what the model does when you slow it up please? A bad tip stall will manifest itself as a sudden snap roll in the direction of the stalled wing, usually unrecoverable if it occurs at low altitude and therefore typically ending in a nasty crash. Is this what you mean by "coming in sideways" or are you meaning that the model yaws into side slip?

If it tends to side slip then you probably have adverse aileron yaw... The simple cure to this would usually be aileron differential.

Steve

[rofujiyama]

Hi Wm,
My library of magizines is minimal nowadays, and I was 4 years old in 1960,lol.
Anyways after more thought, on low speed tip stalling, I feel even more positive that you dont have enough vertical side area aft of the CG, to provide adequate yaw dampening to keep the wings working.
When i was younger I kept cutting down the span of a constant chord wing, to see how much i could remove from the tips to get more speed, but still be able to maintain a tight ,high G pylon turn and still fly stunt manuevers and land dead stick easily , and the lower limit for aspect ratio was ~5 to 1, but then the super light funfly models came out with even shorter aspect ratios, and i finally remembered I do have a highly washedout constant chord A/C which all fly without tip stalling at low speed,and then I though about an arrow , and what would happen with out the fletchings(tail feathers) at low speed, and if it had tiny wings on it and no tail feathers ,at low speed, sooooo even though some washout "may" help the handling I think steve and ollie's initial response are both correct, I think the simpler solution would be enlarging the rudder/fin area.
Roland