Wing construction - RC Groups
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Sep 14, 2017, 12:08 PM
Even that isn't fast enough
Discussion

Wing construction


I am helping my son's High School team design a pylon racer. The rules are fairly simple with the major points being we must use the supplied OS .46 AXII and the wing must have 570 sq in. There is no weight minimum so we would like to be as light as possible. I am curious to ask the question to the pylon community if you could build a Q500 with no minimum weight would you still go with a foam wing or would you go with a built up wing?
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Sep 14, 2017, 06:43 PM
Scott
Pylonracr's Avatar
I would probably go with a foam core. The weight should be close either way, but given the choice foam is easier.
I would go as high of aspect ratio as the rules allow. Good luck with your project.

Scott
Sep 14, 2017, 11:40 PM
Even that isn't fast enough
Quote:
Originally Posted by Pylonracr
I would probably go with a foam core. The weight should be close either way, but given the choice foam is easier.
I would go as high of aspect ratio as the rules allow. Good luck with your project.

Scott
There is no limit on aspect ratio. The decision has to be made based on strength vs weight vs gain in efficiency.
Sep 15, 2017, 12:21 AM
Scott
Pylonracr's Avatar
I think a decent starting point would be a root chord of 12", tip chord of 7" and a span of 60". That would be your area with a nice aspect ratio and a fairly nice taper. Run strip ailerons on the inner 2/3 of the span leaving the outer third clean. Cut the cores from the lightest foam you can get (1 pound density), top and bottom spar made from 1/8 x 1/4 balsa with 12k carbon tow top and bottom of each spar. Sheet with 6 pound or lighter 1/16" balsa using laminating resin and vacuum bag.

Scott
Sep 15, 2017, 11:27 AM
Even that isn't fast enough
Scott,

We are close to the same page was leaning toward slightly less taper and a span of about 64". I have access to a vacuum system but have never used laminating resin. Have always used Poly glue and lots of lead bags for sheeting foam wings.

FYI, the rules were different last year with similar wing requirements to q500. Tail size was required to be larger. We went low wing and retracts. Have decided to try to do a lighter build this year.
Sep 15, 2017, 03:05 PM
Scott
Pylonracr's Avatar
Should work just fine. I was kind of shooting from the hip with my dimensions.
By the way, that looks like quite the shallow V on the tail of that plane. I believe the standard for the Q500 planes is around 100 degrees for stability. Maybe Kevin or someone that designs these things will chime in with the reason.

Scott
Sep 15, 2017, 03:31 PM
Even that isn't fast enough
Yes, it was shallow. There was some math used to come up with the size and angle based on the required size of a standard tail. An Aero PHD even checked the math the kids did. When we built it I got scared for them and had them add the sub fin to ensure yaw stability. Plane flew very well, looked cool, and we won but was probably not the most efficient.
Sep 15, 2017, 09:07 PM
Registered User
Most people that build pylon racing quickies do not do too much math. It's more monkey see, monkey do. So most quickies have the v-tail angle at around 110 degrees, and plus or minus 5 covers most designs. But these angles were more or less copied from sailplane designs where the wings are long and you need a lot of yaw power.

Most sport planes tend to have roughly 1/3 of the horizontal tail area as the area needed for the vertical area. Translated to a V-tail, this works out to be an angle of 120 degrees. However, the vertical tail area is roughly proportional to the disk area of the prop, and racing airplanes use small diameter props with lots of pitch, so while I have flown with a 130 degree tail, and it flew well, the ground steering upon landing was soft. So I stuck with 120 degrees. On top of that, I generally use a very small area for the tail, of around 13%. But 15% is about as low as I recommend.

As to the wing, I would just stretch out a standard Quickie wing to 58", It is difficult to sand a proper radius and shape to a constant chord wing for the full span, but possible with the use of a good gauge. But for a tapered wing, it becomes nearly impossible for an inexperienced modeler to do. The first third of any airfoil makes or breaks the performance of any wing.

Use at least 3/32" or even 1/8" thick balsa for the top sheeting and 1/16 or 3/32" on the bottom with a strip of 0.007" by 1/2" carbon glued directly to the inside skin on the bottom sheeting only at about 3" from the leading edge for sheeted span. Carbon works well for tension, and thicker balsa sheet works well in compression, so this is why you put thicker sheeting on the top surface and carbon on the bottom.

I would also build the plane as a high wing, as they are generally faster due to better performance in the turns.
Sep 15, 2017, 11:10 PM
Scott
Pylonracr's Avatar
WOW,
At the risk of sounding like a jerk I am going to disagree with most of the above post. The 100 to 105 degree tail angle was generally found to be the best compromise between better turning (shallower angle) and sufficient yaw authority for landing and takeoff (steeper angle). Your previous design was a nice compromise with the lower fin, but yaw control to land will be better with a steeper angle.

Very bad advice on the wing. If you are trying to build a light wing and not an aerodynamical cinder block, don't even consider that thick balsa. Note that I suggested a balsa top and bottom spar laminated with carbon tow. The spars are there for a reason. Since this is a school project, the calculation is complex polar moment of inertia. Have the students do the math for practice, you will find that this spar arrangement is stronger than required for 10g loading by an order of magnitude. Remember to factor the wing skins as stressed skin design.

One final thought, since I failed to mention this in my first post. Your last aircraft was a quickie style. If the rules allow, copy a QM40 instead. They are much faster and much more aerodynamic. There is one of these in my hangar right now. http://www.cmadracing.com/Sweet_Vee.html
I kind of scaled the wing dimensions up to your requirements. Notice the high aspect ratio and taper? They are there for a reason. The fuselage will be more work than a Quickie style but it will be worth it.

Best of luck

Scott
Sep 15, 2017, 11:33 PM
Registered User
I vaguely remember the high aspect ratio wings in pylon for some reason.
Sep 16, 2017, 12:00 AM
Scott
Pylonracr's Avatar
Kind of oversimplified, but high aspect ratio wings make the most lift with the least drag. This is why gliders look like they do. Spanwise strength comes into play with powered aircraft, so like everything else aircraft, compromise is the name of the game. All else being equal, a higher aspect ratio wing will be faster. Again, oversimplified.

Scott
Sep 16, 2017, 01:27 AM
Registered User
Just running a few numbers and assuming a reasonably clean design, your design could easily approach a 20 to 25 g load in a 50 foot radius turn if it approaches 130-135 mph. A slight dive, a gust of wind, or tighter turn can easily push you above these loads.

As to increasing the thickness of wood for increased compressive strength, adding a 1/32" thickness in the top skin adds about 3/4 oz with 6 lb wood. I usually used 7 lb wood when my Nelson powered quickies were running in the 160-170 mph range with a 52" wing, with 3/32" tops and 1/16" bottom skins. At those speeds, we would hit 35 to 40 g's. Even with the "heavy" construction, my airplanes were usually 2 oz. under the minimum weight back when the minimum weight was 3 1/2 lbs instead of the current portly 3 3/4 lbs.

Assuming that the wing skins stay attached to the cores, the usual failure mode is compressive failure and buckling. Increasing the skin thickness by 50% increases buckling resistance by a factor of 2.25 to 1. I have used about every method for skinning a wing, and currently think the Gorilla glue, a mist of water to the cores and a wisp of aqua net on the inside surface of the skins is about the lightest method with more than enough strength. The hair spray somewhat seals the wood from soaking up the glue, which you want to penetrate the foam.
Sep 16, 2017, 12:31 PM
Registered User
Quote:
Originally Posted by Pylonracr
Very bad advice on the wing. If you are trying to build a light wing and not an aerodynamical cinder block, don't even consider that thick balsa.
What's the thickness of the wood have to do with aerodynamics? The air only sees the outside shape of the wing, it couldn't care less what the construction is. It's not like he's saying use the same foam core and add a thicker skin. There currently aren't cores so they can be cut to accommodate whatever construction chosen.

That being said, thicker top skins are among the easiest way to vastly increase the strength without additional work.
Sep 16, 2017, 04:15 PM
Scott
Pylonracr's Avatar
The thickness of the skin has nothing to do with aerodynamics, it has to do with weight. I prefer to use a laminated spar and thin skins. The result is a much stronger wing that is significantly lighter. If you want to put a couple of carbon strips glued to the skin and use thick wood go ahead. It will work, it will just weigh more.

Scott
Sep 16, 2017, 11:54 PM
Registered User
Thicker skins allows you to use lighter density wood while still increasing strength.

I've built Q500 wings that were under 15 ounces complete with servo, that handled Nelson speeds without issue. I never had to use more than carbon laminated between the skin and core, and center section glass cloth for reinforcement. Never had a failure, but had numerous wins and held the last 428 Q500 National Record set by a non-molded composite airplane.

Careful wood selection and careful application of glue will do more to save weight and give the needed strength no matter the construction method.


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