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Old Aug 05, 2002, 03:32 PM
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Scratch Build design, need suggestions

I'm pondering about building a scratch built, at about 120". I want the plane's wingloading to be about 4-10 oz per sq ft. I thought about some other designs like the Allegro Lite, but I really want something original... I've got the general design scratched onto a piece of paper, and its the cross between a full-house/ pitcheron plane, and a floater. I wont release the name I thought for it until it's built (It's too good! ). I don't really have dimensions for it, but I would like some reccomended airfoils for the wing and tail feathers. I just need general ideas and nothing too advanced for the design. 90% pilot ability, 10% plane. You may have seen my post in the HL forum, I gained structural ideas there and a few others such as wingloading, versatility, etc. So, what shall it be? I heard of the MH-43 (I think..), but Isn't it best for higher wing loadings like 10-17 oz/ft.? I'm considering a Mark Drela Airfoil...
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Old Aug 05, 2002, 09:37 PM
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Punta Gorda, FL
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I'd be curious to know how you plan to implement flaps (implied by "full house") on a pitcheron wing.

How do you plan to deal with adverse yaw without aileron differential?

Airfoil selection depends on considerations of wing loading, planform chord distribution and lift coefficient range. There are several ways of getting a nearly elliptical lift distribution over a wide range of angles of attack using blended airfoils, washout and taper. Airfoil selection is dependent on the specifics of the wing design.

A more specific statement of design objectives is required for a rational design approach.
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Old Aug 05, 2002, 11:09 PM
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Maybe I used the wrong word...pitcheron.. Where would adverse yaw come from? There's adverse roll with a V-tail, but why couldn't I use aileron differential? Why wouldn't I use aileron differential? I know that this isn't necessarily going to have full house ability, it wouldn't have any problem with controlability.

Pitcheron, hmm... I may be using the wrong word, but this is how it works in my head. The whole wing gets rotated to act like an aileron. You could have aileron differential on a full moving wing! If there were adverse yaw from having the whole wing move, I have a rudder... I'll come up with some dimensions, but I know that I don't have to be too critical for a plane to fly right. I am more concerned about the airfoil selection related to the lift, w/l, and all the other important info. Next post I make will relate to dimensions, w/l (already stated), etc.
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Old Aug 06, 2002, 05:04 AM
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Design

Adverse yaw is proportional to span divided by airspeed. Rudder can be used to overcome adverse yaw but, on a large slow plane like you are proposing, it will take a lot of rudder power at slow speeds. Aileron differential would reduce the rudder power needed. If you try to apply aileron differential in a pitcheron wing by rotating one trailing edge up more than the other trailing edge down, the average decalage of the wing will be reduced. When the average decalage is reduced, there will be a strong nose down pitch introduced. This is one of the reasons pitcherons are usually only successful on fast slope soarers of relatively short span.
Since the rudder compensation for adverse yaw varies with airspeed, it will take a lot of pilot work load to make coordinated turns. With the model circling in a thermal at near the limits of vision, coordinated turns will be even more difficult because you can't see clearly how much rudder compensation is required.

To see how taper, twist and sweep affect the lift distribution and, in turn the induced drag, with changes in angle of attack see:
http://aero.stanford.edu/WingCalc.html

To compare airfoils for your configuration and wing loading see:
http://soaring.cnde.iastate.edu/calcs/frames.shtml

To see how your paper design will perform before you spend time and money building it see:
http://my.athenet.net/~atkron95/pcsoar.htm

See:
http://www.charlesriverrc.org/articles_modeldesign.htm
for many helpful articles and programs to help you do design work.
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Old Aug 06, 2002, 04:11 PM
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Why I forgot about differential on previous models, I don't know... I'm running through the programs you posted, they're helping. I felt that not having ailerons and flaps would reduce the amount of weight on the wings/wingtips. That would result in more thermaling/spinning stability. The design also employs a high dihedral compared to other planes of this size, making it more inherently stable. Here are the initial specs, more will be coming once I really start to narrow down things:

W/S 124" (two 60" built-up wings, 4" center mount on fuse.
Wing root chord 10.34"
Mid wing chord 8.28"
3/4 out on wing chord 6.21"
Next 15 inches is semi eliptical, similar to F3B ship or F5B
lightly upturned wingtips

Dihedral with one wing level to horizon is 16 degrees
Leading edge is 15" behind tip of nose
L/E of V-tail is 43.45" behind T/E of wing

1/4" higher than wing
V-tail has 90 degrees of separation
each is 20" long
base chord 6.2"
light rearward sweep
semi-eliptical w/ lightly downturned tips (maybe)

wing has fuse blended into wing
tail feathers also blended into fuse
fuse is slender!

I think that the decalage could be kept where I want it by mixing the elevator with the pitcheron roll. The tendancy to pitch down would be compensated. The adverse yaw would be increased with the dihedral, so something needs to be done. I could opt for an inverted V-tail, but now it sounds draggy by needing all the compensation... I'll get somewhere with this.
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Old Aug 06, 2002, 04:21 PM
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Airfoil selection for wing

this is my first try with the simulator, tell me whats good, and what's not good:



I'd figure the purple, blue and light blue have their advantages.

(If image doesn't show, right click image and select show picture)
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Old Aug 06, 2002, 04:29 PM
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With AR put in as 15, Sweep 0, and taper .4-.6, and twist 0, is this good?

Wing Analysis Program
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Old Aug 06, 2002, 04:57 PM
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Design

Your wing planform without twist or sweepback will result in a very efficient lift distribution over a wide speed range. There may be a slight tendency to tip stall but that can be overcome by using an airfoil with very gentle stall characteristics like the S3021.

The image comparing three airfoils didn't come through. Which three airfoils did you compare?

I stand by my warning about poor handling with pitcheron roll control in your configuration. Use contest balsa, tapered spar caps and tapered shearwebs to save weight at the wing tips. The weight saved can be allocated to ailerons. Aileron span should be 40 to 60% of the wing semispan and 20 to 25% of the wing local chord.

Your model will be easier to fly in thermal circles with 10 degrees of equivalent dihedral per side. The long tail moment arm will also contribute to spiral stability. See Mike Garton's soaring column in the September Model Aviation.

Your tail area seems a bit excessive on that long tail moment arm. You can save a bit of drag and weight by reducing the area about 20% and increasing the included angle from 90 degrees to about 105 to 110 degrees.
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Old Aug 06, 2002, 05:07 PM
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Here is the image, It loaded before, but now it wont... This will load. I guess I'll skip the pitcheron and go for ailerons in that case, as well as everything else. I'll leave the pitcheron alone for now. I read the article, very good. I originally thought about using 19 degrees, but saw how much 16 degrees was in the column. I also figured that the adverse yaw would be even worse with more dihedral. I wouldn't have a problem with more dihedral now. S3021, I don't think that's in the image...
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Old Aug 06, 2002, 05:14 PM
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Airfoils

I still can't get the image. Please, just name the airfoils and I can take it from there. Please reread my previous post. I edited it since you last read it.
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Old Aug 06, 2002, 06:47 PM
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Clark-Y_B
E231
S6083
SD7032_D
SD7037_D

When reducing the area of the V-tail, where should I reduce the area? Preserve the Aspect Ratio, reduce or increase it?

Using the formula for finding the coefficient of lift in page 111 in MA, I came up with .699. Basically .7. So, does this mean I look for an airfoil which has the least amount of drag at the coeffiecient of lift at .7, as well as learing about which airfoil has a better or worse stall characteristic?
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Old Aug 06, 2002, 07:20 PM
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Airfoil

If you use any of the airfoils you have listed you should do something about tip stall because they all have poor stall characteristics at the low reynolds numbers associated with the wing tips of your planform. You could greatly reduce the taper from half the semi span to the tip or you could add lots of washout in the tips.

A better solution would be to use an airfoil like the S4233 or the S3021 which have much better stall characteristics and will work with your present planform. The S4233 has the advantage of greater spar depth for stronger, stiffer wings. The S3021 has a flat bottom from the spar aft which makes it far easier to build accurately. If you use the S3021 you could thin and decamber it toward the tips just like the Allegro Lite or Bubble Dancer wing using the AG 3- series airfoils. This would save weight at the tips and add more tipstall margin. BTW, the AG3- series airfoils will give similar (and maybe even a little better) performance to the S3021.
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Old Aug 06, 2002, 07:24 PM
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Check my last post that's been edited, if you haven't already.
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Old Aug 06, 2002, 08:38 PM
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When reducing the area of the tail there are tradoffs. A high aspect ratio tail will be a little more efficient but it will cause higher torque loads on the tailcone of the fuselage. With the long tail moment arm, the torsional stiffness of the fuselage can become an issue. So, the choice boils down to a question of what you design the tail cone of the fuselage for.

You need to consider the entire speed range of the model in selecting an airfoil.

Thermals lean downwind. So, working a thermal in the wind means the plane will end up down wind and the harder the wind blows the farther down wind it will go. Also, the harder the wind blows the more difficult it will be to get back to the field. This makes it highly desirable that the airfoil have low drag at lift coefficients in the range of 0.1 to 0.3 or 0.4. At high wind penetrating speeds the parasitic drag of the fuselage and tail are a big component of the drag budget. A good airfoil selection will do less for the wind penetration if the tail is over sized, the fuselage crossection isn't minimized or the plane isn't streamlined.

The airfoil also needs low drag at high lift coefficients (0.8 to 1.2) to minimize the sinking speed when circling slowly in thermals. In the low speed range the drag budget is dominated by the induced drag which is more than half the total drag of the model. The induced drag is proportional to the square of the lift coefficient and inversely proportional to the aspect ratio. Getting all this sorted out is best done in a performance prediction program like PC Soar. High maximum coefficient of lift airfoils need high aspect ratio wings. Lower maximum coefficient of lift airfoils can do with lower aspect ratio but require lower wing loading to achieve good minimum sinking speed. Such airfoils tend to be thinner and put greater demands on the design of the wing spar for strength and stiffness.

To quote Don Stackhouse,"Everything depends on everything else." This is only a slight exageration.
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Old Aug 07, 2002, 01:06 AM
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I will likely use the S3021 as per your suggestions, it sounds good. I just got the PCsoar on my computer, dusted off my DOS skills, and I've got it figured. I like how all the info is organized in it however old it may be.
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