Airplane stability (engineering notes)

[davidchoate]

I have been FINALLY getting a rudamentary understanding of prpeller choice. I read a vaery good post on RC forum about watts per pound, wing area, wing loadind, Figuring out stALL SPEED, AND SPEED AT WHICH PLANE SHOULD FLY, bUT ONE QUESTION. wHAT IS cUBE lOADING?

[David Lewis]

Increasing the wing area gives you more bang for your buck than reducing the weight.
Of course you’d like to do both, but both variables are not on an even footing:

Wing cube loading (or cubic wing loading) = weight/wing area^1.5

The dimension is M/L^3 (e.g. ounces per cubic foot)

Cube loading factors out scale effect and makes direct comparison
of effective wing loading between different size models possible.

A larger model can carry a higher wing loading more easily than a small model can,
so as model size goes down, wing loading must ALSO go down to keep it apples-to-apples.

Note that for small models, wing area is not a precise variable.
Some square inches work harder than others depending on planform
and airfoil, so the raw number by itself can be misleading.

[davidchoate]

Thanks. Another question I have regards Biplanes. I understand the rule of thumb is a propeller should be around 1/4 of the wingspan. Thats for full, and model scale. So on a Biplane how would One figure out the Wing Span in a way to insert it into the Propeller size formula? Also I am curious about multi- bladed props, and once I FULLY comprehend how propellers affect flight performance I will have to understand more because I like Scale models, and most scale Planes use 3 0r 4 bladed props. P.S. I know You maybe noticed the tail appears crooked in The photo of My Model. Don't worry. It was'nt glued yet. And I used MonoKote. You are right. It's heavy. It added 2 oz. to the Plane. I am using a lighter film from now on. The Plane is large enough that 2oz. wont make a big difference.

[David Lewis]

I would just use the biplane wingspan for prop diameter calculations for rubber power because I think the rule is intended to keep torque manageable during the initial power burst.

On electric power, you can go to a bigger prop (because you have a throttle) and in fact on slow flyers, you get an increase in efficiency as prop diameter goes up because 1. disk loading goes down and 2. you have accelerated propwash hitting the wing. A big prop with lots of downthrust reduces the power-on stall speed and increases rate of climb. Downthrust causes propwash to hit the wing at a high angle of attack.

Multi-bladed, scale diameter props work well on electric power. You want torque absorbed by the prop equal to torque produced by the motor at maximum power times the gear ratio: Tpr = Tmo * n
The torque output at maximum power is taken from the motor data sheet.

[davidchoate]

Let Me be honest. I have never been good at math. I failed Algebre, but was Advanced Placement in Physics. Its so pointless, boring, and just not fun to Me. But when I had to apply it to the phsical world(and allowed a calculator) it was not as boring, but I admit that when its time to start finding the wing area to find the stall speed to do the math to pick a motor/prop to suit Me; I'd rather have some one else do it.You must find it enjoyable as You seem to have learned a lot. Is there any ways that I can find some way of making it more enjoyable. ? Or should I just Keep asking some one else to do it.

[David Lewis]

I haven't found a way to make it easy or not boring. From what I've seen, learning math is mostly not fun and is tedious for most students. All I can tell you is keep plugging away.

[TimsCustoms1982]

I'm starting from scratch with no knowledge of avionics or airfoil design or ANY of the math there in, but it is in no way boring to me. It is my desire to be independent and "learn how to fish" if you will, so I can "eat for a lifetime", that keeps me entertained. If I have to ask someone to do it for me, I make sure that they teach me how so I can do it on my own. The net is a wonderful place for learning new things or finding our answers... But, just as it hasn't always been there, it wont always BE there, so learning all I can NOW while it IS there is what makes it fun for me and every time I figure something out on my own I get that little smile to myself that says "HA gotcha..."

[davidchoate]

David. I am starting a $00 series FW109. I figured the wing area at about 96sq. inches. I'm assuming the center section is not counted. It is a tapered wing, and I just kinda guestimated it. Do You happen to know what it is? I dont know why they do not put it on the plan. I understand being multi-purpose kits that the wing loading without AUW is not feasible, but they could at least put Area. Dumas even on their RC kits does not Print the Area or loading , and espescially on their RC models Its a little important to know. And For us math ignorant people. A glossary of the symbols would be very helpful. Thanx.

[TimsCustoms1982]

Hey David,

Wing area is actually a quick one.

You take numbers for ONE wing, you are correct the fuselage isn't counted.

You should be able to measure these numbers easily from the plans.

Take the root cord, add it to the tip cord, and then divide that by 2.

Then, taking that "average cord" multiply that by the span of a single wing.

Finally, double that number.

That is your total wing area for the 190.

[David Lewis]

TimsCustoms1982 formula is correct, but you need to project the root chord onto the centerline so that fuselage center section is included. Lift from B to D is generated by the fuselage due to endplate effect. That's why the fuselage portion needs to be included in wing area calculations. The blue curve marked "load" shows how lift is distributed over the wing and fuse.

moment%2520diagram.gif

You can also take the wing area of the full scale and divide it by the square of the scale factor. (If the scale fidelity of the model is poor this method may not work.) In my "Cantilever wing spar design" thread, I calculated the Messerschmitt Bf-109 kit #401 wing area to be 97 sq in (= .0627 m^2) but I encourage you to double check my figures.

fw 190.gif

I estimate the wing area of the Fw-190 to be 27887 square inches (dark gray region). The Guillow's Focke-Wulf Fw-190 kit #406 lists the scale as 1:16.
27887/16*16 = 109 sq. in.

[davidchoate]

Thanks. Simple enough. But although ythey are "multi-purpose Kits". It seems they should give ypu at least the wing area.

[David Lewis]

I agree but many model airplanes were designed by men with little formal technical training, and in the old days, model airplanes were considered basically toys for boys. Could be the typical buyer was not terribly concerned with scale fidelity, historical accuracy, flying performance or engineering specifications.

Also I've found it doesn't hurt to verify the wingspan, scale, and whatever other data is listed on the box and plans.

[davidchoate]

True. But even Dumas has a newer line of about 1/2A RC kits, and they do not have wing area, weight, or wing loading on their plans. The old 1966 Plans in Guillows kits I laugh when they show the mods for RC. That funny tail Yoke and all. And also David, On biplanes, is it unusual that the top wing appears to be at a more positive attack angle? Hope I used the right term.

[David Lewis]

Dumas models are a little bit ugly in my opinion. Lou Andrews (designer of most of the 300 series) was a professional artist, and it shows in his designs.

[davidchoate]

Yes. I agree. They need a lot of modification to get a good scale appearance, but they are light I think due to the "box Type" of build method.