What if I want to design an airplane?

What if I want to design an airplane?

Here is some information I have found online, most of it has the original site addresses with the information.

OK, so where would I start.

I have a brushless motor that pulls about 9 amps. I have some foam board that is 20" by 30". I want a 'trainer' type airplane to do casual or sport flying.

One site said to keep trainers at about one pound per square foot of wing area. Another site says I should have 50 watts per pound for trainer/casual flying,  and 75 watts per pound for sport flying/sport aerobatics,  and 100 watts per pound for aggressive aerobatics,  and 150 watts per pound for all out performance & 3D.

So for a sport flying trainer I need something around 50 to 100 watts, about one square foot of wing area, and weighing about 16 ounces.

Would my motor work?  Watts = amps x volts.  I plan on using a 3 cell 11.1 volt battery. So my motor at 9 amps using 11.1 volts is 9 times 11.1 or 99.9 watts, that's close enough to 100 watts for me.

What about wing area, what kind of square footage can I get from my material?   One square foot is 144 square inches.  One site said a good aspect ratio for

a wing on a trainer is 5 to 1, that's span to chord.  So for my 30" foam I can easily get a 30" span wing.  With a 5 to 1 aspect ratio it would have a 6"

chord.  That's 180 sq in, or about 1 1/4 sq ft.  So my build weight could be 16 oz to 20 oz.

I think I can do this with what I have.  So what are the numbers I need for the measuremnts I need to make?

My Airplane Design:

 Note: this information is from a Chuck Cuningham design articles, valid site as of 10/24/2013, posted as PDF's at:   http://www.rcuniverse.com/forum/scratch-building-aircraft-design-3d-cad-174/11560594-cunninghams-rcm-design-information.html   A summary of the article is listed below my number calculations.

Pick a wing span of 30", aspect ratio should be 5 to 1 so the chord is 6"  and the wing area is 180 sq. in., the balance point is about 1 1/2" to 2" back

from the leading edge, and an airfoil thickness about 3/4" at 1 1/2" back from the leading edge.

Fuselage is 75% span so 22.5" long fuselage.

Nose length is from back of prop to wing leading edge, should be 20% fuselage length so its 4.5"

Tail length is from wing trailing edge to leading edge of horizontal stab, should be 40% of fuselage length so its 9".

Horizontal Stab area is 22% of wing area or 39.6 sq in.

Horizontal Stab aspect ratio should be 3 to 1, say 'c' is the chord, then 3c is the span of the horizpntal stab, so 3c * c = 39.6.   3c * c = 39.6   3 times c squared = 39.6   c squared = 13.2   c = 3.633   3c = 10.899

So make it 11" long with a 3.75" chord.

Elevator is 20% of the area of the stab.

Verticle Stab area is 1/3 Horizontal Stab are or 13.2 sq in.

Verticle Stab is about as tall as it is wide, so 3.75" tall and 3.75" wide, adjust shape, height, width to look right and keep the area.

Rudder is 1/3 to 1/2 of the verticle stab area.

To keep things lined up make the elevator and rudder hinge line also line up.

That's about what I need to build my trainer.

Do my fuselage calculated numbers add up?

With a 22.5 fuselage length does it add up to a 4.5 nose lenght + 6 wing chord + 9 tail length + 3.75 horizontal stab length? Not quite, the pieces add up to 23.25, that's .75" more than originally calculated fuselage length.  What's the difference?  Well the nose length is from the

back of the prop which may stick out in front of the fuselage.  And the horizontal stab might stick out past the end of the fuselage.  I don't think I will

worry about it, it's close enough for me.

Did I build it?  Yes I did.  Bad balancing and bad piloting put it into the ground in about 10 feet.  The engine mount got knocked off so it's in for

repairs.  I am totaly new to RC flying and here I am trying to build my own design, maybe not the best idea in the world.  So in the mean time I have also

built the FT Flyer.  I am currently waiting for electronics to arrive for that and then I will try that first before I go back to my own design.  I have

ordered from both HobbyKing and HeadsUpHobby.  The HobbyKing order evidently is from China and will take up to 45 days to arrive.  The HeadsUpHobby order

will be here in 3 days and shipping only cost $5.  So far I think I like HeadsUpHobby.

----- Summary of Basic Airplane Design Article:

Wing:

Balance point 25% average chord len back from leading edge, no more than 33% back from leading edge Aileron area 10% of total wing area. Aileron length 8 times its width. Strip ailerons are 1/8 the width of the cord of the wing. Barn Door ailerons are 1/4 the span and 1/4 the cord. Landing gear axle even with leading edge of wing for tail dragger. Landing gear main axle just aft balance point for tricycle gear.

Airfoil:

Airfoil high point 25% back of leading edge. Airfoil thickness 12% to 15% of chord, including covering.

Fuselage:

Fuselage length is 75% wing length. Nose length from prop to wing leading edge is 20% fuselage length. Tail length from wing back edge to horizontal surface front edge is 40% fuselage length. Fuselage height 10% to 15% of fuselage length.

Horizontal Stab:

20% to 22% of wing area. Has a 3 to 1 aspect ratio. Elevator is 20% of the area of the stab.

Verticle Fin and Rrudder:

Verticle stab is 33% of horizontal stab. Verticle stab is about as wide as it is high. Rudder is 1/3 to 1/2 of that area.

Thrust Line: 2 to 3 degrees down thrust. 2 to 3 degrees right thrust.

A thrust offset of 3 degrees is about 1/4 inch over 5 inches.

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Summary of Basic Biplane Design Article:

For biplane design all calculations and percents would be the same as for a monoplane - fusalage length, horizontal stab, verticle stab, etc. Then some of those calculations and percents have to be modified because it is a biplane. This describes what differences there would be for a biplane and how to figure them out.

Determin wing area for a 'normal monoplane'. Increase this area by 25% ( biplane wings are 80% effective ) Make both the wings equal or make top wing 58% of total area and bottom wing 42% of total area. Chord to span is still 5:1 to 6:1 - so -   5 C = S and C x S = area so 5 x C x C (C squared) = area. Ailersons are 12% of total wing area. Ailerons are on bottom wing only. Top wing is forward of bottom wing by 25% of 'total' wing chord. Total wing chord is from leading edge of top wing to trailing edge of bottom wing. C.G. is 25% of this total wing chord back from leading edge of top wing. Wings are seperated by 'a total wing chord'. Horizontal stab is 17% of total wing area. Stab length to chord is 3:1 ratio. Elevators are 25% of horizontal stab area. Vertical stab is 1/3 horizontal stab. Rudder part is 1/2 vertical stab. Fuselage length is as if it were a monoplane of 'normal wing area' - i.e. 75% of the mono plane wing span. Nose length is 20% fuselage length. Tail length is 40% fuselage length. Wheels are placed under leading edge of top wing. Engine has 2 degrees down thrust and 2 degrees right thrust.  - 3 degrees is about 1/4 inch over 5 inches.  - 2.5 degrees is about 1/4 inch over 6 inches. Wing thickness is 15% to 18% of chord. Wing Dihedral ( wing tip higher than wing root ) is about 2.5 degrees. Angle of incedence can be zero degrees - if wanted use 3/16 to 1/4 rise for 5.5 inch chord.

----- Battery flight time calculation from an article I read:

To calculate flight time for a battery you have to do a simple calculation. The calculation is: battery mah times 60 divided by motor draw in milli amps, motor draw is usually listed in amps so you need to convert amps to milli amps

ex. 9 amps = 9000 milli amps.

  Example:     battery is 500mah     Time of 60 min     motor draw is 9 amps     (500x60)/9000     500x60=30000     30000/9000=3.33     so your flight time would be 3.33 minutes     (give or take a couple minutes)

----- Where should I place my components?

Don't forget Archimedes “Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.”

Once you know the CG, and the components you will put into the plane, and the weight of the components, you can start to do some calculations. Like with a teeter totter, weight 'A' times length 'A' equals weight 'B' times lenght 'B', and the CG is your teeter totter fulcrum.

----- A Home made wind tunnel can be made from wine bottle boxes, a small house fan, and a kitchen scale.

http://hsc.csu.edu.au/engineering_studies/focus/aero/aeronautical/Wind_Tunnel_final1.html

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