Scratch Building Project - Need Advice!!! - RC Groups
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Feb 25, 2002, 10:20 PM
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Scratch Building Project - Need Advice!!!

I've been flying the wings off of my TM, but have been wanting something to fly when the wind's up a bit. I spied an old sailplane wing in the basement and got an idea (that's dangerous!). It was a double dihedral wing, so I cut the end section off of each wing, joined them, and wound up with a span of 39". It's a tapered wing and I put around 4-5 degrees of dihedral in it for stability. The fuse is my own design.

It appears that the airframe will weigh about 8-9 oz. complete. I want the airplane to have ample power; I'd like to be able to fly it leisurely at around 1/2 throttle, then be able to pour the coals on when I want to. I'm thinking a geared 400 would do the job, but what ratio? 2:1, 2.5:1, or more?

Any recommendations or opinions on how to power this bird would be appreciated.

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Feb 26, 2002, 08:05 PM
Registered User
Try two Astro 010!!!
Just kidding.
Nice scratch plane.
Feb 27, 2002, 01:36 AM
Faster Faster!!
RCFlybry's Avatar
Not sure about geared speed 400's only used them as direct drives on pylon racers but Wow! I love that design. Keep it up it's looking really good.
Feb 27, 2002, 06:31 PM
Registered User

Motor & GB ordered!

Thanks RCFlybry,

I just ordered a Graupner 2.33:1 BB Gear Box and a 6v speed 400, along with a couple props. Hopefully, this will power it adequately. I've got the wing and tail surfaces covered and am installing servos now. I'll post more pics soon.

I saw an E-Flite Maxx 25 esc at Horizon Hobby for $29.95. That's pretty good for a 25 amp esc! Has anyone ever used one of these?


Feb 27, 2002, 06:58 PM
Registered User
I had one once and it made my Rascal crash!!
Feb 27, 2002, 07:16 PM
Registered User

Two words: "Pilot Error"
Feb 27, 2002, 09:01 PM
Registered User

Getting Closer!

Here's another pic of my scratch built project. Wing and tail surfaces are covered, and servos and pushrods are installed. I'm awaiting the power unit to finish up the front end of the fuse.

A couple observations regarding scratch building: First, it's more expensive than buying a kit or ARF, unless you have a LOT of extra goodies laying around the hobby room. It's amazing how the little trips to the LHS add up!!! Second, if (when) I crash this plane, I'll take it much more personally, having spent so much time on her. Lastly, it's really rewarding to see it come together, and I'd highly recommend everyone try scratch or kit building sometime in their hobby life.

Feb 28, 2002, 12:30 AM
Heli Bouncer
Looooeeee!'s Avatar
That is a nice looking plane... but... Do you think you have enough dihedral for a low-winger. It may not turn so well if there isn't enough. Gosh and you already covered the wing too..
Maybe you can fit strip ailerons onto the trailing edge.
On the sp400 with the GR1700 2:33 gearbox, try using an APC 8x6, as the pitch speed will be close to what you want with this sleek of a plane. Good Luck!

Feb 28, 2002, 06:15 PM
Registered User
Hey Looee,

Yeah, the dihedral is pretty much set, but I appreciate the concern! If this thing flies even half decent, I may retrofit alerons. Right now, my goal is to get her into the air and back down in one piece!

I have an 8 x 6 prop coming and I'll try it first. I have no idea how this plane will perform, but I hope I can handle it!!!

Feb 28, 2002, 08:23 PM
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Ben Lanterman's Avatar


Your dihedral angle is too small too work well. A wing with that dihedral would be useful only on a cabin configuration.

You mention that with the glider it was a double dihedral configuration. The reason was that it needed that much to work OK. Also keep in mind that the glider is probably a shoulder wing configuration with a thin fuselage. Cl/beta (rolling moment due to side slip) is a function of basically two things in your case. The dihedral angle and the position of the wing on the fuselage (along with the fuselage depth). You have a deep fuselage and almost no dihedral which is the worst case.

Your airplane probably won't roll with rudder well enough to be controllable. You should use at least 8 to 10 degrees, perhaps with polyhedral also. In general if you look around and survey the number of low wing, deep fuselage, rudder controlled airplanes flying today, they all have significant dihedral. And there will not be too many of them.

Years ago when there was a class of aerobatic airplanes controlled by rudder only and rudder-elevator control (known as Class I and Class II) I believe over a number of years of designs only one low wing airplane was used in competition and its dihedral angle was so severe to be pretty homely.

It's time to hinge the ailerons and assure yourself a nice flying airplane. I am flying a series of profile F-4 models that duplicate the tip dihedral similar to the full size only I have enlarged the tip area and dihedral a lot. The center of the wing is flat. Even with sweepback (enhances the dihedral effect) they will not turn with rudder only. The best looking turn is with the rudder coupled with the ailerons through transmitter mixing. It makes low speed high angle of attack maneuvering better.
Feb 28, 2002, 08:52 PM
Registered User

OK, now I'm scared!


You're scaring me! I just checked the dihedral again and I probably have 5-7 degrees, although in my picture it looks almost flat. I'm not sure what you mean by fuselage depth. Please explain. I really don't want to install ailerons just yet. Would enlarging the rudder help?

Mar 01, 2002, 10:08 AM
Registered User
Ben Lanterman's Avatar
Hi Scott,

That's me, spreader of fear. Kidding aside, in your case the depth of the fuselage is the distance between the top of the canopy and the wing.

Think of it using two airplanes. A Piper Cub type of model and yours. The Cub will have a lot of "pendulum" stabililty. Imagine the lift vector of the wing is always up from the center of the wing/airplane up through the canopy even if it is banked. If you break the lift force down into a horizontal and vertical component and multiply the vertical component (located at the center of the wing but pointing up) times the distance between the vertical component and the CG (which is low because of the under slung fuselage) then you get a restoring moment, it will try to go back level (I am not too sure I followed that!).

Now visualize a low wing model in a similar bank. The CG is above the wing due to the deep fuselage. With a bank angle now the moment produced is destabilizing and the bank angle will increase. With an increase in dihedral angle it causes the moment arm to decrease. The dihedral needed is one that will make the moment restoring, made the moment arm go to zero and to have the opposite sign.

Increasing the rudder throw will not change any of this, The rudder effects only yaw (and roll like the example below).

Controlability is another thing. The ability to roll when the rudder is applied is due to a aero parameter: rolling moment due to sideslip angle, Clbeta. A cabin wing airplane with dihedral usually has a large value for this. The low wing with no dihedral has very little of this, even changing sign.

With the Cub when you give rudder command the airplane yaws (beta) and then the Clbeta term kicks in and you get a nice roll and results in a pretty nice turn. The pendulum stability will restore the airplane to level when the rudder is released.

With the low wing no dihedral when you give rudder command the airplane yaws (beta) the Clbeta term (which is low or the wrong sign) kicks in and you don't roll much if any, might even reverse. The lack of pendulum stability will cause the bank angle to increase.

The answer to your problem can be fairly easy. Make a profile model glider in smaller size with the dihedral angle you have now. Keep the fuselage profile scaled fairly accurately. Balance it so that the vertical CG is the same as your model's CG. Do this by turning the fuselage on its side and balancing. This needs to be accurate.

Toss it and see what happens. This test is better than all my words put together. The dihedral that will make the glider roll stable and able to respond to a bank angle by self leveling is the dihedral that your model should have.

Good luck, this stuff is fun,

Mar 01, 2002, 05:35 PM
Registered User

A different view

Hi Ben,

You're very knowledgeable when it comes to aerodynamics. Where did you learn about that?

I understand what you are saying about the pendulum effect and the inherent stability of a high winger, but I'm pressing on with this design for now. I have seen other planes (not many!) of a similar configuration and am naive enough to believe it will fly. I've posted another pic from a different angle. Let me know if this changes your opinion at all.

Mar 01, 2002, 05:55 PM
RPV builder & operator
Pierre Audette's Avatar
Yep, it may be tough to get it to turn. If you do go ahead the way it is, you may get it around by yanking & banking. Pull up a bit and stall turn. Basically use gravity to help make the turn. Makes it interesting flying!
Mar 02, 2002, 03:51 PM
Registered User
GG you're done!!!!!!!!

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