gosk8ing

Hey,

Just scratch building a chipmunk and have used a NACA 2415 for the main wing and a NACA 0010 for the tail plane. I've set the main wing to 0 degrees. What angle should I put the tail plane at? I was thinking somewhere between 0 and -2 degrees.

Also, for the engine thrust line I was thinking 1 degree right and 1 degree down. Does that sound right?

Cheers

buzzard bait

For a scale model I would go with the scale wing incidence, which is usually a little positive relative to a datum line that parallels the fuse. Otherwise the plane has to fly at a bit of a positive angle, which is draggier, and it affects the angle at which the plane lands. Also, wing incidence relative to the fuse is quite noticeable, so you run the risk of marring the look of the plane if you change it.

For the stab, you're taking a guess at what will work without having to trim the elevator. Generally, same incidence as the wing seems to be the best bet. Even with the stab above the wing, the air flow around and behind the wing probably gets diverted downward enough that the stab is going to be a little bit negative relative to the air flow.

If the wing is a little positive relative to the fuse, then if the engine is at zero relative to the fuse, it is negative relative to the wing, and you have a bit of downthrust even if it doesn't look like it. Downthrust and right thrust are guesses, and I'm sure you won't hurt anything with a degree of each.

Jim

AA5BY

Any chance that the stab can be set in a cradle that would allow slight shimming later?

gosk8ing

The plans are drawn from a 3 view I have. It comes from "Airplane Model News" from 1950. The main wing, the tail plane, fuselage reference line and the thrust line is all set to 0.

Chad Veich

My Feedback: (60)

I have a Ki-61 Tony of my own design which is set to 0-0-0 and it flies just fine. A degree or so of down and right thrust in your Chippie may help on the ground a bit and certainly would not hurt in my opinion. Don't forget to throw in a degree or two of wash-out as well.

Lnewqban

gosk8ing,

Your wing will have CL=0.22 at zero degree.

If that suffices the lift that is needed for level flight at cruise speed, then, your fuse is going to look and fly horizontal.

That NACA 2415 may give you a downwash of -2 degrees, which may be enough to push the stab down to compensate for nose down moment and static margin and keep things level.
If not, and you install the stab at zero, some elevator trim will solve the problem.
For the next built Chipmunk, you will know the proper angles.

gosk8ing

Thanks for the advice. I think I'm going to leave the hori stab and thrust line level with the fuselage reference line and have the wing at +1 or +2 degrees. Haven't decided yet. 1 degree looks like a far bit.I'll also have to include some washout because the t/c ratio is less at the wing tip.

buzzard bait

Now that I see the drawing I would just go with the full scale set up. If you put in more positive incidence you are probably going to have to use downtrim in the elevator.

Jim

AA5BY

I agree with Buzzard Bait. Because it is a low wing, it is very easy to shim the back of the wing for some positive if it is needed. Another thing to remember is incidence numbers of 1-2 degrees are talked about on WWI craft, biplanes and Cubs and what have you but the ball game changes with planes like the chipmunk that fly fast. For example, I've an old D&B .60 size P-51 with a HB .61 that pulls it as they say in Texas, right smartly. The wing is backwards meaning that it pins in the back and bolts in the front because of the scale radiator. Just a little tensioning difference on the bolts makes a significant trim difference at the speeds it flies.

I'd be hesitant to put 2 deg in any plane that flies fast.

highhorse

My Feedback: (2)

+1

Because it isn't symmetrical, the wing's gonna generate lift, even at 0 degrees.

Fwiw, I would build it at 0/0. That will be pretty darn close.

Lnewqban

I also agree.

Having the lift coefficient curve shown in post #6, it is not difficult to make an estimate of the wing angle for regular level flight.
Nothing too exact, but better than a blind guess.

CL = (64.4 * weight in pounds) / (0.075 * (cruise velocity in ft/s)^2 * wing area in sqft)

For my 0.40 planes with wingload of 20 ounces/sqft, I have calculated CL = 0.14 ~ 0.17

Consider that washout will reduce the AOA towards the tip, hence the CL value.

This thread explains typical CL values for RC models:

http://www.rcuniverse.com/forum/m_87...tm.htm#8704122

Downthust may not be necessary, but some right thrust always helps with taildraggers.

gosk8ing

Now looking back at the 3 view and pictures of the chipmunk, it looks like the 3 view has been simplified. +1 degree would look more scale. Still haven't decided yet. It looks like the plans use a different airfoil at the tip rather than washout. I think I'll use 2 degrees of washout.

Thanks again

ARUP

Here is a neat pic from the book 'The Globe/Temco Swift Story' by Stanley G. Thomas. The incidences were changed to get the fuse aligned with the airflow. I had a Balsa USA Bristol M1c which was the opposite in that it flew tail low. I had to live with it or do drastic 'surgery' to correct! It was a fun flying airplane, otherwise. For your Chipmunk you might consider putting the wing on tubes with adjusters to 'tweak' incidence after test flights.

buzzard bait

It sounds like you really want this to come out right, but don't entirely trust the 3-view. If you're going to go to all the trouble of working up your own plans and building from scratch, then I suggest getting more sources for 3-views. What everyone who is experienced at doing this says is that 3-views can be shockingly different. So collect what you can and then do exactly what you just did...compare with photos (or visit a museum).

Bob Banka has lots of 3-views. I think it would be well worth getting a few. Got to the website, download the catalogue, and look for it under De Havillland and DHC (not DH). He's got at least half a dozen. http://www.bobsairdoc.com

If you go with some incidence in the wing, consider matching it in the stab. The full scale surely had an adjustable stab anyway, so pre-adjust it. Keep us in the loop. Nice project. I have an Airsail kit, and this discussion makes me think I should pick up some 3-views too.

Jim

buzzard bait

One more thing...Banka has a code that might tell you the source. That way you can avoid paying for another copy of the MAN one, which is probably one of the ones he has.

Panther45

My Feedback: (4)

Hey there

I am the guy from dream-rc-airplanes and have done several chippers for customers. I looked back in my files and I find the wings were set at 0 to the datum and the stabs were also 0. The last one did change airfoils to the tip and it flew with no elevator trim with the wing at 0 degrees and 1.5 of washout in the tips.

My thoughts were that the washout was only outboard of the MAC (mean aerodynamic cord) and that changed the lift of the wing in that area. Since the full scale was so aerobatic, Art put in the washout to be sure he did not tip stall and get a wild rotation at slow speeds.

The gas model stalled straight as could be and did not show any evidence of needing incidence in the wing. I flew it straight done at idle and it did not pull out of tuck so I am thinking the setup was just about right

Hope this info helps

pimmnz

I believe the real Chippie wing had progressive washout and section change from root to tip. The tip section is actually slightly 'undercambered'. Perfect for learner pilots. If you want to aerobat your model you might like to change things a bit.
Evan, WB #12.