HO-229

How does one determine the angle of incidence for a scratch built design?
Does the designer need to partially build/estimate the weight/velocity of the aircraft first?
The full scale aircraft has a 4 degree root cord incidence?

AERORICH73

Hi: My aircraft design training tells me that the angle of incidence is a measurement taken between the wing cord and the horizontal axis of the fuselage. With the horizontal axis level, a wing incidence meter is attached between the leading and trailing edges of the wing(cord length) when the wing is attached to the fuselage. A more indepth discussion can be found in Andy Lennon's RC Model Design manual.

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highhorse

My Feedback: (2)

I think he's asking how to determine what the incidence SHOULD BE during design of a scratch built, not how to measure what it IS.......

The answer to your Q, ho-229, is yes. Perhaps someone can point you to some software???

Bad_Daddy

If the full scale has 4 degrees, you already have your answer.

highhorse

My Feedback: (2)

Sorry, but no.....not so simple. Apologies for calling you out on this, Big Daddy.[&o]

It's completely dependent upon the C/L and and anticipated cruise speed at a given weight. The target incidence is the one that produces X lift to fly X weight at Y speed over a Z wing. Exactly NONE of those variable are the same as the full-size and the ratios between them will be different as well. Not even an exact scale wing will produce an apples-apples comparison due to the effect of reynolds numbers.

IMHO.........

da Rock

Almost everything said before is true, but there is a bit more.

You basically do need to estimate or decide upon all the parameters needed in the formulas. Your second question suggests that you already know something about the formulas because you've listed some of what they requier. But then you also need a bit more.

For one thing you need to decide on the airfoil you're going to use and then find the plot or charts for the sucker. And then work them out for your wing design (since they're usually a report of the test results for the profile, not a wing) and that includes span and size (one place Reynolds effect comes into the mix) and a few others. Sound like an impossible task? Well, in recent years a couple of labs have produced really decent model airfoils and published their work. So something pertinent does exist. Finding it actually is the impossible task sometimes.

So yes, the designer needs to work out his design to know sizes, overall weight, and the speed range he expects to encounter. And then make a couple of choices.

But your 3rd questions suggests that there might be an easier option. If you're designing a model of a full scale, you can simply build it scale. If it might be judged in competition someday, you'll benefit from that possibly. And our models are very forgiving. They actually fly with almost any airfoil. And very few modelers have the skills to duplicate any profile with much accuracy, so a lot of this is moot.

What I know for sure is that it's fun to come close. Using a profile that gives the general impression of the full scale and plugging it into the fuselage within my ability to duplicate the full scale's incidence has always produced a pretty good flyer. What has often been the real problem has been pitch control using the scale numbers. The airfoil and incidence is no problem compared to that one.

And if fidelity to scale isn't a primary consideration, there really aren't any real problems.

BTW, have you started figuring out the tail? moments and areas?
And what's the model going to be?

HO-229

Da Rock,
The aircraft is a C2A Greyhound 1/8th scale "true to scale"
Full scale areas are as follows:
Tail= 165.13 sq ft
Wing= 700 sq ft
Flap volume (total)= 34.23%
Aileron volume (total) 12.1%
Root chord airfoil 63A216
Tip cord airfoil 63A414
Vertical stab (outboard) 63A212
Vertical stab (inboard) 63A212
Horizontal stab 64A412 (inverted)
Dihedral (centerline to tip, 65% cord) 2deg 30'
Aspect ratio 9.29

My concern stems from other models that I have built & I did not get it right & the tail dragged, in doing so I had to change the thrustline

Thank you everone for your thoughts and advise

Regards
Dave

da Rock

Then the airfoil plot should be relatively easy to find.

What I'd suggest is looking for one of the recent model airfoils that look similar and use it and it's supporting data. Or make an assumption or two about the NACA ones basing the guessing on the model ones.

We operate in less favorable Reynolds and it's often expected that we'll have to fudge for things like less lift generation and such.

And one thing to consider is if you design for detachable wings, some of those arrangements allow for trimming wing incidence. Bolt on wings that can be slightly pivoted on mainspar tubes for example.

da Rock

Sounds like a fun project.

I have a friend from highschool who flew a similar a/c years ago. Interesting story about the 4 rudders and the navy.