It's so nice to know that if I just follow the plans, it's supposed to work! But inquiring minds want to know anyway:

I'm following the 71" Astro Hog plans that I've blown up to 89.75"
My nearly completed 17 1/2 lb model has the Moki 2.1 engine with zero degrees on the datum line.( zero up or down thrust as specified, but I added 1 1/2 - 2 degrees right thrust too) With the datum line level and at 0, I have the stab at an attack of +1 1/2 degrees to it, and the wing at +2 1/2 degrees to the datum line as well. (as specified) If it makes a difference in the whole picture I'm describing to you, I have cut the wing dehydral in half too.
(Think it was 12 degrees dehydral down to 6. I don't remember)

Also: I might be up another 1/2 or 3/4 of a degree higher on the wing too then the plans specify (+3 -3.25 total degrees instead of maybe the called for +2.5 degrees total.) Yep, you do all that fussing during construction, and then you put that laser meter on to find you still got off the mark somehow when fitting the wing to the cradle !!!

Anyway, what effect do these angle of attacks have on a bird? What charachteristics does it cause, versus if it didn't have these incidences? Is an extra +1/2 to +3/4 degree on the wing a biggie?

-Randall

Not a biggie. What you've effectively done is build in more decalage and more downthrust. If you work the balance point back for better stunting then you'll need some down trim on the elevators for sure. Not a lot but it'll be noticable. And the tendency for the nose to come up will be damped by the extra downthrust in relation to the new wing angle. But in the end what you've done is no worse than many other Astro Hog builders have probably done and they all flew fine.

Bruce,
The extra 1/2 degree of positive incidence I might have added by accident to the wing was just an added question. The main question and what I'm curious about is what the + 2 1/2 degrees on the wing, and the +1 1/2degrees on the stab do. This is called for on the plans, and I do not know enough about aeronautics to know what they do.

It sounds to me that you have indicated these both produce down thrust?

Hi Randall. The Astro-hog is one of the most forgiving flyers, so minor incidence errors should only require minor trim adjustments. I believe it may have been the first successful low-wing R/C model.

In theory, down thrust is required for symmetrical airfoils, while flat-bottom types require little or none, since they produce lift at zero angle of attack. Your extra wing incidence may well be equalized by the positive stabilizer, which is strange, because most conventional aircraft (tail at the rear) require some negative incidence on the stabilizer, to counteract the pitch-down moment of the cambered airfoil. One full-scale light airplane required nine degrees negative stabilizer for straight and level flight. Some of our older aeronautical fellows probably know which aircraft that is.

Bruce; are you related to D B Matthews?

First you have to forget about the fuselage center line except as a purely arbitraty reference. I'ts the angles between the airfoil center line, the stabilizer center line and the engine thrust line that are important. So let's look at these.

First, the relative angle between the airfoild CL and the stab CL is known as the decalage angle. In this case it's the stab angle of +1 1/2 minus the wing's +2 1/2 to 3 (depending on what you find you've built it at) for a decalage angle of -1 to 1 1/2 degrees. You WANT this angle to be negative (as in stab negative relative to the wing) or you will need to add some up elevator to artificially increase the angle so the model will be stable in pitch. Now how MUCH positive is up to how you like to fly. Pattern models apparently fly at very close to zero or AT zero but they would need fairly constant minor inputs to maintain pitch heading when truly at zero. You probably don't want it that close. The angle you have will probably fly quite nicely but you'll also want to do the "dive test" as used by sailplanes to fine tune the CofG and resulting elevator trim so the model is stable without being TOO stable and making it hard to perform certain maneuvers. So that's decalage in a nutshell.

The downthrust angle is a little harder. I like to consider it relative to the airfoil CL but others like to consider it relative to the stab CL with the wing angle then being positive and the thrust line being negative. Yet others like to "split" the decalage angle so the wing is slightly +'ve and the stab slightly -"ve with the datum line sitting at a happy "0" (whatever that is).

And just to confuse the issue of geometry if you have a design that has a LOT of positive incedence in the wing it has the effect of LOWERING the thrust line at the same time it increases the downthrust angle. It does this by altering the angle that the fuselage flies at. See? An example of all this is Hank Cole's old time rubber model designs. Frank Zaic, in one of his yearbooks, determined that the 6409 airfoil gave it's best performance at +6 degrees of Angle of Attack. Now you must realize that the airfoil is still pointed nose down during the glide but the AIRFLOW is hitting the leading edge at that magic 6 degrees as the model settles earthward. So Hank set up many of his later designs with the wing sitting at +6 degrees relative to the fuselage center line and didn't add in any additional downthrust relative to that center line. His models all LOOK like they're diving during the glide but the intent was that the fuselage is pointed directly into that oncoming 6 degree airflow so there is minimum fuselage drag. But this setup automatically provides 6 degrees of downthrust. As a result it's a toss up whether the flier needs to add a tiny bit of down or UP thrust to the original design when it comes time to trim the model. Not that it will ever really BE upthrust but just that it's up relative to the fuselage centerline.

See how confusing it gets? Any officially trained aerodynamics types out there care to shed some light on this?

Hi Bruce;

It appears that you have been getting a practical education in aerodynamics since 1965, and that is probably better than most "official" education.

The French have another meaning for their word "decalage," and that application has a lot of dynamics, too. Are you familiar with that principle?

Do you know which production light airplane required nine degrees negative stabilizer?

Did you ever notice that Piper Cubs fly with a slight tail-up attitude? Perhaps Hank Cole had a hand in their design. With the incidence Randall has, his fuselage may appear to fly slightly tail high, also.

You might say that's merely coincidental, and as you pointed out, the relationship of the wing and stab incidence determines pitch stability, so Randall's Astro-hog should fly just fine.

Oh, I'm sure it'll be just fine. A 1/2 degree or so isn't going to make all that much difference on this type of model. If it was something more competition oriented in the finely tuned way that most of the events drive the designs into these days then it would be a much bigger issue.

Nope, no relationship other than perhaps some common blood WAYYYYY back in the past. The spelling with 2 T's isn't that common though so there may be something closer than I think but he's not on my Christmas card list or anything.....

And there ARE a few full sized aircraft that seem to defy logic. I don't know which light plane you're talking about but I do know that the Fokker triplane looked like it had negative decalage. Or perhaps it's the thick flat bottom airfoils that make it just look that way.

Guys,

You know....when I sit here reading all that tech talk and specs discussion you two had flying back and forth between each other, it had me singing that Wizard Of Oz Tune to myself. (-That tune performed by Ray Buldger the Scarecrow.) But I do now understand the general drift of what you are explaining, and how these charachteristics work.
Nice to know I can rely on you smart guys. They sure didn't teach me this stuff in my High School Shop Class!
l
I have an even tougher question for you: It seems my extra +1/2 to +3/4 incidence didn't screw anything! How'd that happen?!!
(-not having to tear up and correct what I just completed !)

Thanks,
Randall

It didn't botch anything because there is no "perfect" setup. Most of us fly our models quite happily as long as the setup is within a range that we find comfortable. That range gets very narrow in the case of contest requirements but for the average sport flier it's a lot broader than most realize.

In your case it's more than likely going to amount to a "tempest in a teapot" in terms of it's effect. You may find that if you end up moving the Center of Gravity back to sharpen the aerobatics of your new toy that you need more down elevator trim than you would have required if the angles had all come out perfect but that's about the total outcome.

Some day after flying it for a while you might just try shimming the wing to the correct angle with a bit of plywood under the leading edge and, after re-trimming slightly, see if it flies any better for your style. For normal flying you won't notice a thing but for some attitudes, like wingovers or trying to fly knife edge, you may notice that it's a bit different. Not neccessarily better but different.

If you'd built if with a 2 or 3 degree error then I'd be singing a different tune because that would take it well out of the acceptable range.

That help?

Hello Bruce,

I was just kidding with you when I was asking "How come I don't have to tear everything up and redo it". Murphys Law must have been out to lunch this time. Lucky Me for a change!

Thanks Again. Appreciate all the information and detailed input.
-Randall

The light aircraft with lots of decalage was (is) the Piper Vagabond. It was designed to be the entry-level everyman's airplane, perhaps a successor for the Cub. It was introduced about 1947, when most thought private aircraft would become as common as automobiles.

I had the pleasure to fly one on a few occasions. With it's short nose and side-by-side cabin, it was quite an attractive airplane. I found it was not as desirable as the Cessna 120 and Luscombe Silvaire, which were its comtemporaries, but they cost a lot more than the Vagabond.

I've collected some three-views and data with the idea of modelling the Vagabond one of these days.

Interesting and educational thread.

One thing Randall mentioned that wasn't commented on was the change he made to the Hog's dihedral. He halved it.

I did this to a low wing sport plane once and the result was negative coupling between rudder and aileron. This was evident in knife edge when the plane wanted to roll towards its' belly. The effect was slight and a little oposite aileron mixed into rudder fixed it.

Later on after a mishap requiring a new wing which I built to the designer's specs, the plane no longer exhibits this adverse tendency.

Do you guys think that my experience was a normal result of less dihedral on a low wing airplane?

Thanks,
PAW

After reading my own post, I realized I got it backward. The plane wanted to roll toward the canopy requiring me to mix in aileron in the same direction as rudder.

I guess this makes sense when you think about how a rudder only trainer banks with just rudder input. Rudder only trainers have lots of dihedral.

Hi PAW; You are correct about rudder-only requiring more dihedral. It's the dihedral that makes the plane bank with rudder only. The original Astro-hog had lots of dihedral, for roll stability, since it was the first (successful) low-wing R/C aircraft. With modern radios and servos, less dihedral is required (in the hands of an experienced pilot).

This is not unusual, though. If you look at most low-wing personal aircraft, Piper Archer, for example, you will see that they have quite a bit of dihedral. The wing tips are up about cabin level.

Happy flying!