RE: Pattern airframe design theory and discussion
 

To slow the thing down - add wing area -(and stab area)
this added wetted area also can produce more induced drag -which is what really slows things down
a bit more fuselage does little in that respect.
as for being too light to snap- No way .
get out the Depron and do some fast building and testing .
we have done stuff with flat wings - that fly - honest -as smoothly as any pattern plane and snap in the blink of an eye -and hold heading perfectly.
The little Knuffel design is really a flat plate rip off of current Europan FAI pattern stuff.
The past year spent building and destroying these things has been the most informational period -regarding what really happens - I have ever spent.
Now I have to go back and redesign my big stuff- --lower weight - more power - slower flight
Funny but it takes MORE power to fly well at slower speeds .

RE: Pattern airframe design theory and discussion
 

Mike,

What about using split ailerons like the Ultra Stick ARF? For normal rolling, you could use the inboard ailerons to reduce adverse yaw, then on downlines you can split them up and down (I think the glider guys call this CROW) with mixing.

RE: Pattern airframe design theory and discussion
 

klhoard:
That sounds interesting. But would it work well??

RE: Pattern airframe design theory and discussion
 

Well, if you want drag on the downlines, you have to have it on the uplines as well. Using Crow would allow you to get rid of the drag while going up. Just one more switch to keep track of!!!

RE: Pattern airframe design theory and discussion
 

Modern large-bodied, thick airfoiled planes with a 4stroke motor have no problem slowing down on the downlines. On my Mantis with 140DZ I rarely use idle during looping maneuvers (double-I, cuban, avalanche, vertical 8,...). The downside of this being that it takes far more power to fly the upward segments of these loops and the uplines of tall maneuvers than a slippery plane would require. But, that's why we are seeing 160DZs in 10lb planes, right?

RE: Pattern airframe design theory and discussion
 

Eventually we'll end up with a 2M foamy with a 10.9 lb engine up front!!

RE: Pattern airframe design theory and discussion
 

I have never seen a thick airfoil on a modern design. typically they are 12%to 10%
for best efficiency -the thinner the better -
as for the engine - what is used makes no difference - 4 stroke /two stroke / electricmotor - it is the prop disc and low speed of same which adds any braking.
An electric using Back EMF is ideal.
The myth of thicker airfoils adding more lift and drag is simply that - a myth.
First - they have less lift
however on heavy models the lift is more constant at different angles of attack.
The old 3D fun fly models are very numb to angles of attack due to the ultra thick foils
they are also very low on wing loadings - if done correctly.
The thick wings allowed for very light construction due to improved beam loading.
I suspect you will see lighter wing loadings with thinner sections and more relative low speed thrust on future designs.

RE: Pattern airframe design theory and discussion
 

Yes, moving the CP inboard is done a number of ways but first and foremost, by a high taper. Too high of a taper and you get unwanted consequences. Not enough and you lose the effect. I think the Tempest is about as far as you can go without getting too many wierd side effects. It's a 20" root and 7.5" tip. I also used a completely built up structure and used small light aileron servos (9411SAs) and hollowed the tips. The panels are about 13 ozs flying each. Most of that is inboard. All this does is dampens the snap exit. I have noticed the wings getting kicked around a good bit in very turbulent crosswinds, so I know I'm close to the edge of the limit for our purposes. On my new one I went back the other way a bit, 18.5" at the root and 9" at the tip. I used 11% at the root and 12% at the tip. It's pretty much a straight NACA 0011 and 0012 with the high points at about 31%. That's pretty far forward for a modern design. For comparison the Patriot has the thick point around 50%. It's the wierdest pattern airfoil I have seen, and it works.

And yes sweep the tips forward, the aerodynamic center is moved forward as well.

Why don't all planes have the wing/stab mounted on the thrust line? Well, design parameters a-plenty. I'm not the one to ask this question. The only plane I know of that is true 0-0-0 is the Patriot, and it's pretty true in the rolling manuevers. But there are so many more aerodynamic and other considerations, I couldn't possibly list them.

Pattern planes don't have a standard airfoil, they vary from plane to plane. Everybody thinks they have a better idea that works better. Some do, some don't. The Prophecy wing became the standard for a long time, very thin and double tapered (planform). Then the euro stuff became popular, and again things shifted a little. The truth is, airfoil doesn't really matter a lot. The planform is a lot more important, getting the sweeps and areas correct. You do that and you can use just about any airfoil effectively, within reason. Of course if you get too thick, it flies more like a fun fly than a pattern plane.

Arrrgh my brain is frying. Dick is right about thick airfoils, it really doesn't add much drag. Some, but only a small amount of profile drag, and not enough to use it to slow a plane. If you increase the wing area, induced drag increases and there's a better ticket....but then you get into that wierd snapping wind up thing if you go too far. Any wing will snap, it's just a matter of what it takes to make it do so, and even moreso...predictably. The harder you have to push to stall a wing, the deeper the stall, and the harder it will be to come out straight. That's sort of ok for IMAC, but not ok for us. Pattern guys can't even agree on how to score one properly!

A better way to slow down a plane (as Dick said...again...) is prop disc. If you have a large diameter prop turning the same rpm, your down line will be slower if you can keep the engine from winding up. (a large advantage for electrics, then 4 strokes, and 2 strokes require the most work to get that effect).

I wouldn't add that to any book of "truth". I'd add it in the "tested and somewhat proven theory" dept. Notice there is still a NASA test center for flight? Cuz we don't have all the answers. We're only beginning to understand the questions.

This is just some of the theory I'm playing with. In my experience, it's been pretty repeatable so far. But we're always pushing the envelope. Always trying new stuff. Look at the planes now: they'll be totally different in 2010, if it even takes that long. A Prophecy and Typhoon 2000 were the stuff 5 years ago, and look how old they look today. It's never ending, and that's good for us that like to play. I can't say I agree with the construction techniques and the thinking that we all have to fly composite planes made in europe, but I do like the way the designs have gone. I just don't think they should cost NEARLY as much as they do.

But that's just my opinion, I could be wrong......

And still nobody wants to address the anhedral stab thingy.......why was it so popular, why is it gone, and why is Naruke doing it again (and even Nat Penton?)

-Mike

RE: Pattern airframe design theory and discussion
 

Wow -- Lots to digest in here!

I sense a reasonable understanding of horizontal and vertical flight -- and the compromises across thrust line, wing and stab incidence and relative heights. This seems to be governed by centre of lift versus centre of drag etc., and the moments at work can be reasonably easily drawn. Don't get me wrong, I am not suggesting it is easy -- just a little easier to get our heads around -- there is still a lot unknown.

The one I can't construct a mental picture of is the interaction between rudder and wing dihedral - in flat flight but also in knife-edge. I gather the rudder hinge line plays an important role on drag when the rudder is deflected -- and hence the angle of the hinge line and the portion above versus below the stab impacts pull towards belly or canopy in knifeedge, in addition to the combination of thrust line and wing/stab incidences. The centre of lift must have something to do with the aerodynamics of the fuz on its side (which I assume could be quite different to the wing in horizontal flight, and quite different across the thinner models of 5+ years back and the bulkier ones today). But what are the dynamics of the interaction between the rudder and dihedral that lead to adverse and proverse roll on rudder? Any ideas (or a full, simple explanation!!) appreciated.

RE: Pattern airframe design theory and discussion
 

Not really PepsiMike. Aesthetic appeal is just that, and no more. Aerodynamic design doesn't have to be aesthetically pleasing, it's just nice to have. Vice versa is also true. When on occasion we get both a great aerodynamic design that is aesthetic, then its elegant. ELEGANCE, there's a name for you

MattK

RE: Pattern airframe design theory and discussion
 

Hubb, regarding light (low wing loading) vs. poor snap capability, that simply isn't true. I have read some stuff that talked about reducing wing areas and such, (increasing loading) to improve the snapping ability of a design. That is simply wrong thinking. Higher wing loading will be more difficult to stop precisely.

In addition, if the stab area wasn't reduced a corresponding amount to the wing reduction, getting the model to initiate a snap would actually become worse and feel more sluggish.

A lighter model will initiate a snap more precisely and stop more precisely all else being equal and assuming there's enough elevator and rudder control auhority. Ailerons are also necessary of course but even with subdued aileron authority the model will still snap when rudder authority is correct .

regds,

MattK

RE: Pattern airframe design theory and discussion
 

Hi David,

there is a perfect explanation with animation for you: http://142.26.194.131/aerodynamics1/...ity/Page5.html
and there is another good source: http://www.centennialofflight.gov/es...ty_II/TH27.htm

What do you think guys about flying horizontal stabilizer (stabilator) on pattern planes? It can be easily balanced statically and aerodinamically too - your servo will longer lasting!

RE: Pattern airframe design theory and discussion
 

[If you increase the wing area, induced drag increases and there's a better ticket....]

Hallo Mike,

Just a small point. You mentioned that is the wing area is increased, the induced drag increase. This is only true if the aspect ratio goes down. With increas area comes lower lift requirement from the airfoil, which leads to reduced induced drag. The induced drag formula is Cdi = k*(Cl)^2/(3.14159*AR). Therefore, an increase in wing area might even result in less induced drag.

Induced drag increases will only lead to high drag in high g situations where the wing works the hardest ( sharp corners etc. ) I do not know if this is desirable.

I agree that the best braking comes from the prop.

Thanks for a great information.

Regards
Attie

RE: Pattern airframe design theory and discussion
 

Now another issue: a plane, otherwise seemingly perfectly trimmed, is pulling to the belly in knife edge. As a rule of thumb, it worked (almost) always for me as a fix, to lower the horizontal stab on the fuselage. Now I have a question: did you ever try this as a fix, and why? What might be reason this works? I have a theory of my own, but I would like to hear from some expert first.

David

RE: Pattern airframe design theory and discussion
 

David, one of the main reasons a model pushes to the undercarriage in knife edge flight is the relatively low location of the center of pressure (CP), on the vertical stabilizer. The vertical downward force (moment) generated in such an arrangement is too small to counter enough of the natuiral down pitch moment any wing generates. The simple change of lowering the stab has a marginal affect on the knife edge, if that is all that is done.

You may try to raise the CP by doing a combination of the following: increase rudder area above the stabilizer, possibly by adding an aerodynamic counterbalance to the rudder. Add a small dorsal fin at the base of the existing fin. Increase the wing incidence. Increase the downthrust.

If your plane has adjustable wing, increasing wing incidence is the easiest thing to do. Re-trim the stab for straight flight. Then check your up and down lines and adjust as required. Always make a note of where you started so you can return there.

Then proceed with other adjustments as required. Take your time. Dorsal fin addition is simple to try, simply tape a plank on the fin base and fly it until happy.

Good luck

MattK

RE: Pattern airframe design theory and discussion
 

You lost me here... what "natural down pitch moment" any wing generates? We talk about symmetrical airfoils here... no moment at all, as far as I know....

RE: Pattern airframe design theory and discussion
 

And you, me. That's like saying a wing generates zero drag. Or a wing generates no wake. Neither is true. Matters non at all whether a wing is symmetrical

Any low wing set-up will generate a natural pitch to belly. Center of drag is below the Center of Gravity.
MattK

RE: Pattern airframe design theory and discussion
 

Ok, I see now what you had in mind.
I have some doubts, though.
The model is normally trimmed so that this effect, very small IMHO, is fully compensated. A model that is trimmed to fly level hands off has no reason to pitch to belly simply because it was rolled to knife edge. On top of that, my last plane that pitches to belly is almost perfectly mid-wing.
The pitch to belly was always, among my models, most obvious in cap-ish designs (high mounted stab). But ONLY after the application of rudder. With neutral rudder (flying sort of ballistic knife edge for a while), the flight path was straight. After the rudder was applied, the belly pitch started. Must have something to do with the rudder deflection and its coupling to other axes, IMHO.

RE: Pattern airframe design theory and discussion
 

Take the stab away. Where will the wing go?

MattK

RE: Pattern airframe design theory and discussion
 

In my mid-wing plane, straight (or not [8D]).

BTW, I did some searching ... and found [link=http://www.rcuniverse.com/forum/When_to_give_up_on_an_airframe%3F/m_2594052/tm.htm]this stuff on the subject (see the long post by George Hicks)[/link]. Worth reading for sure..

RE: Pattern airframe design theory and discussion
 

I have a question about Ultacote.
The pics show my latest efford in a close to 2meter plane with Gator mount/CF LG/1 servo/pushrod installed and its close to 5.5lbs. If I were to cover it in one color, how much weight would I be adding? (Ball park figures are OK) Plane is 75x75

RE: Pattern airframe design theory and discussion
 

Ball park the surface area of the plane then multiply times the weight of ultracoat.

RE: Pattern airframe design theory and discussion
 

Thanks JVB...that sounds like it'll get me close. I was hoping someone had been at the same stage I'm at now and had a idea of the weight it would add...but this will work
Thanks again LShaver

RE: Pattern airframe design theory and discussion
 

Stop the maddness, lets go back to simplicity. Bring back the Dirty Birdi's and Tipo's. LOL

RE: Pattern airframe design theory and discussion
 

I thought I´d bring this thread to life again after seeing the interview with Chip Hyde at Toledo. The subject I´m interested in is the locations of the wing and stab. Chip mentions in the interview why he prefers the wing lower compared to most other guys and the reason he brings up seems to be a valid one to me as a pattern newbie. Have a look at the interview and please share your thoughts on this subject.

http://67.18.81.100/rcuvideos/magazi...8/chiphyde.wmv

Some further thoughts from Chip in this post: http://www.rcuniverse.com/forum/fb.asp?m=1471502

/Erik