Pattern airframe design theory and discussion
 

Ok, it's been a long time coming, but I think maybe it's time for a thread like this. In case you haven't noticed, there's been a recent ground swell in personal aircraft design. It's nothing new, it waxes and wanes. Right now it's on the upswing, and this is a great opportunity for every last one of us to learn and share knowledge. I'd hope we can remove the mystery and voodoo, and share our own thoughts about the question that drives new designers: why. We have a wealth of experienced people here, and I believe we have the opportunity to do something good.

In addition, I would like to lay a few ground rules for this thread.

1- no flaming. This means me especially, I'm the world's worst. Although I do it usually in a sarcastic and teasing manner, it has caused some hard feelings and flat out made some people angry. For that, I sincerely apologize. Troy and Tony, and others I know I have rubbed the wrong way: truce. I don't have to agree with you about everything, and you don't have to agree with me. But we should be able to voice disagreement, tease a little. and still avoid open conflict. I will give you the benefit of the doubt, and I expect you to do the same. I would however ask that any opinions be clearly stated as such, and if you believe it's fact, please back it up with something substantial other than personal experience. Personal experience falls under opinion. Can we do this, or do we just want a war? Please, keep it civil. Feel free to post as much and long as you wish, but check ego at the door and bring honest opinion and fact. All of us.

2- ask questions. What is the advantage of a double tapered wing vs a straight trailing edge? What is the purpose of thin wing tips? Why ist he rudder post slanted? How do I minimize pitching in the design? Admittedly while most of this is trial and error and varies somewhat from one plane to teh next, there are similarities and theories. Ask, learn, share, implement.

3- no political hijacking. Keep your agenda to yourselves.

4- share your design if you feel like it. Got a pic or drawing? post it. Ask for critique, and pleae those that are experienced, be dead honest and temper it with solution. "That's a dog and will never work" is unacceptable.

I don't expect this to be an explosive thread, but it's just an open door. It's up to you to walk through it and utilize something potentially educational and enlightening. I'd love to ask some good FAI pilots why they do some of the things they do, and get an honest opinion. Without feeling like they'll just be insulted because I asked in the first place.

Well, have at it or ignore it, it's up to you.

-Mike

RE: Pattern airframe design theory and discussion
 

Ok I'll start with a few generalities. These are my opinions, which I base in personal experience with both models and full scale aircraft. When something seems constant to both, it's worth forming an opinion.

Pattern planes are the ultimate balance. They are a trade off. if you want stability, you must give up some small degree of manueverability. That's pretty much a time proven hypothesis. It's bordering on law, and in full scale, it is law.

There is and never will be a perfect pattern plane. Anyone who tells you different is either misinformed and doesn;'t know any better, or is trying to sell you something. This comes back to my first though, that everything is a trade off. Every time you tweak some thrust, turn the incidence adjusters, add some mix, you're making one thing more nuetral and another more unstable. Balance is the key.

Set up: If you fly 100 people's different planes, you'll feel 100 different set ups. Pattern planes are a very individual thing. As an example: I have flown 2 airplanes that were set up by Troy Newman. (or at least that's what they told me, proudly I might add). Not picking on Troy, but everybody knows him so it's a good example to use. I think everyone would agree that Troy is a very good pilot right? To me, those planes were unflyable. I don't mean a little off, I mean "oh crap let me get this thing on the ground". After some adjustments on throws and expo, and a touch of mix, they flew great...to me. And I do fairly well in competition. I don't fly FAI, but I do fairly well. in both cases, the people in question said thier planes then flew much smoother. As a side note, niether of them fly FAI either. The moral? What works for one person in one class doesn't work for everyone. And that's not a knock on Troy ( I feel I have to add that disclaimer lately). That's just a fact. I'm positive if he flew my set up, he'd hate it.

Allright, feel free to either ignore this thread or enjoy it and use it. have fun, and be civil and learn, I hope to.

-Mike

{edited for better wording, I suck at that sometimes}

RE: Pattern airframe design theory and discussion
 

And now I'll ask a question.

Rudder counter balance. Some say it's the way to go, others say it causes wierd things in certain manuevers. I'd like to know more about this, pros and cons. As much detail as possible. I know a few people have done some experimenting, and I hope they will answer with thier findings. With any luck, maybe we can find a good answer. The top designers probably already know this answer, but if you look, some top end planes have them and some don't. So what's the scoop? Anybody?

-Mike

RE: Pattern airframe design theory and discussion
 

The finer points of pattern plane design is something that many of us would find very interesting but and maybe I'm wrong, would have a great trouble actually contributing to. Folks could give an opinion based on personal preference, but I would think most would have trouble documenting any serious data. That's not knocking the post by the way it's just reconfirming your statement that everyone has their own personal quirks when it comes to setting up- trimming, CG, incidence, throws and flying style etc. What suits you, doesn't necessarily suit another. It's not the airplane at fault, it's the person behind the sticks. I feel if people, myself included, really could get a grip of that we'd be seeing quite a different picture.

So why do we get so many new airplanes? Well the schedules change, evolve would be a better term and designs, in theory change to for fill these new demands. I say in theory. There is only pilot/designer that, in my opinion has truly evolved his planes and thats CPLR and the Alliance-Synergy-Oxalys sequence. The great majority of the others seen to come out with a completely new plane, every couple of years or so. Surely things aren't changing that fast... so why the new designs? Because it's good business.... and we, as (hobby) pilots like new planes, new toys and new reasons to blame our bad scores on an "out of date" design; And that's even better for business!

If any of the worlds top ten pilots where to turn up at your average pattern contest equipped with a 4 generation old airplane, how do you think they'd fare.They'd probably win... by a long margin! I saw something similar with my own eyes 2 years ago at the Norwegian Nationals. The multi time, ex champ coming out of hiding and bringing an old Wolfgang Matt designed airplane, 12 years old or so. The P-03 was never flown better during that event...The snaps (and remember this airplane was never designed to snap (of course it was, all airplanes are) where things of utter, jaw dropping beauty. When asked what made this old design snapped so well, I was told that, at first it didn't...I just work and worked on them. It's not the airplane that makes the maneuver, it's the pilot. Enough said.

Huummmm bummer! Now where the catalog of the latest, new and exciting airplanes!

RE: Pattern airframe design theory and discussion
 

Alrighty.....I'm working on a Excelleron/Typhoon/Epsilon close to 2 meters want to be pattern plane. I'm ready to cut the cores for it and have a question about wing tips.
The wing tips on the Epsilon are very simple and light....one 40 degrees cut and an 1/8" cap.(pics).
I'd like to know if this kind of tip has bad habbits or should I stay with the traditional shape?

RE: Pattern airframe design theory and discussion
 

Is there a formula for the spacing of the wing and hor. stab? How do you know where on a 78"ish plane to put the CG. or hole for wing tube?

RE: Pattern airframe design theory and discussion
 

Doc -the slashed tip is a very old design trick -and properly used - works great.
It simply acts as dihedral
Most new pattern designs are very similar to stuff done over last 15 years - the large cowls don't really add anything- in the way of drag -tho they can screw up prop flow.
The best part is the increased torsional rigidity at front of the model
For those still using the shaker engines - this is a worthwhile - for an electric motor - not of any benefit.
However, the deep fuselages with increased forward lateral area, are good for the sequences and the lighter and stiffer the airframe , the better.
rudder hinge line sweep - a little does not hurt -
thrust line - matter of choice as the maneuvers stand - a bit above true wing C/L - good enough.
All of these, added together - tend to reduce stability - NOT making the plane unstable but making it more neutral.
The reduction in weight and improved thrust is critical
This allows the model to operate at lower angles of attack for any maneuvers.
Said differently, more constant AOA and speed - produces less difference in trim drag --which produces a plane that stays in the line desired. Up/Down etc..
And yes, we have tried all this for our own amazement .
Finally - nothing beats lots of proper practice time as the largest influence in flying pattern.

RE: Pattern airframe design theory and discussion
 

Dick - when are you going to write a book, or should I say treatise, on this subject. You've written loads on this subject over the years; including columns in at least 2 magazines (that I can think of) and numerous articles which have appeared in the K-Factor. Put it all together - we will buy it!!
I think Dick has more emperical expertise in the area of model aerodynamics than anybody out there. He's never been afraid to experiment and has taken the time to test lots of variations on theme. Just my opinion... :D
-Will B.

RE: Pattern airframe design theory and discussion
 

My two cent...a picture is worth more than thousand of words....

RE: Pattern airframe design theory and discussion
 

Great stuff so far.

Tph1, wing tube location is hit and miss. Honestly you can get in the ball park, but what type of engine you use, the shape of your wing, where you mount your servos, the construction of your tail, all of these things and a thousand more go into it. Then you have to fly the plane and see where it flies the best as far as balance. So on a totally brand new design, I'd say use a similarly configured model as your reference and then go from there.

Remember that moment=weight x distance, so one ounce in the tail will be equivalent to about 3 in the nose, as it affects balance. The balance position of a wing is somewhere between 25% and 33% of the total length of the plane, again using a number of factors that can push it a couple of inches either way. ALSO keep in mind that the further your stab from your cg, the more longitudinal stability you get with a given stab surface area, and the harder it is to snap.

These are VERY general statements, and meant only as a possible place to start and tweak from there.

As for why we constantly "need" new planes, I'm in total agreement. There's not a thing wrong with the one I'm flying now, but I want a new toy, and hope it flies even better. I know some properties I'd like to achieve so I designed them in, and we'll see if it works. Plus I just like doing things different to prove it can be done. In my case, it's a good dose of new style retro construction.

I also noticed Naruke's new 2 meter plane has anhedral stabs like the old Curares/Tipos. I'd love to hear from Dick about that subject, it was all teh rage for a while then *poof*...now Naruke is doing it again. What's the real scoop?

-Mike

RE: Pattern airframe design theory and discussion
 

Also I'd like to add how much I've learned from long conversations with Matt K lately. Yes there are some numbers and formulas to use, hopefully he'll chime in on a few things.

-M

RE: Pattern airframe design theory and discussion
 

If anyone has access to Lockheed or Northrop/Grumman software I have access to a super computer (really).

In all seriousness, personal experience and "feel" are very important. Clarence (Kelly) Johnson, could tell you from "feel" whether a design parameter was correct - from an airfoil to an engine intake duct.

Not trying to be a smart a*&. Just trying to encourage all types of input and discussion.

Best regards and good flying,
mike
New Patterniac

RE: Pattern airframe design theory and discussion
 

I agree Mike, after a while, you can just sort of look at it and at least get a gut feeling. It's developing that sort of feel that's key for people new at this, and they (we) all need a starting point, and then the guts to actually DO IT. The rest comes from the tweaking.

-M

RE: Pattern airframe design theory and discussion
 

Hi Mike,
Good idea with this thread! I had been thinking about asking about this very topic. How about some of your experience with the evolution of your Tempest? I'd like to know what flight charachteristics in the Typhoon you were unhappy with and the modifications you made to the design to improve them. What were the results of the modifications? This would give us a peek at real world design changes and effects, rather than theory.
Thanks,
Steve

RE: Pattern airframe design theory and discussion
 

Allrighty Steve, I'll give this a shot.

The T2k was a good plane in it's prime. QQ flew it to 6th at the worlds in 99, before he became "the man". So, it was definitely a competetive plane in the top levels of FAI when it was designed. QQ is also an awesome pilot, we can't discount that.

However, as the sequences evolved, so did the designs. I don't fly FAI so I can't tell you all of the specifics, but a larger plane with a smaller wing was needed. Well, it flew the newer sequences easier, so that's the way things have gone.

The T2K had an enormous amount of wing and stab area, and was a full 2 meters square. It had a major pitch to the belly, but it flew the new sequences better than say, a prophecy. What it didn't do well was snaps and spin breaks. With that much wing, you really had to wind it up in a snap, and that means you have to work 10 times as hard to stop it on point. It always seemed to be pointed in the wrong direction after a snap, the yaw was ridiculous to get it to snap well. With that much mass away from the CG at the wingtips, it made it hard to stop on track. In short, it was hard to get it going, then hard to stop it.

Also with that much wing area, spins breaks were difficult without a large amount of positive wing incidence. It just wanted to harrier in place. If you jacked up the incidence to make it break better, then it's pitch coupling and down lines became erratic. Also there are now inverted spins in FAI, and positive incidence makes this even more difficult to get a good break. getting a good break is hard enough without fighting the plane.

That is what I identified as fixable with the same fuse. The other problem is it needs more side area with the knife edge and point heavy schedules both in FAI and masters now. That is something I couldn't fix with the Tempest, and why I moved on, but let's stay with the Tempest.

First I moved the wing up to get it closer to the thrust line. This made it roll a little more axially and require less aileron diff. next I changed the wing to a 74" wingspan, and moved teh center of pressure more to the center of the plane away from the tips. This makes the plane snap like no other I have flown. It's effortless. Also I gave it more sweep, which translates to more dihedral and better stability inverted. I also swept the tips forward some. The down side is that for it to behave as I want, it has to be flown a tad tail heavy. Otherwise it does try to pitch to the canopy in down lines. Everything else is groovy, and I could play with thrust and mix and go that route. It may be a better approach, I think. But it snaps so well and tracks fine, I mean I 10d and avalanche at my first contest with a grand total of 9 days practice on the advance sequence, it's just that easy.

Also I reshaped the tail and gave it a bit of sub fin, and movedt he stab up to try and neutralize some of that pitching effect. It did just that, but as I made the plane more tail heavy, the pitching became noticable again, so I mixed it out with about 4%. To me that's acceptable. Over 10 isn't, and 30 CERTAINLY isn't.

I also gave it more area for the elevators and ailerons to make it more responsive. In order to keep the plane tracking right, it requires super high end servos to nail centers. This is not a problem for me, I use them anyway. And I love the response I get from the plane if I want to play, so I left it that way.

Anyway that's how I got to the Tempest. I had tried a standard T2K with clipped wings, and I liekd it so that's what made me go this route. Today though, because of many things and availability of parts being primary, I decided to break and do something totally new.

I hope that doesn't sound TOO nuts, that's what I was thinking at the time. It seems to have worked well, but if I am in error I HOPE somebody will correct me, that's what this thread is for.

-Mike

RE: Pattern airframe design theory and discussion
 

This is my dream's thread....

Thx for take your time, sure I'll learn more every time...

Mike, the way you got to the Tempest is amazing; I was always concern about wing and stab up or down move and effect on plane....

Doc, a friend tell me that wing tip is like trainer tip; it make the plane stable, as Dick say, make dihedral job

tph1, I was trying to get a formula to CG placement too without find nothing about; I guess it's a very personal parameter for every plane and designers have to play with it to match the plane they want, all around the engine; I'm newbie to design, just trying to learn, and maybe I'm making all wrong, but with Extra I'm building, must move CG rearback about 2" for balance, from wood prototype CG placement; I think it will loss longitudinal stability like Mike say and will be very "wild" tail for me (better than unflayable), but since it's a project, I take the risk and see what happend; I'll post the results.

I have Typhoon 2000 and Patriot 3D plans, and CG are in diferent place; distance between wing and stab are diferent too, not to talk about aifoil, engine-wing spacing, double tapered wing vs a straight trailing edge, etc.... too much numbers to work for me at this moment

RE: Pattern airframe design theory and discussion
 

Mike, I think this was discussed on the nsrca list a few months ago, but my experience confirms that making the stab smaller makes it more sensitive/effective near neutral.

RE: Pattern airframe design theory and discussion
 

Adam, do you think so?

It could be 100% correct, I don't know yet, trying to learn. In my theory, if you take a stab of the same size and move it away from the CG, it will make the plane more stable. However if you decrease the area and make the elevator area more sensative, you will get more response while retaining the stability. Whatcha think? Sounds like a winner to me. It sounds like you would effectively increase the stability while simultaneously increasing the manueverability, IF you got the numbers right on your areas and moments. In theory that is.

See, that's why I LOVE this stuff, nobody knows for 100% sure yet, it's all a new frontier, and we're all watching/learning/listening......always.....

Ok here's one for ya. We have a plane, and it's nice and stable in level flight. It has anhedral stabs and we're about to pull into our first high G vertical. We have a crosswind. We start back on the elevator stick, but something happens. What is it? We begin to enter a stall turn and slow the plane. All of a sudden the plane begins to act strangely, porpoising with the wind when we slow to speed. What do we do?

This is a good and relative question because it is based off of true performance, I would love to know the input of better pilots than myself.

Anyways, I love this stuff. Keep it all coming all of you, especially you FAI guys (like you ADAM!!!!) cuz I want to ask you all some pointed questions soon, thanks a billion for even spending time with us!!

-Mike'

RE: Pattern airframe design theory and discussion
 

I wish Nat Penton would post in here....the "Voodoo Guru" allowed me to fly his plane (image 1) after the LARKS meet last Fall. It is the most neutral thing I have ever flown...period. He is a true aerodynamics thinker and tinkers constantly with new theories. His use of an anhedral stab is partly to keep at least some area of the stab in clean (or cleaner) air in all angles of attack. This design also includes flow straighteners that reduce the effects of torque, etc. This plane has zero engine thrust...not sure of other incidence numbers on stab and wings...they are probably at or near zero. Oh yeah, he is also a master at building light...70" (approx.) wingspan, 78.5" length, 8.5 lbs., OS91fx. The voodoo express is strangely stable in all axes.

I hope to build one of my own sometime this year...see 2nd image.

Some people are all about parasitic drag????...I thought, what if we were to mate this with nature's aerodynamic model of the raindrop? See 3rd image.



Where are you Mr. Penton?

This is a great forum.

-Mark

RE: Pattern airframe design theory and discussion
 

Okay. But let's not forget that aesthetic appeal is also a design criteria.

RE: Pattern airframe design theory and discussion
 

so what your sayin' is.:D....

RE: Pattern airframe design theory and discussion
 

It looks more like a cruise missle than an airplane? ;)

I've seen it fly though, very cool. Just odd looking.

-Mike

RE: Pattern airframe design theory and discussion
 

This is a little bit off topic and is more a root cause thought than a definitive design idea. IMHO, the single most important thing that impacts the pattern aircraft designs is the maneuvers that they must perform and the space allowed for them. Looking at the post above you all may be headed for some real goofy looking aircraft if pattern continues on the same path it is on now. No pun intended, but is it possible that the pattern fraternity has somewhat “boxed” themselves into the current designs? Would it be helpful if the pattern rules were influenced so you could have more creative designs, e.g. more space (dump the box), larger more graceful maneuvers and faster more beautiful looking aircraft? I’ve always felt that high performance R/C aircraft should (and some still do) perform more like full scale jets than an Extra or a Yak, etc. If you go to a full scale air show and watch the spectators the most boring thing for them is to see the Extras and Yaks doing their little tight in patterns with smoke. I’ve seen people fall asleep! Just some out of the “box” thinking! Change is good and always brings out the best creativity.

RE: Pattern airframe design theory and discussion
 

Great stuff Mike, thanks for taking the time. I have a couple questions about your Tempest mods:

Moving center of pressure inboard....this is done by increasing the chord at the root and/or decreasing at the tips?
Swept the tips forward.......Why? This moves the COP forward?
Increased sweep for more dihedral and inverted stability..... Not sure I understand. Are you talking about swept back as in jet fighters? Seems opposite of swept tips forward. Or does this mean greater difference between root and tip chord?
Why don't all pattern planes have the wing located on the center line/thrust line for axial rolls?

OK, thats more than a couple of questions! I want to make sure I understand so I can compile this info into my "Little Book of Truths"

Umm, just one more teeny question. Do the majority of pattern planes use airfoils from the same family? ie. NACA 0012, 0013, 0014, etc? Or are there as many different airfoils as there are airplanes? How about changing airfoils from root to tip?

Oops, thats even more questions!
Sorry, I'm starving for knowledge!

Thanks for the info and help.
Steve

RE: Pattern airframe design theory and discussion
 

8178,

there are a couple of things a play here. first, you would not want to fly so far out and wide (big box) that the judges cannot see your manuvers and judge them correctly. second, is the design of the sequences. with sequences that are complex and have many manuvers within a figure the desire is to give yourself plenty of time between manuvers so that you are not rushing into the next one and present them with a nice even pace. make sense? sorry to jump in on this one, but why do some people associate pattern airplanes with speed?

what I am interested in with this thread is learning what combination of design alows for the slow downline, draggy flight. I have heard many comments the fuse design has nothing to do with it (or very little) but what does? is it wing design, wing loading, wide fuse, wide flying surfaces (tail surfaces), 3 or 4 blade props? or just what is the combination of all of these that work? then I guess that if it is too draggy or light then it won't snap properly? interesting reading and its been fun following so far.

Hubb

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