Well, I am making my first R/C aircraft, and doing it from scratch. Dont worry, I know I am throwing myself in at the deep end, with punctures in my water wings. But feel good, since BMatthews talked me down from making a scale Hawker Typhoon for my first model!

But anyway, the situation is I am making an aircraft to go around my 50 year old 3.46cc throttleless engine. It is supposed to be an advanced trainer, with neutral stability, capable of slow speed but high torque aerobatics. I figured this forum was the best place for my remaining queries on its design.

I will note down the current details of the design, and then point out the things that dont seem to go too well at the moment.

Power Plant:
Volume: 3.46cc (0.21 cubic inches)
Offset: 2 degrees down & 1 right.

Wing:
Area: 3125cm sq (484.3 inch sq)
Span: 125cm (49.2 inch)
Chord: 25cm (9.8 inch)
Aspect Ratio: 1:5
Taper Ratio: 0
Dihedral: 2 degrees.

Airfoil:
Type: Semi-symmetrical
CG location: 30%
Point of Max Depth: 30% (2.5cm above, 1.5cm below reference line (1 & 0.6 inch)
Angle of Incidence: 0 degrees.

Ailerons:
Area: 10% wing area
Dimensions: 50X3cm (19.7X1.2 inch)

Stabiliser:
Area: 25% wing area (including elevators)
Elevator Area: 30% stab.

Fin:
Area: 14% wing area (inc rudder)
Rudder Area: 50% fin

Fuselage:
Total length: 94cm (37 inch)
Nose-Leading Edge: 25cm (9.8 inch)
Trailing Edge-Tail: 44cm (17.3)

Landing Gear:
Front Position: Axle 18cm (7.1 inch) below leading edge.
Front Diameter: 7cm (2.8 inch)
Width: 31.25cm (12.3 inch)
Tail Wheel Position: 5cm (2 inch) fore of tail, & 5cm below reference line.
Tail Wheel Diameter: 4cm (1.6 inch)

Target weight:
1.2kg

My query is the stabilisers and the fin. I have made a rough sketch in autocad of the dimensions fo the key features of the aircraft, and I cant help but think the fin deffinately and the stab probably look a lot larger than is the norm for other models of this type. THe DWG file is [link=http://h1.ripway.com/greenskintau/finissues.dwg]here[/link], can anyone tell me if they think the proportions are right/wrong/how to ammend them?

I dont mind the current layut of the stab so much, but I cant help thinking the fin looks like a joke...Any help is greatly appreciated. My thanks to BMatthews for helping me this far.

One important detail about the horizontal tail is it's aspect ratio. You've not mentioned that. It is worth pondering.

Another detail about the H.tail is it's tail volume.

Those two will give you a very good idea how effective the H.tail will be at providing pitch stability and control.

The aspect ratio of your h.tail will affect both the max coefficient of lift possible and the stall angle of attack of the h.tail.

You can hit the books and work out all those values exactly, or.............

Keep the AR around or above 4 and you'll be safe. I just measured my Tiger2/CalmatoSport and it's got a tail AR of 1:3.5 and that's a bit chunky I believe.

I have a very popular ARF that's just a model airplane, not a semi-scale or such. It has a very low aspect ratio tail. It's almost square. I've built a replacement h.tail for the sucker that has an AR of 1:4.5 and plan to replace the chunky one just to prove something. The airplane will very barely snap roll. I think that's because the chunky tail isn't effective at stalling the wing. While it's struggling to pitch fast enough to stall the wing, it's also BLOWING UP the drag back there like crazy. But I haven't taken the time to retrofit the new tail (been pumping out ARFs like they were going out of style). The airplane is very stable in pitch simply because it's got enough h.tail volume. But that shape isn't very efficient at producing pitch changes. The model is obviously designed to be very "safe" in pitch for the average flyer. And I'd like to loosen up the bird a bit. And I'm sure this will do it.

whatever.........

Tail volume is a formula that gives a value that'll suggest how effective the tail will be. It's actually pretty easy to do the math.

You need to work out or measure 4 things:
Wing chord (actually the MAC)
Wing area
H.Tail area
Tail arm

The wing measurements are talked about all the time. You're interested in the mean aerodynamic chord. Wing area is easy, right. You'll also need to mark on your plans where .25MAC is for the wing. You'll need that to work out the tail arm.

The H.Tail area is the same. And you'll need the .25MAC point for the h.tail too.

The distance between the .25MAC of the wing and the .25MAC of the h.tail is the tail arm. There is more than one way to base this measurement, but the 25% deal works for the overall formula just fine.

TAIL VOLUME = (H.TAIL AREA / WING AREA) X (TAIL ARM / WING CHORD)

Figuring the tail volume is pretty easy. I just ran down to the shop and measured the KYOSHO CALMATO SPORT I'm assembling. How long did that take? 3 minutes?

The h.tail is 143 sq.ins area, and the wing has a 10.8" chord and 659 sq.ins area. The tail arm is 32".

(143/659) X (32/10.8) =
(.217) x (2.96) = .642

You usually want to have something between .5 and .8 for your design.

BTW, the Calmato Sport is an almost perfect copy of the Tiger2 design.

BTW, there is an internet application that ought to be a world of help to you. It'll do a lot of the math for you and give an idea what the dimensions you've chosen will do.

http://www.geistware.com/rcmodeling/cg_super_calc.htm

Ah, I have just got back, and see all that! I have just skim read it, but will give it a more thorough reading and implementation when I get back from the dentists. Thanks!

BTW, I'm fairly certain that I AM THE WORLD'S FIRST R/C hot air baboon.

[X(]

There are some VERY basic "rules of thumb" that produce good-flying RC models:

Choose wingspan, and make the wing with an AR of 1:6. Total aileron area = 10%-25% of wing area

Fuselage length = 80%-100% of wingspan span

spinner tip to LE of wing = 80%-100% of wing chord

Horizontal stabilizer/elevator area = 20%-25% of wing area with AR of 4-6. Elevators = 20%-35% of total horizonatal tail area.

Vertical fin/rudder area = 60%-75% of horizontal stabilizer/elevator area. Rudder area = 40%-50% of vertical tail area.

These are very basic numbers and will produce an airplane that will fly reasonably well. They won't give you an airplane that is good at everything. You can use almost any airfoil on the wing you want. Balance the wing at about 30% of the mean aerodynamic chord, and you'll be in the ballpark. Maybe 1 degree of positive incidence on a semi-symmetrical or symmetrical airfoil. Engine and stabilizer can be 0-0.

Draw these in realationship and put almost any shape of fuselage around them.

For a tail-dragger, just have the center of the wheels at or slightly-forward of the wing leading edge. For a tri-gear plane, the mains should have the wheel centers at about 40% of the mean chord back from the leading edge. You want the balance point forward of the center of the mains so that the nose will stay down on an empty tank. Too far back, and you'll have problems rotating for takeoff. Too far forward, and your model's nose either won't stay down, or it will tend to hop.

There you go. Very basic rules of thumb to start out with your first original designs. Once you have the model flying, you can then start to "play" with the parameters to alter how the airplane handles.

As always, there is a LOT of leeway in the parameters to get a good-flying airplane. In the 60's and 70's, many people who designed models for the model magazine construction articles used such guidelines to be able to "crank out" a lot of designs. People weren't so much interested in airplanes with particular performance as they were in having unique models they build themselves. With a bit of creative imagination, you can make a wide variety of "sorta scale" airplanes...mainly adjusting fuselage profile shapes, and wingtip and tail shapes.

Or you can just copy another design, make minor changes, change the name and call it your own. Design by continental drift.

Bax, do you have a bit more about this. Simply getting the right area isn't the whole thing to get adequate pitch control/authority.

A guy at the field today flew a new design that was pretty badly unstable. It had the appropriate area for the tail. Both stab/elevator and fin/rudder were adequate for sure. But both were almost slap up against the wing. He got about 50' on the first flight. The corn saved it. The 2nd and 3rd flights went about 50' each. He took it home in a bag.

You need something that helps establish the stab/elevator that takes into considertaion how far back to place it.

BTW, full scale designers also take into consideration where the horizontal tail is in relationship to the wing's downwash. The up-down location of the tail and it's resulting performance because of that location really does influence how the plane performs.

This stuff actually ain't simple.

With the fuselage length I mentioned, you just put the horizontal tail surface all the way back. Make the elevator hinge line even with the end of the fuselage. It's really NOT that critical. Some people make fuselage sizes in relation to wing chord. The full length of the fuselage would have 75% of a wing chord ahead of the leading edge, and 1.5-3 wing chords between the wing trailing edge and the leading edge of the horizontal stabilizer.

I checked out a Super Kaos...a "classic" design. Span and length are nearly equal. Wing leading edge to nose is one chord. Wing trailing edge to rear of fuselage is 2 chords, so the airplane is about 4 chords long (based upon center wing chord, since the wing is tapered).

It's not so much the areas and length relationships, but where the model is balanced and how much control deflection you have. With most models, a balance point of about 30% back from the leading edge of the mean aerodynamic chord, and control deflections of about 20 degrees or so are good starting points.

As far as whether the stabilizer/elevator is at the top of the fuselage, bottom, a T-tail, or cruciform arrangement....again, it's not really critical for an airplane that flys well. It will make a difference if you're trying to get a specific level of performance. Many times, it's placed for the convenience of the builder when you're talking about a 'sport' model.

If you take a plane, such as the Great Planes Big Stik, you can move the horizontal surfaces around...at the top of the fuselage, bottom, middle, and so on...even make the rear of the fuselage a bit taller to move it higher...you really won't get a lot of variance in how it flies. Even if you turn the fuselage over and make it a low wing, and just flip the vertical tail surfaces to the new upper side, you won't get a lot of difference. For a good-flying airplane, these things are just not that critical.

Now, if you're trying to get a specific type of performance, then they do make differences, but usually only in nuances. You'll get greater effects by changing the wing, tail, and engine mounting angles to give you differences in incidence, thrustline, and decalage.

One plan that will enable you to learn a lot is the RCM plan, "Scotch Lass". It's a basic box fuselage with wing on top. It builds VERY fast and takes a .15-size engine. I've build different versions of it, using the same wing and tail surfaces, but built high-wings, low-wings, stock, and so forth. They all flew nicely. I kept the same fuselage length and wing and tail placements. Balanced at the same point. I even had one version with a slightly-longer tail. Kept the same balance point. Flew just fine.

Not to beat a dead horse, but the basic parameters all WORK...they will give you an airplane that flies nicely. If you need a plane for a specific purpose, then the tuning begins.

I agree with BAX. I've done a good amount of "TLARing" over the years and the basic ratios that BAX has spelled out DO in fact work amazingly well, and constitute the general layout of 95% of the r/c models out there. There's always a few that go outside the box, but by and large...they don't deviate far from the tried and true.

You can skip a great deal of the secret science, and instead concentrate your effort on building the model STRAIGHTER and LIGHTER and that will make even the most homely looking design fly incredibly well.

This is VERY, VERY true. Light and straight are trumpcards. That's why certain people have been building airplanes for other people and are in very high demand. They've learned how to build light and straight. the fundamentals of model building.

Just like the story I heard of a professional drummer in the L.A. recording scene. He didn't have lightning speed, no flashy moves, no highly-complex rythmic abilities, but he was a master of tempo. He could lock in and hold any tempo. He never lacked for work. He provided the most highly-prized resource other musicians wanted in a drummer: He could set and hold a beat. Very fundamental to good music.

Fundamentals...just like in modeling. You'd be surprised what can fly when it's light and straight.

I was going to title this, "WE DON'T NEED NO STINKING FORMULAS", but figured someone's feelings might get hurt. (but then, the title was so kewl, I had to mention it)

The idea is that we don't need to use formulas to "design". And the way we do that is basically to copy somebody else's design. Or we just build using some basic "ideas". (Don't call them formulas, 'cause we ain't using formulas.) And that list of ideas actually lead us to "create" what? Just what those other things are that can be described simply without "formulas". Hey, that works. To build another whatever.

But what if you actually want to understand what's going on aerodynamically? Why do people want to design airplanes? I guess there are those who really just want to say they designed their own airplane and want the easiest way and figure copying works for them. But what if they really do want to explore. Or to understand. And about this idea of "no formulas"...........

Hey, just make your fuselage 80% of the wing span. Hmmmmm..... And use 4 or 5 for your aspect ratio for your tail. Hmmmmmm....

Say, how does one come up with a value of 4 or value of 5 for an aspect ratio? Isn't an aspect ratio the result of a formula? But I guess you could explain it without having to use symbols and pretend it's not a formula. But isn't an aspect ratio sorta like this: AR = S / C

Is the whole idea to not use algebraic symbols when stealing.... uh, designing your airplane and it'll somehow be a better design effort?

Lf = .80 X Sw
"Your fuselage length should be 80% of the wing span" is another way to talk about that formula.

BUT I'M NIT PICKING, right? No, I'm actually making a point. The point is, that the formula (or description in words) for the tail volume of the horizontal tail actually is worth knowing and using.

Not everyone wants to steal a successful model design, change the wingtips, and call it their new design. Some people want to design something new. That might be impossible or very difficult, but that might be the motive. And they might want to learn something from the effort. Sure, just changing a successful model might teach you something, but what? Use the most basic formula for success and what do you learn? That build something to the simplest formula that's like the majority of existing popular models and it'll fly like them.

Take the formula for tail volume for example. What does it tell you? It not only sorts out some design details, but it also tells the designer what dimensions are important to pitch stability and how. Would you have inherently known that Cw (the chord of the wing) has something to do with pitch stability? The wider the wing chord the less pitch authority the tail will have. Did you know that the Aw (the area of the wing) works the same way? Everyone probably would guess that the area of the tail (Aht) is good for pitch stability, but it's only about as good as the distance it is aft.

Tail Volume = (Aht / Aw) X (Tail Arm / Cw) actually tells you something.

And it actually is good to know for understanding what is happening when your new design takes to the air. And it's good to know if your design doesn't do what you wanted. It gives you both understanding and a way to work out a solution if you have a problem or if you only want to enhance something. Say you don't have the pitch stability you were hoping for. From that formula, you can see that if you increase the distance the tail is from the wing, OR increase the area of the tail, OR decrease the area of the wing, OR decrease the chord of the wing, you can change that lack of pitch stability. And you can run the numbers again and again to get a feel for how much you'd have to do each individual change.

You'll actually learn something. And understand what's what.

We actually don't need no stinking formulas, but they sure are useful if you wan't to actually design. and learn what does what......

Which is what most of the designers I know do it for. I'm guessing that greenskintau was sorta figuring he'd learn something with his effort, but I really don't know for sure. Just trying to help him.

Heh, yeah you seem to have guessed right .
Whilst I have researched other designs, to see what people have done to get what abilities in their aircraft, I am not planning on butchering the other designs, into one or something. That would be a Frankenstein airplane, and I hope to achieve a plane that is more or less unique. Whilst this is probably impossible, given the sheer range of designs made already, but I do want to try.
I am designing this aircraft around an engine I was given. It is a 50 year old antique diesel with no throttle, so I am already pressured into certain desgn aspects (straight wings for control on a glide approach since i will have to axe power for landing etc), so am already limited in the direction it takes me. I do however wish to do all the calculations, work out all the proportions myself. And the designing too, I'm really looking foreward to that.

I plan to study Aerospace Engineering at university next year, so this is basically a vehicle for me getting a hold on the principles early. If it dosent fly for whatever reason, I guess I will have to tweak it and try again. I am teaching myself rudimentary AutoCAD skills to do the design work too. The point is I hope to learn throught this project, and having the working flying model of my own creation that I have wanted to make for years at the end is just an awesome bonus.

So any help anyone can offer me as to the features and how they affect the aircraft, how to calculate the best proportions for my specification, anything at all, I would be more than happy to learn from your (much vaster than mine) experience. I also plan to put up regular updates of my progress so the experts here can tell me off when I accidentally design in thirty tail fins or set the wing loading to 1 tonne/square cm or something equally rediculous [X(]

Thanks for everyones help so far, especially you darock


EDIT: and sorry for any typos that I diddnt pick up on, I think someone who knows more than their fare share about computers has messed with the keyboard functions as every time I type "y" i get "t" amongst other things.

Darock is also right. You don't need formulas, and you do need them. It all depends upon the level you want to work at. If you're into building models that are yours, and yours alone, and not trying to get a particular level of performance, the general "rules of thumb" work very nicely...epecially if you're not too concerned about the "hows", and only want the "what works" part.

Using the general guidelines, you can whip out a piece of blank paper and draw a flyable model in an evening. You'll spend more time on the aesthetics than the actual design parameters. The resulting model will fly reasonably well, and your ARF-only friends at the field will consider you some kind of prodigy or genius.

Again, I have to stress that I only presented some very general guidelines that will enable you to build a successful model. If you're going for an airplane with very specific performance goals, then much more stringent "rules" apply. Study similar models of the type you want to design. Then go from there. Even so, after a bit of study, you'll find that those models will all conform to a set of guidelines that can be reduced to a similar set of "rules of thumb" as the very general ones I mentioned above.

Of course, if you go that route, you'll be copying ideas developed by other people, and you may not have a full understanding of the "whys" of how they work. If you're only interested in a flying model...then fine, just copy parameters and design your own aesthetics. If you want to know the "whys", then there's plenty of study for you to dig into.

It all depends upon what you want to do. The field's wide open....

OH...and be sure to build light, straight, and strong.

..and you can spend an inordinate amount of time re-inventing the wheel to arrive at something that is round, and rolls. Even Firestone (and others) can't lay claim to inventing the tire...only making refinements to an invention that's existed for thousands of years. Try as you may, you will not in this lifetime reinvent the airplane.

Any airplane, regardless of the amount of engineering involved is a nothing more than a series of compromises that work together to perform some basic task. Not every task....just some.

It's important to have an objective. What the design is supposed to accomplish...before you can improve upon anything. Even then...best case scenario....your design will only be optimized for THAT set of parameters and nothing else. This is why fighter jets make such lousy trainers, and Airliners really turn in a pitiful aerobatic performance.

When you set out to do this...plan on building TEN of your designs...each with some alterations because the best layed plans on paper will always leave room for improvement, and there is no software in the world that can predict with certainty how well a particular airplane will fly.

Where r/c planes are concerned...it interesting to observe that variations on the same basic platforms that have existed for 30-40-50 years are still at the forefront of sport model design. Change the materials...change the cowling...add a canopy....remove a canopy....add some area...make this a little bigger, and this part a little smaller.....the basic layout remains the same.

Hmmmm.

Not to poo-poo your enthusiasm for getting in there with both hands and making something better than it was...just be sure you have realistic expectations.

Let's look at that a little closer.

I've had a couple of Ultra Sticks. They flew like Ultra Sticks. I had a CAP232. It flew like a CAP232. My Cub flies like a Cub. I expect the Skybolt to fly like a Skybolt. My P40 flies like a model P40. Not one of those is "the same basic platform".

I'm good enough at flying now that I can tell what a models flight characteristics are. I can tell that they fly differently, and what differs with each and how much it differs. And darned if any one of them was made up with the one size fits all list of simple formulas. But they all fly decently and some fly like magic.

But if I want to get an idea of why the P40's pitch authority is what it is, I have a fairly easy way to do it. And if I wanted to build a P40 version like the later P40N that was measurably different than the P40D (which is what most models are scaled from), I would have a fairly simple way to give myself some idea of how the two might differ in flight. And I'd have a clue before I hacked the first piece of balsa or uncorked the CA. And if I didn't like the clue, I'd know how to fudge some dimensions or areas to get what I thought would be more apt to give me what I was looking for.

Does that work? You betcha. example????? I wasn't entirely happy with the way my UltraSticks stalled going into inside or outside snaps. So I did a bit of number twiddling and could see right away what the UltraStick was lacking and what would probably help get the sucker to fly more like I wanted. So I drew up a different horizontal tail for it. Building a tail wouldn't take an evening but I decided to look around at the existing designs to see if one was pretty much like an UltraStick but had the numbers I wanted. I found it. Bought it. Flew it. And now fly it most of the time. That turned out to be what I consider the ultimate solution to a problem. I was FORCED to buy another model. I cried all the way to the LHS.....

So let's look closely at the idea that all models are the same basic platform. Wait a minute..... we just did....... didn't we?

Got any pictures of your latest design?

Get out a tape measure....and you tell me if most all airplanes aren't based on the same BASIC set of parameters. You can't obfuscate the simple truth that they are.

But...you can believe differently. That keeps the hobby shops in business, and from that...we all benefit.

Most airplanes are designed from a set of performance parameter demands. The designers take the requirements and start applying the formulas with the requirements included.

They're not designed "on a basic set of parameters". They're designed with a set of formulas.

And that's the truth.

Here is what that set of formulas can help a clever designer to create. The Blohm&Voss on the right was reported by it's pilots to be a very nice flying airplane.

When I worked at Lockheed a secondary job was to teach a course to aeronautical engineers. It wasn't a course in AE but I am pretty sure that I know how airplanes are designed as I later on worked with a number who were doing just that.

They weren't whipping up anything as crazy looking at that Blohm&Voss on the right of the two picture examples above, but they were doing some pretty interesting exploring. They were tasked to "think outside the box". And they were doing just that. And to see if their wild and crazy ideas were worth pursuing, they'd apply the formulas. They didn't work from a list of soundbite simple ideas that would create yet another Ugly Stick.

whatever..... this spin off has had enough said.

RaceCity,
I edited out all the personally directed content. Stick to the topic.
Review this:
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This isn't a contest.
It's never been a design challenge.
It's a discussion of a topic, not opinions of other posters.

DaRock