CrateCruncher

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As part of my R/C design education I decided to select three of the most beloved classic sport/pattern planes I could think of and attempt to unravel their secrets. I've been compiling measurements such as wing load, airfoil thickness and tail moment and comparing and contrasting to see if it can help to explain why these planes perform so well despite their age.

As an example, I discovered after plotting the airfoil of the Kaos that its thickest dimension is 18% of the chord length. This is a very thick symmetrical airfoil and helps explain why it can land very slow and stable yet tends to bounce about in windy conditions. It also might help explain why adding horsepower doesn't seem to add much to maximum airspeed.

The DirtyBirdy also has a thick airfoil but its a more modest 16% helping to explain the faster reputation and better behavior in wind. The big difference between the Kaos and Dirty Birdy seems to be the large flying stabilizer of the DB vs the smaller flat stab of the Kaos. I have no idea yet how this effects flight character however.

I have very little information on the UltraSport 60 and am hoping someone with a kit or plans might be willing to share some information.

My ultimate goal in this project is to design a stylish sport plane around a popular .75 or .90 2-stroke that reflects the same fine handling tendencies of the planes I'm comparing.

WEDJ

Good luck. We've been doing this for decades................


Seriously, most good classic pattern planes have very similar moments and areas for a given wing loading / size. It works. Changes come from different airfoils, fuse side areas, tail moments, adjusting incidences and distances from the centerline, etc. A lot of different combinations work well.

rainedave

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I guess the Don Lowe Phoenix series represents a linear progression of mods to a basic design as well as any of the "series" designs of the classic era do. I think Lowe tried everything Nick lists and then some (like wing sweep and wing area).

David

CrateCruncher

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Well I guess I have to agree I'm a little slow in doing this but I didn't appreciate these planes until I started flying a Kaos recently. Many people at the field I fly at have never even seen a Kaos much less built their own airplane. I like the plane very much but would like to build something unique that I know will fly well without too much prototyping. The first thing I would change on the Kaos is that boxy flat-topped fuselage. While its easy to build, its homely looking and can't knife-edge for beans.

I hear the UltraSport 60 holds an effortless knife edge but I've heard some "Flipper" comments. I like to build complex shapes in balsa.
Mike.

rainedave

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There's nothing wrong with the Magpie approach, borrowing elements from several designs to come up with your own. I actually made the stab on my Super Kaos using the built-up, sheeted ribs construction of the DB/UFO, so it's sort of a hybrid.

David

CrateCruncher

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Rainedave,
I've been reading your UFO thread and admire the quality of your work. Were you able to shift your Kaos CG back with the airfoil stabilizer? How does it fly compared to a stock Kaos?
Mike.

pimmnz

A symmetric sectioned stabiliser is the same as a flat plate, aerodynamically speaking. There is no change in cg when going from one 'tother. Cg's are always highly subjective, fly it where you like it.
Evan, WB #12.

rainedave

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I just pulled the OS .40 Surpass out of my S K Jr. (which honestly flew it well) and crammed an OS AX .46 in the nose. Same prop - APC 11x6 - but probably 1500 to 2000 more rpm. I had to add 1.75ozs of lead inside the tail of the fuse (muffler?) and I balanced it exactly according to the plans.

David

WEDJ

OK, take the Kaos wing & stab, keep them separated as is. Keep the engine the same distance from the LE. Now start to sketch some fuselage sides. So long as you keep the rudder AREA the same (or even a bit larger) you should have a solid-flying plane that you can call your own.

Photos will be required

WEDJ

Further thought - If you raise the wing towards the thrust line, you need to decrease the dihedral.

Hey, how about making it a shoulder-wing, and put a tad of annhedral in it? Would look like the old Prather Panther

CrateCruncher

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Thanks for all the great ideas everyone! I had noticed the DirtyBirdy CG was at the mid-point of the root rib and falsely assumed it was due to the flying stabilizer contributing lift. But looking closer now I see the DB stab is at 0 incidence and the CG is further back because of wing sweep! The Kaos has none. Thanks Pimmnz.

Personally I've never been a big fan of dihedral. Seems whatever benefit there is detracts as soon as the ship goes inverted. I'm still trying to understand the role of sweep and how it can substitute for dihedral. A mid-wing, symmetrical airfoil seems the most neutral but would mean longer main gear. Would the stabilizer need to be raised an equal amount?

WEDJ, I've been thinking of just making a nice oval-shaped fuse for my Kaos and keeping everything else the same. (I wonder if this is how the UltraSport got started?) The graceful yet sturdy fuse construction of Platt's Duellist might be a place to start. I want as little frontal area as possible yet keep as much side area as I can for knife-edge. To me, this sounds like an oval cross-section with maximum width at about 25% of length from the nose.

Question: The engine downthrust in Bridi's designs is to keep the plane in trim by compensating for changes in lift with airspeed/throttle? Does this sound right?

wind junkie

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Just a quick note: I don't believe the DB has a "flying stab." You meant to type "built up" or "airfoiled" stab, correct?

Flying stabs are used a lot on gliders and jets. They are also called "stabilators". The whole surface moves on a flying stab and there is no hinge for a separate elevator.

rainedave

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They also didn't have the means of mixing in a little rudder with the throttle stick to the extent that we do today.

David

CrateCruncher

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Oops! Yes, I misused the term. How could I forget the role of the flying stab in breaking through the sound "barrier"?

Jon Wold

Ultra Sport 60 plans: http://www.flickr.com/photos/aerowold/1030846783/

CrateCruncher

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Thanks for the link Jon!

In a thread covering an UltraSport 60 build its been mentioned the wing is EXACTLY the same wing as the Kaos wing. Go figure. I guess that magpie thing has merit!

kdc

You might also want to check out the Southern Eagle which is similar to a Kaos etc but restyled and moderised ( 2 aileron servos in wing )
You can see a copy of the original article and a plan if you go to www.rcmmagazine.com and look for requested articles.

Can you post a list of the main dimensions of the models you are investigating for everybody's reference?

rustypep

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I agree the wings are the same. I bought a Kaos 60 kit back in the late 90's when Tower was selling them and that kit even had the formers needed to build the turtle deck for the US. The manuals were pretty much identical as well.

tommy s

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There were a lot of Kaos clones after Joe Bridi sold the rights to the Kaos
design and not all of them used the original wing airfoil, some of the later
ones used a thinner airfoil.

tommy s

rustypep

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True but the replacement wing was the US 60 wing set at the time I bought my kit.

CrateCruncher

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KDC,
Here is what I have so far:

CrateCruncher

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Based on the comparison between planes three major differences emerged in my view:

The first and perhaps most important is the shape of the airfoil. The kaos has a thickness 18% of its chord and a leading edge radius of 1/2" compared with 16% and 3/8" for the DB. This gives the Kaos a very high-lift airfoil for nice slow landings and smooth yet slow aerobatics. It also causes some trim changes and erratic behavior in turbulent air. The DirtyBirdy has a larger wing area but thinner airfoil with a sharper leading edge radius. Combined with a higher overall weight this helps the plane fly faster with less drag and tolerate gusty conditions better. Notice the Dirtybirdy needs only 2 degrees downthrust while the Kaos requires 3 degrees to offset the higher lift wing.

The second characteristic I notice is no sweep in the Kaos wing compared with 4 degrees of sweep in the Dirtybirdy. In addition to adding a pleasing profile to the eye, Bridi also changes the balance point of the latter aft by about 16% of the root chord from the leading edge. This shift creates a longer thrust moment as well as a shorter tail moment when compared with the Kaos. I have to think this would make the Dirtybirdy more responsive with less control surface deflection and drag.

The third major difference I notice is the fin and stabilizer. The DB is 50% larger in surface area than the Kaos. Could this be to compensate for a shorter tail moment? The elevator of the two planes is the same suggesting the airfoil section of the Dirtybirdy may contribute to the effectiveness. One odd thing is the Kaos rudder is 50% larger in surface area than the Dirtybirdy which seems counter to trend in the two planes. I guess this may have been changed in the prototype to try and achieve better knife edge performance but who knows.

These are my feeble attempts at a design analysis. I initially thought it would be instructive to add the Ultrasport but the plane is so similar to the Kaos I decided to leave it out for simplicity. This is only intended to foster interesting discussion. Mike.

flywilly

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Here's a few more thoughts regarding the DB vs the Kaos: The original Kaos was designed back when the cross-flow scavenged engine still ruled; The DB was designed after the Webra Speed was released and around the time the OS .60FSR started production - significant power increases! Also, the pattern maneuver schedule was continually changing: The touch and go (as well as take-off and landing, obviously ) was part of the schedule at the time of the Kaos, but eliminated by the time the DB was designed. More rolling maneuvers appeared in the schedule (3 rolls, slow roll, 4-point/8-point roll plus more rolls in other maneuvers like the top hat and figure M variations) and more airspeed was desirable for better vertical performance. The DB wing was designed for better rolls (more leading edge sweep) while still retaining good looping characteristcs (straight trailing edge). As for the fin/rudder - in a word 'speed'. Back in those days you needed enough rudder to do stall turns, spins and keep the nose up during rolling maneuvers <by the way, the knife-edge loop is now included in one of the new FAI F3A schedules> which the design change to the DB fin/rudder accomodated.
I built 2 Super Kaos 40s, 1 Super Kaos 60 and 4 Dirty Birdies back in the '70s (in that order). The Super Kaos .60 had an OS 60FSR pumper in it and was a rocket. It also had a pronounced tail waggle in level flight until I added some area to the vertical fin. Shortly after that the radio failed it flew through several trees at full power (the trees won []). I loved my DBs (still do!!). My last one I built with swept wings to improve the roll characteristics. I just took the existing wing planform, cut some foam cores and then cut the root face to permit the sweep (I have no idea how much, probably about like a P-8 <Phoenix 8>). I liked the way it flew!!
That's my 2 cents...
Will Bartlett

kdc

Thanks for the data CrateCruncher.
You may be interested to look at the Super Kaos and Super Kaos Junior original articles which are also available from the RCM 'requested articles '( along with the Southern Eagle ) at www.rcmmagazine.com
The Southern Eagle wing looks very similar to the Super Kaos etc but has 2 degrees of washout, which is said to be important.