maxpower1954

The Babcock Aeronca is finished, but I have a few concerns.

It seems pretty heavy to me at 26 ounces. The wing span is 37", and the area is 240 sq. inches so the loading is 15 ounces a square foot. A modern electric model this size would weigh 16 ounces or less and have a wing loading around 9.6, just like my electric Ranger 42 turned out.

How does this compare to similiar size single channel models like the Minnie Mambo, Jr. Falcon and Lil Esquire back in that era?

The power loading comes out to about 90 watts per pound, which for you non-electric guys should be fine but I'm thinking of a heavier, more powerful Park 400 motor because it already needed 1.5 ounces of nose weight to balance - I hate useless dead weight in an airplane!

Here are some pics - delightfully funky and cute at the same time! Has anyone actually seen one of these Babcock ARFs sucessfully fly 50 years ago?
I included my Robbe Seabee on one picture because it's another 1940s general aviation design, and it's a modern ARF for comparision to the 1958 version.

Russ Farris

ukengineman

Hi Russ, very interesting and unusual project. Regarding wing loading, my 1951 Sparky (UK single channel model) weighs 35 oz and has 360 square ins area which I make a loading of about 14 oz/sq ft. It flies superbly with electric power and rudder/elevator and is only just under your wing loading. Power loading is 70 watt per pound and it is very lively. Good luck with the test flight. You can see my Sparky here:-
http://www.rcuniverse.com/forum/m_80...tm.htm#8051114
Alan

BobHH

My Feedback: (18)

Russ, I flew one of these many years ago at the Charlotte, NC field for a fellow single channel. As I recall he had an Ace pulser and Cox Baby Bee .049. Flew great! The large rudder was plenty enough and she would thermal after the engine quite. Can't recall the weight though.[]

Bob Harris

Duke58

Hi Russ , that's a real funky little plane!! can't wait to see the flight video.

Earl

squeakalong

My Feedback: (8)

BOTH of those planes (photo 1) are very cool projects! Good luck on the maiden!

Soft landings.

Joe

maxpower1954

Thanks guys, I always try to make my vintage projects something unique and not commonly seen.

Bob, I might have known you have flown this baby, and it gives me comfort to know the one you flew did well. I am upgrading the motor to an E-flite 450 to get rid of the dreaded nose ballast.

One thing I am wondering about is the effect of such a sharp leading edge on the stall characteristics. I probably will attempt an ROG first... I'm the best hand launcher in these hear parts (a lost skill these days) but that lag getting to the stick can be critical on a heavy model or EDF. If the Aeronca flys OK, I'm bringing it to the Celebration of R/C in Muncie, along with the Twin Skylark 56 and the Shoestring.

Russ Farris

Deadstik

My Feedback: (8)

Russ,

Congratulations on a very unique project. I do agree that the plane may be 5-9 ozs. more than a Jr. Falcon/Mini Mambo I'd be willing to bet it flies just fine. Reason being is that I have a yellow Cox Fairchild 24 which weighs a TON and even so flies beautifully with an .049 (queen bee I think). The Fairchild has a very sharp LE but flies beautifully and is capable of some really wild aerobatics. I hope you have good luck with yours.... always great to see history take to the wing again.

Dan
Carolina Custom Aircraft

maxpower1954

The Aeronca flew today - the good news is, it's still in one piece. But it was the most difficult first flight I have ever experienced!

Winds were light, about zero to five mph. I taxiied around a bit, since I went to the trouble of engineering a steerable tailwheel. Finally I headed into the wind and hit the throttle. It lifted off beautifully in about 30 feet, in perfect trim and started a nice shallow climb. About 75 feet up I started a right turn - the response was immediate and violent! It rolled into a 60 degree bank - I applied opposite rudder, and was rewarded with a near snap roll the other direction. I backed off the power and things got better. The problem was it has a very strong roll/yaw coupling, better known as Dutch Roll.

This is caused by insuffcient vertical tail area, and I have never flown a model airplane with Dutch Roll as horrific as this. At half power, it was barely controllable. A few times, the bank angle was close to 90, and I nearly lost her. I came very close to cutting the power and ditching it, but I knew I wouldn't have learned a thing about what was wrong. After about five minutes of this torture I set up for a landing - power off, the Dutch Roll completely disappeared; it flew just like any other airplane and I made a perfectly normal landing.

I reduced the rudder travel on my computer TX by maybe 30% and took off again. Better, but it was obvious it had very serious directional stability issues. Anything beyond a gentle bank was almost unrecoverable. Landed safely again, thank goodness! The good things: It had plenty of power and would maintain altitude at 1/2 throttle, an electric model benchmark. The pitch trim was perfect; the CG was dead on.

I'm currently flying 3D, electric ducted fans, glow singles and twins and that little Aeronca gave me more excitement than anything I have ever flown in 40 years!

Some boring history...there are many full-size airplanes that ended up with vertical tails enlarged over the original design. The Boeing 707 is an example; three were lost in training accidents in 1959 due to Dutch Roll during engine-out practice. As a result, every 707 then produced went back to the factory and had a larger vertical tail installed, along with a ventral sub-fin. End of Dutch Roll problem.

Looking at the Aeronca, the fin does seem small to me. Maybe it was barely adequate with an .049 and small rudder travel, as it was designed for. But my setup produces 15 ounces of static thrust on an 8-6 prop for a 25 ounce airplane. Who knows?

But I think the fix is in! I took a plastic hotel key, cut it to a nice shape and taped it to the bottom of the fuselage under the stab as a sub-fin. I'll try again, and once I get this problem solved (hopefully - otherwise it will become a ceiling hanger) I'll borrow a technique from the scale model rocket guys and make the final version out of clear Lexan so it won't be obvious. Russ Farris

HighPlains

My Feedback: (1)

Russ, you always have interesting problems and I will offer the solution. You need to use a smaller diameter prop. Simple as that.
(I don't know why all my "solutions" to your pro(p)blems seem to be prop related!). Anyway it goes back to the fact that a tractor installation of the prop is destablizing, and the bigger the diameter, the worse the effect.

maxpower1954

Yeah, I know that High Plains (the fact that the problem vanished power-off made that obvious) but since it's an electric the smaller props (I tried a 6 x 4 and a 7 x 3.8) didn't pull enough watts to give me a comfortable margin over stall speed. It's a completely different animal than glow... I'm confident that adding some fin area will cure this problem. I do think it was marginal to begin with. I have also noticed that the full-size Champ has a dorsal fin missing on the Babcock version.

I'm pretty sure this is the first Babcock Aeronca Champ to take to the skies in many years. The real one, BTW is a great little airplane - I prefer it to the J-3 (sorry Dan Thompson!) Russ Farris

HighPlains

My Feedback: (1)

You would have to get a motor designed to run at much higher RPM (larger Kv number) to use the 6x4. Or much closer to square prop 6x6 which might be hard to find.

BobHH

My Feedback: (18)

Or Russ you could fly it like it was designed for in the old days with a real genuine honest to goodness glow engine!!! Say .049?[sm=spinnyeyes.gif]

Bob Harris

UStik

Sorry guys for chiming in here, it's so interesting that I just can't control myself. Having flown [link=http://www.koralpe.biz/topsy.htm]rudder-only models[/link] in the 1960s I remember only models with small vertical tails for better spiral stability (having no elevator). On the other hand, the rudders were even smaller because their effect was indeed violent as long as they were hit by propwash. Full rudder gave sorts of aerobatics while with engine out (out of fuel) all it gave was a very wide turn which wasn't even a 180 till the model hit the ground. The models were set up to need a blip (with the bang-bang R/C) of rudder to start a turn whereon they (at least some) maintained the turn (after lowering their nose and gaining some speed to replace the missing up elevator) until commanded otherwise. Wing loading was nearer to 20 oz/sqft than to 10.

So my question is: What is different on this very nice Aeronca except the huge rudder? Wouldn't it need only a blip of very small deflection to start a turn? If the rudder effect is right power-off then it has to be far too strong power-on. And isn't the power and speed too much? Those Babe Bees weren't that powerful, and a smaller higher-rpm prop would give even more propwash. Today flying only electrics I'd say 30 to 40 W/lb is well enough for such a model. Besides, I thought a Dutch roll is an oscillatory motion, maybe forced by control input but after that self-maintaining, even power-off. No offense meant, but all I can see are the effects of over-powering and over-controlling the model. Please correct me if I'm wrong and please explain why a big prop might destabilize the plane. (Do you mean P-effect?)

jaymen

Has anyone considered that due to the excessive weight, the plastic of the structure may be deforming and causing the problem? I know with free flights we have tried to make too light this has happened.

maxpower1954

Flight test day number two report - complete success! A totally different airplane.

The only changes were the addition of a sub-fin, and reducing the size of the prop to 7.5 (because I knocked a blade tip off on landing yesterday.) Even on the take-off roll, it went straight as an arrow without any correction from me, with no wind. It now flies the way I expect a rudder airplane to fly, the improvement was dramatic. It has very strong stability in all axis - you have to hold it in a bank, and when you release the rudder it rolls quickly back to level flight. I just nudged the controls to herd it around the sky and actually began to enjoy flying it. It does fly faster than a modern small electric and I don't consider it a park flyer by any means. Things were going so well I was tempted to loop and roll it, but suddenly my better judgement kicked in (where were you when I was 19 years old???) I declared mission accomplished and landed. Slight mishap when a wing tip caught in the tall grass and spun it around, popping off the wing and breaking the wood engine mount, which I designed it to do to save the plastic firewall. No other damage - this airplane is much tougher than I thought it would be, and the structure is very stiff and non-flexing.

I think I'll decrease the prop diameter till it pulls around 64 watts/lb, but I don't consider it to be overpowered by any means - probably somewhat better than the .049 was. I plan on increasing the rudder and elevator travel slightly, and doing a little tweaking on the sub-fin size before making a permanent one but at least now I have an airplane that flies well enough to bring to the Celebration of R/C. Thanks for all the ideas, guys...Russ Farris

UStik

Glad to hear the model flies well now! This historic marvel would deserve to make do without the ugly sub-fin, though, and I can't believe the original model needed it to fly OK. You modified several parameters at once and I suspect the only effective modification was to follow the advice to use a smaller prop with more pitch. In effect, you've done just that by clipping the blade tips.

Looking at the second picture of the plane above I think it was meant as a semi-scale model and not as a Class I aerobat. With the decent decalage and the small engine it was flown like a free-flight model. After hand launch a steep climb followed without much use of rudder, if any, letting the model climb in big circles. When the engine (sans throttle) ran out of fuel a slow glide was commenced near minimum sink rate for a long flight, and now the controls were used to get the impression of a near-scale flight. Hence the big controls, to be effective at slow speed without prop wash.

With the sub-fin, of course the weather-vane effect is bigger and the model strives to return to straight flight from a turn. At the same time, the rudder is less effective. But that overshooting in turns was also due to a quite high flight speed and would vanish if the model is flown slower, both due to less power and a more rearward C/G. I'd make sure to have the model balanced as recommend in [link=http://www.rcuniverse.com/forum/fb.asp?m=9099756]this post[/link] in the other thread, it's really golden advice.

With 15 oz/sqft wing loading the model isn't quite a parkflier (10 oz/sqft) but it's not much faster (10 m/s instead of 8 m/s) in cruise flight, either. The good old flat-bottom airfoil gives 1.4 maximum lift coefficient even at the model Re number and the required 0.7 cruise lift coefficient is not bad. The model needs 6 W propulsion power in level flight to overcome the drag, and each 1 m/s (200 ft/min) climb rate requires 7 W in addition. As a rule of thumb, assume an overall drive efficiency (including all losses from battery to prop) between 30% and 50%. For a 300 ft/min climb about 20 W propulsion power are needed or about 50 W electric at 40% efficiency, correct choice of prop and motor provided. I mean for the whole model, not per pound. With more power, you may simply waste energy (inefficient drive) or have a livlier (but less scale-like) model. But the smaller drive would save some weight and make it an even better model. (Just use the battery to balance it and not the motor.)

Besides, make sure to use a high-pitch prop as was advised earlier here. These parkfliers have geared inrunner or outrunner motors turning quite slowly for better efficiency. I don't think the prop has to be especially small in diameter, but it should have about 6" pitch. The 4" you considered (or used?) are used for slow fliers (6 m/s) or 3D models with low wing loading and more than 1 thrust/weight ratio. A parkflyer flying on its wings needs speed but not much thrust, hence the low power. And as opposed to the original glow-powered model, it should fly most efficiently in cruise. Both motor and prop are overloaded and not that efficient in climb, but in cruise flight the prop turns slowly at maximum efficiency and has to have the high pitch. I'd try a 9x6 or a 8x5 SlowFly for a start (pure guessing, but could also calculate a suitable prop choice). I fully agree that half throttle in cruise flight is right, but I mean a slow cruise flight with rearward C/G (at a much lower power level), needing not much elevator throw, either.

Now I looked at the drive you mentioned in the other thread: E-flite Park 300 motor. With a 8x3.8 SF prop and a 3s LiPo it would be nearly perfect for cruise flight, meaning it would give about 8 W propulsion power at 37% peak efficiency and 10 m/s flight speed. That's not really good because the motor has a too big kv (1380, 800 would be better) and the prop too little pitch / too much rpm. At full power the static thrust/weight ratio would be too big (0.72). A 6x5 SF prop would give 7 W power at 40% efficiency and only a 0.47 static t/w ratio at full power. Perfect! (Figures are approximations, of course.)

maxpower1954

Ustik, those are very interesting observations. I don't completely agree with all of them, but that's OK! You obviously have a great deal of knowledge of the subject, and I appreciate your taking the time to give some feedback.

,Two mods hardly represents "several" , the sub-fin and slightly smaller prop were it. Probably the smaller prop helped, but the added area certainly had a greater effect. I suspect if I only shortened the prop,
it would have had minimal results. And I don't plan to experiement to find out...

The CG is 1/4 ahead of the plans reccomendation, and the elevator is faired neutral to the stab. Pitch trim is stable in all regimes of flight. That giant stabilizer which was normal in those days does a great job...I'm not going to fool with the CG. Also, I can't use anything larger than an 8" prop due to ground clearance issues. I'm getting 10 minutes plus with my motor/prop setup, on 3S 1320s. This is just a demonstration model for vintage events, not an everyday flyer.

Here's what I think on the directional stability issue. As designed for an .049 and single channel flight, the vertical tail was adequate...barely. Look at the side profile picture in my first post on this thread. This design has a considerate amount of side area forward, and the tail certainly looks on the small side to me. BTW, my final power set-up is an E-flite Park 400 outrunner, cut down 8-6 prop for 64 watts/lb. and 12 ounces of static thrust (for a very scale like flight) I've already reduced the size of the sub-fin by a substantial amount, and the final version will be clear plastic. I don't agree that reducing the size of the prop by 1/2" is what changed this little airplane from a tiger to a kitten! On that first flight, it gave every indication of an under-sized tail. And maybe I wasn't clear in my initial report, but even at substantialy reduced power, like half throttle or less, the slightest gust would send it careening into a classic Dutch Roll, which was very difficult to stop without reducing the power to idle. The sub-fin turned it into just another airplane, and an easy flying one at that.

I once had an aeronautical engineer tell me that sizing the vertical tail was the one place that often didn't turn out the way that was predicted - even mighty Cessna has changed the size of it's new SkyCatcher three times (each one larger than the last) because of two prototype crashes during stall/spin testing. Here's a partial list of full-size airplanes that had the tails enlarged, from memory - either before they went into service or later models of the same aircraft...I never have heard of a tail that was reduced in size!

Douglas DC-3
Boeing 314
Boeing B-17
Boeing Stratoliner
Boeing 707
Boeing B-52
Martin 202
Cessna 400 series twins
Piper Comanche 400 (big horsepower increase from original)
Beech Bonanza (larger V-tail, C-35 on)
And later versions of the Champ, which went from 65 to 90 hp had a dorsal fin

I wouldn't assume that Babcock got it exactly right either; at the very least it was close to the minimum, with little or no extra margin.

Well, you get the idea - in my case, it was quicker and easier to add a sub-fin than experimenting with down-sizing power and props since the little beast was unflyable as it was, and it was a method I knew would work. I wanted to bring it to the big VRCS meet in Muncie next week, so I had a deadline for a quick fix. I wonder what Chuck Hollinger, the designer of the Babcock Tri-Pacer and Aeronca would have to say about all this...sadly he passed away in 2006. Russ Farris

UStik

That's fine with me, as I said before no offense intended and no indoctrination, either. I just tried to attract your attention to some aspects typical for such models and just not for full-scale airplanes. Actually, I'm still trying...

What I meant with "several parameters" where parameters, not just prop and fin. You modified the prop's diameter and pitch, the lateral area and its longitudinal and vertical position, and the tail airflow.

You may well be right that what you observe is a true Dutch roll, though still something inside me is refusing to believe that. Anyway, the problem increases with power, speed, and mass of the airplane. That's why I would use a drive as small as possible and that's why I found that the Park 300 with 6x5 would be perfect. You could avoid to touch the rudder during full-power climb and use rudder without problems during (less than) half-power cruise. That's what I'd do with that vintage model converted to electric.

What I'd never do is set the c/g ahead of the plans recommendation. Usually there's already a "safety margin" in them and a more aft c/g is actually better. The safety margin is there to allow for small differences between each copy of a design and to have a start. The best c/g position for an individual model has to be found by test flights with c/g set back step by step. I think your model is trimmed for too fast flight (more than 10 m/s) and that's why you think the 400 motor is right. Another indication is the 10+ minutes flight time, which should be 25+ minutes instead with that battery. The big stabs give the damping needed for rudder-only models and allow for the rearward c/g position needed for those free-flighters. (Please, please re-read [link=http://www.rcuniverse.com/forum/fb.asp?m=9099756]this post[/link].) And a forward c/g would make a Dutch roll problem even worse while a more rearward c/g might even cure it.

I suppose the model was once flown to altitude like a free-flighter and only then (power-off) flown with rudder. Seems it was seldom equipped with an elevator servo. But even if it had an elevator those models were set up and trimmed for a pleasant control behavior. Once I read what a glider designer recommended for designing the vertical tail. He said make it as small as you think is possible, then try and cut away a bit, try again and cut away until it hurts. Power-off or even with small power, Dutch roll is not an issue so they aimed for good spiral stability and turn behavior without ailerons (dihedral instead) and even without elevator (decalage instead). Such big dihedral and decalage are quite "unnatural" and not found on many full-size airplanes (as well as such big stabs).

As to the sub-fin, it's actually not that big so maybe it's not simply the additional lateral area that helps. Often the very common dorsal fin is needed to direct the air flow and avoid separations so the tail is effective. Those ventral fins as you added to the Aeronca are sometimes used to enhance the spin behavior for the same reason. Indeed may make a big difference here, considering the round fuselage below the vertical tail.

I wish you wouldn't be afraid of some tinkering with drive and c/g. You could do it how it was once done with the free-flighters and get a lighter, slower, tamer model. Anyway, I wish you all the best with the model and hope you all will enjoy the demonstrations (a video would be great so I could do as well).

boberos

Hi Russ,

Bring the Babcock plane to Muncie next week.
We would enjoy seeing it.

Bob G

Doc.316

Maxpower;
So, do ya like the new tail on the Skycatcher? Good to see someone is at least paying attention to it. Sizing the tail is indeed one of the hardest parts of designing an airplane.

The fix on your airplane looks good. It will barely be noticeable.

Steve

WacoNut

If you look at the later model Aeroca Chiefs that were manufactured they had a Dorsal fin added in front of the vertical fin. I am sure this would also help your model.

HighPlains

My Feedback: (1)

A tractor propeller of a typical airplane is a destabilizing influence on the directional stability

http://www.centennialofflight.gov/es...ty_II/TH27.htm

http://books.google.com/books?id=D-c...age&q=&f=false

http://www.djaerotech.com/dj_askjd/d...shtractor.html

Doc.316

That is correct a tractor prop is more destablizing. Also more h.p. or watts. It makes you wonder if the electric power system wasn't putting out way way more power than the original airplane with an engine.

maxpower1954

I did some final test flights on the Aeronca, before departing for Muncie tomorrow.

I added a temporary dorsal fin, very similiar to the one used on the 7DC and Chief and reduced the size of the sub-fin. Same rock-solid flight as the previous larger sub-fin only, at all speeds. Then I removed the dorsal - WOW...while not quite as squirelly as the original configuration, it was close. I did some experimenting with power, and even power-off it exhibited poor directional stability, with over banking tendencies. Left to it's own devices, it would eventually return to wings level provided you had enough altitude. Not pleasant...back in the shop, I made the dorsal permanent out of yellow scrap plastic from the kit - it looks like it belongs there. I cut a sub-fin out of clear plastic, slightly larger than version #2, it is practically invisible (think Wonder Woman's jet.) I now pronounce the "Yellow Peril" safe for public debut!

I'll be the first to admit my single-channel design experiences are limited to flying a friend's Testors Skyhawk back in 1970, but I've had many three channel models through the years (I learned on a Royal Coachman) and am fully aware of the limitations of rudder vs. aileron ships. I do believe the fin area was marginal to begin with, made worse by my somewhat overpowering this thing, and my modern flying techniques. I remember Bob Aberle electriyfying a Goldberg Ranger 30 (my first free-flight as a kid) and having very similiar issues with it, and he's an expert and I'm a modestly talented amateur (IMHO, of course!) I treat it like a free flight, with occasional interference from the pilot (what, no four point rolls???) and it fufills this mission very well, minus the noise. An interesting and satisfying project...now, if I can just get my hands on a Babcock Tri-Pacer...Russ Farris

maxpower1954

Hey Doc.316 - Steve, do you work for a certain major GA company in ICT? I owned a F/S 1952 Cessna 170B for ten years - sold it a year ago. The grand daddy of 35,000 172's - Russ Farris