|Sep 13, 2012, 12:55 PM|
Britkit build off - KK Dandy
Iíve wanted a Keil Kraft Dandy for 57 years, ever since I first saw the ad in the April 1955 Aeromodeller [Photo 1] and knew I had to have it. As a young boy I couldnít even afford the price of the kit, never mind the Allbon/DC Bambi diesel it needed. But I promised myself at the time that I would build one some day, which now seems to have arrived (these things do take a while sometimes).
The Dandy is an interesting model, for several reasons. It didnít sell very well for Keil Kraft, probably because the Bambi diesel it needed was financially out of reach of most kids at the time, and the more experienced modelers who might be able to afford a Bambi (and successfully build and fly a Dandy) probably had more lofty ambitions.
Hereís what Model Engine News (http://modelenginenews.org/cardfile/bambi.html) had to say about it in their article on the Bambi:
ďOnly one kit appeared on the market for the Bambi. This was the KeilKraft Dandy, a free flight sport model boasting a massive 21" (53cm) wing span. The first advertisement for this kit appeared in KeilKraft's regular position on the rear cover of the Aeromodeller of April, 1955. It appeared again the next month, but as far as I can tell, this was the last time it was featured. Notice in the bottom right corner of the page that KeilKraft identify themselves as the sole UK Distributor for DC Engines. This provides a clue as to why they might go to the effort of kitting a design for an expert-level engine, requiring expert attention to weight during construction, followed by expert level balance and trimming.Ē
I find it particularly interesting that this was (I believe) the first ever kit for a micro sized power model. So it could probably be considered the grandfather of all the hundreds of other such models that we see today. (Somebody please correct me if Iím wrong here.)
Anyway, apart from all that, itís a very cute little model that just "looks right", and one we donít see around much. In fact, Iíve never heard of anybody building one, and I think that needs to change.
For this build, Iíll stay with the original size. Electric and RC, of course. A ďDouble DandyĒ of 41Ē span would be very nice and a lot more practical, but that would be a different model (maybe to come right after this one?).
I got the plan package from Derek Scott (www.model-plans.co.uk). In addition to a copy of the original kit plan and set of building instructions, Derek provided a scan of all the printed/diecrushed wood supplied in the kit (ah, memories of Keil Kraft) and a picture of the kit box [Photo 2]. Just a note and comment here: the small low resolution illustrations like this that I have included in this post are not intended as any kind of free distribution of what Derek provided to me, and I donít think they can or will be used for anything like that. Derekís prices are more than reasonable, his service fast and friendly, and he is providing a very valuable service by making hard-to-find material such as this available to us. Letís support him in his activities so he can continue to do that.
The Dandy was designed by Albert Hatfull, who needs no introduction to most of us here. Albert drew the Dandy plan, and probably wrote the building instructions, himself. You can see his signature ďairfoilĒ logo near the top right corner of the plan.
I have a question for you guys. As shown in the enlargement of part of the plan in Photo 2, Albert writes ďAll up weight 2 oz bareĒ. What does ďbareĒ mean? Uncovered? Without fuel? Almost 2 oz? Something else? Just curious.
Albert goes on to say, ďFor best performance do not exceed an all-up weight of 2-1/8 ozsĒ. Since the Dandy is a pretty simple, basic model and the build itself is rather trivial, I will make one of my main objectives here be to come out lighter than that, including all electric power and RC components.
Time to get started!
|Sep 13, 2012, 01:32 PM|
Hi Harry, great to see your build log start, especially for such a rare and interesting model.
I imagine the "bare" weight quoted refers to the model finished, ready to fly but less fuel. I must say that I will be full of admiration if you manage to get it anywhere near 2.1/8 ounces including radio, my guess would be nearer 3 or a bit more. Given that you will almost certainly have more power available than the Bambi would have provided (and it will be a good deal easier to start!!) I really don't think this will be a problem.
The problem with such a small airframe and sparse structure is to see where you can save weight over the original, even allowing for the fact that the original kit wood was probably not the best it won't be easy, as there is so little wood there to start with. Given that the FF version weighed 2 ounces plus, and that your motor, ESC and lipo will probably weigh as much as, or more than, a Bambi, even with the very lightest radio it is hard to see how it can be 2 ounces - but I'll be very happy to be proved wrong!
I have posted a link in post 1 of the Build Off thread and will be watching with great interest. Good luck!
PS, it is strange that you should have been enthralled as a boy by the Dandy; on seeing that self same advert I really fancied it's partner, the other AEH model then called Anzac but subsequently re-named Bantam. Like you, some 56 years passed before I finally built one, for electric and radio, last year, and a great little flier it has turned out to be. Hope your Dandy goes as well.
|Sep 14, 2012, 11:43 AM|
Thanks for the input, George. I agree that my target of coming in the same as (or lighter than) the FF version may be a bit ambitious, but modern equipment is so unbelievably light and compact that maybe itís not totally impossible. Weíll see - itíll be fun trying, anyway.
First step is to find a power train and some RC gear. For me, getting all those things lined up before starting to cut balsa saves a lot of hassles later on.
Bearing in mind the 2-1/8 ounce (60 grams) total weight objective, and the Dandyís small size, all the gear will obviously have to be pretty ďmicroĒ. I do a bit of indoor RC (a guy pretty well has to, with our short flying season here) and I have some lightweight bits and pieces in my parts boxes that Iíll be using for the Dandy.
First, power train. An Aeromodeller engine review for the Bambi gave its maximum output as 0.0063 HP, which is 4.7 watts. Assuming a 50% efficiency for a small electric motor, that would mean about 10 watts in. For a 2 oz model, 10 watts in would be 80 watts per pound, which should be enough for the way I intend to fly the Dandy (use power just to get the model to altitude, and glide around from there).
At the 7.4V of a 2S lipo, 1.35 amps would provide the 10 watts needed. I have a HK A05 outrunner, and that looks like it will fit nicely. Weight is 4.2 grams, and based on the ď3 watts per gramĒ guideline for estimating realistic power input capabilities of motors, it should be good for about 13 watts in. (HK, as usual, is just a tad optimistic with its rating of 30 watts.)
I did some static testing with various props, and found a good setup to be a GWS 4025 (about 1.5 amps and 11 watts). Weight 1.2 grams. The Bambi also used a 4Ē prop, so the 4025 would be ďscaleĒ. I could increase power if necessary, by going to a 4040 or 4540 prop (13 and 14 watts). An E-flite EFLM1933 (0.8 gram) prop adapter completes the motor setup.
For the battery, I decided to save a few grams by making up my own 2S packs from individual lipo cells. By doing that, I could eliminate the overly heavy wires, connectors, and wrapping that commercial packs typically have. I used 240 mAh single cells (http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=7568). Actual weight of each cell, 5.1 grams. Using micro connectors, a dual-purpose main/balance lead configuration, and minimal wrapping, my 2S packs weighed 11.5 grams. HK calls those cells 240 mAh, but I measured the actual capacity as 200 mAh. Thatís OK, because my packs still weigh a good bit less than commercial packs of that size. The expected current of about 1.5 amps is only 7.5C, equivalent to about 8 minutes of full-throttle operation. If it turns out that I have to cut some weight, I could easily go to smaller lipos (a similar setup with 100 mAh cells would save about 5 grams).
For the ESC, I will use the good old rock-solid reliable Turnigy Plush (Hobbywing) 6A. Removing the heatshrink and replacing the wiring and connectors with smaller, lighter ones reduced its original 5.7 grams to 3.5 grams. Every gram counts!
So, my intended power train (motor, prop, adapter, lipo, ESC) weighs 21 grams. A Bambi weighed 5/8 oz (18 grams) without prop, hardwood engine bearers, and fuel. So, powertrain-wise, Iím probably just about exactly the same as Albertís original. And, I will have a much longer motor run and very much easier starting than he did!
Next, airborne RC equipment.
Receiver choice was easy - HK R415 Orange micro (http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=18827), weight 2.1 grams, $10. Iíve used this receiver in several models, and it has proved to be totally reliable and excellent in every way.
For servos, two HK5320 micros, 1.9 grams each (actual weight, with wires, connector, and arm). Amazing little servos, $3.55 each. http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=17540.
Pushrods will be 1 mm carbon fiber rods. Weight, with short wire ends and nylon control surface horns, should be about 1 gram.
So total airborne radio system weight (receiver, servos, and pushrods) will be 7 grams. (Power from the ESC BEC, weight already accounted for in the power train setup.)
Therefore, to meet my weight objective I basically need to build the airframe 7 grams lighter than the original. Possible? Maybe, with things like careful wood selection; use of minimal amounts of thin CA instead of balsa cement; the lightest practical covering material; and possible other minor savings here and there.
Photo 3 shows the gear described above. Total of those items is 28 grams (1.0 oz, a little less than half the target total model weight).
Sorry this got a bit long-winded. With the model being so simple, the build description itself will be shorter than most.
|Sep 14, 2012, 12:21 PM|
Gordon, I did seriously consider that but decided the additional flexibility, safety, and fun potential of having elevator control would be worth the 2 gram, $3.55 penalty.
|Sep 14, 2012, 01:49 PM|
Well Harry that looks like a pretty good set-up and I think you have at least a shot in shaving 7 grams off the airframe weight so that you meet your target. Covering is going to need to be light though, what are you thinking of?
I agree entirely that for the very small weight (and cost!!) penalty it is well worth fitting elevator control, if only because it makes initial trimming that much easier and safer.
If you hit your target weight, it might well be an indoor flyer as well. My Lil Bee weighed in at 70 grams, albeit with a lot more wing area, and I used to fly it easily in a one badminton court sized hall. I'm currently re-fitting it with a little brushless and lipos in place of the original Falcon geared coreless motor and very temperamental 8 x 50 Mah nicad battery. Not that I have any indoor flying sites here, but we do get a lot of glass calm mornings and evenings so it will be nice for the garden.
|Sep 14, 2012, 04:49 PM|
Wimps, the both of you.
By carefull adjustment of the thrustline there are no issues to Rud/throt operation......don't know what the worlds's coming to..pah?
Call yerself aeromodellers? Tsh
My mini-Matador has been a consistent performer for the last few years on Rudder/Throttle all powered by a single Lipo....11 watts hot off the charger, normally 9 (count them!)
|Sep 15, 2012, 01:15 AM|
Crikey Gordon, were you a monk in an earlier life - they were into self-flagellation were they not? Yes of course you CAN fly these converted F/F models perfectly well without an elevator, I still have a Pushy Cat set up like that and it flies fine. "Graduating" from F/F contest power models (or as Applehoney would say, regressing) I spent several years flying "proper" (escapement and pulse rudder) single channel models without the benefit of an elevator, some of these (eg. Cox Special powered Gasser) were very hot indeed and by virtue of very marginal trim could be flown in all weathers. However, when I got first Galloping Ghost and then proper proportional I quickly decided that rudder only flying was a bit like banging your head against a brick wall - nice when you stopped!
My point being that, with an elevator, you can fly these vintage converted F/F models in a bit of a wind without it becoming a war of attrition to keep it in the field. These are not models that can be safely trimmed like my Gasser was, but with the benefit of being able to lean on the stick when a bit of penetration is required, I can happily fly even sub six ounce park fliers like my Witch and Sportster in 6 or 8 knots of wind.
In the case of Harry's tiny model, I doubt he will be wanting to fly it in much of a breeze, but having pitch control will still make life easier - and at my age I am all for making life easier!
|Sep 15, 2012, 01:37 PM|
http://www.rcgroups.com/forums/poll.php?do=showresults&pollid=5448. Fairly decisive!
Of course, the size and type of model that little servos like this can be used with is limited and a bit of common sense is needed. With all the hundreds (thousands?) of servos available these days, it’s hard to know what type and size to use for a particular model.
My own feeling is that most guys (me included) sometimes use servos that are unnecessarily large and heavy for the slow, lightly-loaded, gentle types of oldtimer models we fly. Perhaps that’s a carryover from the old days, when only “standard” servos were easily available and those were sized to be capable of handling just about anything.
Recently, I’ve been experimenting with using much smaller servos than I’ve been used to, and so far haven’t run into any problems.
There are several online “servo calculators” around. I have no idea how accurate or relevant they are, but ISTM that they have to be better than just guessing. Here’s one I’ve been playing around with: http://mnbigbirds.com/Servo%20Torque%20Caculator.htm. It seems to confirm my feeling that not much servo power is needed for our oldtimers.
After becoming thoroughly disillusioned with Hitec when I received my third DOA servo from them, and having stripped the gears of two others during normal flying activities, I decided to look at what HK had to offer. They have hundreds, in every size, and I’ve spent hours reading through specifications and reviews trying to sort out the junk from the good stuff.
I also bought a servo tester http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=4573 and built a test rig so I could measure things like centering accuracy, repeatability, torque, current draw, etc.
To cut a long story shorter, here are the various small servos that I have tested and found to be “above average”, and that I will be using in my own future models. They are all well under $5 each. FWIW, YMMV, etc.
1.7 gram: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=17540
2.1 gram: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=14839
3.7 gram: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=5459
5 gram: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=663
9 gram: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=662
Note: all manufacturers seem to have adopted the unfortunate approach of quoting servo weights excluding cables, connectors, and arms. Pretty annoying and stupid, because most servos don’t work very well without those components. Add 10 to 20% to the above weights to get realistic.
I'd be interested to know about other people's thoughts and experiences with servos for our oldtimer models.
How’s that for hijacking you own thread?
Back to the Dandy. I’ve nearly finished CAD tracing the plan, and will soon be able to start making some balsa dust.
|Sep 15, 2012, 02:24 PM|
So although I fully respect (and, in a strange kind of way, admire) those who choose to go ďretroĒ with the technology, itís not really for me. But itís all good.
George, Iím planning on using Ultralite film (AKA Solite, Litefilm, etc etc). Itís about the lightest ďstandardĒ film covering available, and I used it with good success on my previous model, the Martian Spaceship. There are of course lighter coverings, but those tend to be a bit specialized (indoor models etc) and not really suitable for a ďgeneral purposeĒ model like the Dandy. I really do want to try your mylar-and-tissue procedure one of these days, but I donít think the Dandy would be a good project to learn that on.
I do hope the Dandy will turn out to be suitable for indoor flying. We have access to a 1930s-era hanger (part of a small local aviation museum) which has a nice big floor area but a very low ceiling. It would be nice to fly the Dandy in that, if I can.
Iíll probably be test flying it from the paved street in front of my house. I occasionally fly my very small models there after midnight, when the winds and traffic have dropped to nothing. Itís kind of neat to do that, under the street lights, with nobody else around and me at perfect peace with the world.
|Sep 16, 2012, 07:10 AM|
Good luck with this project. A very good chioce and interesting model here.
The lesser know / forgotten models are always so rewarding to bring back to life. Its what makes this forum so intersting, I'll be watching with great interest..
|Sep 16, 2012, 05:58 PM|
One more step before I reach for the balsa knife.
With these old designs, I always do a quick CAD tracing (basically just a building outline) mainly so I donít destroy the original plan. Or, if Iím working from an electronic plan, it lets me correct the inevitable inaccuracies of manual drafting. In both cases, doing this provides an easy way to build a rescaled version if I want to do that later.
It looks like Albert drew the plan on his kitchen table, judging by how delightfully crude and full of character it is. Photo 3 shows my CAD tracing of his plan, and the various parts to be cut out. Not very many of those!
I always try to keep oldtimer models as close as practical to the original design, without getting obsessive about it. Too many ďimprovementsĒ, and itís not the same model any more. So I try not to change things like planforms, airfoils, rigging angles, and so on, unless they obviously wonít work for RC.
But I will take advantage of the fact that we no longer have to hack up the lines of a modelís nose to accommodate a bulky, unsightly chunk of iron and aluminum, and we can have a nice, smooth, well-shaped nose area that the original designers could only dream about.
With the Dandy, I kept the length of the nose about the same (actually, I made it ~1/4" longer so the battery would fit) but gave it the shape I think Albert would have used if he didnít have to carve it up for the Bambi. I increased the vertical fin area slightly, to compensate for the added nose area (and because Iíve found that many FF designs adapted to RC fly better with more fin area). Other than those two minor adjustments, everything is per the original.
Another question. Iíve always found it strange that so many of the oldtimer designs donít mention CG anywhere, and Dandy is no exception. So I would appreciate any advice you guys who have more experience with this type of model than I do may be able to offer. Dandy has a fairly large (36% of wing area) lifting stab, an undercambered wing airfoil, and a wing incidence of 6į (which is probably too much, but Iíll leave it at that). My guess for the CG position would be about 60% back from the wing LE, and Iíll probably start at around 50% (bearing in mind the old ďtail heavy flies onceĒ thing). Any thoughts?
Maybe Iím overthinking this whole thing.
Finally, on to the building!
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