Electrifying the Keil Kraft Chief
If you were a schoolboy or an RAF Air Training Corps cadet during the fifties and early sixties, these type models were in every model shop. Names like Veron, Mercury, Frog and of course Keil Kraft would put your imagination into overdrive. Radio control was in its infancy and very expensive, therefore some of these models had very cunning devices to keep them under control. Devices like de-thermalisers, where the tailplane pops up at a pre-determined time, (normally 5 mins for competitions), and the model descends in a series of gentle stalls in order to stop you loosing the model in a thermal. They were so light that they seldom damaged themselves on landing. Or automatic rudders, like the Chief, which activates after launch to get the model to fly in a gentle turn. Late summer evenings was normally the best time to fly these models when the wind has dropped and thermals died down, all ways of loosing your masterpiece.
Of course there were plastic models from manufacturers like Airfix & Revel, but these models did not fly very well. When finished they just gathered dust and some times literally flew of the shelves if mother was careless with a duster. They were, however a very good way that a young air cadet could get to know the names of all the parts of an aircraft and learn Port from Starboard.
We soon progressed to flying models. From a very young age I remember looking at the Keil Kraft Leaflet a thousand times and in particular the Chief, this model had a massive 64 inch wingspan! With a tape measure spread over the floor at 64 inches it seemed huge. Today it would be considered a 'tiddler', especially by members of the Large Model Association. However at that time the Chief was just about the ultimate Keil Kraft glider. As we get older though, the models get bigger, some of us progress to 1/1 scale. Those are models that you have the inconvenience of having to sit in them to fly and the disadvantage of seldom walking away from accidents!
As a schoolboy I built many Keil Kraft models, (part of my aeromodelling 'apprenticeship'), but not the Chief, but now thanks to Outerzone plan service I can build one now. The Chief with its long nose that seemed tailor-made for a 3s Lithium polymer battery and an electric motor, is an excellent candidate for Micro RC and electrification.
The Chief is of course a 'free flight model', yes, you just launch it and let it go! Therefore, as with all free flight models the Chief is self stabilising thanks to the polyhedral wing. That is, if a gust of wind hits from the side or causing it to pitch or roll it will endeavour to stop this from happening and correct itself. The same effect can happen if we try to turn the model with radio control, it will try to prevent this from happening and the tail will start to wag from side to side. The outcome of this is called 'Dutch Roll', a common problem with rudder steered aircraft. A simple fix for this is to fit ailerons. Not an option, I think, with the Chief. Alternatively you could reduce dihedral/polyhedral, which gives it it's stability, by about 50%, (wing root and wingtips).
For covering I used 'Airspan', this material is almost indistinguishable in looks from tissue, but much stronger. It shrinks well around shaped parts, much better than 'Litespan' does. Airspan does not necessarily require doping. If you need to use dope one coat should be enough, it will however become more brittle and more easily punctured. I covered the nose section with some Solartex off-cuts, this is the bit that thumbs can penetrate when launching!
You could, of course build and fly the model free flight just as it would be done in the fifties. Why not! I decided however, to take advantage of modern R/C equipment and motor/battery technologies that are currently available, which would be considered science fiction in the fifties.
Other R/C modifications:
1). With the extra weight from the battery, servos, RX, ESC and the motor, (double the normal weight) the balsa wing spars need to be replaced with spruce and webbing just to be on the safe side.
2). Elevator, (snake) and rudder, (closed loop), system fitted for R/C control.
3). Interior of nose lined with liteply, to hold the Li-Po battery and motor, with access hatch fitted.
4) Underside hatch to access servos, if you wish.
5). The original had 'crash proof plug-in wings' with no retention, (not good). For our heavier model hook and elastic retainers are necessary. With a two-piece wing I found that when under load the dihedral increased causing 'tail wag', and the start of 'Dutch roll'. Better to convert the wing into one piece, with a suitable wing joiner built into the spar and change the fuselage construction accordingly. (recommended).
I joined the two wing panels together and secured the wing to the fuselage with one Graupner plastic bolt.
R/C & power conversion specifications:
|New weight:||31 oz ( free flight original 14-1/2 oz)|
|Wing loading:?||8.66 oz/sq ft (free flight original 4.05 oz/sq ft), still good.|
|Motor:||960Kv 480 brushless outrunner.|
|ESC:||Eflite 30amp Pro|
|Propeller:||10x6 Graupner CAM Folding Prop|
|Servos:||Hitec HS65HB x 2|
|Battery:||2200mAh 11.1v 3S Li-Po|
|Covering:||White Airspan, (nose area covered with Solartex)|
Flying the Chief:
With this power configuration, at full power the thrust is very noticeable, the aircraft needs little effort to hand launch and will climb at quite a steep angle. You will need to decrease power. At the desired height the motor can now be cut and the prop will fold. In this case a folding prop with a brake on the motor is a necessity, a windmilling prop makes an excellent airbrake. With the present wing configuration, (50% reduced dihedral/polyhedral and a one piece wing), turns are very smooth, with little or no tendency to yaw from side to side. Final glide is long and flat. With no airbrakes you will need to ensure that you have plenty of room to land, especially in calm conditions. Longest flight so far is about 45 mins, using an occasional blip with the throttle and little assistance from thermal. Despite the extra weight the Chief is still best suited to calmer wind conditions or perhaps on a ridge in a gentle breeze.
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