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ARCHIVE > The (Jet)X Files

The (Jet)X Files 19 for July 2004
by Roger Simmonds

Reprinted from SAM 35 Speaks, July 2004

Afterburning

After my request for an assessment of Benson-Ball’s miniature jet engines in May’s article, John Scott-Scott kindly provided an informal analysis of the X-J-E/20 three-stage axial turbine illustrated in the 1954 Aeromodeller Annual article (see (Jet)X Files, March 2004). John begins: “If we assume a 6.5" diameter and 26,000 rpm, the mean blade speed (at about 5.3" diameter) is about 600 ft/sec. This is quite fast and would impose very high stress on the blade roots and discs. In comparison, the Armstong-Siddley Python, on which enormous sums had been spent in development, had 11 stages at a lower blade speed. The bearing locations and arrangement do not lend themselves to high speed or high temperature operation; even today, ceramic bearings cope, but only just. The combustion chamber lacks a ‘proper’ flame stabiliser, so burning would be wherever it suited, but very likely it would only be stabilised on the turbine stator. Would that jet engine design could be so simple, but, particularly with axial compressors, life is not like that! Miniature axial flow bladings are notoriously inefficient; all today’s model gas turbines use centrifugal compressors for good aerodynamic reasons and they also run faster”. In summary, therefore, as a ‘doodle’ the sketch is fine, but in reality it is no more than that, so we must conclude that Mr Ball used some form of ducted fan”. John also has doubts about the R/C system; “Ball claims to have had a 10 channel radio fitted! Even our early reed sets had not evolved by then”. John concludes, “It’s a good story … the shapes (of his model aeroplanes) are worth replicating with modern R/C and fan units”.

 
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Im tempted to leave the BB saga there: as one correspondent said, If true, its the greatest aeromodelling story ever told; if only partially true, why the exaggerations?.

Benson-Ball was not alone in claiming a working model turbojet in the early years post-WWII, and the advert on the right comes from a 1948 Model Airplane News. The design boasts a centrifugal compressor, which is a step in the right direction, but I wonder how many were made from the plans and I doubt it could have been either self-sustaining or provide a usable thrust. However, I’m sure it made a wonderful whine when turned over!

I recently came across the Gamages Jetex advertisement [below right] in Meccano Magazine, May 1949. It is interesting as the earliest I have found that includes the Jetex 50. Incidentally, what became of Meccano Magazine? I seem to remember it featured simple Jetex models well into the sixties when the mainstream Aeromodeller and Model Aircraft had all but abandoned Jetex propulsion.

Somewhat stung by Stan Pearson’s remark, “jet planes with stringers? I ask you!”, I have been looking for a jet/rocket plane where a fabric covered open structure is not incongruous. There was of course the rocket-assisted Opel Sander Rak-12, a model of which for Jetex 50 was published in Model Aircraft, March 1956. This design has appeared in SAM 35 Speaks at least once before, and Chris Strachan has made a 13'' version for the Rapier L1. Chris is quoted (Aeromodeller, Nov 2002), “Since building the model I have done a lot of searching for information on early rocket aircraft. This model is wildly inaccurate. The Rak was very high aspect ratio, and it should have about twice this span”. There was a rather better realisation of the Rak-12 (also for Jetex 50) by Walter A. Musciano in the US.

I thought it might be possible to go back even further than 1929 when the wonderfully named Fritz von Opel first steered his Rak-12 down a ramp and up to 50 feet and 90 mph, and remembered the Rumanian-born Henri Coanda claimed to have made the World’s first ‘jet-propelled’ flight in 1910. Hmm…

I looked for data about Coanda’s prototype without much success, and the rather poor illustration [below] (from a 1950’s Royal Air Force Flying Review) is the only one I have been able to find. I have cleaned the photo up a bit and hope the definition is adequate.

Coanda

Coanda apparently believed the conventional ‘dragging’ motion of a propeller could be bettered by ‘creating a vacuum in front of the aeroplane into which it could be drawn’ (?), and this remarkable aircraft, which also included a cruciform tail unit and retractable undercarriage amongst its idiosyncracies, had a very strange motor at the sharp end. Coanda called this a ‘turbo-propulsor’. It was, perhaps, more in the nature of a ducted fan, (and a model would be more suited to EDF), but he did plan to ignite fuel in its innards and derive some jet reaction. There was, apparently, one short test hop, or rather, ‘test crash’, though this was without the benefit of jet augmentation. After this disappointment, Coanda, a poor pilot by his own admission, went on to more significant and lucrative research. The contemporaneous advert shows only the wing planform of the aircraft. Does any reader have any more details? By the way, ‘Aéroplanes sans hélices’ sounds like a good alternative title for The (Jet)X Files!

I received an interesting email from John Harvey; “There is no ‘Vintage Rocket Society’ as such (see April’s column), but SERFS (Southern England Rocket Fliers) fly everything as long as it is safe and fun. The Estes catalogue has lists of model lift-off weight against motor that will indicate a suitable motor for the Wilmot Mansour Dan Dare Space ship. There are other motors, Quest for example”. The Estes website is somewhat daunting, and Grif Ingram recommends Ye Olde Rocket Shoppe website, which I will peruse for a motor equivalent to the 50R (about 1500 mN for 5 seconds), as I quite fancy Ian Gedde’s Coccinellida (Aeromodeller, Nov. 1956).

John also comments on the IMA target drone (see April’s ‘Gliderscope’); “I have one in good condition, and now that a variety of high power motors are available I am sure I could fly it as it was meant to be … but getting it down safely, that’s the problem, I don’t want it to be a ‘one off’ spectacular!” I hope John remembers the original was launched using a bungee powered ramp.

  Rotojet
Rotojet model gas turbine
- Model Airplane News, 1948
Jetex ad (Gamages)
- Meccano Magazine, May 1949 (p. iv)
Coanda ad
‘The only propellerless aeroplanes’
- Royal Air Force Flying Review, 1950
After seeing the Draken for Rapier L2 in April's column, Sten Perrson sent me plan of Sigurd Isacson's SAAB J-210, (the Draken's precursor). He writes, "I was thrilled that Isacson's Draken has taken to the air again in the hands of Chris Strachan … as test flying of the ('full size') J-210 proceeded changes were continuously incorporated so that the final version resembled the full grown J-35 more than the original J-210. Sigurd experienced some directional stability with his model and recommended a 'non visible' forward fin".

Isacson's J-210

Sten comments, "As with other Isacson Jetex designs, the J-210 is only a 'stand-way-off' scale model, designed for the comparatively inexperienced modeller, but it is still able to capture the character of the prototype, particularly in the air".

Isacson's MiG 15 has appeared in SAM Speaks before, but not, I think, his NA F-86 Sabre, so, for the sake of completeness, here it is.

Isacson'sF-86

The Jetex 50 might be a little too forward, and the fuselage could do with a few more stringers, but this is a nice model with some similarities to Albert Hatfull's KeilKraft design. Note the minimal dihedral and the tailplane trim tab.

  Isacson's logo
Isacson's designs may be 'stand-way-back' scale, but, as can be seen from Chris's Draken (right), resplendent in the silver and red trim of the prototype, the spirit of the 'real thing' has been effectively captured and it flies very well indeed. If any reader would like a copy of the J-210 (or Sabre) plan, please let me know.

Remember, however, that these fifties' plans are derived from second generation or even third generation photocopies, and may have suffered copying distortion over the years, as early photocopiers were notoriously bad in this respect. Look carefully at the Sabre plan: you will see that the two sheets from which the plan was reconstructed not quite match. Sometimes the distortion is not this obvious, and it was only after I had made both wings for my Skyleada Hunter that I discovered the wing roots mismatched by 1/8th of an inch at the root rib. This was too much to be 'fudged', I'm afraid, even by an experienced 'bodger' like myself, so the starboard wing had to be remade using the reverse of the port wing. For a glacially slow builder like me, this represented a day's work.

  Strachan's Draken
Chris Strachan's J-35 Draken
- photo by Chris Strachan
Jason Willbourn, who has also made a Skyleada Hunter, didn't complain of distorted wings, and perhaps his plan was OK, but it is best to check the accuracy and matching of components before building, and redraw if necessary. Small jet planes achieve quite high velocities, and accurate alignment of flying surfaces (and thrust line) is important. For models with a horizontal keel ( la Bill Dean), aligning wings and tail is comparatively straightforward, the keel providing a useful datum, but where there is a vertical keel and mid wings, making a set-up jig similar those included in the Jetex tailored kits (see right) makes good sense. So that nothing is wasted, this can be incorporated in your carrying box (you do make these, don't you?) afterwards.

What Sten says about Sigurd Isacson's models is also true of the Jetex 'Flying Scale' models from Veron, KeilKraft and Skyleada, and perhaps the rude remarks I have made about some of these are unfair. Amerang no longer make any KK kits, but Easy Built kits are still listed in the Sams Models catalogue.

Easy Built Jetex kits

I am not sure of the origin of these transatlantic offerings, but they may be fifties' designs adapted and slightly enlarged for the Powermax Jet-X 50, which was rated at 0.7 oz (220mN) thrust. For example, the Easy Built MiG 15 (which is not Bill Dean's design) is quoted as 16 3/8" span, whereas the KK MiG 15 was 15" span. The Easy Built model is thus around a third (163/153) larger, about right given the 0.5 oz thrust of the early (ribbed) Jetex 50. Interestingly, and as John Emmet observed in his SAM Speaks column in the eighties (when the Easy Built range first appeared), the 'Vulcan' is actually an Avro 707.

  P1-A alignment jig
Jetex 'Tailored' P1-A alignment jig
On the right is Howard Metcalfe's Easy Built Swift. Howard writes: "It is 14.5'' span and weighs 38g with a used Rapier L2. It is finished with Airspan heatshrink covering and airbrushed in artist's acrylics. The air intakes and canopy are modelled on the speed record holder rather than the first prototypes [it is light blue with hand painted roundels]. I have lengthened the air intakes and added soft 1/16" stringers in between existing stations to fill out the rounded form better. The canopy framing was a masking nightmare; I used electrician's tape cut to 0.5 mm width or less, which sticks well but comes off easily. The paint doesn't creep underneath, and most importantly, it can be persuaded round fairly tight bends. The Rapier L2/L2/HP motor is set at 5 degrees downthrust with the nozzle just ahead of the cg. The model is balanced to give a floaty glide so that I can let it turn quite tightly without fear of spiralling in. The surfaces are a little out somewhere, as although it will turn happily either way, the predominant turn is to the left. To counter this the motor is set for two degrees right thrust. Launches are with an elastic band (to get a fast clean launch), done slightly to the right of the wind with the wings banked to the right about thirty degrees. The model goes off in a climbing right turn for a circuit or so, then stalls slightly and settles into a left turn which can tighten up and stay level or open up into loops depending on how frisky the pilot is feeling. If I launch it into any wind it just loops into the ground at my feet. Oops!"

The Easy Built range, then, is well worth exploring. Though the kits appear rather unappealing and a trifle basic at first sight, with a little care and detail modification they can be made into very nice looking models.

Catapult Jets

Stan Pearson need make no apology about these sorts of models, which have a long and honourable history. For example, A. G. Piell published a nice article, 'Catapult Solids' (Aeromodeller, Jan. 1954) that featured the 1/72nd model below.

Piell's Hunter

Note that Mr Piell also prefers a plain finish for his Hunter. The AM staff were clearly quite taken by his designs: "Two models sent to the editorial offices were soon put through their paces, amazing all with a rate of climb second only to the real thing. What's more … they glide!" As well as the Hunter, APS offered Piell's (1/72nd) plans for the Sea Hawk, DH Vampire and NA Sabre, and the 'Vickers-Supermarine' types 508, 510 and Swift.

  Metcalfe's Swift

Metcalfe's Swift
Howard Metcalfe's Easy Built Swift
- photos by Howard Metcalfe
These diminutive models were fired off using a well-stretched loop of 3/16" rubber held somewhat dangerously between finger and thumb. AM advised; "launching should be round 45 degrees, but a varience in the angle of bank will be found advisable for best turning trim, whilst a warp in the elevators can be arranged for high-speed aerobatics". What the AM article neglects to state is that catapult models (especially the larger ones) can be trimmed coherently to give good flying characteristics that are more predictable than those of Jetex models, allowing subtle aerodynamic effects to be explored. Though kits, especially in the US, were advertised as, “being suitable for catapult or Jetex”, the two power modes are actually incompatible, and models really need to be set up for one or the other. To start with, catapult models need to have stronger wings, weight is of little disadvantage, and airfoils need to be carefully designed to give optimum launch height/gliding characteristics.

Stan is our own doyen of the genre, and has me a selection of photos from his vintage collection of 1/48th catapult jets that I have been waiting an opportunity to publish.

As can be seen, these are true scale 'flying solids' devoid of stringers, though still capable of an excellent performance (Stan tells me he lost several OOS in thermals). It would be good to see catapult jets at our meetings, so if any reader has any copies of Mr Piell's APS plans I would love to hear from you.
  Pearson's MiG 15

Pearson's Meteor
Stan Pearson's MiG 15 and Meteor
- photos by Stan Pearson
top

Acknowledgements

- Article:
    Roger Simmonds
- Illustrations:
    Roger Simmonds, Chris Strachan, Howard Metcalfe, Stan Pearson

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