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ARCHIVE > The (Jet)X Files

The (Jet)X Files 15 for March 2004
by Roger Simmonds

Reprinted from SAM 35 Speaks, March 2004

Afterburning

Soon after writing last month’s discussion of camcorders, I received a copy of Mike Stuart’s latest CD video. Mike has put this together from the many recordings he and others have made at a number of 2003 meetings, for example the ‘Millenium Dome’, (I’m glad they’ve found a use for it at last), Old Warden, (including the very hot SAM 35 Gala), and Peterborough Flying Aces. Contact Mike if you would like a copy; the cost is a very reasonable 2-50. As well as many Rapier rocket cavortings, the selection includes the more leisurely flights of some very nice rubber powered scale models and excellent footage of Steve Glass’ indoor EDF Skyray.

 
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2003
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Nov
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2004
Jan
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Apr
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Jul
Aug
Sep
Oct
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It was good to reprise the glory of Mike’s XB-51 [below right], Graham Potter’s F-117 Stealth Fighter [below right], Chris Strachan’s SU-11 [pictured on the Jetex.org homepage], and Pete Smart’s many models, especially the Douglas Skyrocket and twin engined Tupolev Badger. Watching these (and other) recordings has emphasised the impact the new modified Rapier L2 motors have made on our activities this year. I’m not sure if Graham has ever tried an L4 in his F-117, but this impressive model needs a ‘250 mN’ L2 HP at the very least. From the evidence, delta winged designs like the SU-11 and Lightning really seem to benefit from the extra power, and the ‘250mN’ and ‘270mN’ L2 HP motors go some way to replacing the early (and discontinued) L3, and have even begun to challenge the L4. The (lower thrust, longer burn time) L2 LT has also proved useful. For example, Pete’s scratch-built F-86 Sabre has been a quite feisty flier; however, it is now tamed by the L2 LT, and, flying in a much more predictable fashion, has recorded several nice long sorties. As Pete is heard to say, "we seem to have found the right motor for it". His diminutive Fairey Delta 1 also flies well with this motor. So if you have a model that appears underpowered by an L1, or quite intractable with a ‘standard’ (90-140 mN) L2, inclined (if that is the right word) to tightening up in turns, first in that direction, then in the other, try an L2 LT. You may be pleasantly surprised.

Though I love the L2 LT, others find it a frustrating motor, and it is perhaps more prone than others to misfire. An effective cure is to abrade the solid propellant to create a small amount of easily ignitable powder; a procedure that has suggested another strategy recently debated on the ModelJetPlane discussion group. A small watchmaker’s screwdriver (no more than 1mm diameter, to match the thickness of the fuse) can be used to make a 2-3mm hole in the propellant core. This has to be done carefully so that the ceramic nozzle is not damaged, but it has two useful effects: a) ignition is facilitated, b) the initial area of burning propellant is increased. The effect of (b) is to boost the initial thrust; this helps prevent the slight stall and shallow dive into the ground commonly seen at the beginning of a flight. Hitherto, I have only sought to scrape away at the propellant, but the creation of a ‘bore hole’ may allow the use of the L2 LT motor with larger models than would otherwise be possible. The higher initial thrust will help accelerate the model away and get it ‘on the step’, after which the nominal thrust will maintain flying speed. One doesn’t get something for nothing of course, and the overall burn time will be slightly reduced. Nevertheless, exploring ‘core boring’ gives Rapier fliers something else to do, and I look forward to doing this with my Bell XS-1.

  Stuart's CD
Mike Stuart's CD

Stuart's XB-51
Mike Stuart's XB-51

Potter's F-117 Stealth
Graham Potter’s F-117 Stealth
- photo by Graham Potter
Feedback

I received a very nice letter from John Scott-Scott, who is happily both a SAM member and active in model engineering. John says, "you might like to know that at least one of your ‘historic references’ is still going". Indeed I did. John started rocketeering early: "I had some flights with the ‘Astral Rocket Plane’ [right], so bad that I fitted pendulum ailerons to stabilise it when it tried to Dutch roll". He adds, "it’s these ‘tricky’ subjects (I think he means all ‘Jetex’ type models) that bring out the best in we modellers". John has obviously not seen some of our video recordings or heard the language we can use.

‘Dutch rolling’, and, incidentally, tip stalling, are common problems with swept wing scale designs. A couple of degrees of wingtip washout is an well known and effective cure for the former, but it has taken the average modeller some time to realise that (perhaps counter intuitively) reducing dihedral and hence side area can help with the latter. Howard Metcalfe’s KeilKraft Hunter and Pete Smart’s Douglas Skyrocket (a Richard Crossley design) [right] both fly very nicely with anhedral. Pete’s aforementioned Sabre has less dihedral than the old KK kit, and from my observations of recordings of my own KK MiG 15, even this superlative design could benefit from rather less dihedral than the plan indicates.

  Astral Rocket Plane
- Aeromodeller, Nov. 1946 (modified)

Crossley's Skyrocket
Skyrocket built by Roger Simmonds from a plan by Richard Crossley
- Roger Simmonds
Early Wilmot Mansour models

The first Jetex powered models revealed to the modelling ‘Great and Good’ included a tethered car and hydroplane [right]. Details of the latter, (which was 350 powered and inevitably called the ‘Hydrojet’), were published in The Model Boat Book (1950) by M.C. Cowell. This being so, I have some hopes that plans exist, somewhere, for the RTP aeroplane in a similar period publication. The search continues.

  Cowell's Hydrojet
M.C. Cowell's ‘Hydrojet’
- The Model Boat Book, 1950
Jetex ad Joe Mansour also demonstrated Jetex free flight. Colonel Bowden reported at the time: "I saw a number of flights made by a model ‘flying wing’ reminiscent of the De Havilland full size machine. The model flew around in a number of circles gaining height, followed by a good glide to earth". This model may be the one displayed so prominently in an early Jetex advertisement (Aeromodeller, 1949) [right]. If so, it is very much more than ‘reminiscent’ of the DH 108; indeed, it looks like a scale model of the third prototype that briefly held the world air-speed record. It predates D.P. Golding’s 1952 Aeromodeller plan by several years, (being distinguished from this design by hinged elevons and a planked fuselage) and whilst is also similar to Howard Boys’ ‘Powakits’ product, the motor (I don’t know which one it was) appears to be fully enclosed.

Does any reader know anything about this model? It was, I believe, part of the Jetex Exhibition at Southampton Aviation Museum, and looks so much nicer than the other early scale Jetex efforts that I’m surprised it was never marketed. Jetex were obviously proud of this model, which shows that Joe Mansour was (unsurprisingly, given his experience with embossed paper models at FROG) thinking of realistic scale models from the beginning. Unfortunately, he (and modellers) had to wait several years for the development of suitable balsa moulding techniques and the augmenter tube, and until Bert Judge and Mike Ingram had joined him, before his vision could be realised with the innovative ‘tailored’ designs.


Jetex ad
- Aeromodeller, 1949
MEW engines ad What Athodyd next

First, my apologies for the pun, which I was not strong enough to resist. Chris Strachan sent me an advertisement (Model Aviation News, 1950) [right] that included not only the M.E.W. 601 ‘Jet’ engine, but also the M.E.W. 707 Rocket Engine. Chris asks, "Does anybody know anyone who ran the 601, “the most perfect self-contained jet engine on the market”, and survived?" Doug Reefman, who obviously did survive, confessed on the Jet-Ex-Press discussion group: "I bought two from Science and Mechanics for $3.95. My brother and I fired one up in the basement one day, on a string tethered from the ceiling. It was loud and it took off at a high rate of speed, spinning in a circle until the string broke, when it flew into a concrete wall"! The 707 is for “advanced modellers and experimenters”, which is at least some admission of the difficulties with this unit. Its thrust is given as 2 oz., quite feeble for any application, as according to the advert, it “must be operated on a control line or tethered to carry [the] air supply line”.

Given these commercial products, it is no surprise that making pulse jets and even athodyds (ram jets) was quite a popular activity for technically minded modellers who finished the War with relevant engineering experience. These engines are exciting and not particularly complicated machines, though they can be quite temperamental. There are now, inevitably, several Internet sites featuring various types of reaction motor for home construction, but in the good old days designs were printed on paper in the Aeromodeller, and a ‘ram jet test-rig’ was published in the late forties. Does any reader have a reference for this article?

Eric Marsden drew the sketch below right from memory. He writes:

Many of us at that time had not learned the difference between pulse jets and athodyds. The former will run whilst stationary, taking in their own air, whilst the latter require a high-speed airstream, well beyond the velocities of modellers, to function. The athodyd test rig produced a fair amount of noise – but at that time I knew nothing of pressure waves or wave fronts, though I had learned that the ‘Borda mouthpiece’ was helpful for getting a one-way flow, and none of our efforts produced any thrust. At the time we were obsessed with smallness and lightness, and we only learned that engines could be profitably be made much more solidly when flying C/L with engines of pre-war design. I teamed up with a friend who understood pressure waves and had the necessary mathematics, and we built a whole series of pulse combustors from about 1' bore and 15" long to 3" bore and 5' long. We also had to build a large silencer box to house them during runs – the neighbours got a bit huffy over the explosive barks of the larger units, which were as loud as a 3.7" ack-ack gun!


MEW engines ad

- Model Aviation News, 1950

MEW 601
M.E.W. 601 “Jet” Engine kit

MEW 601
M.E.W. 601 “Jet” Engine, assembled
Marsden's athodyd Our most promising unit was a small one – it was valveless, and gave about a pound of thrust. Unfortunately the fuel supply was a rat’s nest of tubing, quite probably, and by sheer accident, tuned to the engine. When my ‘oppo’ wanted to make ‘improvements’ (modifications) I tried to insist he leave well alone, and make a second engine for development. I wasted my breath. I came home one day to find him looking at a modified engine – he had made two changes – which was quite dead. It took several years to sort out the problem, by which time pulse ducts were a guaranteed way of losing flying fields.

Eric adds, "it’s all in the past, and that’s another country." No manufacturer could now market the 606 or the 707, especially in the US, where it is, I’m afraid, difficult to import Rapiers and almost impossible purchase the bulk chemicals to fabricate their own motors.

Benson Ball


Marsden's athodyd

- Eric Marsden
Though I really should know better, it is not unknown for my eyes to stray to the top shelf of the magazine racks at newsagents, or even to have a surreptitious look at the contents. I know there will be little of relevance to the Antique Modeller in any of these ‘glossies’; nor have I quite recovered from the shock of seeing pictures of ‘Vintage ARTF’ aeroplanes. However, there is a contemporary publication devoted to jets (the radio controlled turbojet powered variety) that I did peruse in the comfort of my own home with the doors locked. This generated some reflection: what are the vintage forebears of the sophisticated and (very) expensive examples of the manufacturers’ art illustrated therein? Surely not Colonel Taplin’s pulse-jet powered model? The engine (see photo) might be nine inches long and ‘pulsate at 400 beats per minute’, but the model appears to be a modified petrol engine powered free flight design and would have looked desperately old fashioned even at the time. Perhaps it was just a test bed; I have doubts it ever flew, for, as the good Colonel said, "the photograph was rigged as far as the flight appearance is concerned".

There were more sophisticated models around (see ‘The Dynajet flying wing that rocked Eaton Bray’, SAM Yearbook No 11), but for a true progenitor of today’s beautifully engineered models one must look elsewhere, perhaps to the legendary W. Benson-Ball. W. Ball published an intriguing article, ‘Flight Testing and Development of Model Jet units and Aircraft,’ in the 1954 Aeromodeller Annual. W. Benson-Ball followed with three articles, ‘Experiments with Model Jets and Deltas’, (Model Aircraft, 1960, Oct. onwards).

 
Taplin's pulse jet
- C.E. Bowden, Model Jet Reaction Engines, 1948
(reprinted TEE Publishing, Nov. 1953)
Mr Ball’s first flying wing, made of ‘solid balsa’ and powered by a Brocks’ 6d ‘Fire trail,’ flew in the summer of 1942. Not surprisingly, it “caused quite a commotion and [the] instant intervention of the law”. After that, things moved apace. V E Day saw him successfully fire up his first model turbojet; by the end of 1947 he had flown deltas powered by solid fuel rockets that developed a thrust of eight ounces. Only a little later his radio controlled X-D-7 jet [pictured right] was timed at over 100 mph. Not only are the models years ahead of their time, but the radio control system, which was built by BB’s father, is also in advance of what anyone else (manufacturers included) had achieved in the early fifties. His articles include much data, and many line drawings and (poor quality) photos, so there is no reason (except for their fabulous nature) to disbelieve his stories.

His publications, especially the later ones, are erudite, though the style is a little patronising, indeed, haut en bas, but then who can blame him? His models, he says, "are of course far from a beginner’s project … my constructional methods are a little out of the rut … I have for quite a few years made use of fibre glass extensively … six months’ to a year’s work must go into them". His discussion of delta design is eminently sensible; "the lateral (Dutch roll) and spiral instability so often complained of in deltas is easily overcome … anhedral only must be used, either by tapering the wing thickness on the upper surface, or by incorporating up to 5 in the effective span". Overall, this is all very impressive stuff, enough to give any reader a distinct inferiority complex.

Ball's Model Turbine
- Aeromodeller Annual, 1954 (p.88)

I wondered what sort of impact these articles had at the time. Stan Pearson (who tracked down the Aeromodeller Annual references for me) says: "as to Mr Benson Ball and his Incredible Flying Machines, I think the first reactions were one of disbelief. Perhaps he was some fictitious "Boy’s Own" character designed to thrill a youthful audience". I asked John Scott-Scott for his opinion. John wrote back: "I read of this at the time, but, like many others, I found the reported data rather ‘way out’. It seemed too much for a model project .… he may have been some sort of test pilot with access to MOD information. The text reads like a professional aeronautical engineer’s report … so the rockets plus pulse jets I can believe, but not the turbojets, as they have taken time to develop even for Ministry UAV projects."

‘Commander Benson’ is last spotted in the 1974 Aeromodeller Annual, holding a very high powered shrouded fan delta. The photo caption reads, "Cdr Benson at Mojave Desert test site for his R/C XD 110A which has recorded flight speeds up to 262 mph". Does any reader know anything more about him? I wonder what became of ‘Commander Benson’, and I would love to know more about this most extraordinary man and his achievements.

Now here’s a thought to ponder: a radio controlled XD-7, (which dates from 1947), powered by a small modern turbojet, could be flown with every justification at a SAM Gala. An ambulance, though, would have to attend the shock victims.

Though I admire the engineering skill modern R/C jets display, it is not at all the kind of thing I myself want to do. But that does not mean the designs of Benson Ball have no attraction for me. If we can allow miniatures of vintage gas buggies powered by small diesels and CO2 engines, we could certainly make a scaled down X-D-7 or XD-20 for Rapier power. Reproducing the structure would be quite a challenge, but one would end up with an attractive, futuristic ‘vintage’ model. It’s not often we see the appellations ‘vintage’ and ‘futuristic’ applied to the same model!
  Ball's XD50 W. Ball with his X-D-7
- Aeromodeller Annual, 1954 (p.87)

Ball's XD50
Ball's XD50 W. Ball's X-D-7 and X-D-60
- Aeromodeller Annual, 1954 (p.87)

Ball's Rocket Motor

W. Ball's Solid Fuel Rocket Motor
- Aeromodeller Annual, 1954 (pp.90-91)
top

Acknowledgements

- Article:
    Roger Simmonds
- Illustrations:
    Eric Marsden (Athodyd test rig)
    MAAC archives via Bill Henderson (pictures of Ball and his models and drawings)
    Richard Crossley (Skyrocket)
    Roger Simmonds (other illustrations)

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