Hillson Praga G-AEYM

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The Praga ‘B’ is more closely related, certainly in cylinder head and crankshaft bearing detail, to the original E113, not the subsequent A,B or C models. Students of the pre-war Aeronca will know that the original E113 used Warner Scarab derived cylinder heads supplied by the Govro-Nelson Company of Detroit. Hopefully three photograph are attached to this post, two showing the Praga ‘B’ (early single ignition version), and detail from the Aeronca E107 – E113 Engine Manual.

The later parallel valve E113A, B, C, and JAP J99 cylinder heads were undoubtedly much easier to produce; certainly in casting complexity and machining fixtures and processed alone. The E114 suffix denotes the slightly larger swept volume of the Praga engine; 1904 cc compared to the 1860 cc of the E113/J99.

The Praga also uses roller big end bearings similar to the E107, E113 inc. A and B models. The E113C and the JAP J99 have plain bearing big ends. (Plain bearings might seem a retrograde set but my last JAP big end overhaul cost £180 all in, crank polishing and crack testing included).

None of this delightfully whimsical wind and pips has turned up a photo of G-AEYM. Would a kind person, competent in the use of photoshop or similar, be so kind as to doctor a photo of one of the others, pref. with that naughty Mr Joyce at the controls!

The Czech engine is described as a version of the E.114, not the E.113, so perhaps some of the differences between these two engines may be important?

The only difference described is that the E.114 had a smooth crankcase. Beyond that, I don’t know if there were any other differences between the E.113C and the E.114/Praga ‘B’ and, if so, what they were. Maybe someone else does?

Interesting it wasn’t formally struck off the register until February 1946 – presumably a post-war tidy up?

https://www.caa.co.uk/docs/HistoricalMaterial/G-AEYM.pdf

The Czech engine is described as a version of the E.114, not the E.113, so perhaps some of the differences between these two engines may be important?

However, I suspect that whereas the JAP engine will have had other applications, the Praga did not. It therefore makes more economic sense to sell the Hillsons with the Pragas and the JAPs as a separate deal.

Not having either a Czech Praga ‘B’ or a Jowett copy or the original Aeronca E.113C engine to examine or test, my analysis is based on what I’ve read. Ord-Hume discurses in greatest detail on the subject. He says:

‘While the original Aeronca E.113C twin-cylinder air-cooled engine was manufactured under licence by J.A.Prestwich Ltd as the Aeronca JAP J-99, the British made engine differed in having full dual ignition achieved through altering the mount for the single magneto to form a table on which two BTH magnetos could be attached parallel to the cylinders and driven by a bevel gearbox. The pressed aluminium rocker-box covers, ever prone to leakage, were replaced by cast aluminium caps with the lower edge cut into a notch to allow access to the lower sparking plugs. The heavy crank-shaft bob-weight was altered in shape to make it lighter without detriment to its effect, and other minor improvements made the J-99 an appreciably better better motor than the E.113C.’

I’m assuming, rightly or wrongly, that the Praga ‘B’ was an unmodified copy of the Aeronca E.113C and thus that Ord-Hume’s comments on the E.113C can be applied directly to the Praga ‘B’. If so and if Ord-Hume’s analysis is accepted, that is the reason why I have averred that the J-99 would seem to have been a better motor than the Praga ‘B’ and, on that analysis, expressed surprise that the Straight Corporation would find reason to replace the Aeronca JAP J-99 motors in their five Hillson Pragas with Praga ‘B’ motors within a year.

Why is the Czech built Praga B seen as inferior to the Aeronca E113? Like the E113 it has a better combustion chamber shape than the later parallel valve engines. Having studied mine for an hour or two the detail is delightful. Proper valve train lubrication, hemi head, a much tidier rear cover and oil pump layout. The twin magneto arrangement is very neat, far tidier than the JAP which is a (albeit clever) lash-up using skew gears to a pair of transverse magnetos.

There’s no doubt the JAP is a good engine too but the top end suffers unless modified to close the forward opening in the head and keep the lubricant on the valve gear and not the left side of the windscreen and the underside of the left wing!

Thank you, Chitts, for that additional information. However it’s most perplexing. Why replace the JAP engine with a Praga which, if my analysis in post #9 is correct, appears to be a retrograde step? But seventy six years later, I suspect that question won’t find an answer – for the answer seems likely to have been lost in the mists of time.

Having got home from work this evening I found I was talking from under my hat in post #7. The aircraft fitted with the JAP is clearly G-AEUU, one of the five advertised with a new Praga B engine; to add to the intrigue I checked the paperwork I was sent when I bought my engine. I was sent copies of documents for both engines although I only bought one, as a chap in Prague wanted the other for a Praga Baby project which is well advanced.

The other engine is engine number 532. The logbook for that engine shows it was fitted to G-AEUU on the 20th April 1939 and last ran on 14th August 1939!

So ‘avion ancien’ your theory of replacing the JAP engines with Czech built Praga B engines is proved correct, at least in the case of G-AEUU. Most intriguing indeed?

Continuing briefly to drift, I retain a nostalgia for the old Birmingham Science Museum. I spent many a lunchtime wandering around it when I worked a short distance away. It was a real museum – not an historical theme park costing nigh on £13 to get in. If I want to think tank, I’ll go the the museum at Bovington!

Haw, haw, that did make me chortle!

Well spotted AA :applause:

But I think you’ll have to explain it – there’ll be forumites unable to sleep tonight trying to work it out!!

Interesting to read of the relationship between the Aeronca/Czech Praga/British Praga/Jowett, set me thinking about the old Birmingham Science Museum and they would have probably been interested in a Jowett aero engine.

By coincidence the B’ham S.I. have (or did have) a Gnome-Rhone radial engine that was in a ‘relationship’. Gnome-Rhone built Bristol Jupiters under licence(?) but started to redevelop which Bristol became very unhappy about. They designed and built their version and despite court action by Bristol somehow interested Alvis to build it in the UK. Alvis, in turn, via the Alicedes and the Pelides ultimately came up with the Leonides – a worthy successor to the Jupiter!

Sorry about thread-drift back to the Hillson Praga

Roger Smith.

Interestingly, there is an advert in the June 1st and June 8th 1939 offering five Hillson Pragas for sale. They are G-AEYM, G-AEYL, G-AEUU, G-AEUT and G-AEUP; all advertised as ‘recently been fitted with new dual ignition 40 hp Praga “B” engines’

These would appear to be the five Straight controlled flying clubs’ Hillson Pragas. So maybe they had not just two but three different engines in their short lives before the outbreak of war? Maybe the Czech Praga engines were more reliable than the Jowett licence built versions? Whilst not addressing the subject of Czech built Praga engines, as to the Jowett built copies Ord-Hume (‘British Light Aeroplanes …… 1920-1940’) says:

‘….. the Jowett built Praga ‘B’ , built from non-approved materials and therefore preventing the aircraft from obtaining a Certificate of Airworthiness, proved troublesome and many were re-engined with the Aeronca JAP J-99 engine before the war. In this form, they were eligible for full certification. The dry weight of the JAP was marginally greater than that of the tiny Praga ‘B’ which weighed just 99 lbs.’

and earlier in the same essay on the Hillson Praga, as to its original engine he says:

‘….. the engine [i.e. the Czech Praga ‘B’] was copied and manufactured by Jowett Cars Ltd, one of the associate companies of Hills. The British-made engine was known as the Praga ‘B’ and was, to all intents and purposes, identical with the Czech Praga ‘B’ which, in turn, was a replica of the Aeronca E.114 smooth-crankcased variant of the E.113C motor as used in the Aeronca aeroplane.’

The Aeronca 100 was powered by the Aeronca JAP J-99 which was, effectively, the Aeronca E.113C engine built under licence in the UK by J.A.Prestwich Ltd. but significantly improved in material respects. Thus it might be said that the Jowett built Praga ‘B’ engine was a poor copy of the Czech original, whereas the JAP J-99 was an improved copy of the original original American Aeronca engine from which the Czech Praga ‘B’ derived. And if this is so, why would the JAP J-99s, retro-installed in the five Straight controlled flying clubs’ Hillson Pragas, ultimately be replaced by the seemingly poorer Czech copy of the Aeronca original? Or maybe there weren’t. Maybe what the 1939 advertisements in ‘Flight’ [?] were describing as Praga engines were, in fact, the JAP J-99s. For amongst the improvements that J.A.Prestwich made to the Aeronca E.113C was the introduction of full dual ignition.

Yes, it turns out the motor was stored in the garden shed of one William Joyce for the duration!

Haw, haw, that did make me chortle!

Yes, it turns out the motor was stored in the garden shed of one William Joyce for the duration!

My motor is a Czech-built example and was fitted to G-AEYM in April 1939 and, according to the logbook, was still fitted at the time of the prang. Interestingly, there is an advert in the June 1st and June 8th 1939 offering five Hillson Pragas for sale. They are G-AEYM, G-AEYL, G-AEUU, G-AEUT and G-AEUP; all advertised as ‘recently been fitted with new dual ignition 40 hp Praga “B” engines’.

In the Richard Riding Ultralights books there is a photo of G-AEEU (not to be confused with ‘UU) and it clearly has a JAP engine fitted. It’s the only photo I’ve seen of a JAP engined Hillson Praga, or more correctly a Hillson JAP?

Hmmm, it seems to me that as this was surplus & useless in 1939 – it should likely have been collected as salvage and re-cycled to build a Spitfire. Hence the person with-holding valuable materials at a critical time was a criminal. Not sure whether the current owner can be pursued for complicity in the earlier crime.

That’s confirmed by Jackson (‘British Civil Aircraft’), who said:

‘Production of the British model, known as the Hillson Praga, began at Manchester in 1936, the Praga B engine being built under licence by Jowett Cars Ltd.’

and as to reliability, he went on to say that:

‘…..trouble was experienced with the Praga motors, and all five [being those supplied to the Straight controlled flying clubs at Ipswich, Ramsgate and Weston-super-Mare] were fitted with 40 h.p. Aeronca J.A.P. J-99 Mk. 1A two cylinder engines during 1938’

One might deduce that G-AEYM was one of the Pragas so modified. So maybe the Jowett built Praga engine had been removed from the aircraft before it suffered its terminal accident in 1939. If so, that might explain why it remained in a good condition – and possibly why the chap in Kent had more than one of the engines (if five had been removed and substituted with Aeronca J.A.P. engines).

Seems the engines were produced.There’s an article on the Praga in the Air-Britain Archive of September 2011 in which it states –

“Hills laid down a production line of 35 examples (of the Praga)…Of these 25 were registered in the UK, 3 in Australia and 7 were not completed….”

“In a number of cases, the Jowett-built Praga engines gave trouble and were replaced by 2-cyl 40hp Aeronca JAP J-99 engines.”

It came from a well known vintage aeroplane owner in Baginton, Coventry. He acquired it (and another) from a chap in Kent around twenty years ago, previously storage history unknown. It has done 107 hours from new; I pulled the cylinders off for a quick look and it’s as clean as a whistle inside although the piston rings were stuck. Now soaking in Duck Oil until my next home run later this year. A good clean and careful reassembly should have it in running condition. It doesn’t appear to have suffered any damage from the prang at Weston Super Donkey.

Gosh. Where’s that engine been lurking for the past seventy six years? G-AEYM is recorded as having been written off following a crash at Weston-super-Mare in September 1939.