Pobjoy potterings
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Pobjoy potterings
On a slightly different note I managed to obtain two crated Pobjoy engines from a back garden in South London (Streatham) (pre interenet and web sites) but only because the owner of said engines had some vegetables delivered wrapped in some pages of the then popular Exchange and Mart. He happened to unwrap the package thus exposing an advert in the Aviation section for Pobjoy Spares. Ten years later we finally concluded a deal !!!.
POBJOY....... Just as matter of interest are those two engines now flying/airworthy...??
Last edited by Planemike; 26th Aug 2021 at 07:59.
Thread Starter
Pobjoy potterings
The other motor had been in the bottom crate and this had rotted allowing the cylinders to rest on the ground. Guts inhibited and ok but not needed yet, and has the very rare Rotax mags which run at half engine speed. No I engine ran fine on Australia attempt until rotor arms gave up over Gulf of Oman requiring a short 'holiday' in IRAN.
PJ......... Thank you for your comprehensive reply. Ten years to strike a deal....!!! You were both patient & persistent...!! Worth it in the end of course!!
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Trying to strike deal
The matter came to a head (or rather cylinder head) when no 1 cylinder on the original R engine suffered an exhaust valve failure on take off from a local strip, although I managed a hurried landing the valve penetrated the piston and rather spoilt my day. I was in another protracted 'deal' to buy a genuine 42 Willys Jeep (in a shed on a local farm) but had to 'overshoot' to try to get a replacement motor for the Swift. This required another attempt for the Streatham Pobjoys which took a couple of years and the Jeep money. The R engine has no proper valve lubrication so with the Niagra I trusted with its oil fed rocker heads the problem would fade away. It was better but during a ferry trip to Redhill for yet another show there was another 'bang' and I landed straight ahead in a pasture near Dorking. This time the piston held so it was not a serious situation for the innards. To cut this brief, I decided to consult with a serious manufacturer of vales for the racing car world and was invited up to Godalming for a investigation. This was an absolute superb visit as they decided immediately that the material of the original valves was totally useless for reasonable use, and that was the main problem rather than lubrication. They fixed me up with a replacement set in a modern material (and slightly better tulip design) with the classic statement (they will never break) and they never have since.
This became a PFA/LAA approved mod eventually many years later when they found out.
PJ.... Interested to hear about your adventures "commuting" in the Swift from Cornwall.. Pleased you sorted your valve problems, others in PFA/LAA will no doubt be grateful for your efforts....
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Swift flutterings
The Swift was frequently the oldest machine at a show so always made welcome, and with the folding wings always could be found 'shelter' in our delightful weather which included snuggling in with the fire engines on several occasions.
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Thanks joli, ordered a s/h copy via Amazon!
Pobjoy, certainly the Swift was the oldest aircraft at one of the first shows I ever attended, 'CDC was probably there as the second oldest!
Pobjoy, certainly the Swift was the oldest aircraft at one of the first shows I ever attended, 'CDC was probably there as the second oldest!
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Pobjoy Probings
The two engines in question had been purchased 'post war' by a former RFC/RAF pilot who intended to put them in a Piper Cub, (That would have given a Cub some grunt) but no doubt the authorities would have scuppered that. The were both Niagra's as alluded one BTH and one Rotax, in crates, and complete less exhaust systems. The bottom crate had rotted so several of the cylinders were corroded. The BTH version therefore went in the Swift (and is still there) with the Rotax kept for spares. As for the R type indeed these were supplied with aluminium valve covers to keep dust out, but devoid of lubrication other than grease. All types had valves of low service life so were replaced with a modern material that is reliable. The other nuisance is the collector ring exhaust which would be fine (but expensive) in regular use so I substituted stubbs, and never had to change them. Remember I would do many hours in a display w-end so needed reliability. Great engine albeit (needs fettling) and watching of mag drives, but excellent power to weight ratio and thrust plus the afore mentioned FUN in operating such a fine piece of engine engineering. (a sooty face with white eyes) (behind the goggles) indicates a Pobjoy with stubbs), especially after a long flight. But the pilot will be smiling. PP
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My R had once been in a Fahlin SF1,I suspect it had been sent back to Pobjoys at Hooton Park for an overhaul.
The power to weight ratio of a 7 cylinder original Pobjoy was incredible and the 'Thrust' with the reduction box well ahead of a basic American flat 4.
In fact the reduction box on a Pobjoy is a work of art in itself, and is built with the precision of a swiss watch.
Once you get over the valve issues, the rather speedy running of the BTH mags was countered by the half speed Rotax version.
Cooling and the basic oil system is not an issue but I suspect the cost of the unit was rather high and the market for 90hp catered for by a manufacturer that also made light aircraft (DH) . With the factory destroyed by bombing and Pobjoy killed in a post war accident there were no more designs from this talented designer.
The Pobjoy was essentially the equivalent of a 'racing' unit with weight (lack of) a major factor. The rotating parts are minimal but strong and 'polished'. The pistons are only just big enough to hold the rings and are both sculptured and polished. The aforementioned reduction drive gives it incredible thrust and is totally reliable. The associated two blade prop looks like a railway sleeper rotating ahead of you. The combination gives the Swift a very nimble performance.
The power to weight ratio of a 7 cylinder original Pobjoy was incredible and the 'Thrust' with the reduction box well ahead of a basic American flat 4.
In fact the reduction box on a Pobjoy is a work of art in itself, and is built with the precision of a swiss watch.
Once you get over the valve issues, the rather speedy running of the BTH mags was countered by the half speed Rotax version.
Cooling and the basic oil system is not an issue but I suspect the cost of the unit was rather high and the market for 90hp catered for by a manufacturer that also made light aircraft (DH) . With the factory destroyed by bombing and Pobjoy killed in a post war accident there were no more designs from this talented designer.
The Pobjoy was essentially the equivalent of a 'racing' unit with weight (lack of) a major factor. The rotating parts are minimal but strong and 'polished'. The pistons are only just big enough to hold the rings and are both sculptured and polished. The aforementioned reduction drive gives it incredible thrust and is totally reliable. The associated two blade prop looks like a railway sleeper rotating ahead of you. The combination gives the Swift a very nimble performance.
Last edited by POBJOY; 26th Sep 2021 at 19:49. Reason: content
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Bit late on the drift now so may as well finish off. The J3 Cub 'as was' with its A65 hp cont was in fact perfect for Army use and the equivalent of an airborne jeep. In fact I am a big fan of the A65 as a no nonsense unit. Like the Merlin the reduction box on a Pobjoy is the final piece of the power train even though the Pobjoy version is probably rather over engineered (good job to).
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I am glad so many people still admire the quality and power ratio of these beautiful units that are such a delight to operate. They are indeed a tribute to a standard of design and production excellence that still deliver the goods. As you will see from the 'cutaway' an errant valve (usually a top one) dropping into that rotating mass does rather spoil your day and render the guts to scrap. PP
Hmmm... I doubt much will be left after nearly 90 years.......!!! Could even be buried...
Re: Alf Knowles referred to in Msg 16. Is he the he the Alf Knowles of Luton Duet G-AYTT fame...??
Re: Alf Knowles referred to in Msg 16. Is he the he the Alf Knowles of Luton Duet G-AYTT fame...??
Last edited by Planemike; 30th Sep 2021 at 09:27.
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The exhaust valves** on both the R and Niagara are equally unreliable, however the increased lubrication and 'sealing on the later engines did give them a better service life. **It seems that 40 hours were considered to be 'life'. My 'Valve man' in Surrey was quite dismissive of material quality and the lack of a decent tulip shape, but in all fairness they were over 50 years old with little history. PP
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Power to cost ratio
The drawback with our 7 cylinder set up would be the cost issue. It would only be viable with utilising a cylinder that could be used elsewhere (motorbikes) (industrial units) that were mass produced. Even then for a typical 150 hp unit the flat four does rather fit the bill. Plus the radial exhaust system would always be a high maintenance/cost item.
