Radial engines
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  1. #1
    Fellow Frogger! Decca's Avatar
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    Default Radial engines

    A question for the bright ones amongst us.

    Why do radial engines always have an odd number of cylinders??
    Is it timing problems, cant link to crankshaft, vibration?



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    Gone Fishin' Ray Bell's Avatar
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    A good question to put to the boys in the Technical Forum of the Atlas F1 fora...

    www.atlasf1.com > Forums > Technical forum

    There might be something already there, do a search.

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    Fellow Frogger! Decca's Avatar
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    Ray. didnt find anything via the search.
    But very interesting site frequented by some pretty clued up guys.
    have added it to my favourites

    Saddened to hear about David McKay. Have lots of magazines from mid 60's onwards.


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    1000+ Posts Wildebeest's Avatar
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    Default Radial engines.

    Quote Originally Posted by Decca
    A question for the bright ones amongst us.

    Why do radial engines always have an odd number of cylinders??
    Is it timing problems, cant link to crankshaft, vibration?




    Decca
    Has it got something to do with one piston/rod assembly, the "master" rod has all the other rods attached to it. Possibly to balance the odd numbered engine?
    I've never wondered about this. As 7 or 8 year olds living in Kalgoorlie, we had the luxury of a WW2 aircraft dump to play in. The planes were Vultee Vengeance dive bombers. They had twin row 14 cylinder Wright Whirlwind /Cyclone? radials. Apparently the Vultee V was a good diver, unfortunately pulling back out of it wasn't so flash
    We went back one weekend to play and found the "scrappies" had torched them all. Just blobs of alloy and magnesium left.
    Last edited by Wildebeest; 8th June 2005 at 10:57 PM. Reason: add name.

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    Gone Fishin' Ray Bell's Avatar
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    Originally posted by Decca
    Ray. didnt find anything via the search.
    But very interesting site frequented by some pretty clued up guys.
    have added it to my favourites

    Saddened to hear about David McKay. Have lots of magazines from mid 60's onwards.
    Yeah, poor David's passing was noted with a thread in Froggy Chat as well...

    Did you post in the Tech forum? There must also be aircraft-based fora around as well.

    Oh, yeah, any time you want to get right into the Atlas Nostalgia Forum you'll find an incredible amount in there.

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    ade
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    Default I recall reading ......

    I recall reading something about avoiding purely axial motion between two opposing cylinders, a little like what happens with a yoke crank. Of course that was years ago i read that and that memory is as grey as the cells that contain it! Another vague memory is that there was a later day radial that did have a single bank of even cyclinders. But then again the memory fades.....

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    Fellow Frogger! Decca's Avatar
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    Ray, somehow missed the AF post on David Mckay

    I didnt post on Atlas as AF keeps me occupied at the moment, but may in the future.


    ade, that may be the reason, although there are plenty of boxer engines about. But the extra conrod connections onto the master/main conrod may foul things up. ??? dont know.. just summising

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    Default Radial - odd number of cylinders.

    This is a good question and I didnt know the answer - but it was easy to find on Google:

    It is quite logical when you look at it:

    There is basically only 1 big-end journal on the crankshaft with all conrods attached to that big-end.
    So all the cylinders on one side of the engine are approaching top-dead-centre while the opposite side are at BDC.
    The firing order is every 2nd cylinder in sequence - and the cylinder in between is on its exhaust stroke (being four stroke engines).
    When the crankshaft comes around for the second revolution the other cylinders then fire - this requires an odd number of cylinders (or the same ones would fire again).
    I.E. - Firing order for a 7 cyl engine: 1, 3, 5, 7, 2, 4, 6, 1, 3, ....etc. (cylinders are numbered consecutively in direction of rotation - like a 7 hour clock face).

    A bit like doing up a 5 stud wheel nuts, if you tighten every 2nd one you end up tightening them all in a fairly even order.

    Cheers
    Fordman.

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    1000+ Posts Gamma's Avatar
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    http://www.radialengines.com/faq.asp

    This link should answer all questions.
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    1000+ Posts edgedweller's Avatar
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    it seems to me that an uneven no of pistons would ensure that the stresses developed by repeating a short pattern of stress that continuously progressed the starting point for that pattern shares that stress much better than an even no of pistons which would start a pattern, that by its nature, didn't progress and would repeat stress in a permanent pattern and create a harmonic that could shatter the engine

    ed ge

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    Gone Fishin' Ray Bell's Avatar
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    Well, that's (sort of...) the reason there's always an imperfect ratio in hypoid diffs, but I don't know about radial engines...

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    1000+ Posts Gamma's Avatar
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    Remember there are two types of radial engines.
    -one where the crankshaft moves, (cylinders remain static).
    -the other type where the crankshaft is fixed and the cylinders move with the Prop.

    The odd number of cylinders ensures even torque delivery, (very important for flight).
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    1000+ Posts HONG KONG PUGGY's Avatar
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    Quote Originally Posted by Gamma
    The odd number of cylinders ensures even torque delivery, (very important for flight).


    That kind of s self-explanitary I think, would make an ineteresting flight though. Self created turbulance
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    1000+ Posts Gamma's Avatar
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    Yes it sort of is self explanatory HKP.

    Its got to do with prop, pitch vs. revs vs. torque and some PITA thrust equation which I cannot remember.
    Is there and aeronautical engineer in the house. (Never one around when you need one).
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    1000+ Posts Wildebeest's Avatar
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    Default Radial engines and tennis?

    Radial engines and forward firing, through the prop, machine guns on WW1 fighter planes was fairly crude in its early operation, some of the bullets made it through the prop the others struck the deflectors on the blades.

    Along came the famous French WW1 fighter ace Roland Garros. He invented a synchronised firing through the prop method. Well you would wouldn't you if you were flying them yourself?

    Roland Garros had the Paris tennis courts named after him. He was named Roland after one of Ettore Bugatti's sons.

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    ade
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    Default While we are on the subject...

    While we are on the subject, I know there was a Swiss car earlier last century that had a 7 cyclinder radial engine by the name of Turbo and believe it moved shop to Italy later. Have there been any other cars that have used radial engines?

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    Gone Fishin' Ray Bell's Avatar
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    I seem to recall one, a racing car of the twenties...

    Can't remember what it was called.

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    Most replies have come up with the solution re the question of an odd number of cylinders for radial engines. The key of course is regarding the firing interval per Otto Cycle (720 degrees of rotation).

    This still remains a common question within aircraft engineering examinations (Power Plants), and the radial engine website fairly well covers the answer.

    Remember, if you see the specifications for a radial engine which says that it is a 14 cylinder, then it has two rows of cylinders, with 7 cylinders per row. Pratt & Whitney's 'ultimate' radial piston engine was the R4360. This had 28 cylinders (4 rows of 7 cylinders each) for a displacement of 4,360 cubic inches. The cylinders are arranged in a 'corn-cob' layout, which was an attempt to overcome the achilles heel of this engine, being insufficient cooling of the 3rd & 4th rows of cylinders.

    With radial engines, each row of cylinders will have a 'Master Rod' & cylinder, to which is attached the remaining number of 'Articulating Rods' to allow power transfer between the remaining cylinders and the crankshaft.

    Our organisation operates several aircraft with radial engines - two DC-3's with 2 x Pratt & Whitney 14 cylinder engines (7 per row), two Lockheed P2 Neptune aircraft with 2 x Curtiss Wright 18 cylinder engines (9 per row) and a Lockheed Super Constellation with 4 x Curtiss Wright 18 cylinder engines.

    The P&W engines on the DC3 develop 1,200 HP each, the CW engines on the Neptune aircraft 3,700 HP each and the CW engines on the Constellation 3,400 HP each. The non-availability (commercially) now of 115/145 AVGAS means that we limit our max power for the Neptune & Constellation engines to 3000 HP.

    With reference to radial design where the cylinders rotate around the crankshaft, these engines are referred to as rotary engines. This was a WW1 French design ( eg the Gnome engines), with the principle design benefit being improved cooling of the cylinders as the power outputs were increasing.

    The major disadvantage was the gyroscopic effect caused by having such a large mass spinning around the longitudinal axis of the aircraft. The pilots of the day could turn sharply in one direction, but had considerable difficulty turning in the opposite direction! Such examples have long been confined to aviation museums.

    For those interested, the Curtiss Wright R3350 engines (18 cylinder) are turbo-compound engines. This means that, apart from being supercharged (2 speed), they also have three Power Recovery Turbines (PRT's) per engine. These PRT's utilise the energy of the exhaust gasses (6 cylinders per PRT), & recover 150 HP per PRT at take-off power. This additional energy is transferred mechanically via a fluid coupling & dampner to the crankshaft.

    The PRT's function using the 'blow down' principal. This means that the exhaust gasses move from a region of high pressure (the cylinder) to a region of low pressure (the atmosphere). Thus, as altitude increases, the atmospheric pressure decreases, which improves the efficiency of each PRT as the blow down pressure differential increases.

    These engines are 3,350 cubic inches, and, with the final power figures being 3,400 HP + (depending on the model), were considered the ultimate in commercial aircraft piston engine design. Also appealing were the BSFC's (Brake Specific Fuel Consumption) figures. In cruise, figures of 0.5 & less are achieved - that is 0.5 lbs of fuel per hour per horsepower.

    Cheers,
    Kim.

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    Gone Fishin' Ray Bell's Avatar
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    Of course there's nothing like the Napier Nomad engine, is there?

    Off topic, not being a radial, but a fascinating engine...

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    Ray, have to agree with you - the Napier Nomad was an aero engine that had me fascinated from my earliest youth.

    It probably explains also why I was also captivated by the visits of Avro Shackleton aircraft to the Richmond (NSW) RAAF base during my air force years. These aircraft were the last link with the Avro Lancaster, and featured 4 x Rolls Royce Griffin engines with contra-rotating propellers.

    The Griffins were the ultimate development of the Rolls Royce Merlin, being a V-12 engine featuring fuel injection and several other refinements to bring their horse power up to 2500 & more. The last mark of the Spitfire also featured the Griffin engine. Every Shackleton that took off sounded like a squadron of Spitfires!

    But yes, the Napier Nomad...............

    Kim.

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    1000+ Posts Wildebeest's Avatar
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    Default Radial engines and other things.

    Kimdeb,
    Your experience with aircraft and also being around when Lancasters etc were having their "Last Hurrah" makes you very fortunate.

    My earlier post about my childhood in Kalgoorlie and the Vultees continued on into the '50's when we were lucky to see Lancaster, Lincoln, Mustangs and a Sea Fury or two with their carrier hooks on the tail, flying into the local aerodrome, that's what they called them then, not airports.
    They were quite accessible to see up close on the hard standing.
    I wagged school to get a look at the Sea Furies. They had radial engines that were started with a pistol. I scored one of the expended oversize shot gun type cartridges. I wonder where that went to?

    We were lucky kids to experience all this and later came the classic car thing.

  22. #22
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    Yes, it is a case of "where are they all now?", or, wish we had 20/20 foresight at the time to set about preserving some of these priceless classics.

    The Sea Fury was a very fast piston engined aircraft, and was fitted with a Bristol Centaurus radial engine, producing around 3000 HP from memory.

    This engine was unique in that the induction and exhaust functions of the Otto Cycle were controlled by sleeve valves, as opposed to the more conventional poppet type valves such as we see even today on motor car engines.

    The sleeve valve arrangement was quite complex due to the internal gearing needed to rotate the valves. However, it had the benefit of precise control of the induction & exhaust cycles, as well as making the engine much quieter than other radial engines.

    Cheers,
    Kim.

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    Fellow Frogger! Decca's Avatar
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    So it's all to do with timing. ( also applies to comedians)

    I didnt find much info at all on the web. The radial engine url given by Gamma didnt really say much. But was interesting that the cylinders counted counterclockwise. ???

    KIMDEB
    Funny you should mention the Connies. I asked the pilot of the connie at the recent Avalon airshow about the odd number of cylinders and he didn't realise that was the case.

    So when you have more than one bank/row of cylinders do you have to synchronise the timing between banks?

    When I lived in Rabaul, I took my step father (who during the war serviced aircraft) to an area that had many war time aircraft wrecks. He spotted an engine and said it was from an Air Cobra?? and said they had an engine in front and one behind the pilot with a connecting shaft under the pilot's seat which connected the two engines together .. apparently.. this is what I think he meant. Did these engines have their timing synchronised?

    questions, questions, questions.


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    Decca,

    Re your Connie question, pilots sometimes have to be forgiven in regard to detailed engineering type questions. The Constellation is one those aircraft from an era in which the pilot could not fly the aircraft without a Flight Engineer. In fact, the pilot does not even start the engines - that function, and many others are handled by the Flight Engineer.

    Thus, the Flight Engineer undergoes a very rigorous engineering school before being exposed to the aircraft operations. I have been there, and it is quite demanding.

    Re the Air Cobra, the engine was located behind the pilot. A shaft then took the power from the engine to the propeller at the front. So, there was only the one engine.

    Re timing of twin row radial engines, the crankshaft for each row is interconnected. Therefore, the firing sequence for the total engine is inclusive of both cylinder rows, as the engine cylinders are numbered around a 360 degree circle which includes all cylinders.

    Hope this helps,

    Kim.

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    Fellow Frogger! Decca's Avatar
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    Kim, thanks for the reply. Supplementary question if I may

    Re timing of twin row radial engines, the crankshaft for each row is interconnected. Therefore, the firing sequence for the total engine is inclusive of both cylinder rows, as the engine cylinders are numbered around a 360 degree circle which includes all cylinders.

    does one magneto/distributor feed both rows? I would had thought the case as motors are very compact

    As the second row is staggerred to get air flow. firing times are between the first rows firing times, which makes for a distributor with a lot of lobes. or are they set up that cylinders number one in each row fires simutaneously?

    When two rows are joined ..is the crankshaft joined so as the two master bigends are opposite? or is each row considered a balanced unit and it doesnt matter


    Decca
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