Remanufacturing A Rotary Airplane Engine

If someone tells you they have seen a rotary engine, the chances are that you will immediately think of a Wankel engine, as you might find in some of the more exotic Mazda sports cars. But there is another rotary engine that has a prior claim to the name, and it can be found as the power unit for many early-twentieth-century aircraft. In these rotary engines the cylinders are arranged radially around a stationary crankshaft, and it is the engine itself that rotates. They have the advantage of extreme simplicity, smooth power, and a low parts count, at the expense of total loss lubrication, a relatively large rotating mass, and some difficulty in controlling their power. These rotary engines were largely obsolete by the 1920s, but  recent upsurge of interest in WW1-era aircraft has led to the creation of a small demand for them. New Zealand based Classic Aero Machining Service have stepped in to fill that gap and are remanufacturing the Gnome radial engine, the most numerous design of that era.

For anyone with an interest in internal combustion engines, the Gnome is a fascinating study. It’s a nine-cylinder design that runs a four-stroke Otto cycle, but instead of the two or more valves you might be familiar with from your motor vehicle it has only a single valve. The so-called Monosoupape design uses its valve for both fuel and exhaust, opening it on the inlet stroke as well as the exhaust stroke. The simplicity of a single valve and no carburetor is thus offset by a difficulty in varying its power , so rotary engines would frequently reduce the number of firing cylinders in lieu of throttling back.

The CAMS Gnome is a faithful copy of the original, but with modern metallurgy and the addition of an electronic ignition system. The original castor oil is still used — it seems classic aviation buffs like the smell — but becuase it is notorious for leaving sticky deposits in the engine they are evaluating modern alternatives. They have some technical details on their website, and there’s a good chance you my hear one of their engines one day at an air show near you.

Thanks [Itay] for the tip.

64 thoughts on “Remanufacturing A Rotary Airplane Engine

  1. The wankel also uses its motor oil as lubricant for the rotor. Not total loss, but skip checking your oil or go too long a service or if the oil passage gets blocked you lose your motor!
    Best thing for old wankel motors is add premix oil! Probably wouldnt be bad for this either.
    In these you got to make sure your oil isnt pooled in the bottom cylinder or instant hydrolocked!

    1. Pre-mix on old motors is a waste of money, and is unnecessary. Rotary (wankel) engines are designed to use oil and have an oil metering pump that injects oil into the housings in relation to load and rpm. If you only use pre-mix that is worse as when your foot is off the gas no fuel is being injected and thus no oil is also being injected. Just service the engine regularly instead of band-aiding it with pre-mix or other “hacks”.

    1. No, it’s a Gnome Rotary.

      The crank is stationary the engine (cylinders, pistons…) rotates about the crank, yes it’s a huge spinning lump of metal attached to the front of an aircraft, which is why that’s not done any more but was effective in WW1

      A standard (“modern”) radial rotates the crank and the cylinders are fixed in place with the prop attached to, driven by, the crank so you only have the mass of the prop and crank spinning in front.

      1. And hardly anyone below bothered to notice the difference between “just a radial cylinder arrangement” and the much more unique rotary block engine shown here – or even watch the first few seconds of the video before jumping in to point out “someone is wrong on the internet”. Gheesh! I admit a little disappointment at that. I like Jenny’s picks of oddball stuff!

      2. From what I understand the rotating mass resulted in the extremely slower turning of the plan in one direction vs the other. when being attacked from behind the attacker would have an advantage of knowing which direction the plan would likely turn.

        1. Actually, the Sopwith Camel could do a 270-degree turn in one direction, assisted by engine torque, faster than any other aeroplane could turn in any direction. This was well-known by the operators of these aeroplanes and was used to great effect by Camel drivers.

          This is one of the reasons why Camels shot down more enemy aircraft than any other type during the first world war, and that includes competitors such as the SE-5/5A and the wonderful Dolphin.

          If you want to read an excellent account of air fighting, written by a bloke who was there and flew Camels, look for a book titled “Winged Victory”, by V M Yeates. You can get it from Amazon, either as hard copy or an e-book.

    2. I first learned about rotary engines via the work of the British 20th century young persons author W.E. Johns, and his stiff-upper-lipped flying hero “Biggles”. When I was a kid they were still quite popular, and as someone who read a lot I devoured them along with everything else.

      Johns wrote from a position of experience, he’d been an RFC pilot in the first world war, and though he had to research contemporary aircrews for his WW2 books his tales of WW1 air operations bear the mark of authenticity. His descriptions of the Sopwith Pup and Camel in particular are spot-on because he flew them in action. He explained the difference between a rotary and a radial engine, and also described the Camel’s ability to do super-quick turns (IIRC to the right but not to the left) because of the gyroscopic effect.

      https://en.wikipedia.org/wiki/W._E._Johns

      Search “The Camels are coming” and you should be able to find a text version.

    1. I like the diagram, but according to this there is likely 2 valves per cylinder, one being a pop-it valve located internally on the piston. That could be difficult to diagnose any problems. Here is a similar design that I ran into on youtube https://youtu.be/tZmd7k33JWE?t=227

      I have seen on of the ww1 rotating engine airplanes flying at the air show.

    2. Thanks for that link. I knew WWI engines rotated like that, but I never really thought enough about it to question how they got fuel/air into the cylinders. I immediately thought it must have been through the pistons… and the animation confirmed.

      It’s interesting to see how they came up with clever solutions for some problems, but then totally missed what today seems like obvious solutions to other problems. I’m sure manufacturing capabilities imposed restrictions, but still, rotating the entire block and cylinders just seems insane today!

      1. Another huge advantage rotating the engine had was cooling the cylinders: they never had engine overheating problems, which were a recurring problem with WWII aircraft.
        More disadvantages than advantages, but the cooling was great.

    1. Definitely a rotary. Radials have stationary heads and rotating cranks. This has rotating heads and a stationary crank. The prop is bolted to the heads in a rotary. Driven by the crank in radial.

  2. So am I the only one who calls this type of piston engine a Radial Engine, not a rotary? To my understanding there are 3 types of internal combustion engines, based on how they mechanically produce power. They are Turbine, Piston, and Rotor. This is clearly a piston engine.

    1. Radial, while it describes the layout of the cylinders, is more commonly used for the modern variant where the cylinders (engine) is fixed in place, and the crank rotates.

      Rotary is used for the historical layout where the engine spins and the crank is fixed in place (prop attached to engine instead of attached to crank).

      You could call it a rotary radial, or radial rotary, but just calling it a rotary is easier, also why you should refer to a wankel rotary as a wankel not a rotary if you’re just using one word, in aviation circles.

    1. Normally you would be correct, but not in this case. The Gnome is BOTH radial (cylinder layout) AND rotary (the entire engine rotates). It actually has more of a claim on the term “rotary” than a Wankel engine does.

      1. I’m a pilot and never heard any pilot or engineers call it a rotary engine. 🙄 . The entire engine rotates? The only thing rotation is the crankshaft . The wankel does rotates the crankshaft and rotors and that’s why it’s a rotary engine because it has rotors . Not rocket science mate

        1. This is a Gnome rotary. The engine rotates, the crank is stationary. The prop is attached to the engine.

          This is a historical WW1 era engine used famously in the Sopwith Camel amongst others. Yes, it’s a pretty silly design for such a large scale engine, but it was an effective engine for the time. As you can imagine it produces some interesting handling issues having a massive chunk of iron spinning around in front of you.

          It is worth pointing out that rotary engines of this style are still used… can you guess where… I’ll give you a hint, almost every single modern electric RC aircraft! BLDC motors used on aircraft are almost universally “outrunner” where the prop is attached to the motor body which rotates about the shaft which is fixed in place to the chassis!

        2. As DainBramage and James have mentioned, it is indeed a rotary engine. The crankshaft is stationary and the cylinders spin with the prop. WW1 dogfight manouevres took into account that it was way easier to bank and turn to one side than the other.
          I’m quite astounded that as a pilot you’ve not heard of this arrangement, but perhaps they don’t teach aeronautical history when learning to fly.

          1. Rotary engine is determined by the device transforming chemical energy into rotational energy ie a rotor, in the case of wankel a 3 lobed rotor instead of a piston as in this radial piston engine. It matters not whether the cylinders turn or the crank turns. this is still a radial piston reciprocating engine not a rotary engine imho

          2. @Todd3465 Then why don’t we call a plain old 4cyl engine a rotary aswell? It converts chemical energy into rotational.

            I’ll tell you: Because that’s not why rotary engines are called rotary.
            The Wankel is called a rotary, because the piston rotates.
            The Otto is not called a rotary, because the pistons do not rotate.

            A radial is not called a rotary, because the pistons do not rotate.
            A radial rotary on the other hand is aptly called a rotary, because the pistons (and cylinders and valves) do rotate.

        3. If you are a pilot you must not be much of an aviation enthusiast. It is a rotary and yes the crankshaft is fixed and the engine rotates. It was to solution to air cooling early aircraft engines. No you are wrong. You do not know what you are talking about. Just Google Gnome rotary engine and you will be educated.

  3. Never mind the rotary engine, I want to see more of that back-to-front pickup truck! (c:

    “…at the expense of total loss lubrication…”

    Anyhow, I’ve not read up on rotary aircraft engines, why does it lose oil? These engines were in long-range bombers which flew for hours in a single mission, how is it managed? They must’ve large oil reserve tanks in addition to fuel then?

    1. You are confusing Radial and Rotary.

      This is a Rotary engine, the whole engine spins, the crank is fixed. It is a total-oil-loss design because simply the oil mixed with the fuel and incomplete burned thus being spat out the exhaust valves all over everything – this is why pilots of the day wore scarves incidentally, not because it was cold but because they didn’t want to be breathing oil!

      Radial engines came along later, where the engine is stationary and only the crank rotates, it is much easier to have oil pumps etc on a radial engine so the oil is not burned (excessively) and kept inside the engine.

      Rotary engines were not used on large aircraft just on early fighters, but Radials certainly were for many many years.

      1. “this is why pilots of the day wore scarves incidentally, not because it was cold but because they didn’t want to be breathing oil!” And not jut that, since the gnome was lubricated with castor getting too much of the exhaust could make you have to “exhaust” as well, not a pleasant prospect in the cockpit.

    2. you did see the video attached? probably not. these engines are two stroke types, hence use a oil/fuel mixture and no oil carter. maybe a short catch up is in place?

    3. The later Skyraider, which had a radial rather than a rotary engine, had its maximum distance and flight time limited by how big the oil supply to the engine was, because it too lost oil. https://en.wikipedia.org/wiki/Douglas_A-1_Skyraider
      But rotaries like this were unusual in that (in many designs) the fuel and air used for combustion went into the crankcase and then was pulled into the combustion chamber through a lightly sprung, untimed valve in the piston body: when the combustion chamber pulled a vacuum because the exhaust valve was closed, the intake valve was pulled open and a new charge of fuel/air was pulled in. Since the crankcase was being used for fueling, it didn’t have any way to be sealed for oil, so they just flowed oil right through and then right into the combustion chambers, so it came out the exhaust.

  4. Aviation mechanic and pilot chiming in. These things didn’t have throttle control either so this is why you hear pilots blipping the ignition to intermittently shut down the engine to loose speed for landing and other things.

  5. I just love the internet experts here who didn’t bother to read the article or watch the video or know any aviation technology and history, asserting that they are calling this engine by the wrong name.

  6. I am a licensed aircraft mechanic in the USA. the folks stating this is a rotary engine are spot on. They were mentioned briefly in my mechanic schooling. They have tremendous torque issues which the pilots of these aircraft (the ones that survived their first flight) used to their advantage. They would turn abruptly, due to the enormous gyroscopic effect of having such a huge mass of rotating machinery. Pushing forward on the stick would, in addition to the expected effect of causing the plane to enter a dive, would turn sharply to the right. This made it extremely manueverable in aerial combat. Pushing on the edge of a gyroscope, will move the spinning mass as though the force had been applied ninety degrees later. Google “gyroscope” for a better explanation than mine. Or try it with your leaf blower. Try to turn it in any direction and it will fight your efforts and try to move in an unexpected direction.
    Whoever remarked about the public’s propensity for pointing out possible errors on the internet before getting all of the facts has it right.

    1. BS! You know nothing! Read the text and watch the video. And if you have enough money, buy LJK Setright’s book “The Power to Fly – The Development of the Piston Engine in Aviation” (George Allen & Unwin Ltd, 1971). This has detailed explanations of how ROTARY engines work v RADIALs and THEY ARE NOT THE SAME THING!!!!!

      1. The problem with such open Forums is that there are always one or more ‘Know-all’ contributors who can’t stand an opinion or fact that contradicts their view.

        To cement his opinion as being correct to the forum labels such as ‘BS’ and/or ‘You know nothing’ follow in reply without knowing who I am or what I do.
        Remember: “When the debate is lost, slander becomes the tool of the loser”.

        In fact they themselves are mostly best described by their own attributes they are spreading.
        A problem also is that the moderators condone such comment which is why that forum is not for me.

  7. From the Video – Engine #1 went to TAVAS in Caboolture, Australia and installed in a replica Eindecker. My BiL is tangentially involved and explained the Eindecker to me – it doesn’t have control surfaces like aelirons and elevators – it’s a “wing-warper” where control cables pull directly on the outer trailing edges of the wing and tailplane to effect control.

    TAVAS have a couple of flying WW1-era replicas:
    http://www.tavas.com.au/the-aircraft.php

  8. There is a guy who sells plans for model radial engines and is working on a model Gnome rotary engine- ageless engines. Google it if you want to see some incredible model engineering. I believe his grandfather started the company.

  9. tell me again how this isn’t a rotary engine. I hope half of these comments were in moderation while the other experts posted, otherwise the commenters not only didn’t watch the video and understand the concept, they didn’t read the comments. they just wanted to snipe the article, poor show.

    1. I’m also more interested in (almost) no one else mentioning that the intake is via crankshaft and radial transfer ports on the cylinder, or vacuum poppet valves on the piston and that the poppet valve is just for exhaust, not intake *and* exhaust as wrongly mentioned in the article.

  10. The first time I ever heard a gnome rotary engine was during the dawn patrol at Sun ‘n’ Fun. Pretty interesting to wake up to an aircraft alternating between screaming 100% throttle and dead silent since killing the ignition was the only form of throttle control.

    Kermit Weeks “The Wizard of Orlampa” flies rotary engine aircraft at his Fantasy of Flight museum East of Tampa.
    https://youtu.be/098sgInsvj0

  11. Thanks for posting this , Jenny. I had already seen the video you included. Anyone interested in this subject should also look at these:
    Kermit Week’s Sopwith Pup restoration: https://youtu.be/098sgInsvj0
    Making a propeller for an SE5 : https://youtu.be/jn_90hoEc04

    This whole cottage industry is fascinating. The use of the most modern technology to manufacture new machinery according to either old plans or from 3D scans of old parts has become an actual thing. This is one of the best examples. An engine made to the original design, but with the latest techniques and using modern tech to improve reliability. In my opinion this completely in line with the originals. If the manufacturers in 1914-18 had had access to the tech we have they would have used it. This particular rotary engine design, according to LJK Setright’s book (see elsewhere in this thread!), it was built under license by numerous manufacturers, who all had ideas about how to make it better.

  12. Since for some reason there are so many “um, actuallys” in the comments who refuse to recognize that the term “rotary” has been used to describe Gnome-type engines for over a century, and that usage long predates the invention of the Wankel…

    Here are a couple pages from one of my prized books, “Aircraft Engine Maintenance” by Daniel J. Brimm and H. Edward Boggess, published 1939, when radials where extremely common and there were still a few rotaries kicking around. They initially distinguish them as “rotary radials” and “static radials” here, and for the rest of the book they follow the common convention of “rotary” vs “radial”. https://imgur.com/a/SK6t2RZ

    I blame the misinformation on Hollywood… you have to go to great effort to see a WWI rotary engine in action. Almost all original or replica WWI fighters that are currently flying use unoriginal radial engines, since they’re so much cleaner and easier to use. So, when you rent a replica Nieuport 17 to film for your WWI documentary, it probably has a modern Rotec or Verner radial engine, not the original Le Rhone rotary, and they never seem to bother CGI-ing in the correct spinning cylinders, because only dweebs like me would notice.

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