The Merlin and Allison both have prop speed reduction gear cases integrally built in the front of the engine. The only fairly successful airplane that used a detached gear box would be the Bell P-39 where the engine was a mid fuselage mounted affair with the output shaft running under and almost between the pilot's legs. They had a distinctive sound and you knew when was was coming over. The DO-335 had a shaft driven rear prop but no detached gear box. A question was posted asking if there had been wind tunnel tests on this Bugatti and as far as I can determine there have been none. They mention that the aerodynamics was looked at by a professor in Brazil . ?????. My aero guy buddy at the Kite Factory said that he would make sure that he was far away from the test flight especially the point of impact.
It is obvious that the props did not have a variable pitch mechanism on the Bugatti 100T if you look at the six-bolt attachment hubs on the front of the transmission. They are there to attach to a fixed-blade pair of props.
According to the 100T rebuilding website, one engine turns clockwise and one anti-clockwise. I found an exploded view of the gearbox, maybe exploded is the wrong word to use. From the lower renderings, it looks like they were considering variable pitch. The inner prop shaft end bearing appears to be located at the outer prop shaft at the inner prop. I don't know what type of bearing they use, it looks like a plain bearing like a car's mains have. Since plain bearings have a long life if they are lubed correctly with clean oil, they should last for a long time, unless one of the props gets out of balance, that would be bad. The shafts that holds the gears driven by the driveshaft are really short so it should have a straight rotation instead of out of round rotation which can happen on long unsupported shafts. Since it uses strait cut gears, it might not matter anyway since a helical cut gear has different bearing loading loads than what they are using. They must also not be too worried about noise since the straight cut gears are louder although in this case, those open pipes (x2) probably drown out all the other noises besides prop noise. It's a beautiful plane but I would build a scale model of it to test its flying characteristics as well as coolant air flow. They have the coolant flow coming in through one of the is eleron the correct turn for elevators in a v-tail configuration? What if they the break up the wind and there isn't any airflow into the coolers??? What happened to the exhaust outlets? I saw apic of them fitting them and now I can't find it. It must be there, I'm just laming out on the search function; that and I don't use facebook. Who is going to be the first pilot? I pray that this thing doesn't cost any lives. Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login
If they were going to use the constant pitch props in the lower illustration, there would have to be some arrangement made for prop control - which I do not see in the drawing. I think it is more likely they will be using the solid wood props shown in the upper picture. But, like I said before - this transmission, and the type of props, are the very least of the problems with this very scary design.
The aren't using constand speed props. On their 'production' gearbox, it clearly shows that they are using just wooden props without any way to change the pitch. I just showed the other gearbox to show that they were considering it. Maybe they were worried what happened to Howard Hughes might happen to them.
Yup. There have been about as many manifest failures in contra-prop designs as successes - if not more. An exception might be noted with the big Russian turboprop Bear bomber - but they only turned 750 RPM. However - if they were much worried about the Howard Hughes accident perhaps they should have thought better about building this thing at all. One other question I had - are they actually trying to use vintage Bugatti engines? If so, how in the world did they find a counter-rotating one?
They surely would not have the torque required for flying. Aero engines usually (I believe) sit on like 2000 rpm so they are unstressed and reliable ... you could not have these motorcycle engines sitting at 8000 rpm for hours on end. And those vintage Bugatti engines would have been much happier to run at 3000 rpm sort of range. Pete
They are NOT using Bugatti engines. The Suzukis will run for hours on end at 4-5k rpm; easily. You need acceleration torque why? They'll run at pretty much a steady speed. Props are geared. Shoot, you don't even need to run an engine backwards, do it with gearing.
Yes aware of that, my point was in reference to the original design and engine torque characteristics. Bugatti versus Suzuki. True but that is unusually high rpm for a plane reciprocating engine. Never said they needed acceleration torque, I said they need torque and a Suzuki motorcycle engine at usual aero engine rpm of around 2000 rpm will have none ... even at 5000 will have little. There is a reason why a Spitfire engine is massive, not 1300cc (even 2 x 1300cc). True, but the design of the gearbox that drives the counter rotating props requires one engine to be running backwards otherwise both props will rotate in the same direction. Pete
A Couple of neat pictures from last years EAA Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login
Prop pitch is controlled by controlling oil pressure to the prop hubs. It looks like there are two oil lines that would be in the right places to do that in the pictures above. The first is at the back of the inner shaft, there is an oil line that is supplying oil there and the line for that runs behind the bulkhead. That would control the front prop. There is also an oil line on the outer nose case about half way down the outer shaft that is likely the oil supply for the rear prop. So it looks like there is a provision in the design for controllable props. If they use fixed pitch props they can just cap off those lines. With regards to engine speed, what you want to look at is piston speed, which is really comes down to stroke x rpm. Shorter stroke engines can spin faster, longer stroke engines spin slower. Many high performance engines have used geared designs, from the V1710, to the Merlin, to some big radials. The amount of power and the flight speed and density altitude sets the prop diameter, and from the prop diameter you back out a prop tip speed near Mach 1. That sets your prop shaft speed, it has nothing to do with the engine. Bigger engines need to turn slower, but the mechanical limitations of the stress in the crankshaft and piston assembly are such that they allow a higher rotational speed and therefore (since torque directly related to displacement and power = torque x rpm/5252) you make more power if you spin the engine faster than the prop. Bigger, slower turning engines don't require gearing, but also are heavier than if you run a smaller displacement engine nearer to its mechanical limits. For some kind of racing or high performance aircraft it makes a lot of sense to gear the prop. But for that you also get higher piston speeds, and less engine life, but as I said, if you want short term speed, you want higher rpm and that all makes sense. For a real world example, a 540 cubic inch Lycoming makes between 250 and 300 hp and weighs about 535 lbs (if my memory is correct) at about 2500 rpm. A 427 cubic inch Corvette LS7, fully dressed, makes 505 hp at 6,000 rpm and weighs just shy of 400 pounds, and that includes a flywheel which an aircraft version doesn't need. Weight of the LS7 with a similar level of dressing to the Lycoming, (no exhaust manifolds or flywheel) is closer to 350 pounds. So you can see there is an incentive to gear an engine in a high performance application where life is not a primary concern. Even with a gearbox the power to weight ratio available from a geared package can be almost twice as good as an engine spinning the prop directly. Been that way since before WWII, and that's why those fighter engines were geared. That said, there has also been a sad history of poor life from geared engines, which is why I'm not a fan of them. Since, in almost every case, the prop is the flywheel of the system, the gearbox gets whacked by high forces from the firing pulses, and unless the cylinder count is 12 or above, the gears actually see reverse loading (the prop is driving the engine between power pulses) which is hell on gear teeth. The original earl versions of the V1710 in the P40 had a planetary gearbox with a life of 20 hours or less. Later versions had a more conventional pinion and bull gear setup, that was more reliable. Geared engines make more power, but don't have a good history of living very long. There are a lot of geared engines in the homebuilt world, most of which don't live very long either for all of the usual reasons.
Possibly - you cannot tell really what it has underneath the spinner in those photos above. Except that the spinner openings do not look like they have clearance to vary the pitch much (if at all). However, this thing still looks unflyable to me at for at least several other reasons. Maybe it will end up as a display piece and just get taxied around a little and shut down before it overheats too badly.
Not implying that it has variable pitch in the display article, just noting that there is likely a provision for it in the design. Otherwise there isn't much reason to put oil into the center of the shaft, that's all. And yes, it is most likely a stability and controls engineers worst nightmare, but then so is the GB... I just pray that it doesn't end up in a ball at the end of a runway or a smoking hole in a farm field...
Exactly. We had a guy at the little airport where I kept my Citabria who bought a real, live, Formula 1 racer - simply because it looked so pretty. He stalled and spun and made a smoking hole at the side of the runway on the very first takeoff. (did not have the experience for this thing, and several people told him that - but it was just too pretty)... I hope something similar does not happen here...and I have more confidence in that Formula 1 racer being able to theoretically fly than this thing. I suspect that almost any pilot with the experience and maturity to actually attempt it would just walk away after study of the problems involved...leaving it to someone less experienced and more enamored of the shape.
Okay I think for some reason I'm not coming across clearly. While some might fly a plane with a converted car engine, I wouldn't be too keen. There is a reason why piston engines designed for flight rev considerably slower than car engines and it is all about reliability. Yes the internals might be stressed the same but with the higher revving engine there is a lot more going on that has to keep going on dead right for the plane to stay in the air. Yes Suzuki make great motorcycles (I used to own a GS500) but I would have not thought they would be the first choice for an aero engine ... definitely superior though than a Bugatti engine. I'd rather a current generation Chev v8 turning over at 2 to 3000 rpm, maybe supercharged to produce the necessary torque. Pete
Remember the Pond Racer of the early '90s? It was powered by two Nissan automotive engines that gave a lot of trouble and ultimately resulted in the loss of the aircraft and the death of the pilot.
Recently in Australia a 3/4's Spitfire replica crashed resulting in the death of the pilot, due (possibly) I believe to engine issues. It was powered by an Isuzu truck engine ... http://www.news.com.au/national-news/pilot-killed-in-replica-spitfire-plane-crash-at-salisbury-in-adelaides-northern-suburbs/story-fncynjr2-1226599160065 Pete
It was one of his, but there was nothing really wrong with the airframe. I"ve seen some 3/4-scale P-51 replicas with Chevy 454 engines that ran quite well. I think it all depends which engine you choose, and how well it's cooled.
