# Jet engine design question

All jet turbine engines have a set of guide blades on the case, and a rotating shaft with the turbine blades.
Would it be possible to build an engine with a static shaft, and a rotating
case ?
Possible advantages of such a designee might be a more compact engine, with easier ducting of a bypass air fan.
--
Jonathan

Barnes's theorem; for every foolproof device
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"*All jet turbine engines have a set of guide blades on the case, and a
rotating shaft with the turbine blades.
Would it be possible to build an engine with a static shaft, and rotating case ?*"
The flow between the tips of the blades and the case is critical to th efficiency of the engine. This is one of the reasons why the stators ma be made up in units with connecting bands between the tips and th roots, but the rotor blades aren't.
The flow is from root to tip thanks to centrifugal force. If you sto that flow by rooting the blades in the case, then you get an increase vortex at the root.
The clearance between case and blade is also critical because under th temperature and centrifugal force exerted on the blades they tend t creep, to get longer. By rotating the extra mass of the case yo increase the forces enormously (you have placed all the mass at th point of greatest velocity / acceleration instead of at the point o least velocity, the centre). As a result you will find that you wil need exotic materials like metal matrix composites a lot quicker.
Lastly, one of the purpose of the case is to contain the fragments whe the turbine suffers a catastrophic breakage. Rotating the case require another case, a much stronger and heavier case, to contain the muc larger bits and greater energy if that should break up. Which isn' good news if you are tryingto build a lightweight engine.
Technically it's possible, but why?
Rotating the case (cylinder) can be done with a reciprocating engin where the speed is a lot lower and you are replacing the mass dedicate to smoothing out the power impulses with an equivalent mass doin something useful .
regards Robi
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On Sun, 14 Aug 2005 10:07:37 +0000 (UTC), "Jonathan Barnes"

Interesting question: the rotary turbine would be supported through a front or rear stinger. I guess the forward stinger would avoid one or two problems of hot gas impingement - the stinger would feed fuel and control. The rotation rate of tunbines being high, placing more mass on the rotating periphery would increase the moment of inertia, it's true. That looks like the biggest objection
Brian Whatcott Altus OK
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Brian Whatcott wrote:

That looks to me like a pretty big objection. Not only would there be a bigger moment of inertia, but also (assuming the same RPM), more centrifugal force. The most massive part of a normal jet engine's rotating stuff is the shaft, which is a relatively small diameter compared to the whole tubine assembly. In order for a rotating shell to have enough strength, it would need to be considerably more massive than it is now, and therefore stronger, and therefore more massive, etc.
In a conventional design, the rotating stuff is held together at the shaft; and the failure (like a broken turbine blade) of any one component doesn't automatically compromise the rest of the turbine. It makes a serious and dangerous mess, of course; but doesn't necessarily turn the entire engine to shrapnel. With a rotating periphery, the shell of the engine would be held together by the tensile strength of a circle. Any failure at any part of the circle would probably allow the whole thing to fly apart, making a really big and really dangerous mess. And, of course, in a conventional design, all the moving stuff is contained by shell, which offers some protection from projectile vomiting when an engine gets sick. Not so when the shell contains all the energy.
I've sometimes wondered about the gyroscopic effect an engine has on an aircraft when the aircraft changes attitude. Obviously, designs are made to deal with that, or there'd be broken aircraft littering every street. But if the shell were to rotate, and to be heavier, and to have more inertia, etc., then it seems to me that any change in the aircraft's attitude would want to twist the wing's off, or, in the case of something like a fighter jet with a centrally located engine, would cause the whole craft to yaw dramatically in some real unpleasant ways.
In the early part of the 20th century (No, I don't remember any of this first hand), there was a biplane called a Neuport (NewPort?) that used a radial engine with a stationary crankshaft, and a rotating ring of cylinders. I have to imagine that this had some of the same problems. A bit of historical research might be interesting.
KG
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Supposedly there are old military videos of external rotor engines -- either rocket or turbojet -- flying apart in flight. Hoop stresses are all they had to work with to restrain the centrifugal forces.
I've heard of the air force "ringing" the blades in a conventional engine rotor so that hoop as well as radial stresses can restrain the rotational forces but I don't see how that wouldn't make matters worse. Adding more material to the outer tip of high speed rotors merely increases the centrifugal forces you were trying to overcome in the first place.
Maybe the ring can be cooled so that it has a higher tensile strength . . .
< In the early part of the 20th century (No, I don't remember any of
< this first hand), there was a biplane called a Neuport (NewPort?) that
< used a radial engine with a stationary crankshaft, and a rotating ring
< of cylinders. I have to imagine that this had some of the same
< problems.
Tip speeds and ave. operating temperatures in reciprocating aren't nearly as high as those in turbo so that wouldn't be the problem. Centrifugal force increases with the square of tip speed.
The rotating cylinder arrangement seems like the rpm would be a fraction of a conventional reciprocating engine -- higher torque but very low rpm.
Bret Cahill
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The fixed crankshaft, rotating cylinders reciprocating engine is a
arrangement known as 'Rotary' .
Thousands of these engines powered WW1 fighters.
http://www.aviation-history.com/engines/rotary-theory.htm tells you how they worked and why they were replaced by differen designs.
(Rotary engines have little in common with the modern rotary o Wankel)
Robi
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wrote:

A radial engine suspended by its stationary crank was called a rotary. This design was adopted because it helped reliability quite a bit - the cylinders cooled anytime the engine ran. The aircraft you have in mind was I think the Nieuport. A sample rotary was also French: the monosoupape.
Brian Whatcott
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A Mazda mechanic said the worst thing about Wankels is overheating. When you run low on water with an ordinary piston engine you pull over and get some water. With an aluminum block Wankel you call a tow truck because your engine is already shot.
Wankels should be air cooled by rotating the outside casing. There are no bearing, balancing or seal problems, no trouble injecting fuel or air from the center stational part.
A reliable 10 kW Wankel would work well with hybrids where space is at a premium because of its small size.
Bret Cahill
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On Sun, 14 Aug 2005 10:07:37 +0000 (UTC), in sci.engr.mech "Jonathan

Actually only true for axial flow turbines, there is a whole other class of radial flow machine, fyi. ---------- Ed Ruf Lifetime AMA# 344007 ( snipped-for-privacy@EdwardG.Ruf.com) http://EdwardGRuf.com
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rotating
with
Interesting question, however, and this is simplified so don't be too critical of my comments------The turbine shaft is what rotates the compressor--compressor has a similar set of stator vanes (guide vanes) as the turbine. In the compressor, the stator vanes are ganged together and are scheduled, that is, a different inlet angle as a function of compressor speed. In between the two are the combustor, diffuser and exhaust casings What about them and all the misc external components that are attached to the casings? Look at an engine up close and it looks like plumber's worst nightmare. There are many issues, for example, turbine speed is one of the control system variables and at high speed it can be one of the controlled parameters. Does one try to control casing speed? If so, turbines can run in the 17,000 RPM speed range with less that a mil P-P (.001 inches) vibration amplitude; balancing that mass would certainly be a challenge. Installation, have you ever seen an engine installed in an aircraft? The clearances between the engine and airframe are tight, in many cases you're lucky if you can even get a finger between the two. Can't picture a rotating casing in that environment. Besides that, bay cooling would be an issue as air is directed through the nacelle etc., etc. Broad coverage of a very complex topic--jet engines are a state of the art machine and one that is not fully appreciated. MLD
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Any radially symmetrical flow machinery can be turned "inside out." NASA and others going back to Goddard have already patented "exoskeletal" axial flow gas turbine engines. The idea was the centrifugal force would hold high temp. ceramic rotor blades in compression. Ceramics might deteriorate after an hour or so but that's a long time for a missile.
At least 2 major problems with the NASA engine other than high specific angular momentum:
1. The bearings were on the outside where tip speeds were too high for conventional bearings. You can always double up bearings but this isn't desirable on aircraft.
Maybe the bearings should have been located in the center with the rotor mounted on spokes?
2. The blade length was small compared to the overall diameter of the annular flow design. Turbomachinery tip clearances typically need to be 1% or less of the blade length. It might be difficult or impossible to keep the tip clearances down to a reasonable level once it got up to speed.
On the plus side you never need to worry about rotational strains in a rotor blade causing it to touch any part of the stator.
Is a reciprocating engine possible where the cylinders moved eccentrically and the pistons were fixed in a radial aircraft type engine? The valves and spark plugs would be located on the pistons . . .
Could a Wankel casing spin on a fixed rotor?
There are an infinite number of engine designs. Only a very few basic designs are worthwhile, however.
Bret Cahill
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