Rotaries

Spinning Jenny · 21 January 2004, 07:42 AM

Some years ago, a friend of mine gave me a book that the Hartford (CT) library was going to dispose of as obsolete inventory.

The title of this book is "The Airplane Engine",written by Lionel S. Marks, B.S., M.M.E. professor of mechanical engineering, Harvard University. The publisher was NcGraw-Hill Book Company, 1922.

This book is a engineering summary of WW1 engines, including the Curtiss OXX-3, Fiat 650, Benz 230, Simens-Halske double rotary Clerget 130 and a few more.

Marks makes a statement about the forces on the connecting rod in a running rotary that I can't seem to understand.

I'll include the actual text from the book-
" If the speed of the engine is increased the condition is soon reached when the resultant force on the piston pin is negative throughout the cycle. In that case the connecting rod would be under tension all the time ( a very favorable condition, permitting considerable lightening of the rod)
and the connecting rod might be replaced by a chain except that it is under compression before the engine has attained its full speed."

Are there any enginering types out there who could explain how a connecting rod on its power stroke can still be under tension? No math please!

chip55 · 21 January 2004, 12:21 PM

Hello
I'm not an expert but here goes my 2 cents worth.
I wouldn't put it as under tension during the power stroke , I would call it pressure from the combustion force against the pin and rod and the crankshaft to make the engine mass rotate. I don't see how a chain, being flexible, could go this.
I have seen specially designed chains in my work that have some stiffening properties that enable them to flex one way and not the other, (a lobe on the link that keeps them from flexing one direction or the other for electrical and air applications in robotics) but I don't think this was available then.

be well
chip55

http://igus.com/echain.htm

jumpinjan · 21 January 2004, 05:06 PM

Maybe something like Conservation of Angular Momentum, just like an ice skater would bring in her arms (the mass of the piston), that puts a torque on her body (torque on the engine & prop mass) and she will revolve faster? The difference with the engine & her is that the exploding gas in the cylinder is putting energy back into the system or doing work so the engine should sustain itself rotationally? I'll tell you this, those people that were building airplanes back in the 1916 were pretty clever!
Jan

Wind In The Wires · 21 January 2004, 05:19 PM

I think what he was saying was that there is a rotational speed above which the centrifugal force trying to throw the piston away from the center of the engine would be greater than the force on the top of the piston exerted by the combustion of the gas/air mixture. The engine would continue to run because the explosive force would try to drive the cylinder head away from the piston, forcing it to rotate in a direction which would increase the distance between the piston and cylinder head. Find a cutaway view of a rotary and it will become clear.

By 1922 the rotary engine was pretty much a dead issue and that idea never got pursued.

BTW, Adams-Farwell automobiles (about 1903-1916) were powered by 5 or 7 cylinder rotaries. They also featured tiller steering from the front or rear seats.

baldeagle · 22 January 2004, 05:27 AM

AT the Musee des Arts et Metiers in Paris they have a rotary engined motorcycle.

(and a really cool voiture helice)

chip55 · 23 January 2004, 01:41 PM

Hello
I posted this several times before, but it may be of interest here.

be well
chip55

Hmm It always comes out smaller each time

Langdon · 23 January 2004, 04:18 PM

Quote:

Originally posted by Wind In The Wires@Jan 22 2004, 12:49 PM
[b] I think what he was saying was that there is a rotational speed above which the centrifugal force trying to throw the piston away from the center of the engine would be greater than the force on the top of the piston exerted by the combustion of the gas/air mixture.

Although surely this could not be correct. The crankshaft is stationary, being bolted to the airframe and so the conrods and pistons are as well. There should really be no centrifugal force on the pistons unless it is caused by sideways movement. The pistons go up and down in the cylinder because their centre of axis is different to the rotational centre of the engine.
My understanding is that the pistons exert a sideways force on the cylinders thus causing the engine to spin. Possibly the faster the engine spins the more the downward force of combustion moves to a sideways force?

Langdon

FlyXwire · 23 January 2004, 05:05 PM

The pistons are reciprocating within the cylinder barrels that are attached to the rotating crankcase.

The pistons are therefore rotating as well, and are also subjected to centrifugal force.

Langdon · 23 January 2004, 07:05 PM

Yes you are correct - it takes a bit of getting your head around - they are pulled around the trust block by the cylinders and therefore have the same gyroscopic force applied as that applied to the cylinders. It would be interesting to know at what RPM this would cancel out the powerstroke effect on the connecting rod and if the engine could withstand such an RPM.

Langdon

Wind In The Wires · 23 January 2004, 08:37 PM

Which brings up another question. The first rotary engine I'm aware of is the Balzer/Manly that powered Langley's Great aerodrome. Were there earlier rotarys?