TALESPINNERS - March, 2003

Hangar Flying
with Stu Faber

WHICH ENGINE ?  The April issue of Custom Planes carries an interesting article on choosing a VW engine.  However it also discusses several general principles of hp, rpm, torque, prop size, pitch, and thrust.  If you are using a setup that a number of other builders have had success with that is probably the way to go.  But if you are on your own there are ways of comparing various engine and prop combinations at least in theory: Horsepower = rpm x torque, 3600 rpm x 117 foot-pounds of torque = 421,200.  5252 is the constant, so 421,200 / 5252 = 80 hp. ( / means divided by - as in a fraction).  Torque = 5252 x 80 hp = 420,160 @ 3600 rpm, 420,160 / 3600 rpm = 116.71 foot-pounds of torque.  Looking at horsepower and torque it looks like a VW engine of 76 hp and 121 ft-# of torque should easily replace a Rotax 582 which is rated at 65 hp and 53.3 ft-# torque.  But the Rotax uses a 2.58:1 reduction gear and can turn a 72" prop against the VW 62".  The longer prop will have about 25% more static thrust.  In general a long slow turning prop is more efficient.  Then there is prop pitch.  A prop needs to allow the engine to turn to its maximum permitted speed to develop full available power.  Prop length often governs rpm due to tip speeds.  The "rule of .65" says hp x .65 equals prop pitch.  As: 76 hp x .65 = 49 inch pitch.

NEW TECHNOLOGY
     NANOCRYSTALS are super durable particles which raise the strength of metals and plastics by as much as 300%.  Purdue Univ. has developed a cheaper way to use them.  Useful in all mechanical devices including airplanes.

     VIBRATIONS set up in materials by high frequency sound waves can be used to diagnose problems in everything from people to machines.

     ZIRCONIUM is a composite material that shrinks when heated and can be used to strengthen materials that are weakened by heat.  All the above from Kiplinger Letter 31 Jan 03.

AIRCRAFT ENGINE ENCYCLOPEDIA.  The book is an international listing of 435 airplane engines designed and built, up to the 1921 date of publication.  The variety of engines was surprising.  Nearly every configuration one could think of was tried.  Some were two stroke, some 4, and a couple 6, and don't expect me to explain 6 stroke.  General configurations were many: Rotary, horizontal rotaries with right angled gear boxes, radial, in-line, V-8s and V-12s, fan types with all cylinders above the centerline as well as W types with 3 banks of in-line cylinders, Y types, X types, opposed, and a number of barrel types.  Hardly any V-6s.

   EXAMPLES
      EDELWEISS (Swiss ?) This was a really unusual 6 cylinder radial engine.  Not enough details but front and rear pictures show a round engine made up of a cylindrical case outside the cylinders.  The pistons were attached to the outer casing and the cylinders moved back and forth with 2 external connecting rods to each cylinder.  6 spoke like bars front and rear of the external case held the crank centered in the case.  The rear "spokes" also double as intake manifolds from the hollow crankshaft and carb.  2 models were built, a 6 cyl. 456 c.i. and a 10 cyl at 760 c.i.

     WHERRY (Britain) was a barrel engine with cylinders in pairs pointing in opposite directions and parallel to the prop shaft.  One crank geared to the prop shaft for each pair of cylinders.  Other barrel engines used wobble plates or a plate running in grooves.  Some were rotary for cooling.

     SIEMENS-HALSKE (Germany). In order to reduce air resistance due to the spinning engine and to tame the gyroscopic effect, Siemens built 9 cyl rotary of 726 c.i. and 110 HP at 900 RPM.  A set of bevel gears and pinions in a gear box on the front allowed the crankcase and cylinders to turn one way and the crank shaft to turn the other.  Thus the propeller turned about 1/2 engine speed.  There is a cutaway drawing but it is too small to see how the gears worked.  Several others also.  In a larger similar engine the prop was on a crankcase extension.

Copyright © 2003 by Stu Faber.  All rights reserved.