Sense of Propellor rotation

[Re-Heat]

True, but of course if the spools are counter-rotating, the passage of the air throught the fan and core are "straighter", and do not need to run at such a high rpm...

[barit1]

the passage of the air throught the fan and core are "straighter"

Every time air passes through a stage of compression OR turbine, it is turned ("swirled") in the direction of that rotor. The stator stages straighten this vortex at every stage, but the next stage swrils it some more. This is true of any gas turbine, c/r or not.

The aero advantage of counterrotation shows up between two turbine rotors. Where a "normal" engine has a row of stator or nozzle vanes to introduce the hot gas to the next turbine, a c/r engine can eliminate this stage. Less weight, less pressure drop, lower EGT, more efficiency.

[Landroger]

Barit1

Now you're talking. There is no reason for me to know that, but now I do its blindingly obvious and a very good reason why designers should go to the lengths of contra rotation.

Roger.

[Low Flier]

US built engines rotate clockwise where UK built engines rotate counterclockwise

Not quite true. For example, on the Beechcraft Duchess the left hand engine rotates clockwise and the right hand engine rotates the opposite way. This eminently sensible arrangement eliminates the critical engine problem. I dunno why more piston twins don't have that counter-rotating arrangement.

The Rolls Royce Continental engines, despite being UK built, rotate clockwise.

[Big Pistons Forever]

Anyone know why Piper made the Aerostar 700 P engines counter rotate outwards rather than the more logical inward counter rotation used on the PA 31/34/39/44 aircraft ?

[Tinstaafl]

Lowflier, you're correct that counter rotating engines eliminate the critical engine issue, however the Vmc reduction is generally small. It carries with it the added expense of unique parts imposing separate inventory costs and reduced economies of scale. A rather expensive way to solve a 'problem' that is not that big a deal.

Consider a typical twin with engines that rotate in the same direction. Vmc & other limits will be based on the worst case engine failing. As long as you operate in accordance with those limits then you're protected i.a.w. the certification requirements - no different from counter rotating, non-critical design. If it's the non-critical engine that fails then you will have 'gained' bit of extra margin over certification limits.

[Feathers McGraw]

Now I know it was single-engined, but the other rhyme I've heard goes:

Don't give me a P-39
With an engine that's mounted behind
It will tumble and roll
And dig a big hole
Don't give me a P-39

Not sure about direction of rotation for an Allison V-1710....

[barit1]

The Allison V-1710 was most interesting in that both left- and right-hand versions were made. P-38's had #1 prop turning left-hand, #2 turning right.

And the engines could be field-converted to swap rotations. The camshafts were turned end-for-end, starters and mags swapped, and presto the crank turns the opposite way. Not sure how they managed the oil pump, although a couple check valves could have handled reversing oil flow.

The P-39 prop (gearbox in the a/c nose) was right-hand, so the engine proper turned left-hand.

[Feathers McGraw]

From Wikipedia:

"Another feature of the V-1710 design was its ability to turn the output shaft either clockwise or counter-clockwise by assembling the engine with the crankshaft turned end-for-end, by installing an idler gear in the drive train to the supercharger and accessories and by installing a starter turning the proper direction. So, there was no need to re-arrange the ignition wiring and firing order, nor the oil and Glycol circuits to accommodate the direction of rotation."

So it would seem that the idler gear fixed the oil pump rotation too.

[barit1]

I need to make one correction on my previous P-39 post: The P-39 gearbox appears to be an internal spur gear type. Thus with the prop turning right-hand, the engine also turns right.

And I've found several web references to the RH <> LH conversion of the V-1710, all saying that the CRANKSHAFT (not camshafts) was swapped end-for-end. This is at odds with earlier information I had. For me, the subject is still open; If someone can find the V-1710 T.O. online, that would settle the matter.

[jh5speed]

... and for what its worth, the A400M with its TP400 engines has opposing sense for inboard and outboard engines

i.e. from rear: cw/acw/cw/acw

[Brian Abraham]

Barit, the P-39 prop as you note turns clockwise (viewed from the cockpit) but the crank turns in the opposite direction. The reduction gear casing consists of two aluminium alloy castings which support the airscrew shaft, thrust bearing, reduction gear and pinion gear. The reduction gear is an external spur gear bolted to a flange on the airscrew shaft. The airscrew shaft is supported at the front end by a ball thrust bearing and at the rear by a large roller bearing. The pinion gear is mounted between two collar bearings and is driven by the extension shaft through an internally splined flexible coupling. Reduction gear teeth are lubricated by an oil nozzle supplying three jets of oil directed on the teeth. A combined pressure and scavenge oil pump is mounted on the front of the reduction gear housing and oil is supplied from a separate external oil tank. On the rear face of the casing drives are provided for two gun synchcronisers and an airscrew governor.

From "Allied Aircraft Piston Engines of WWII" by Graham White

[AerocatS2A]

Originally Posted by eckhard

The trickiest part [about prop swinging] was taking a passenger for a flight and having to brief them on the operation of the cockpit controls. Always made sure we had chocks installed and brakes on for that exercise!

One way to tackle that problem, for a Tiger Moth at least, is to start it by swinging the prop from behind. There are a number of advantages to this, the blades are moving away from you rather than toward your legs and you have immediate access to the front cockpit controls. I spent several years swinging passenger loaded Tiger props both from in front and behind and didn't have any dramas.

[barit1]

Brian, thanks for that drawing, which I consider definitive, at least for the production P-39.

I was basing my "internal spur gear" statement on a photo of a museum display. It included the engine, driveshaft, gearbox, and 20mm cannon with magazine. The thing that caught my attention was the gear housing which was perfectly circular, strongly suggesting an internal spur gear. I also found that some early V-1710's did in fact use an internal spur gear - and that led me to my statement. It's possible, perhaps, that prototype P-39's had the internal spur arrangement.

If I can find the photos I'll post the link.

Edit: See P-39Q engine/GB/prop/cannon photo. Upon closer inspection, the circular gear housing is NOT concentric with the prop axis, and likely is consistent with the gear drawing in your post.

[Brian Abraham]

barit, the 39's gearbox is circular, and as far as I can tell remained unchanged throughout the aircrafts life.

The through the prop gun was usually of 37mm calibre, although the D-1 and British models had 20mm.

[Ex FSO GRIFFO]

I'm with 'Aerocat', for the 'Tiger' and similar -

From behind, you are -
- On the 'down' swing with the left hand, and thus 'away' from you -
- Your bum against the lower wing can be the brakes, just in case ...
- The Mag switches are 'right there' with easy reach...Port Side..
- And as stated, the throttle control is easily reached as well.

And 'difficult' taxying can be done from outside as well, with the rear cockpit 'flap' down, the throttle is firmly within your grasp, and you can 'grab' the fuselage when 'you are de brakes'....