Which way do propellers turn?

On a 2 engined plane, would 1 turn clockwise and the other counter? On a 4 engined plane, would the 2 on one side go clockwise and other 2 counter? Did all nations props turn the same way?....I ask because of reading about some take off , and landing characteristics of some aircraft ( Beaufort ). I am guessing engine torque caused problems during high/low engine revs. So would having props spin in different directions help?

 

If we take the Mosquito as an example, engine torque certainly caused 'swing' problems which could be fatal. As successive Marks of Mossie were upgraded with more powerful Merlin engines, the problem worsened. 'Handed' ( ie counter-rotating ) engines were finally fitted to the Mosquito NF.36 which first flew in May 1945. The post-war DH Hornet fighter always had 'handed' engines.

I can only assume ( although I've never seen a full discussion on this ) that technical and logistical problems are the reason that this wasn't tackled earlier - under pressured wartime conditions I assume that the pilots were left to 'get on with it'.....

 

I think the P-38 was the only twin engine plane of the war that had counter rotating props.

 

All US 4-engined bombers had all 4 engines turning clockwise (viewed from the cockpit). The B-25 and B-26 had both engines turning clockwise.

The need for counter rotating props in twins is more apparent in low powered civil aviation twins than in high powered military twins with lots of rudder authority.

Clockwise turning propellers cause forces that make the aircraft want to turn/roll to the left. This makes losing the left engine 'critical'. The right engine is trying to turn the a/c left from propeller forces, also being offset to the right of the a/c center line adds to the left turning tendency, and when the left engine quits, the windmilling propeller adds drag to the left side of the aircraft. So, when the left engine quits, three major factors are trying to turn/roll the a/c to the left. The problem is worst at low airspeed/high angle of attack, when the aircraft is already in a high drag and marginal flight condition. Many civil twins barely have enough power to fly on one engine, much less to be able to climb on one. Losing the left engine on takeoff has condemned many a light twin to a slow roll to the left into an inverted crash.

By making the right engine turn counter-clockwise, you've taken away one of the three major left turning tendencies described above, and thus the left engine is no longer critical. Losing either engine still results in the a/c trying to turn/roll into the dead engine, but by taking away the prop effect, you've improved your odds.

High powered military twins usually can cope well with losing an engine, although if heavily loaded, losing one on takeoff is still hairy. The twin rudders on the B-25 certainly helped in engine-out situations. The B-26 had a bad reputation and scared a lot of pilots. Because of its short wing it was called the Baltimore Whore (no visible means of support). Jimmy Doolittle talked in his biography of taking a B-26 around to bases in N. Africa to fly demonstrations at B-26 bases. He would fly down the runway and shut down the left engine, then pull it up into a loop to prove the B-26 was very controllable on one engine.

The P-38 was originally designed with both engines turning inward so that neither engine was 'critical'. The prop wash caused buffeting on the tail, so the rotations were reversed so that both turned outward - technically making both engines 'critical' although it still flew relatively well on one engine.

 

That is excellent info. A lot of nuts in there. Thanks everyone. Worried I had asked a dumb question. Now am glad I did. Such a seemingly simple thing is actually pretty complicated.

 

I was waiting for " props turn the other way once below the equator".... When I first saw the Russian Ka25, I was like, how the heck does that thing fly with no tail rotor?...Now it makes sense.

 

Oddly enough, I heard that when the P-38 was delivered to the RAF, it was modified so that the props would both rotate in the same direction. If true, this sounds a bit silly to me.

Another silly story I heard ... was that the F-15 was the first fly-by-wire fighter, completely controlled by a computer program. But the early versions had some bugs .. one of which was, the first time the F-15 crossed the equator, it decided to fly inverted!

Both the above tales are apocryphal. I would welcome confirmation or debunking from wiser heads than mine.

 

I believe that is correct. From what I recall the British requested it to simplify the logistics. More damaging was the lack of supercharges which really cut into the performance of the models sent to Britain.

 

The F-15 flight controls were conventional hydro-mechanical. The F-16 was fly-by-wire. The only reason it would have rolled inverted crossing the equator would be if so ordered by King Neptune in order to become a Trusty Shellback.

 

Oh well, I guess that F-15 story was just too funny to be true.

 

The "story" is true, but it was the F-16, not the F-15. Also, the error was caught during flight simulation testing and not in actual flight.

Regarding another "Ooops!" Wasn't it the F-22, that had all it's systems crash when it crossed the International Date Line? From what I heard, that was on a real flight.

 

Whoa. Very interesting stuff there, Takao.

At least that's one thing WWII pilots didn't need to worry about!

Back to the propeller spin topic, regarding the British request to have only P-38s with same-way spinning propellers:

Thanks. But the other funny thing is that I believe the P-38 had both engines turning the same way - so the only logistical difference would have been a reversing gear and a set of other-way propellers. To me, saving on two parts doesn't seem worth the loss in handling ability. But ...

Well, if the British were silly enough to delete the superchargers, I guess they could be silly enough to delete the counter-rotating props as well.

Silly British. They sent all their cleverest people here, to Australia.

 

They didn't deleat them the US refused to send them. Superchargers were considered high tech at that point and the US was guarding them pretty closely. One of the main reasons the two countries cooperated so closely in electronics is that the British gave the US a bunch of info including samples of the magnetron which made it kind of hard for the US not to reciprocate in the electronics area. I don't know if it was the timeing (I suspect it was) or just different ricebowls that affect export aircraft so much (the P-38 was not the only aircraft so affected).

 

Interesting, lwd. I assume you are referring to the turbo-superchargers (which we now know as turbochargers) on the P-38. The British were quite aware of mechanical supercharger technology, which was at the heart of the Spitfire's Merlin engine and its derivatives.

Turbo(super)charger = driven by exhaust gases
Supercharger = driven by engine shaft

 

Governments are not always the most rational when it comes to such decisions.
Sometimes it's because some low level beuacrat simply interprets something different that what a "reasonable" person would. Note that the rules that resulted in the oil embargo against Japan didn't in fact call for an oil embargo but an individual in charge of implementing them chose a very strict and exapansive interpretation of said rules.

Thanks I'm not all that up on infernal combustion engines and tend to be rather sloppy with my descriptions there of. I probably should be more careful in that regards but will likely continue to make mistakes of this sort. PLS correct or elucidate if you notice them.

 

Did the Axis have superchargers? I know they had nitrous and water injection.

 

A supercharger (or turbocharger) is basically an air compressor, that feeds higher pressure air into a piston engine - such as on a WWII fighter plane, or an ordinary automobile. It effectively increases the power of an engine by forcing more air and fuel into the cylinders than would otherwise be available. A "mechanical supercharger" is driven by the driveshaft of the engine. A "turbo-supercharger", or "turbocharger", is driven by the engine's exhaust gases.

In WWII, there were two advantages to supercharging the engines of an aircraft. One was greater performance, allowing more power to be produced by an engine of the same size and weight. The other advantage was high altitude performance; the compressor in the supercharger could thicken the thin atmosphere at 30,000 feet, and let the engine perform almost as if it was at ground level. Non-supercharged engines would lose almost all their power at such heights.

You didn't make any sort of mistake at all, lwd.

There were several different types of superchargers used during WWII (and today as well, mostly in sports cars). Any nation might have an advantage in one particular type of supercharger. So it's entirely possible that the Americans might have wanted to protect the secrets of their own turbo-supercharger, even though the British had a well developed mechanical supercharger.

Yes they did - at least the Bf109 had a mechanical supercharger, and other planes no doubt had them as well.

The supercharger was well-known, civilian technology by the late 1930s. Every country knew about it.

The differences were in the details - the best method of supercharging, the best pressure, the best materials, and so on. Everybody knew about how to make basic superchargers. The exact details of making the best superchargers were often secret.

 

The reason the US was touchy about exhaust driven superchargers had more to do with the high temperature technologies that went into making them than anything else. Everyone pretty much knew the idea and could see its merits. The problem in making a turbosupercharger was getting materials that could withstand the heat and run reliably. So, even if the US had given Britian this technology they really couldn't have done much with it as their industry didn't have much capacity to manufacture the necessary materials to make it. This is a major reason the British jet industry was producing just one or two engines a year for much of the war also.

On handed engines: One way to get around the torque problem to a point was to offset the tail fin of an aircraft slightly (maybe by a half to one degree or so) such that it countered the torque by aerodymanic force when in flight. This limited the torque problem to take offs and landings when the tail and rudder were not effective. In these situations the pilot could usually apply slightly different amounts of brake to steer the aircraft until it gained enough speed to raise the tail and get some of the necessary force applied on it by air flow.