The P-Factor Factor (1 Viewer)

[mikewint]

I have read that, apparently, Camel pilots could turn more tightly in the direction of the torque. This would have been true of any rotary engined design with the exception of a few German fighters such as the SSW D.III/IV who had the SH.III rotary. The SH.III was geared so that the prop to rotated in the opposite direction that the engine did. This largely, though not completely, eliminated torque and gyroscopic precession. I suppose you could argue whether that was a good thing or not since it eliminated the torque-turning feature but it probably reduced casualties among novice pilots and reduced the their learning curve.

I've also read that rotary engines absorb (waste) about 10% of their power output do to the air drag of the rotating cylinders....about 10% more than equivalent inline engine. Have you heard/read this?

The torque on the airframe exerted by the rotary engine, is: Torque (at prop shaft) + 10%? So, with equal power expended to rotate the same prop to make the same thrust, the rotary powered aircraft will still exert an additional 10% torque on the airframe. I would think that they, the aircraft riggers, would include some trim in the wings to help counter the engine torque. The pilots inputs just to keep the wings level would then be minimal at cruise and really only needed at possibly lower speeds with less air flow?

 

[gumbyk]

I have read that, apparently, Camel pilots could turn more tightly in the direction of the torque. This would have been true of any rotary engined design with the exception of a few German fighters such as the SSW D.III/IV who had the SH.III rotary. The SH.III was geared so that the prop to rotated in the opposite direction that the engine did. This largely, though not completely, eliminated torque and gyroscopic precession. I suppose you could argue whether that was a good thing or not since it eliminated the torque-turning feature but it probably reduced casualties among novice pilots and reduced the their learning curve.

I've also read that rotary engines absorb (waste) about 10% of their power output do to the air drag of the rotating cylinders....about 10% more than equivalent inline engine. Have you heard/read this?

The torque on the airframe exerted by the rotary engine, is: Torque (at prop shaft) + 10%? So, with equal power expended to rotate the same prop to make the same thrust, the rotary powered aircraft will still exert an additional 10% torque on the airframe. I would think that they, the aircraft riggers, would include some trim in the wings to help counter the engine torque. The pilots inputs just to keep the wings level would then be minimal at cruise and really only needed at possibly lower speeds with less air flow?

Not sure about that - the torque either goes to the cooling losses as you describe, or to the prop. Don't forget, with a rotary engine, the 'prop' shaft is bolted to the airframe.

 

[mikewint]

the 'prop' shaft is bolted to the airframe.

I think that we're having a terminology problem. Do you actually mean the stationary crankshaft? Unlike stationary aircraft engines, in which a turning crankshaft drives the propeller, in rotaries the whole engine spins around a stationary crankshaft. The prop is bolted directly to the engine and spins with it. The primary reason for the rotary configuration was to provide adequate cooling. Because the cylinder heads are perpendicular to the slipstream and whirling through the air, the engine stayed within safe temperature limits. Engine cooling was a challenge in the first decade of the 20th century, even with water-cooled engines, and rotaries presented a viable solution to the problem.
There are also a number of fallacies concerning rotaries that keep getting printed: #1.They were two-stroke engines that mixed the oil and fuel, #2. They did not have throttles and, #3. they were tricky to fly because of the gyroscopic and torque forces from the spinning engine.
#1. Regardless of configuration, all rotaries are four stroke engines. A common fallacy about rotaries is that they are two-stroke. That misconception probably arises because of confusion over how they are lubricated. Regardless of make, rotaries share a total-loss oil system. The air, fuel and oil are delivered to the engine through a hollow crankshaft, creating the impression that the oil mixes with the gas as in a two-stroke. In fact, the oil and fuel do not mix. Castor oil, often the famous Castrol brand, was the preferred lubricant precisely because the oil maintained its lubricating qualities even under pressure and high heat. It did not ignite with the fuel as it passed through crankcase, cylinders and out the exhaust.

#2. Many books on WWI aviation state that rotaries had no throttles and always ran at full speed; the only way to slow them was to temporarily turn them off via a coupez or "blip" switch that cut the ignition. While this is semi-true in fact all rotary engines had a way to control [engine speed] except the 100-hp Gnome. All had carburetors except the 100- and 160-hp Gnomes.
So if you limit your discussion to the 100-hp Gnome it fits the typical description. Naturally it proved very unpopular with pilots, since they couldn't adjust speed by increasing or reducing power. Canadian born ace and 1st Pursuit Group commander Harold Hartney, who flew various planes powered by the 100-hp Gnome, stated in his memoir Up and At 'Em that it was a horrible engine.

Manufacturers quickly developed ways to regulate engine speed by manipulating the relationship among the carburetor, mixture control (or fine adjustment as it was called at the time) and sometimes the ignition. Different types of engines used different systems. Only the Gnome engines did not have carburetors or throttles. The only direct control pilots had over the fuel and air mixture was the fuel flow fine adjustment lever in the cockpit. Gnomes were often called Monosoupape engines, French for "single valve." The single valve both induced air for the fuel/air mixture and vented the exhaust after the mixture ignited. In place of a valve for inletting fuel, the 100- and 160-hp Gnomes had inlet ports toward the bottom of the cylinders. Because the Gnome power plants did not have a throttle or carburetor, pilots could vary the speed of the engine only by interrupting the ignition. The 160-hp Gnome improved on the 100-hp version's simple on/off switch by providing a variable ignition timing selector.

#3. According to pilot memoirs and modern pilots flying exact replicas , the often-repeated tales about tricky aircraft handling due to the gyroscopic and torque effects of rotating engines—that the spinning mass of the engine made for very quick turns to the right and slow turns to the left—are exaggerated. "When you hear the stories about rotary engines being hard to fly, the problem was with the inexperienced people flying them," stated one pilot. "When I made my first flight in a rotary-powered aircraft, I landed and then realized that I hadn't noticed any gyroscopic effects. An experienced pilot automatically compensates for those things. Turns to the right might be a little quicker, but that is because the rotary engine tends to pull the nose down [in that direction], and you make a quicker descending turn than you make a climbing turn."
Others agreed: "There are small gyroscopic effects but nothing close to the exaggerated tales often repeated in print and in documentaries. You adjust for them much the same way you would if you were flying in mildly gusty conditions. The torque reactions are most notable during takeoff and gliding in for landing when 'blipping' the engine."

Other factors had a more pronounced effect on aircraft handling. Pilots noted that all the early rotary-powered planes had a lot of adverse yaw (the tendency of the plane's nose to point in the opposite direction of a bank when starting a turn), and all were tail-heavy, leading to a certain amount of instability in their handling.

 

[pbehn]

I've also read that rotary engines absorb (waste) about 10% of their power output do to the air drag of the rotating cylinders....about 10% more than equivalent inline engine. Have you heard/read this?

Ive not read much about them. From looking at how they work the energy of the combustion is transmitted to the cylinders by the pistons acting sideways, the friction losses must have been high. Wiki made me laugh.
Clerget 9B - Wikipedia

• Fuel type: Gasoline mixed with Castor oil lubricant
• Oil system: Castor oil mixed with fuel

 

[mikewint]

Ya, lots of misconceptions. The main reason for rotary engines was cooling, and when engine makers figured out how to cast cylinders with adequate cooling fins, they were able to eliminate all the complexity of ignition and fuel/air mixture that they had with rotary engines. Castor oil was another consideration in Rotary engines becoming quickly outdated. They required huge quantities of it and the logistics of supplying the large quantity of Castrol required to keep a squadron of rotary-powered airplanes in the air were daunting to say the least. Most rotary engines consume about five or six quarts of oil per hour.

 

[Ascent]

The other thing to remember about castor oil is that it's a laxative. With a total loss oil system the pilot would be getting big mouthfuls of it.

You can imagine the effect on them.

 

[gumbyk]

The other thing to remember about castor oil is that it's a laxative. With a total loss oil system the pilot would be getting big mouthfuls of it.

You can imagine the effect on them.

Don't have to imagine it, I've spoken to pilots who've flown behind them, and after a weekend its bad enough...

 

[XBe02Drvr]

Feathered is the blades at 90º to the plane of rotation - you don't have the prop in this condition with the engine running

Free turbines like the PT-6 can and do idle with a feathered prop. If you're doing a single engine turn in East Podunk, Maine, and you don't want to blow away that un-chocked, un-tied-down TriPacer sitting across the ramp, you feather your right prop and goose the left just a bit, then feather it as you coast through the last 90° of turn to your spot at the pax gate, making sure you stop with your nosewheel cocked right. No propwash blasting back across the ramp at the "milkstool". And when you're done, you start the left engine in feather and just ease it to the edge of feather and play with it there til she starts to roll. "Nice little TriPacer, now, stay,...stay,...stay,...good girl!".
Cheers,
Wes

 

[XBe02Drvr]

I doubt that this would produce a brake effect. Braking requires a Negative Pitch. At flat pitch the propeller is going round but producing zero thrust.

In flight it produces a very pronunced braking effect. In the 1900, if we wanted to go down fast we would ease the power levers smoothly back to flight idle. As torque dropped almost to zero, the blades would flatten attempting to maintain governed RPM until they were against the low pitch stops and it was like having two barn doors out there. Certainly not negative thrust, but a huge braking effect, nonetheless. Done smoothly enough, and with well-timed trim adjustments, we could kill 30-50 KIAS and pick up a 1500-2000 FPM descent with a level deck angle and no change in prop noise. Pax seldom noticed.
Cheers,
Wes

 

[soulezoo]

A little OT but reminds me of an event while deployed in the Middle East. KC-10 flight crew preps a/c for a mission. It's only 125* on the ramp so the ACC (Aircraft Commander/Pilot) directs FE (Flight Engineer) to start the APU to run the pacs (air conditioning). There's a local order instructing that the APU can only be ran for 15 minutes due to over heating. I warn pilot, don't do this, you'll not be able to start the engines. Well, the brains of the outfit decides he'll just start the #2 engine and keep the APU running (cannot start the #1 or #3 wing engines due to temporary "tent" hangers around plus a bunch of Apache helicopters parked nearby-- the aircraft is nominally towed to the taxiway for engine start due to this). So the aircrew buttons up, we're in the middle of the tow to the taxiway when the APU overtemps and shuts down. Pilot (who was a reservist that flew DC-10's commercially), "no problem, I'll start the other engines from #2". Except that is when he realizes that the KC-10 differs from the DC-10 in that the air pacs are omitted on the #2 engine for the KC-10 and he cannot start the aircraft. We waited another half hour on the ramp waiting for the APU to cool down enough to start up so that engine start could take place. (The only aircart on the ramp was unavailable at the time) The group commander was quite displeased....

 

[mikewint]

The other thing to remember about castor oil is that it's a laxative. With a total loss oil system the pilot would be getting big mouthfuls of it.

Another one of those WW myths. While what you posted about MEDICAL grade Castor oil is true recall that:
1. Pilots wore scarfs around their necks. These served several purposes, neck chafing due to constant "head on a swivel" lookout for enemy aircraft and the wool collars of flight jackets. (b) Simple warmth. (c) Pulled up over their mouth and nose to protect against fuel and oil blow back and clean goggles of the same.
2. The most common thing that happened was with a Camel (and some other aircraft) in a fast dive often the gauge (tube) showing fuel pressure burst covering the pilot with fuel and forcing them to use the gravity tank to fly to safety.
3. While some pilots recorded being sick and having problems with digestion after flights and the trouser "brown-out" problem. It had more to do with being a symptom of a psychological problem originating from the combat stress of almost dying on a regular basis. As anyone who has been in a life/death situation can attest to. It's a reflex and part of the Fight/Flight syndrome
4. The half cowls on the the planes were there for the sole purpose of deflecting the oil away from the pilot. I'm sure that some oil got through but I a don't think mouthfuls of oil made it into their faces. Such heavy spray would have made seeing very difficult
5. The open cockpit and helical slipstream would have also carried exhaust fumes back to the pilot. The inline exhausts quite often would have gone back towards the pilot (look at how long the pipe was on the Brisfit to avoid this) and engine exhaust has all sorts of nasty stuff in it. Fokker DVII has the exhaust placed nicely to blow back on the pilot.
6. The reason for the binge drinking in the mess and the general smashing up of it was release of nerves and tension and to stop them thinking about friends who hadn't come back. The weird drinks were just a result of young, public school boys exotic tastes (or more likely drinking the mess dry of all other alcohol) and cherry brandy/blackberry doesn't counter the effects of laxatives.
7. When pilots came back with black marks on their faces in WWI it was from gun soot - they could have inhaled a lot of that and gun oil from extensive shooting but unlikely to be castor oil.
8. I think you'd be more likely to ingest raw Castor oil as ground crew working/fueling the aircraft after it returned and swinging the props to start the engine.
9. But having said that flying in formations behind other aircraft you certainly could be inhaling the oil from the other planes in front of you (and their exhaust).

 

[gumbyk]

Free turbines like the PT-6 can and do idle with a feathered prop. If you're doing a single engine turn in East Podunk, Maine, and you don't want to blow away that un-chocked, un-tied-down TriPacer sitting across the ramp, you feather your right prop and goose the left just a bit, then feather it as you coast through the last 90° of turn to your spot at the pax gate. No propwash blasting back across the ramp at the "milkstool".
Cheers,
Wes

Sorry, I should have clarified that I was talking about pistons...

 

[gumbyk]

Another one of those WW myths. While what you posted about MEDICAL grade Castor oil is true recall that:
1. Pilots wore scarfs around their necks. These served several purposes, neck chafing due to constant "head on a swivel" lookout for enemy aircraft and the wool collars of flight jackets. (b) Simple warmth. (c) Pulled up over their mouth and nose to protect against fuel and oil blow back and clean goggles of the same.

Not from the pilots of rotary powered aircraft that I've talked to. After three or four days of flying they start to feel the effects.

 

[pbehn]

#3. According to pilot memoirs and modern pilots flying exact replicas , the often-repeated tales about tricky aircraft handling due to the gyroscopic and torque effects of rotating engines—that the spinning mass of the engine made for very quick turns to the right and slow turns to the left—are exaggerated. "When you hear the stories about rotary engines being hard to fly, the problem was with the inexperienced people flying them," stated one pilot. "When I made my first flight in a rotary-powered aircraft, I landed and then realized that I hadn't noticed any gyroscopic effects. An experienced pilot automatically compensates for those things. Turns to the right might be a little quicker, but that is because the rotary engine tends to pull the nose down [in that direction], and you make a quicker descending turn than you make a climbing turn."
Others agreed: "There are small gyroscopic effects but nothing close to the exaggerated tales often repeated in print and in documentaries. You adjust for them much the same way you would if you were flying in mildly gusty conditions. The torque reactions are most notable during takeoff and gliding in for landing when 'blipping' the engine."

Other factors had a more pronounced effect on aircraft handling. Pilots noted that all the early rotary-powered planes had a lot of adverse yaw (the tendency of the plane's nose to point in the opposite direction of a bank when starting a turn), and all were tail-heavy, leading to a certain amount of instability in their handling.

With respect to the guy who said point 3, pilots in WW1 had about 15 hrs flying experience, very little to do with combat manoeuvers and he didn't have some Fokker shooting at him. Also there were many engines fitted to a Sopwith Camel ( for example) all with different weights and power.

Torque effects lasted to the end of the prop age here from a test on the Tempest against the P47

61. At speeds up to 300 I.A.S. there is little to choose between the two, the Tempest having a slight advantage to the right and the Thunderbolt to the left. Above 300 I.A.S. the Tempest become increasingly superior.

68. In comparison with other British contemporary fighters the aiming qualities of Tempest II are good. Like all single-engine aircraft without contra-rotating propellers it is sensitive to changes in speed which involve continual rudder trimming or application of heavy foot loads to prevent skid. This is a major source of inaccuracy in ground attack.

 

[mikewint]

After three or four days of flying they start to feel the effects.

I am going to maintain that the reports of trouser brown-out are grossly exaggerated and are attribitual to many causes besides the presence of Castor oil. Now there are no absolutes involving any thing human. So were some pilots affected...most likely yes but the vast majority were due to any number of other causes...bad water quality, bad food, fear, anxiety, poor sanitation.
Look at any old newspaper or magazine from that period and check out the ads...70% at least are for nostrums to cure poor digestion and its relieve its symptoms.
I'd also point out that the human gut is filled with bacteria which produce methane and CO2, amongst other gasses. At altitude, reduced air pressure makes gasses in the gut expand with embarrassing, often painful, and potentially fatal effects. Sudden changes in air pressure, like fast climbs to high altitude, make the problem worse. In extreme cases, intestinal ruptures are possible. During WW2 US aircrew (at least in the 8th and 9th AF) experienced a similar trouser brown-outs and were given low-starch rations precisely to prevent this problem.

 

[Frank G.]

I guess that everyone knows what "P-factor" is but first I'll explain it. A prop driven aircraft with a right turning prop (as viewed from the cockpit) generates a spiral of powerful propwash that circles the fuselage in a clockwise direction. When the propwash gets to the vertical fin it pushes the fin over to the right, and the nose tries to go left. This is most noticeable on takeoff and the climb that follows, when the power level is high and the airspeed low, which makes the propwash strong and the directional righting effect of the fin low. In WWII this effect was called "torque" but in fast it was not the gyroscopic effect of the prop.

The way you correct for P-Factor is by applying right rudder. Most airplanes have at least the fin offset to help counter P-factor. Even the Ercoupe has some P-factor, even though steps were taken in the design to reduce it but using two fins outside the propwash and by pointing slightly to the right.

I just read where a P-38 pilot used P-Factor to an advantage. They were escorting bombers in Jan 1944 when they were attacked by 20 or so FW-190's from behind. He had just read an intelligence report that a P-38 pilot was able to evade a German attack by a right corkscrewing climb. So he tried that same tactic. What happened is that he climbed, which of course slowed the airplane a great deal and the Germans climbed too, firing away. The difference was that the P-38 had no P-Factor to contend with,and the Germans had to try to get their noses far enough to the right on him to hit him. He saw one FW-190 after another behind him try to get the nose around, at a high angle of attack and relatively low airspeed, and then flip over to the left when the combination of factors led to a stall and a spin.

When he got home he found that the P-38's left wing had so many holes in it that it had to be replaced. But nothing vital had been hit.. , .

 

[Frank G.]

Hello,

I'm new to this nice site... have enjoyed wandering around it before registering. Lots of great info presented.
After reading your posting on P-Factor I thought it might be a good idea to help clarify a few points about it and the other left "yawing" forces acting upon a propeller-driven airplane. Before doing so please know I've been in the aviation game for just about 50-years. And, yes, I've done my share of pilot-training which I loved.

With conventional and tricycle geared airplanes there are forces acting upon them that will make them yaw to the left (for right-hand propeller rotation from cockpit view). These forces can vary in intensity and occur at varying times (dependent upon airplane action). These forces are:

1. P-Factor
2. Spiraling slipstream
3. Engine-created torque effect
4. Gyroscopic precession

I'll just address these forces as they relate to the take off. Discussing how they affect an airplane in flight is a bit more complex because control use should be added to the mix.

One of my 'favorite' airplanes is the puggish-looking AVIA S-199. Recently I wrote an article about it for our IPMS Chapter newsletter (I made a model of one flown during the 1948 Israeli War for Independence). In the article I discussed its reported nasty take-off characteristics. If you don't mind, I'd like to share excerpts of that article which I suspect are relevant to yours and others postings about P-Factor, etc.

· The Jumo 211 reportedly produced much more torque than the DB605. Torque is actually a roll force which applies a downward load/force on the left main landing gear tire during take off. It acts like a hung or dragging brake and can yaw the airplane to the left unless corrected for by right rudder application. This affects tail wheel and conventional gear airplanes alike.

· The S-199's big, wide, propeller also produced a lot of P-factor. P-factor creates a left yawing force resulting from greater lift created by the descending propeller blade as it rotates; this is especially noticeable in tail wheel airplanes. This is corrected for by right rudder application. This force becomes an influence on tricycle geared airplanes as the pilot applies back-pressure at Vr (rotation to a takeoff attitude at a prescribed airspeed--pitch up).

· The S-199's massive prop also created a whole lot of spiraling slipstream. This is caused by the rotating propeller's tip vortices that corkscrew aft along the fuselage. This "corkscrew" impacts the left side of the vertical stabilizer/rudder unit producing a left yawing force which is corrected for by applying right rudder. This affects the tricycle and conventional geared airplanes. Its affect is a bit more pronounced with tail wheel configured airplanes because they do not have the benefit of a nose wheel which helps dampen out some of the yaw force (think P-39/63... a couple more of my favorites!).

The propeller essentially a rotating wing. Most "normal" wings create wingtip vortices because of the way air flows over and under it. The higher pressure air below the wing moves outward toward the wingtip trying to move into the lower pressure air flowing over the top of the wing at its tip. The result is a vortex formation, AKA, vortices. These vortices trail behind the producing wing creating what is commonly known as wake turbulence. This stuff can be quite nasty, especially when produced by larger aircraft like an airliner or bomber. However, with propeller-created vortices, they corkscrew aft along the fuselage.

· The S-199 had another take off left yawing problem: Gyroscopic Precession. As one begins taking off in a tail wheel airplane the pilot usually applies forward pressure on the stick as the airplane gains airspeed. This is done so you can see the runway better during acceleration to lift-off speed rather than having to look out the sides of the cockpit to maintain runway alignment.

When placing the airplane on its main gear during takeoff, the pilot normally applies forward control pressure with smoothness. Even when doing it with care, the pilot will have to apply additional right rudder to compensate for the mild left yaw inducement. However, if the pilot applies a to rapid forward pressure on the stick during the early phase of the take-off roll when under full power, the airplane will tend to yaw to the left (or right depending upon propeller rotation). This is caused by the induced gyroscopic precession (pilot action creates a force 90-degrees to the rotating gyro's (prop's) plane of rotation. This makes the rotating mass want to "move" 90-degrees opposite to its plane of rotation... in the case of the S-199, to the left). Since the rotating mass is attached to the engine's crankshaft, and the engine is attached to the airframe, the resulting left yawing force is applied to the airplane.

The bigger the propeller (mass), the greater the left yawing action. As an observer, I've witnessed this: one fatal accident in a Tempest and one serious injury accident with a PT-17; and as a flight instructor surprised by my tail wheel students whom I was about to sign off for tail wheel flight operations.

I hope the above helps in the understanding of the left-yawing forces upon a propeller-driven airplane. If the prop rotates to the left, everything is the opposite... that's what bit the Tempest pilot who was used to a Corsair.

Cheers,

Frank G.

 

[pbehn]

I hope the above helps in the understanding of the left-yawing forces upon a propeller-driven airplane. If the prop rotates to the left, everything is the opposite... that's what bit the Tempest pilot who was used to a Corsair.

.

Great post, apart from the above "bit" this is part of the human condition, learning something is one thing, you can learn somethings so well they become almost reflex actions. Unlearning a reflex action and replacing it with another action can be difficult.

 

[gumbyk]

Great post, apart from the above "bit" this is part of the human condition, learning something is one thing, you can learn somethings so well they become almost reflex actions. Unlearning a reflex action and replacing it with another action can be difficult.

And that's why I don't talk about which way an aircraft will swing, or what rudder inputs you need. Both the aircraft that I fly regularly have 'backwards' turning propellers. Just use whatever you need to stay straight.

 

[pbehn]

And that's why I don't talk about which way an aircraft will swing, or what rudder inputs you need. Both the aircraft that I fly regularly have 'backwards' turning propellers. Just use whatever you need to stay straight.

I am British and worked all over the world driving right-hand cars in UK and Europe also left hand drive all over the world including UK. Just when you think you are the master of it you set off in an "English" car in the town you were born in and drive on the wrong side of a road for 50 yards until you see some "idiot" coming towards you on a Sunday morning, and no, no alcohol had been taken.

At one time I owned a Norton 500cc motorbike a Suzuki 250cc motorbike and raced a Suzuki 250cc, all different gear change systems between up down and right left. Within a month I sold the road bikes and bought a rear set linkage, it wasn't a question of going a bit quicker but staying alive.