Propeller Effects

[Minotaur]

I have dragged this topic over from another thread, but I feel that it is worth some clarification.  The reason is that most commonly, any effect encountented by prop effects is slang labled as "torque" by the average player.  This is much the same way as the term "compression" is often misused.

 

By HiTech:

<snip>

Just so everyone knows the effects from the prop. They are 5
1. Vortex / slipstream
2. Gyroscopic
3. Torque.
4. PFactor.
5. Thrust

<snip>

Thrust is the easy one to understand.  No problems here.

Torque comes into play only when a change in prop/engine RPM occurs.  Basically this change in RPM, attempts to cause the rest of the A/C to rotate in the opposite direction.  For a CW rotational prop, torque causes CCW rotation on throttle up.  IE: The A/C must provide counter torque by means of pilot control to remain wings level.

Gyroscopic forces are in effect at all times with the engine running.  These forces are resisting the change of motion for the plane of motion perpendicular to the axis of rotion of any rotating component.  The major rotating components are the engine and the prop.  Any force applied to change this plane of motion is transferred to a point 90 degrees in the direction of rotation.  IE: You apply force to the left front side of props plane of motion and this force is applied (translated) to the top front of the props plane of motion.  This is for CW prop rotation.  

Vortex / slipstream I am not completely sure about this one.  I do know that thrust air flow is not laminar.  This thrust air is created by a rotating propellor and also rotates in the same proportional speed and direction.  This vortex of thrust air flowing over the A/C has a significant effect .  In cases where the difference between A/C speed to thrust air speed is the greatest.  IE: High thrust air speed and low A/C speed.  

PFactor again I am also not completely sure about this one.  However; I think that it has something do with prop rotation.  This "something" being that the propellor blades rotate upward on one side of the A/C and downward on the other side.  This somehow this causes thrust air to strike one side of the A/C with a greater amount of force than what strikes the oposite side of the A/C.  Particularly the vertical part of the epinage.  As is in the case of vortex air, this has greater effect at high thrust air flow rate with low A/C speed.

At one time there was a very good write up about these "Prop Effects" posted up on the HTC webpage, but I am unable to locate it now.

Keep in mind that I am not 100% on any of these prop effects and I am simply giving this thread a starting point.  Those more knowledgable please chime in and help me out!  

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Mino
The Wrecking Crew

"Best is the trash talk. Severly and viciously going after your enemies, their mothers, and their shabby sheep."
StSanta

[Tyro48]

Slipstream: Causes Yaw due to one side of the prop slipstream hitting tail surfaces before the other side does.

Mot sure where the PF came from however "P" factor is: Your prop is like many different wings attached to your crankshaft each blade of the prop making its own thrust, so when your in high angles of attack the downward blade produce more thrust than those moving upwards and this also causes the aircraft to yaw.

 So basically thorque is yaw which is a total of reactive forces, slipstream, gyroscopic precession and "P" factor.

[Jigster]

And what's perplexingly weird, there is very very torque on some planes, but a significant yawing effect (Typhoon) and on others there is alot of torque (109G10) with little yawing effect.

Some planes still feel right (P-51 has proportional torque and yaw) while some feel like they have neither (F4U) Without combat trim, auto takeoff, etc you can firewall several planes you only need to trim the nose up a bit, and apply slight rudder pressure till the tail comes up and the plane will fly it's self down the runway.

- Jig

[Jigster]

As an aside, I checked the Typhoon and Yak again, with all the autos off and neutral trim.

These two have the right torque but it is extremely week in both yaw and torque

These, and all of the planes have no immediate reaction to going from stall and idle to full throttle other then a nose pitch up (due to the force need to keep the nose up at that speed) None roll on their own axis, but all that I've looked yaw sligtly left, pitch up, and roll slightly left in what looks like a low speed high yoyo.

I'll post a film when I get home this evening.

- Jig

[This message has been edited by Jigster (edited 12-09-2000).]

[LLv34_Snefens]

When looking at a plane with counter rotating propellers, will all 5 forces still be present?

Thrust will of course, but how is it for the others. Do they reduce ,enlarge or simply dissappear?

[Zigrat]

jets are so much easier to model, stupid propeller

[wells]

Torque from accelerating the propeller mass is non-existent in every sim I've ever flown.  It's always only an 'aerodynamic' torque, that being resistence to rotation of the blades, which for any constant rpm can be figured by

Torque = Horsepower * 5252 / RPM

The slipstream effects that I believe are being modelled are an increase in angle of attack on the left wing with a corresonding decrease on the right wing, which causes a *right* roll, that counters the torque effect  mentioned above.  Depending on how much wing area is involved and the slipstream velocity, it appears that this effect is overpowering the torque effect in some planes.

Given that the ailerons on say, an F4u at 100 mph can make some 20000 ft-lbs of torque, the  aerodynamic torque of 3900 ft-lbs, which comes from 2000 hp at 2700 RPM is not very large and is easily countered with ailerons.

[bod]

The gyroscopic forces can be very spectacular. The aerobatic maneuver "Lomcevak" uses the gyroscopic force of the propeller to cartwheel in the air. It starts off as a roll with the tail describing a cone (much like these spinning toys when they start wobbling). here is an avi videa showing an Extra 300 doing a Lomcevak:
 http://www.tote.com/tumble1.avi

[bloom25]

Thanks for moving this topic from my thread Mino. I wanted to discuss it a little more, but I was afraid A) My thread was too long, and B) My thread was degenerating into a flamefest.  

I guess I'm looking for an expert opinion here:  Is the torque in AH 1.04 at more realistic levels as compared to 1.03.  1.03 was pretty harsh on takeoff and landing for planes like the f4u, typhoon, and even the spit.  Based on what I've read, this is realistic.  In 1.04 the torque is still there, but it is no longer a factor to be worried about.  I know that in 1.03 if I was flying a hog (offline of course   ) and immediately after starting the engine I went immediately to wep the plane would be uncontrolable.  Just about everything I read about *any*  WWII fighter will say something about sudden increases in throttle during takeoff and landing.  In 1.04 I punch wep immediately after the engine starts and I start to roll.  There isn't a single plane that doesn't straighten out perfectly once the tail comes off the ground.  To me this doesn't seem right.  Why would the tail coming off the ground reduce torque effects so much?  Is this realistic?

I think part of the fun (for me at least) was getting some of the bigger planes off the ground when heavily loaded with bombs.  In 1.04 once the tail comes off the ground you have nothing to worry about.

 



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bloom25
THUNDERBIRDS

[Hans]

Well, maybe you need to split these up into two different groups.

Three of those YAW the airplane to the left (P-factor, Slipstream, and Gyroscopic)

Torque ROLLs the plane to the left (when you throttle up).

Slipstream is caused by the propellor blade vortex.  They corkscrew around the fuselage and add preasure to the left side of the vertical tail fin.  If there is an equal sized fin under the tail, this will cancel out the yaw motion, but adds more preasure to roll the plane, plus it is usually impractical and gets scraped on the ground in bad landings.

P-factor is when the airplane is pitched up at low speed, but moving horizontally. There is an angle for the propellor to work with.  The decending blades on the right side of the prop arc are at high angles of attack, make more thrust than the ascending blades on the left.  Yet more power to YAW the plane left.

Gyroscopic comes into play when you pitch up to get off the runway.  You've just tilted a spinning disc, and it will react in a 90 degree angle, yawing you to the left.  When you dive it yaws you to the right.

Torque is a whole different animal.  You gun the engine and get that prop applying more force to the air, it will resist you and try to roll you the other way.  Props usually "roll" to the right, so you will roll to the left when you make a drastic throttle increase.

Hans.

[This message has been edited by Hans (edited 12-10-2000).]

[Lephturn]

Just a supposition....

What if all the forces are modelled correctly for a given throttle setting when flying, but with no modelling of the forces resulting from throttle changes?  Wouldn't that produce the exact effects we see in the game now?  The planes seem to have all the right forces acting on them when in flight at full throttle, but they don't show the reactions we are expecting when violently moving the throttle.

Also, how does propeller RPM fit into this?  Regardless of how the engine SOUNDS, due to simplified engine management are we really changing the RPM of the engine or just increasing manifold pressure?  Maybe we don't see the results because "firewalling the throttle" in the game is not the same thing as doing it in real life?  It could be that all the forces are modelled including engine speed changes, but the simplified engine management doesn't let us really change the RPM of the engine to get the effects we are looking for.

Thoughts?

------------------
Lephturn - Chief Trainer
A member of The Flying Pigs  http://www.flyingpigs.com
 
"A pig is a jolly companion, Boar, sow, barrow, or gilt --
A pig is a pal, who'll boost your morale, Though mountains may topple and tilt.
When they've blackballed, bamboozled, and burned you, When they've turned on you, Tory and Whig,
Though you may be thrown over by Tabby and Rover, You'll never go wrong with a pig, a pig,
You'll never go wrong with a pig!" -- Thomas Pynchon, "Gravity's Rainbow"

[Mattibaby80]

I was under the impression that gyroscopic effect was most apparent on WWI fighters with rotary engines.  With a rotary, the prop is attached to the propeller and the whole spiel  spins around the crankshaft.  The mass of moving metal caused the aircraft to nose down in right turns and nose up in left turns.

I don't know how this relates to radial engines.  I didn't think gyroscopic effect was really that big of a deal on WW2 fighers.  Does the radial work the same way as the rotary, or are they essentially the same kind of engine with different names?

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Meine schwester hat keine kartoffel salat?  Du bist eine lustige bube!!

[funked]

"but they don't show the reactions we are expecting when violently moving the throttle."

Huh?  Get in a Spit 9 with no throttle damping and do some violent throttle motions on the runway.  The plane jerks to the left pretty suddenly.  Not sure what more you guys want.  I don't fly the other planes much so maybe the Spit is the only one that does it?

[This message has been edited by funked (edited 12-10-2000).]

[funked]

PS Hans, I think you have the gyro yaw directions reversed.  Remember angular momentum is conserved and the net angular momentum vector of the engine/gearbox/prop system points forward.

[bod]

 

I didn't think gyroscopic effect was really that big of a deal on WW2 fighers

It is mainly during start and stop (low speed) and when the plane is stalled that the gyroscopic forces are noticable since they are relatively weak compared with the aerodynamic forces under normal flight.