Where's the torque?

[Nealius]

Alternate title: Is my takeoff assist actually off?

 

I was under the, possibly mistaken, impression that the Mossie would have a lot of torque with two massive, non-counter-rotating engines. On takeoff we're supposed to give right rudder trim and lead with the left throttle until rudder authority is available.

 

But I'm not experiencing much torque at all in the Mossie. I have takeoff assist 0, and auto-rudder off, yet this is the behavior I observe:

 

1. No rudder trim needed on takeoff

2. Leading with left throttle does nothing; assymmetric thrust in general does nothing unless one engine is completely out

3. No rudder needed at all on takeoff in 0 wind if your tailwheel is straight

4. No rudder needed when making drastic power changes

 

I'm hardly using the rudder at all and it feels so wrong for a stick-and-rudder prop. Is this expected behavior, or is my takeoff assist/auto-rudder setting porked? I ran a slow repair to make sure, but nothing changed.

 

Edited September 21, 2021 by Nealius

[grafspee]

No torque at take off, all of it goes  in to groung. Only p factor contribute.

 

[Darrylx444]

Yeah, I noticed that too. Feels kind of weird to me, even compared to similar twin planes in my other WW2 combat sim.

[Nealius]

5 hours ago, grafspee said:

No torque at take off, all of it goes  in to groung. Only p factor contribute.

 

 

I always get torque and P-factor confused. What's the difference between the two? 

[GAF_flyer]

27 minutes ago, Nealius said:

 

I always get torque and P-factor confused. What's the difference between the two? 

Torque is basically the engine trying to turn the plane instead of the Prop. Or more scientifically speaking the equal and opposite reaction to the propellers turning.
P-Factor is the slipstream of the propeller impacting the tailplane. shouldn't be much of a factor in a Mossy in low Wind condition, as the props are not inline with the Vertical stabilizer. 
You also have the effect of the down moving prop-blade producing more lift than the up moving, shifting the center of thrust slightly to that direction. Since the Mossies engines both turn clockwise (from the pilots perspective) that induces a  left yawing tendency and also makes the left engine the critical one. 

[Bozon]

16 minutes ago, Nealius said:

 

I always get torque and P-factor confused. What's the difference between the two? 

Torque is rotation force applied through the shaft, in our case the plane is torquing the prop in one direction and in response the prop is torquing (trying to rotate) the plane in the opposite direction around the prop axis.

In the mosquito the props rotate clockwise from the pilot pov which means torque is trying to roll the mossie counter clockwise (left).

 

P-factor is when one side of the prop disc is producing more pull then the other side. This creates a yaw tendency on the frame of the plane. P-factor is created when the plane is pitched (usually up) relative to the direction of the air flow. In that case, the blades traveling down have a higher angle of attack relatively to the blades traveling up on the other side of the disc.

In the mosquito this means that increasing the pitch relative to the airflow will create left yaw tendency. Inducing a pitch up/down movement (pull/push stick) makes the nose swing sideways a bit as a secondary effect. Increasing/decreasing throttle at a high angle of attack (slow speeds usually) also creases some yawing motion by changing the yaw force from the P-factor, even when the engines are perfectly synchronized.

 

There are also other effects involved in the final result of secondary effects.

[tweet]

Check the "Special" tab, Mossie section, in the Settings/Options menu. Auto rudder and rudder assist are available though I can't remember if they're on by default. That is almost surely the case if you don't need to use rudder trim and the rudder for takeoff.

Edited September 21, 2021 by tweet

[Nealius]

16 minutes ago, tweet said:

Check the "Special" tab, Mossie section, in the Settings/Options menu. Auto rudder and rudder assist are available though I can't remember if they're on by default. That is almost surely the case if you don't need to use rudder trim and the rudder for takeoff.

 

 

As stated in my first post, I have these turned off and am still experiencing little need for rudder on the takeoff roll. Once in the air and bringing gear up, I need a lot of rudder to track the runway as I accelerate. That and landing are about the only two times I need rudder. Adverse yaw seems to be lacking compared to other warbirds as well. 

[FlyingTaco21]

ive notices it quite a bit but only after the tail raies.  aircraft will draw right  and then then when the tail lifts there is a strong left tendency a left roll when off the ground 

 

[Nealius]

Okay, now I see. This should be "where's the p-factor?" I can see the torque with some left rolling tendencies, but I'm not really experiencing any P-factor at all. Is this normal for twins?

[Art-J]

^ Not when both engines turn the same CW direction it isn't! As long as the airplane AoA is non-zero, port prop pulls more closer to the fuselage, while starboard one pulls more closer to outer section of the wing. Total thrust is not symmetrical in relation to CoG.

 

I want to believe it's modelled to some extent, but how strong it should be in the Mossie - I'm not qualified to judge. 

 

On a sidenote, I know you use combined axis for props, but do you use it for boost as well? I use both Warthog HOTAS throttles as separate boost levers, but I struggle with moving them together to obtain the same power on both sides. When I match the physical levers, the virtual ones seem to be slightly offset. When I match the virtual levers, the physical ones have to be slightly offset. Windows game controller panel suggests both are calibrated correctly, but I don't know what to think of it. Maybe that's the reason why I actually have to work quite a bit to keep the plane rolling straight during takeoffs.

Edited September 22, 2021 by Art-J

[Nealius]

I use the split throttles on the Warthog for boost. I haven't paid attention to the game throttles, but if I link my Warthog throttles there definitely is a discrepency in the boost you get. I just leave them unlocked and go by the boost gauge, but the offset between the two throttles is so minimal I can't really tell.

[Art-J]

Thanks. Glad to hear it's not just me then. I tend to glance at RPM gauges more often, as they're just bigger - but then, it seems they sometimes show a bit different average value even if boosts are supposedly the same. Which makes me scratch my head, because I use the grey trim slider for combined RPMs, so theoretically, as long as prop governors on both engines are good, both tachos should show the same RPMs too... but they don't always do that.

[Nealius]

I've seen the same too. One engine might be ~50rpm out of whack with RPM levers linked and same boost settings.

[ARM505]

They're constant speed propellers, so RPM and boost won't really be linked at normal power settings (ie, once stabilised after a power change, RPM should go back to where it was set). And, a comment based on real world experience. Based on flying many, many twins, theres normally always a slight throttle stagger, even if it's just a few millimeters, to get the same indicated power setting. The same for the propeller levers. What we get in sims, where the engines are at precisely the same power setting with the same throttle lever position is actually not very realistic feeling to me. Because of this, I noticed quickly with the Mozzie, the RPM levers need to be 'tweaked' indivdually to match the RPM's - which is realistic. Now, for bonus points, we should have to tweak it on the sound! Trying to tune out the 'WaWaWaWaWaaaaWaaaaaaaaWaaaaaWaWaWa' from the props is integral to propeller powered twin flying!

[Rhinozherous]

How is the bigger distance between the wheels affecting torque?

Could this be the reason why she is not that affected by it on ground, but more in air?

[Bozon]

3 hours ago, Art-J said:

^ Not when both engines turn the same CW direction it isn't! As long as the airplane AoA is non-zero, port prop pulls more closer to the fuselage, while starboard one pulls more closer to outer section of the wing. Total thrust is not symmetrical in relation to CoG.

 

I want to believe it's modelled to some extent, but how strong it should be in the Mossie - I'm not qualified to judge. 

 

The magnitude of the P-factor depends not only on AoA but also on speed. It is zero when the speed is zero and very small until the speed builds up, but at that point the rudder is already effective - so for the most part it is not significant on takeoff runs. It’s most significant effect is supposed to be when you rotate, but then it is also mixed with other effects so it is difficult to separate.

 

[DD_Fenrir]

Gyroscopic precession is another factor that needs to be taken into account when lifting or lowering the tail. 

[Bremspropeller]

21 hours ago, GAF_flyer said:

P-Factor is the slipstream of the propeller impacting the tailplane.

 

P-factor is the uneven pressure (thrust) distibution across the prop-disc, caused by the different local aoa of the ascending and descending blades respectively.

Slipstream is, well, slipstream.

 

 

I think I'm usung too much rudder-trim on take-off, as the aircraft wants to skid to the right. I'm usually putting the pointer onto the T in "Trim".

I might have to experiment a bit.

 

She certainly seems not amused when heavy, and not using flaps for take.-off. Not sure it it's just the increased mass swinging around when all gassed-up (and with bombs).

 

Also, two divisions of nose-down trim is still not enough to make her accelerate to 170mph after gear-retraction.

 

Does anybody know what the blue-line speed is(best single engine rate of climb)?

Red line (VMC) seems to be 170'ish mph at take-off rating.

Edited September 22, 2021 by Bremspropeller

[Art-J]

@Bozon - I beg to differ. The blade AoA is higher when the plane is stationary, blades are on their fine pitch stops, then governor limits the prop RPM by making the pitch more coarse. so that's when local blade AoA differences are highest. Then, as the plane starts rolling and airflow vector from its movement kicks in, the governor changes pitch accordingly so I don't see how speed would be a primary factor. And certainly not when the tail comes up and the P-factor diminishes to zero. That's when the gyro precession kicks in, however, as Fenrir noted.

[Bozon]

2 hours ago, Art-J said:

@Bozon - I beg to differ. The blade AoA is higher when the plane is stationary, blades are on their fine pitch stops, then governor limits the prop RPM by making the pitch more coarse. so that's when local blade AoA differences are highest. Then, as the plane starts rolling and airflow vector from its movement kicks in, the governor changes pitch accordingly so I don't see how speed would be a primary factor. And certainly not when the tail comes up and the P-factor diminishes to zero. That's when the gyro precession kicks in, however, as Fenrir noted.

Without airspeed there is no P-factor, regardless of angles. When you rotate for lift off the AoA is pretty high (you are slow) and the P-factor change is quick - from zero when running with the tail raised to a high AoA with a bit of speed, and may require rudder input. Though as I mentioned there are several other secondary effects involved, especially when rotating for lift off.

[grafspee]

The biggest p-factor is present on take off roll. Once speed builds up it rises but rudder efficiecy often surpass it making less of a problem unlees you are in bf109. In mossie both p factor and gyroscopic effects are present but plane construction mitigate those effects. For example right engine tries to yaw plane left, in case of single engine fighter this force is applied very close to cog most of the mass is very close where force is applied. In case of mossie right p factor tries to swing left engine and whole fuselage back, yaw axis is placed at right prop. This is alot of mass and couple meter arm to the left engine. Even wheels prewent yawing plane along right prop axis, wheels would skid if that would happen. So both this effect are greatly reduced at take off

Edited September 23, 2021 by grafspee

[Wrcknbckr]

Remains why OP isn't experiencing any cross-coupling on take-off.
-On the ground if you rev-up I'd expect to see a rolling tendency to the left (due to engine torque)
-During rolling and speedup there should be a tendency to yaw left due to (little) more pression on left gear (suppose that's in the gear model).
-lifting the tail (pitch down) causes a momentary yaw to the left (gyroscopic effect)
-Pitch up causes yaw to the right (the gyroscopic effect)
-Once you're off the ground the aircraft wants to roll left, counteracted by aileron.
I would expect these effects to be present in a flight model. P-factor I'm not so sure. It's complex physics and I doubt there's data available. I would expect input from SME's and some hand waving/tuning (WIP?).  I'm not even addressing prop-wake issues.

I'm curious to know how engine torque is counteracted in cruise flight; differential aileron as base? structural asymmetry? tabs? continuous roll trim?

 

46 minutes ago, grafspee said:

 This is alot of mass and couple meter arm to yhe left engine. 

P-factor's main contribution is an off-set of the thrust vector which luckily is only a fraction of the prop radius (or did I misunderstand).
 

[Yo-Yo]

33 minutes ago, grafspee said:

The biggest p-factor is present on take off roll. Once speed builds up it rises but rudder efficiecy often surpass it making less of a problem unlees you are in bf109. In mossie both p factor and gyroscopic effects are present but plane construction mitigate those effects. For example right engine tries to yaw plane left, in case of single engine fighter this force is applied very close to cog most of the mass is very close where force is applied. In case of mossie right p factor tries to swing left engine and whole fuselage back, yaw axis is placed at right prop. This is alot of mass and couple meter arm to the left engine. Even wheels prewent yawing plane along right prop axis, wheels would skid if that would happen. So both this effect are greatly reduced at take off

 

I think your explanation has a mistake: P-factor is shift of the point where prop thrust is applied. So, if you shift both engines thrust right you shift their resultant force to the same distance relatively the aircraft symmetry vertical plane. So, the resultant moment will be the same in comparison of one engine of double thrust.
What is really reduced is downwash effect on the tail. 
Thus, Mosquito trim curves are different from the one-engine planes: it requires almost constant right aileron input, but less rudder input. At take-off as the rudder is not downwashed the plane has no stability and controllability, though the one-engine plane has no stability but can be effectively controlled.

Just now, Wrcknbckr said:

Remains why OP isn't experiencing any cross-coupling on take-off.
-On the ground if you rev-up I'd expect to see a rolling tendency to the left (due to engine torque)
-During rolling and speedup there should be a tendency to yaw left due to (little) more pression on left gear (suppose that's in the gear model).
-lifting the tail (pitch down) causes a momentary yaw to the left (gyroscopic effect)
-Pitch up causes yaw to the right (the gyroscopic effect)
-Once you're off the ground the aircraft wants to roll left, counteracted by aileron.
I would expect these effects to be present in a flight model. P-factor I'm not so sure. It's complex physics and I doubt there's data available. I would expect input from SME's and some hand waving/tuning (WIP?).  I'm not even addressing prop-wake issues.

I'm curious to know how engine torque is counteracted in cruise flight; differential aileron as base? structural asymmetry? tabs? continuous roll trim?

 

P-factor's main contribution is an off-set of the thrust vector which luckily is only a fraction of the prop radius (or did I misunderstand).
 

P-factor is modelled since the early 2011 Mustang. It is very noticeable at tail lift - initially you need certain amount of right rudder, as the tail is being rised you need to add right rudder to counteract gyroscopic moment but as the plane has low AoA the amount of right rudder becomes lower.

[Bremspropeller]

16 hours ago, Bozon said:

Without airspeed there is no P-factor, regardless of angles.

 

You always have considerable net flow through the prop disc - even when standing still. Hence you also have P-factor at zero airspeed.

The only question is the angle of the flow-field, which may be tilted in line with the engine's waterline. Then there'd be no P-factor, but not because there's no air-speed, but because there's no difference in blade AoA.

Edited September 23, 2021 by Bremspropeller