That pitching up ( to loop ! ) due to prop effects...

Yesterday, while I was making comparison tests with another flight simulator in order to answer some questions regarding the modelling of phugoids, I found, again, something really weird with the flight dynamics of the K4, using version 1.5.1... I had experienced this in the past, but forgot about it somewhere along the patch history...

I tried to achieve straight and level flight, and the speed at which the K4 appears to be in almost perfect equilibrium in terms of it's yaw and roll trim settings.

I was flying at even lower than 1,0 ATA, had finally achieved more or less S&L flight, of course having to set the pitch trim fully forward ( which, for a start, still seems unplausible to me... no matter what I've been reading on several posts at this forum... ), and as I was about to perturb my K4 in pitch, to check the pitch stability response, I found that I was still flying uncoordinated, my ball a bit to the right so... I started trying to coordinate...

As I kept pressing the right pedal, trying to bring the ball to the center, the K4 started pitching up, at a good pace, forcing me to use a forward stick to avoid reaching a high pitch up attitude.

I tried it again, very softly trying to coordinate, with really slight right rudder input, but this turn around I did not add fwd stick, just to check what would happen... The pitch up was so aggressive that I just didn't loop because it faled on it's back at almost 80º pitch up !!!

I think it was once mentioned this was a result of p-factor, and I confirmed the logic behind this claim but heck!!!! Isn't this a bit overdone, really ? Beware I was never even above 1,0 ATA !!!!

The doesn't sound right for a 109 indeed... but it also doesn't seem right for properly updated 1.5.1. I tried to reproduce it on my machine, version is 1.5.1.47285, autumn map, standard air start with default loadout and fuel as given by fast mission editor, no trim adjustments in "special" options tab and obviously, no auto-rudder.

My observations don't quite match Yours, and I don't know what the reason for it is. First of all, I can't get a "hands free" attitude with ATA between 0.9 and 1. The plane always rolls to the right. Also, the ball is very much centered at this setting. Another thing is, in order to maintain level flight with such low ATA, I have to trim about only +1 down. If I apply full trim down, about 1.3 ATA and 520km/h IAS is required for level flight, with ball running a bit to the right. Small left rudder input applied to get it back to center does cause a pitch change (down), but it's small and slow, requiring only a tiny backward stick input or trimming out.

Did You use any preselected aileron and rudder trim settings we could test? If You did, maybe they cause some unexpected plane behaviour? They're still quite new feature after all.

Well, after reading your post I tried to reproduce it again, wit no success

I use the "Concrete Jungle" Single mission.

Tried at 1,0 ATA and indeed this time the ball was centered most of the time, and I initiated the phugoid tests which were successful.

Yet, using aileron to maintain wings level, if we apply right rudder, the aircraft pitches up and starts climbing. Left rudder has the opposite effect.

Well, after reading your post I tried to reproduce it again, wit no success

 

I use the "Concrete Jungle" Single mission.

 

Tried at 1,0 ATA and indeed this time the ball was centered most of the time, and I initiated the phugoid tests which were successful.

 

Yet, using aileron to maintain wings level, if we apply right rudder, the aircraft pitches up and starts climbing. Left rudder has the opposite effect.

I have experienced the pitch up issue with a centered ball. I experience it frequently, but never to the aggressive amount you said. Typically I'm at about 440Km/h, and centering the ball results in the 109 pulling nose high pretty quickly.

Well, after reading your post I tried to reproduce it again, wit no success

 

I use the "Concrete Jungle" Single mission.

 

Tried at 1,0 ATA and indeed this time the ball was centered most of the time, and I initiated the phugoid tests which were successful.

 

Yet, using aileron to maintain wings level, if we apply right rudder, the aircraft pitches up and starts climbing. Left rudder has the opposite effect.

Is it one of stock 1.5 missions? Shame to admit, I never noticed it :). Will test when I get back from work.

Thx Art-J, although I believe my original experiment might have been contaminated by some unwanted surge in the throttle, and I was not actually doing 1,0 ATA but probably a higher setting.

I will also try it with manual prop pitch.

One thing is for sure:

1) it is opposite what gyroscopic effect might account for, but also, under the conditions we are running our tests in, that effect is negligible....

2) We can explain it based only on p-factor, if we really believe that p-factor can be so important under these same flight conditions...

From any situation where you can achieve, as close as possible, stable S&L flight in the K4, if you maintain wings level using constant aileron deflection and then use rudder, left or right, you will notice that sideslips will cause your pitch to increase if you deflect the right rudder, and decrease if you input left rudder...

Finally I was also able, although I had to use almost permanent roll corrections to stay level, to experiment very plausible phugoids as a response to pitch up and pitch down perturbations, from an as close to S&L starting situation.

Unless I misunderstood Your post, I would rather say the direction of the gyroscopic effect is actually correct. In a plane with a clockwise prop, yaw to the right causes pitching up, yaw to the left causes pitching down.

It's the amount of pitch response, which causes confusion in this thread, isn't it?

SiTh, and Art-J...

A) My initial observation was most probably based on an incorrect assumption - that of being at 1,0 ATA, because I could never reproduce it again starting with any Bf109 K4 scenario, but, I have used the Mission called "Concrete Jungle" from the "Single" missions that come with the K4 install in 1.5.1 for startup. The aircraft is flying low and is supposed to fly along a course between the buildings. I usually climb to around 5,000' to start my tests from there, and setting ATA to 1,0 I just have to use aileron to fly straight at cruise settings. The ball is almost perfectly centered all of the time for those power settings...

I tested the phugoids from there and indeed it works beautifully, both for pitch dn and pitch up perturbations from level flight. After a few cycles I am back to my trimmed speed / AoA.

B) Art-J, if the prop rotates clockwise, pushing the nose to the right ( right rudder ) is equivalent to applying a force from the left, so ( by gyroscopic effect ), if you rotate clockwise 90º you'll get a pitching down moment, not a pitching up ;-)

Ah, yep, You're right, I got vectors confused :). Might be interesting to test how gyro effect works in 190 and -51 then.

But I am sure at these flight states, gyroscopic effect is negligible, so, those pitching moments we observe must have their roots in other factors....

Maybe Yo-Yo comes across this thread and can offer some light into the whys and the hows of these pitch up with right rudder, pitch down with left rudder :-) - is it ( again ) a "side effect" of p-factor ?

There are some funny things going on with the fm's atm I can concur.