Trims and Forcefeedback

[Furio]

In real world, there’s also the “backside of the power curve”. That is, under a certain speed (usually 1.3 times the power off stall speed), fly slower requires more power, until power on stall is reached.

I understand that recreate correct trim behaviour is not easy in a sim.
For example, attitude should change noticeably with flaps extension, with nose going down for a steeper glide path.
For trimming, I believe that the best solution would be to link it to speed, making the plane seeking to keep it, descending, climbing or maintaining speed with power variations without any other control input. That speed should appear as momentary text with power setting.
Something like:
Power: 75%
Trim speed 220 km/h.
The beauty of this solution would be that you’ll end up to use realistically the trim to obtain the speed you want: best climb angle speed, best rate of climb speed, best endurance cruise, best range cruise, all of these speed being listed in any aircraft pilot’s manual.

Perhaps we should suggest this solution for Team Daidalos’ consideration...

Trim can vary by weight, but much more significantly by CG and speed. For best speed you want to be loaded at the aft limit because the stabilizer produces less downforce* at that loading than at the forward limit (the stab always produces downforce in a normally loaded conventional aircraft). Some aircraft can behave oddly concerning trim, for example the 190 which has a single trim setting (based on a given CG) for all speeds above 260mph. Some aircraft nose up sharply when flaps are lowered - some don't. That would be a function of the stabilizer size and distance from the wing and the type of flaps. This, of course, affects trim changes during flap operation. IL2 handles aircraft trim alright in a generic sort of way but my feeling flying the different aircraft is they all seem to be variations of a single flight model. I'm sure that's rather unavoidable. Rather than scoping in on the intricacies of trim I would go after much bigger fish by, first and way-up-front-and-foremost, coming up with a halfway realistic stall. The current IL2 stall makes the plane feel like the CG jumps toward the tail about eight feet during stall making spins universally almost inevitable and difficult (if not impossible) to recover from, regardless of real world performance.




*lift and downforce are drag and the more downforce a stab applies, due to the CG moving forward, the more drag it has and the more weight the wing has to carry - more lift - more drag. A typical figure for stab downforce carried by the wing is 10%. (This is why canards are more efficient - no downforce.)

[Furio]

Originally Posted by zipper

Trim can vary by weight, but much more significantly by CG and speed. For best speed you want to be loaded at the aft limit because the stabilizer produces less downforce* at that loading than at the forward limit (the stab always produces downforce in a normally loaded conventional aircraft).

This is true in theory, but at the price of a reduced stability. P51 emptied rear tank before attempting any aerobatic manoeuvre.

Originally Posted by zipper

Some aircraft nose up sharply when flaps are lowered - some don't. That would be a function of the stabilizer size and distance from the wing and the type of flaps.

This is true for the immediate reaction of plane (more common in high wing design). Fact remains that you must re-trim – PRESTO – nose down and maintain a steeper glide.

Originally Posted by Furio

This is true in theory, but at the price of a reduced stability. P51 emptied rear tank before attempting any aerobatic manoeuvre.

That was because when the fuselage tank was full the CG was AFT of the aft limit - not AT the aft limit. The aft limit is set by minimum stability requirements and the Air Force threw that out the window for range.


This is true for the immediate reaction of plane (more common in high wing design). Fact remains that you must re-trim – PRESTO – nose down and maintain a steeper glide.

Again, different aircraft behave ... differently. The flaps by themselves cause a severe nose down force that is countered by the downwash behind the wing (created by the flaps) landing on the stab. The size of the stab and its position in the downwash are major factors in trim settings. I've flown planes that I've trimmed in the downwind leg and not had to retrim before touchdown. As the planes slowed on base and final rather than retrim noseup I simply fed the flaps down and maintained neutral trim. Not all planes are like this.

ha - my first response got caught in the quote ...

[Furio]

Originally Posted by zipper

That was because when the fuselage tank was full the CG was AFT of the aft limit - not AT the aft limit.

The P51 was just an example. Stability is reduced while the cg goes aft and, beyond aft limit, we cannot talk anymore of reducing stability but of increasing instability. All of this to say that playing with cg for obtaining more speed is something acceptable for racing, but I doubt it can be a wise choice – or a realistic one – in an operational plane loaded for war.

Originally Posted by zipper

Again, different aircraft behave ... differently. The flaps by themselves cause a severe nose down force that is countered by the downwash behind the wing (created by the flaps) landing on the stab. The size of the stab and its position in the downwash are major factors in trim settings. I've flown planes that I've trimmed in the downwind leg and not had to retrim before touchdown. As the planes slowed on base and final rather than retrim noseup I simply fed the flaps down and maintained neutral trim. Not all planes are like this.

You’re right, different planes behave differently, and I also flew with planes that showed pitch-up at flaps lowering. But generally speaking, the effect of flaps in landing position is to generate more drag than lift and – again, generally speaking – planes steepen their approach with flaps down.

Returning on topic, trim is presently simulated with moderate realism, and this is perfectly understandable and acceptable. Any attempt to simulate it with force feedback should try to simulate stick force variation with trim changes. It's no easy task, and I sincerely wish good look to anyone attempting it!

[flyingblind]

I'm looking at building another stick from scratch and the thought I had on trim was to have a means of rotating the pot or hall sensor body so that the center point could be moved in game so setting trim via the hardware rather than software. This may give a more realistic solution? But obviously no good if you are using a bought stick unless you can modify it in some way although I think at least one stick, not sure which one, has trim wheels but again don't know if they control trim via hardware or software.