Pitching moment due to flaps ...

[Anatoli-Kagari9]

Well,

 

yesterday I spent some time testing the reaction of the aircraft to flap deflection under various power settings and configurations. Gear up and down, low power and speed or high power and / or speed, etc...

 

Under all circumstances, including one where I had my aircraft flying S&L at 300 kph, and slowly started deploying the flaps, ending on a stall after it pitched up and started a climb until stalling, make me feel that indeed this is an area requiring fine tuning.

 

My only comparison is il2-bos and the G2 in that sim. There is also an initial pitch ( more like balooning ) effect when flaps are deployed, but then under most power settings, speeds and configurations, it converges towards a stable pitch down attitude. This is particularly true when in landing configuration, and we really need to use "up trim", like we don't in this k4...

 

In the K4, and even on landing conf, with my throttle just a bit above iddle and even when manually setting the prop to fine ( for braking ...), lowering the flaps also results in increasing pitch / attitude...

Edited December 12, 2014 by jcomm

[DB 605]

I don't think Bos could or should used as reference, but flaps causing nose down pitch is confirmed by real life test reports as well so i do agree some work is needed here. But again, its still only beta afterall.

[red777]

This is what I noticed too, with trim nose down, the effect of the flaps always result in a pitch up attitude regardless of power settings.

[Kwiatek]

Such behaviour ( flaps down pitch up) is noticable in most sims. In CLOD TF managed to made pitch down movement with flaps down but it cost some elevator effectivness at slow speeds.

 

Maby these is generally a problem in sims to make correct behaviour in these area.

[PhoenixBvo]

A short explanation by the FAA (Federal Aviation Authority) might be of interest here: Use of Flaps.pdf.

 

It states that, in general:

• the direct effect of flap extension is a rearward shift of the center of pressure, resulting in a pitch-down moment (if trimmed for level flight before extending flaps).
  
• a secondary effect due to increased flow over the tailplane due to downwash however, may produce an overall pitch-up moment.
  

 

The latter mostly applies to high wing aircraft however. So, from this the effect in DCS seems suspect indeed.

[Anatoli-Kagari9]

Excellent source Phoenix,

 

of course, as mentioned - "in general", there are always exceptions.

 

Examples like the C310, B200 amd B1900 are typical of low wing aircraft that exhibit a positive pitching moment due to flap deflection.

 

Yet, the 109s had their trim and flap wheels implemented the way they are to become even more intuitive in their operation... Lowering the flaps could be done with your hand also rotating the trim wheel in the same direction...

Edited December 12, 2014 by jcomm

[PhoenixBvo]

Yet, the 109s had their trim and flap wheels implemented the way they are to become even more intuitive in their operation... Lowering the flaps could be done with your hand also rotating the trim wheel in the same direction...

 

Yes, IMO that's the most persuasive argument in the case of the 109. Messerschmitt engineers put those wheels next to each other like that for that purpose and the simultaneous operation to compensate pitch-down when extending flaps is confirmed by many pilot reports as well...

 

Contemporary example: (Bouncing Clouds)

The most innovative and interesting feature in the cockpit were two large concentric wheels situated on the left sidewall, aft of the throttle. The outer wheel actuated the flaps and inner wheel controlled the pitch trim by changing the incidence of the horizontal stabilizer. Since the flaps inevitably affect the pitch trim, the pilot could ostensibly maintain trim during flap deployment by actuating both wheels simultaneously.

 

By the way, the manual (p.101) actually has an error there, saying:

• To raise the flaps, rotate the wheel counterclockwise.
  
• To lower the flaps, rotate the wheel clockwise.
  

 

In the sim, we rotate counterclockwise to lower flaps. And coincidently, rotating the trim wheel counterclockwise gives pitch-up trim...

[Yo-Yo]

Such behaviour ( flaps down pitch up) is noticable in most sims. In CLOD TF managed to made pitch down movement with flaps down but it cost some elevator effectivness at slow speeds.

 

Maby these is generally a problem in sims to make correct behaviour in these area.

 

Very often the PITCHING moment, i.e. trim changes is mixed to PITCH changes due to increased lift.

So, if you freeze the stick the plane AoA will decrease (nose heavy pitching moment) but CL at this AoA is greater than without flaps, and the plane starts climb increasing its pitch.

 

In the real plane as you experience acceleration for balooning you push the stick. In Yak-18T it's very noticable because of almost instant lowering flaps, and you feel something kicking your butt :).

 

THe only way to judge about the pitching moment is to measure AoA at the same IAS with and w/o flaps.

[Kwiatek]

If i remember my flights it could be that when i drop landing flaps the same time i push stick forward to keep speed and landing glide angle ( eventually trim nose down). I remember that at least in 2 planes when i trim for heavy nose to get correct ladning speed and angle with full flaps down ( one taildragger upperwing, one tricycle lowwing). In aerobatic Zlin 526 F (taildragger, low wing) i dont change trim at all, in final at landing with full flaps as i remember i move little stick forward to got correct flight patch and speed. But the same time i dont remember " balloning " effect casue some times before flaps drop there is need to pitch up a plane to drop more speed ( VFe). It would be need to test a few planes in level flight to check these things correctly not in landing glide.

 

BTW Yo-Yo did you compare take off with G-4 Red 7 and K-4 in DCS? I mean stick postion during take off phases? I dont have yet K-4 but looking at videos there is stron pitch up tendency immidietly with tailwheel rise and plane imidietly got aribone. Comparing to G-4 pilot after rise the tail (stron push stick) he relase the stick then move it little back to lift the whole plane. I think take off power in both planes is quite similar - G-4 probably got little more power but it is much lower weight then K-4 so it could be even that it have better power to wieght ratio at take off ( without K-4 using Mw50).

 

Edited December 12, 2014 by Kwiatek