Jump to content

Sabre flaps


mslechta

Recommended Posts

Hello,

I would like to ask for plane behaviour after lower flaps.

DCS Sabre becoming tail heavy after lower flaps but every planes what I know from my AF career behave oppositely (they are nose heavy).

 

Is it mystake (in game) or does exist any reason (construction, aerodynamic or...) for it?

 

Thank you.

Link to comment
Share on other sites

I've also noticed that the Sabre is extremely tail heavy with flaps down. Even with full nose-down trim I find myself having to fly final with a bit of forward stick pressure. Haven't been able to fly the past couple of days but maybe you are supposed to have a nose-high attitude on final? Not sure myself.

Link to comment
Share on other sites

Hello,

 

After few tests, yes each time that my flaps down my tail is heavy.

 

I'm not sure but during all the flight phases we must trim (pitch up or down) the F-86.

 

So, in final approach when my speedbrakes full open my tail is a little bit heavy and I must trim up.

When my flaps is down my tail is really heavy and I must trim up again for obtain the good AOA.

 

Normaly flaps down increase the lift and flaps up decrease the lift, no ?

Maybe that the F-86 flaps are too powerfull ?

 

AOA8.gif

 

Bye, Skull.


Edited by Skulleader
Link to comment
Share on other sites

I think Skulleader's pic explains it, the lift is greatly increased when the flaps lower and the nose rises. If you watch the vertical velocity it increases also. Can't say if it's over exaggerated or not as I've never flown an F-86 before but is pretty dramatic for sure. You see the same thing on the A-10 when on final, the FPM rises quickly when you lower the flaps, just not as dramatically.



Win 10 64 Pro, MSI Z390 I7-9700K @5ghz Kraken Z63, 32Gb Corsair Dominator, MSI RTX-2070, 1TB NVME 2TB SSD's, TM Warthog, Pro Rudders, OpenTrack w/ IR Clip

Link to comment
Share on other sites

Unfortunately, I don't think we can get the full picture from just that image. It shows how lift works for a 2D airfoil but a 3D wing is different, especially if it is swept. The Change in centre of pressure of the wing with relation to the CoG and wing sweep of the F-86 must be taken into account.

 

If the centre of pressure moves ahead of the CoG sufficiently or if the lift increases enough while the CoP is still ahead of the CoG, it can create a moment due to the increased lift, causing the pitch up. But actual calculations have to be made. I'll see if I can find more info. All I know is that the F-86 borders on unstable in the pitch axis. Thus it stands to reason that pitch up can further increase AoA in a feedback loop.

 

fig041.gif


Edited by SgtPappy
Content in 2nd paragraph
Link to comment
Share on other sites

Now that I think about it the sabre flaps are located on the inboard section of the swept wings so they are quite far forward in relation to the centre of pressure of the wings. The inner most section the flaps may even be in front of the centre of pressure.

 

If the centre of gravity is close to the centre of pressure, and some portion of the flaps is location in front of both, it would make sense for the pitch up with flaps down. I think...

Link to comment
Share on other sites

Now that I think about it the sabre flaps are located on the inboard section of the swept wings so they are quite far forward in relation to the centre of pressure of the wings. The inner most section the flaps may even be in front of the centre of pressure.

 

If the centre of gravity is close to the centre of pressure, and some portion of the flaps is location in front of both, it would make sense for the pitch up with flaps down. I think...

 

Agree, but in the original (online) flightmanual i can not find a Caution , Warning, or Note, about the strong nose up moment

So i agree with youre logic...but is it really a realistic effect.

 

Wish an old :smilewink:pilot of the sabre would give the answer...they would really know !!!

 

Greets

Link to comment
Share on other sites

  • 6 months later...

The pilot's manual procedure provides a small clue: takeoff trim requires a bit of nose up trim, as it should be to compensate for the nose-down moment caused by the flaps. With the present nose-up moment caused by the flaps, using additional nose-up trim per the real-world take-off procedure only makes things worse.

 

It is noteworthy that the Bf109K4 suffered from a similar problem, but after the last patch, it was fixed. Now deploying flaps and gear for landing requires nose up trim as specified by the manual.

 

I would like to see a similar fix for the F-86F.

[sIGPIC][/sIGPIC]

Link to comment
Share on other sites

The pilot's manual procedure provides a small clue: takeoff trim requires a bit of nose up trim, as it should be to compensate for the nose-down moment caused by the flaps. With the present nose-up moment caused by the flaps, using additional nose-up trim per the real-world take-off procedure only makes things worse.

 

It is noteworthy that the Bf109K4 suffered from a similar problem, but after the last patch, it was fixed. Now deploying flaps and gear for landing requires nose up trim as specified by the manual.

 

I would like to see a similar fix for the F-86F.

 

If this is the case, then I would like to see it fixed as well. Is this in the F-86F manual or the E? Just wondering, as I'm going to look into it myself now that I have the F manual.

Link to comment
Share on other sites

I have the F-86F manual. But I have yet to find any text telling the pilot to expect a trim shift when flaps and/or gear are deployed. But the effect is very simple: both landing gear and flaps push to the rear while lower than the CG, which is why most aircraft exhibit this tendency to various degrees.

 

Since the F-86F takes off with flaps fully lowered, it makes sense that you would need nose up trim to help lower the stick force required to pitch up at take off speed.

 

But as currently modeled, the aircraft will pretty much jump off the runway without nose up trim, and tends to pitch up excessively when you adjust trim to make the take-off trim indicator light up.

[sIGPIC][/sIGPIC]

Link to comment
Share on other sites

That is normal because of the position of the horizontal stabilizer in relation the the wings.

 

When you select flat down it also force the airflow that goes around the wing downward and it result into a change in the direction of the relative wind on the horizontal stabilizer, forcing a pitch up moment.

 

%5Bimg%5Dhttp%3A//i.imgur.com/gVzAsDE.png[/img]

 

%5Bimg%5Dhttp%3A//i.imgur.com/3Gnnzfv.png[/img]

 

 

 

If of course you have a T tail configuration like the Mig15 or a very long fuselage like a B747-800 then you will have a nose heavy

 

o7


Edited by Decibel dB
Re format
Link to comment
Share on other sites

  • 2 months later...
Since the F-86F takes off with flaps fully lowered, it makes sense that you would need nose up trim to help lower the stick force required to pitch up at take off speed.

 

But as currently modeled, the aircraft will pretty much jump off the runway without nose up trim, and tends to pitch up excessively when you adjust trim to make the take-off trim indicator light up.

 

I agree. The take-off behavior using textbook trim and flap settings is bizarre and runs counter to standard practices for take-off procedures in any aircraft. You want the plane to lift off the runway, not LEAP off of it. As it is now, the plane will pitch up to a stall configuration if unattended. That's not right.

Link to comment
Share on other sites

Don't take off with full flaps, that's for sure. It is surely a mistake

As Art-J said: From the real manual. With revision from 1971. So if that procedure was wrong, surely it would have been changed by that date.

 

Fox

 

attachment.php?attachmentid=117743&d=1431980944

692829353_sabreflaps.PNG.7e4462cb6c36c909acb4cc26a1aa1de9.PNG


Edited by iFoxRomeo
Spoiler

PC Specs: Ryzen 9 5900X, 3080ti, 64GB RAM, Oculus Quest 3

Link to comment
Share on other sites

Is it written before that what are the condition of the takeoff? ie short or damage runway? I never witness nor flew any aircraft's that takeoff with full flaps apart from some specific operations or on very short field or rwy limited takeoff but as a normal takeoff no. That being says I can be wrong.

 

I will check with someone who flew it and I get back to you.

Link to comment
Share on other sites

I sometimes get an impression that this part of flight modelling has been reversed with MiG-15 by mistake :D. The Russian bird was (in)famous for pitch-up tendency during take-off, docs mention it, crash of restored HA-UTI showed it, yet in "our" rendition it's not really that noticeable.

 

F-86, however, if trimmed so the white trim bulb flashes, and configured with any flaps, pitches up like crazy and requires quite a bit of a stick push.

i7 9700K @ stock speed, single GTX1070, 32 gigs of RAM, TH Warthog, MFG Crosswind, Win10.

Link to comment
Share on other sites

I sometimes get an impression that this part of flight modelling has been reversed with MiG-15 by mistake :D. The Russian bird was (in)famous for pitch-up tendency during take-off, docs mention it, crash of restored HA-UTI showed it, yet in "our" rendition it's not really that noticeable.

 

F-86, however, if trimmed so the white trim bulb flashes, and configured with any flaps, pitches up like crazy and requires quite a bit of a stick push.

 

Yes, I have exactly same impression of the two FMs.

Link to comment
Share on other sites

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...