CG position is correct!

Hi,

Did anyone else notice the little quantity of stick pull needed in order to have the BF-109 pitch up until it stalls? Now I don't need to be an aviation expert or engineer to find out that with this airplane you are able to vary the angle of attack from null lift towards maximum lift (near stall) with just 5 cm (about 2 inches) of stick travel.

The way the BF-109 K-4 feels when handled in pitch gives you an immediate feeling of a plane with very little longitudinal static stability margin left (that's when the CG (center of mass or gravity) is almost near the CP (center of lift/pressure)). Now I don't know why..., but the BF-109 handles almost as difficult in pitch control as the Su-27 (which I had a lot of fun with in the meantime) with ASC direct control ON. It's not that hard but tends to be like it.

I know that the P-51D has a reduced pitch stability margin, but that's only when the central fuel tank is full and it's particularly available for it because of the laminar flow wings (it was among the first and few WW2 aircraft to use laminar airfoils) which had this habit of "throwing" the CP rapidly forward as the pilot starts pulling on the stick (gaining AoA), thus making the aircraft reach a stall AoA very easily and quick, but the BF-109's flight behavior should most probably be nowhere near such characteristics.

I'm not a noob flyer nor trying to say anything against the BF-109's FM, but I doubt that in the 1940's when they built this plane, they did so wanting the pilot to control the AoA range from very little or no lift angle of attack to almost stall angle of attack by just moving the stick that little (which is about 10% of the maximum stick travel). What would they do such a thing for, cause it doesn't make any difference at high speed, and less to say at low speed, because normally you'll reach about the same angle of attack for the same stick travel at any airspeed, so this wouldn't make any sense to have so much stick travel if it wouldn't be useful, right?

Had any RL BF-109 pilot (or someone who knows and can tell) tried this particular aircraft in DCS and felt like the stick travel according to AoA gain is correct?

If so, I'm very happy to understand that, cause otherwise I think something's not right!

Here's a closer look using a track and at some point you'll see the stick's position at stall and how much more stick travel wasn't even used:

BF-109 FM's pitch static stability.trk

From my perspective, either the general center of lift (neutral point) is a bit too forward or the CG is a bit too aft close to the lift center, giving a very little gap between them, thus leaving the aircraft with the feeling of very little pitch or longitudinal static stability when flown.

One more aspect makes it possible for the CG to be too far aft, because when on the ground and rolling at considerable speed (about 150km/h and higher/lower), the plane brakes with quite a remarkable force (similar to the P-51) but doesn't have any tendency towards flipping over it's nose and as far as I know in fact, the BF-109 had a very likely desire to flip forward when the brakes were applied over a certain amount, so it's very likely that the CG is quite aft compared to the real plane.

Another proof that the CG might be too far back is this video:

Try this with our BF-109 K4 and you'll loose directional control (yaw) very quickly because it's very unstable in yaw while rolling on the ground. You can only takeoff and land from and on 3 points (main gear and tail wheel), otherwise it's very easy (and almost impossible to stop) to get a side slip and ultimately flip on a wing and this has nothing to do with the infinite grip of the grass, cause even on tarmac/runway you can't hold a straight line before you actually lift off completely or when you touch down with the main gear unless you also put the tail wheel down and hold it, you won't hold a straight line no matter how good you are...!

Maverick,

I'm not saying it is ok the way it is, but would like to point out, based on Eric's interview, that:

1) Most of the time the 109 was operated at ATA values ranging from 0,9 to a max of 1,2, maybe 1,3 - 1,4 for takeoff.

We tend to fly it at much higher power settings. I now use ~1,4 ATA for takeoff, with manual prop pitch set at 11:45, and then as I get to around 200' AGL I switch to automatic prop pitch, and from there on try to use only between 1,0 and 1,2 ATA, sometimes while cruising even 0,9ATA. With this power settings, you can even, sometimes, have to pitch nose up!

2) Braking full, even during the rollout, was not an issue, as described n many sources in the 109 k4, due to it being tail heavy.

This was actually wrong when the model was released, and you could easily prop strike, but it was solved after a few patches .

This being said, I also find that all of a sudden the K4 becomes too loose on ground, after break away power, but I have configured my throttle axis so that I can easily maintain bellow 1,0 ATA while taxiing. Using rudder and brake to the side you want to turn, and even forward stick on tight turns helps. To taxi along in a straight line, locked tailwheel and stick back is how I do it.

I'm not a noob flyer nor trying to say anything against the BF-109's FM, but I doubt that in the 1940's when they built this plane, they did so wanting the pilot to control the AoA range from very little or no lift angle of attack to almost stall angle of attack by just moving the stick that little (which is about 10% of the maximum stick travel). What would they do such a thing for, cause it doesn't make any difference at high speed, and less to say at low speed, because normally you'll reach about the same angle of attack for the same stick travel at any airspeed, so this wouldn't make any sense to have so much stick travel if it wouldn't be useful, right?

 

Are you using a real stick from the real plane? Is your joystick the same length of the stick used in the real plane? Longer stick, longer travel. Shorter stick (as in computer joystick), shorter travel. You see where I'm headed with this right?

Are you using a real stick from the real plane? Is your joystick the same length of the stick used in the real plane? Longer stick, longer travel. Shorter stick (as in computer joystick), shorter travel. You see where I'm headed with this right?

Did you even watch the track to understand what I meant? It's all about the REAL stick travel of the BF-109 between having it at almost null lift and then at maximum lift (critical AoA), which of course it's the virtual one in the cockpit for this subject, not mine at home! Doesn't matter how much I move mine, I can move it even 1 millimeter if that makes you happy, it's all about the virtual stick's travel between null lift and stall!

I'm using a Hotas Warthog and it doesn't make any sense!

Maverick,

...

1) Most of the time the 109 was operated at ATA values ranging from 0,9 to a max of 1,2, maybe 1,3 - 1,4 for takeoff.

 

We tend to fly it at much higher power settings. I now use ~1,4 ATA for takeoff, with manual prop pitch set at 11:45, and then as I get to around 200' AGL I switch to automatic prop pitch, and from there on try to use only between 1,0 and 1,2 ATA, sometimes while cruising even 0,9ATA. With this power settings, you can even, sometimes, have to pitch nose up...

 

Hi Jcomm,

 

 

So for short, you're telling me that the torque (obtained through the ATA value) driven to/by the propeller modifies the pitch stability derivative in terms of reducing it with AoA and thus having this particular behavior..., that sounds plausible and if there's no other real life proof to attest it, then that would be the answer, hence at low torque on the prop the more stick I should be able to pull before the buffeting starts to emerge, while at higher engine power (higher ATA and torque) the less stick pull is enough to enter into the buffeting range and stall.

 

I now understand that it is correct to be able (in the K4 alone) to use full brakes on the wheels and not have the tail get lifted from the ground as it might be heavy enough corresponding to the horizontal distance between the main gear ground contact and the CG, but what about taking off from 2 points and not 3 (not tail wheel also) and landing on 2 points as well with the aircraft's center line almost horizontal..., is there any masterpiece hand around that can roll the BF-109 just on the main gear in a straight line continuously as they do it in RL? This one still makes me feel that the CG is a little aft and that's why you get such low yaw/directional stability while rolling on the ground on the 2 main gear wheels only cause that would be crazy if they wanted it like that when it was designed.

Did you see real 109 of last series (G and K) balance curves at full power?

Honestly I don't have any access to such data, that's why I was wondering if the CG might not be a little too far aft.

I now understood from Jcomm and might definitely correlate with what you're telling about balance curves (probably stability derivatives) which vary with engine power setting, so I'm all hats down if that's the case for the pitch stability behavior..., but I'm still curious if this plane couldn't have been ridden only on the 2 main gear with the tail lifted to have the plane almost horizontal without loosing directional control that easy, so now I'm pointing my view towards the yaw stability problems when rolling at high speed on 2 wheels on the ground.

Thank you!

As a future spitfire pilot I think the pre-set Trim tab on the BF-109 elevators is a good idea :D

Honestly I don't have any access to such data, that's why I was wondering if the CG might not be a little too far aft.

 

I now understood from Jcomm and might definitely correlate with what you're telling about balance curves (probably stability derivatives) which vary with engine power setting, so I'm all hats down if that's the case for the pitch stability behavior..., but I'm still curious if this plane couldn't have been ridden only on the 2 main gear with the tail lifted to have the plane almost horizontal without loosing directional control that easy, so now I'm pointing my view towards the yaw stability problems when rolling at high speed on 2 wheels on the ground.

Regarding the yaw stability on the ground, may I suggest you take a look in this thread:

http://forums.eagle.ru/showthread.php?t=145051

I believe it has been discussed extensively there.

Regarding 3 point vs 2 point landing, Erich Brunotte mentions at one point that it was a safety requirement in the Luftwaffe to always make 3-point landings, and they would get in trouble if they didn't.

I guess there must have been a reason for that?

For takeoff I believe I usually lift the tail for a while before I get airborne, as I also do in the Dora.

I can check again, but am not at home right now, so can't do it until next week.

The Dora can easily takeoff in 3-point, although I usually opt for lifting the tail, by simply easing on the stick, as I get to around 150 km/h.

The Dora can easily takeoff in 3-point, although I usually opt for lifting the tail, by simply easing on the stick, as I get to around 150 km/h.

That's more or less what I do in both planes also.

I find it easier to avoid stall this way, than when taking off 3 point.

109G2

Thanks Yo-yo...,

I believe you took the effort to mark the important info in red, so I thank you once again for that. This is probably for the G2 (as the bottom suggests) and if you know that it has the same stability characteristics compared to our aircraft (K4), then I'm happy to know the truth now and no longer believe that something could've been wrong with the CG's position.

Now I've got a better picture of how the longitudinal static stability margin and maximum aft CG positions evolve with indicated airspeed at a given engine torque, RPM and trim setting.

The stability margin increase with airspeed can also be confirmed by the fact that when the airspeed is higher (mostly above 350-400km/h), the virtual stick travel (NOT the personal joystick as some might confuse with) is actually greater when pulling until the stall starts to buildup, as the stability curves/slopes within the diagram prove.

Even if the stick travel between the aerodynamic limits seems small, I can't be more happier to know this is true with our airplane!:thumbup:

...

For takeoff I believe I usually lift the tail for a while before I get airborne, as I also do in the Dora.

 

I can check again, but am not at home right now, so can't do it until next week.

Hi Sporg,

I also lifted the tail 1 to 2 seconds prior to takeoff many times with flaps settings varying from null to full in the BF-109 K4, but couldn't maintain a perfect straight line whatever I tried on two points due to the very limited yaw stability (using only the rudder without diff brakes, of course...), at least at lower IAS where the dynamic pressure is quite low for control. With P-51 and FW-190 Dora I otherwise can hold a straight line for long distances because the yaw response and it's stability are much greater for those 2 aircraft in comparison to the 109. I'll check out that forum link you gave me about this subject.

Thanks!:smilewink:

Thank you all..., happy skies!

I just wanted to suggest that when trying to maintain yaw control on the take off run it helps to give quick but strong flicks of rudder to keep it in line, rather than trying to hold a constant smaller rudder deflection. Give it a quick kick back inline. Especially at lower speeds.

I also wanted to point out he that the Emil in the video didn't have a MW50 tank in the rear fuselage. And so we could speculate E series bf109 had less tail heavy character than Gustov and Kurfurst series for this reason. Although G % K series had heavier engine/armament forward of wing also...

I still wonder if the leading edge slats are popping out at too low AOA though. Does anybody else wonder about slats behaviour?

A final note on this subject...

I've been using the K4 mostly on my DCS online sessions.

Exploring the speed and power ranges mentioned on Eric's interview, as I posted before, helped a lot regarding the feel of smoothness and control harmony this aircraft can provide, but I found, during my latest test flights, that setting the prop pitch governor to Manual Control, and operating the throttle and the pitch control carefully to stay within acceptable limits revealed an even more pleasant aircraft.

I have completed a few missions, with a few enemy Ponies shot down, and landing at the end of each mission, using only manual prop control, and I am getting used to it, and actually a fan!

On those flights under several occasions I did have to use tail heavy trim settings to avoid having to pull the stick!

A final note on this subject...

 

I've been using the K4 mostly on my DCS online sessions.

 

Exploring the speed and power ranges mentioned on Eric's interview, as I posted before, helped a lot regarding the feel of smoothness and control harmony this aircraft can provide, but I found, during my latest test flights, that setting the prop pitch governor to Manual Control, and operating the throttle and the pitch control carefully to stay within acceptable limits revealed an even more pleasant aircraft.

 

I have completed a few missions, with a few enemy Ponies shot down, and landing at the end of each mission, using only manual prop control, and I am getting used to it, and actually a fan!

 

On those flights under several occasions I did have to use tail heavy trim settings to avoid having to pull the stick!

That sounds interesting.

Could you reveal your prop settings as compared to ATA setting and rev/min, to get a starting point for experiments?

Would like to play with that as well. :)

That sounds interesting.

 

Could you reveal your prop settings as compared to ATA setting and rev/min, to get a starting point for experiments?

 

Would like to play with that as well. :)

Hmmm,

1,1 ATA 2200 RPM up to 2300

1,0 ATA 2200

1,3 ATA 2300 up to 2500

1,4 ATA only for takeoff, or runaway.... In the extreme event of having to get a foe out of my tail, then I set MW50 and can go up to 1,8 ATA for no more than 15 sec.

Hmmm,

 

1,1 ATA 2200 RPM up to 2300

1,0 ATA 2200

1,3 ATA 2300 up to 2500

 

1,4 ATA only for takeoff, or runaway.... In the extreme event of having to get a foe out of my tail, then I set MW50 and can go up to 1,8 ATA for no more than 15 sec.

Thank you.

But what are your prop settings?

In the manual is recommended 12 o'clock, do you use the same?

Or do you adjust rpm after setting ATA?

Thank you.

 

But what are your prop settings?

In the manual is recommended 12 o'clock, do you use the same?

Or do you adjust rpm after setting ATA?

It depends Sporg.

The only settings I have for certain are my takeoff and landing settings:

TAKEOFF: prop pitch gauge at 11:45, ATA max 1,4

LANDING: prop pitch gauge showing 12:00, ATA as required

For the rest of the flight I adjust prop pitch by setting a given ATA and then adjusting RPM. If I climb or descend, or change ATA, then again I have to adjust prop pitch to stay within the required Prop RPM.

IF I set emmergency power ( mw50 ) then I use only auto-prop.

I never use ATA 1,8 for more than 10 seconds aprox.

It depends Sporg.

 

The only settings I have for certain are my takeoff and landing settings:

 

TAKEOFF: prop pitch gauge at 11:45, ATA max 1,4

LANDING: prop pitch gauge showing 12:00, ATA as required

 

For the rest of the flight I adjust prop pitch by setting a given ATA and then adjusting RPM. If I climb or descend, or change ATA, then again I have to adjust prop pitch to stay withing the required Prop RPM.

 

IF I set emmergency power ( mw50 ) then I use only auto-prop.

 

I never use ATA 1,8 for more than 10 seconds aprox.

Ok, thank you very much. :)

Then I have something to go with.

Ok, thank you very much. :)

Then I have something to go with.

You're welcome Sporg.

Please let us know if you find other good ATA / RPM combinations during your flights.

Right now I am just trying to do it manual as much as I can, even during dogfight.

Why do you fly with manual prop, if i may ask? For fun?

I nearly fly automatic in all situations. Sometimes i choose manual control for landing or take off, but it's not necessary.

Did you see real 109 of last series (G and K) balance curves at full power?

109G2

Thanks Yo-yo...,

 

 

I believe you took the effort to mark the important info in red, so I thank you once again for that. This is probably for the G2 (as the bottom suggests) and if you know that it has the same stability characteristics compared to our aircraft (K4), then I'm happy to know the truth now and no longer believe that something could've been wrong with the CG's position.

 

Now I've got a better picture of how the longitudinal static stability margin and maximum aft CG positions evolve with indicated airspeed at a given engine torque, RPM and trim setting.

 

The stability margin increase with airspeed can also be confirmed by the fact that when the airspeed is higher (mostly above 350-400km/h), the virtual stick travel (NOT the personal joystick as some might confuse with) is actually greater when pulling until the stall starts to buildup, as the stability curves/slopes within the diagram prove.

 

 

Even if the stick travel between the aerodynamic limits seems small, I can't be more happier to know this is true with our airplane!:thumbup:

Maverick Su-35S, I think it's time for Yo-Yo explaining you why these russian chart are make with 1.3 ATA, 2600RPM.

That's not cruising power for Bf-109G2. That is climb and combat power for a G2.