Bf109 K-4 pre-stall buffeting...

[Anatoli-Kagari9]

Whenever I enter dogfight with the K-4, even at speeds above 400 km/h indicated, it's easy to get the aircraft buffeting, in tighter turns.

 

Looking at the slats, they're deployed every time that happens.

 

Is this WIP, or realistically modeled already ?

[NineLine]

You should post a vid, with the controls displayed so we can see what you are doing and what is happening... really hard to say what is going on, I usually only see troubles when I am flying the 109 too hard...

[Anatoli-Kagari9]

You should post a vid, with the controls displayed so we can see what you are doing and what is happening... really hard to say what is going on, I usually only see troubles when I am flying the 109 too hard...

 

Good idea. I do not have any recording software, but I will probably enable Nvidia Shadow PLay and record something one of these days...

Edited January 8, 2016 by jcomm

[NineLine]

Shadowplay works well, and I have not noticed any performance issues with it.

[Kwiatek]

Critical angle of attack is not a function of speed. YOu could reach or pass cAoA both at slow as a high speeds. So if you are in turn and your slats are open its mean that you are closing to cAoA and you could expect stall buffeting.

[javelina1]

.... I usually only see troubles when I am flying the 109 too hard...

 

Yep, exactly for me too. I'm just pulling a little to hard using the Hawg. I'm getting better to learn the amount of control I give, (without having the force feedback).

[Anatoli-Kagari9]

Critical angle of attack is not a function of speed. YOu could reach or pass cAoA both at slow as a high speeds. So if you are in turn and your slats are open its mean that you are closing to cAoA and you could expect stall buffeting.

 

Well, I know that, as you can probably guess since you know I fly for real at more than 3 decades :-)

 

I was just making sure accelerated stall was what I was referring to...

 

Question is.... would the real thing, when slats deployed, show that buffeting the way the K-4 shows in DCS ?

 

I can never know... but I guess it probably is the case....

 

Closest experiences from RL are from a Rallye Commodore 180 GT, and slats deployment actually avoided any "shaking" near stall...

[312_Lobo]

Closest experiences from RL are from a Rallye Commodore 180 GT, and slats deployment actually avoided any "shaking" near stall...

 

AFAIK Rallye Commodore 180 GT has full span slats, while bf-109 has slats only on the outer half, isn't this what makes the difference?

 

You probably know that wing has usually some washout so that wing root stalls first producing some buffeting giving feedback to pilot that plane is near cAoA while keeping ailerons effective. Although Bf-109 wing has no washout, it has slats at the outer half which produces similar effect as washout i.e. when inner portion of wing is at critical AoA the outer part is not due to slats deployment... .

Edited January 9, 2016 by do.st
typos

[Anatoli-Kagari9]

AFAIK Rallye Commodore 180 GT has full span slats, while bf-109 has slats only on the outer half, isn't this what makes the difference?

 

You probably know that wing has usually some washout so that wing root stalls first producing some buffering giving feedback to pilot that plane is near cAoA while keeping ailerons effective. Although Bf-109 wing has no washout, it has slats at the outer half which produces similar effect as washout i.e. when inner portion of wing is at critical AoA the outer part is not due to slats deployment... .

 

Thx,

 

I thought the 109 wing had washout, even if I agree it would be more complex to implement together with the wing tip slats...

 

But your point is good, and the stutter is probably just ok. I keep training and getting better...

Edited January 9, 2016 by jcomm

[Sporg]

Whenever I enter dogfight with the K-4, even at speeds above 400 km/h indicated, it's easy to get the aircraft buffeting, in tighter turns.

 

Looking at the slats, they're deployed every time that happens.

 

Is this WIP, or realistically modeled already ?

 

You probably have this under control, but I found buffeting to happen easier during turns, if I did not control the rudder very well (keeping the ball centered throughout.)

[Anatoli-Kagari9]

You probably have this under control, but I found buffeting to happen easier during turns, if I did not control the rudder very well (keeping the ball centered throughout.)

 

Thx Sporg,

 

I've been able to pretty much iron it out by using a 10% curve on my pitch axis. Practice is the other mandatory ingredient ... :-)

[BSS_Vidar]

Here's a train of thought about stalling and getting pre-buffet from a wing.

 

You can stall a wing at any SPEED, any ATTITUDE, and at any ALTITUDE.

 

Even in a straight down dive... if you pull hard enough to reach the Critical Angle of Attack (CAoA), the wing stalls or departs. Of course, in a scenario like that, the plane AND the pilot would have to survive the ordeal to get to that point.

 

If you're in a turn fight and start experiencing buffet, that's your hint that you're at the limit of that wing. Slats help and maybe a notch of flaps to change the AoA reconfigures the wing, thus changing the CAoA.

 

Hope that helps.

 

V

[Anatoli-Kagari9]

I know V,

 

but indeed it appears slat deplyment is almost unnoticeable in the 109s, most of the time the pilot will only become aware of it if visually confirming the deployment, and pre-stall buffet almost inexistent too...

[Hummingbird]

The real 109 gives lots of warning before the stall via buffetting and stick vibration.

 

The reason MTT didn't put full span slats on the 109 were two fold:

1)Because the inboard part of the wing always stalls later than the outboard part to begin with

2) Because slats placed straight in the path of the propwash wouldn't work in a power on situation, the accelerated air from the prop would keep them shut at speeds where they were supposed to be open. Also the propwash already acted as a set of slats by itself, accelerating the air over the inboard wing section, making it stall at an even lower relative airspeed than in the power off condition.

 

The slats were therefore placed where needed, on the outboard wing section. Here the slats helped increase the CLmax by keeping the outboard part of the wing from stalling before the inboard part by raising the critical AoA & Clmax by approx. 25% in the covered area. And since the inboard part of the wing already stalls at a higher AoA and CLmax than the outboard part(usually ~15% higher) the slats end up increasing the overall CLmax of the entire wing by ~10-15%. This becomes esp. true with power on as the lift & critical AoA of the inboard wing section is then increased even further by the prop stream further raising the overall CLmax over the already 10-15% power off gain.

 

Hence how at full power the Finnish were able to fly their G-2 level at a speed of just 130 km/h, flaps & gear up.

Edited January 12, 2016 by Hummingbird

[Yo-Yo]

The real 109 gives lots of warning before the stall via buffetting and stick vibration.

 

The reason MTT didn't put full span slats on the 109 were two fold:

1)Because the inboard part of the wing always stalls later than the outboard part to begin with

2) Because slats placed straight in the path of the propwash wouldn't work in a power on situation, the accelerated air from the prop would keep them shut at speeds where they were supposed to be open. Also the propwash already acted as a set of slats by itself, accelerating the air over the inboard wing section, making it stall at an even lower relative airspeed than in the power off condition.

 

The slats were therefore placed where needed, on the outboard wing section. Here the slats helped increase the CLmax by keeping the outboard part of the wing from stalling before the inboard part by raising the critical AoA & Clmax by approx. 25% in the covered area. And since the inboard part of the wing already stalls at a higher AoA and CLmax than the outboard part(usually ~15% higher) the slats end up increasing the overall CLmax of the entire wing by ~10-15%. This becomes esp. true with power on as the lift & critical AoA of the inboard wing section is then increased even further by the prop stream further raising the overall CLmax over the already 10-15% power off gain.

 

Hence how at full power the Finnish were able to fly their G-2 level at a speed of just 130 km/h, flaps & gear up.

 

Slats do only one trick - they prevent ailerons from stall preserving their control ability. Either wind tunnel tests or modelling show very insufficient changes in CL max.

The STOL :) planes like Fi-156 or Yak-12 or An-2 have FULL SPAN slats. Or Su-27 or Su-25 or MiG-29 where slats extend AoA range.

[Hummingbird]

Slats do only one trick - they prevent ailerons from stall preserving their control ability. Either wind tunnel tests or modelling show very insufficient changes in CL max.

 

Incorrect. But you are welcome to show any tests that show no increase in overall CLmax from using outboard wing mounted slats on a high power prop aircraft. It is specifically in the POWER ON condition that the slats really make a significant difference.

 

It's pretty logical also really, as that the inboard wing section features a higher critical AoA and CLmax than the outboard section, if you increase both the critical AoA & CLmax of the outboard section it will naturally increase the overall CLmax of the wing as a result. The slats are actively preventing the outboard section from stalling at the point were it normally would without decreasing the AoA, thus the actual critical AoA & CLmax of the inboard section can be reached at the same time that the outboard section is still creating lift.

 

The STOL :) planes like Fi-156 or Yak-12 or An-2 have FULL SPAN slats. Or Su-27 or Su-25 or MiG-29 where slats extend AoA range.

 

The Fi-156 & Yak-12 both feature slots, not slats, hence they don't have any problems with the propwash keeping them in at speeds where they need to be out as slots are permanently locked out to begin with.

 

Automatic slats were put only on the outboard part of the Bf-109's wing for the exact reasons I explained, to increase the overall CLmax of the wing to make up for the lower wing area. If all that was wanted was improved aileron effectiveness then simple washout would've been applied, esp. since its a lots simpler and less expensive to make than automatic slats and comes with no weight penalty.

Edited January 12, 2016 by Hummingbird

[NineLine]

that show no increase in overall CLmax

 

Just as a small note, he didn't say no increase.

[Hummingbird]

Just as a small note, he didn't say no increase.

 

Well he actually says: "slats only do one trick", which is false.

 

By allowing the outboard wing section to still provide lift whilst the inboard section reaches its CLmax (which is 1.53 for the 2R1 btw) you will automatically increase the overall CLmax, and that's exactly the reason MTT put the slats there.

 

Here's a NACA experiment with a similar planform wing to that of a 109:

 

 

 

Note the increased CLmax to 1.8 with flaps & gear retracted on the second example once the desired result of keeping the outboard section unstalled is reached.

 

The 109's wing stalls in exactly the same manner as the second example in the NACA report:

 

 

 

Add to this that with power on the inboard wing section stalls at an even later relative AoA and higher CLmax, which the slatted section has no problem with considering it can keep going for an extra 25+%.

[NineLine]

Well he actually says: "slats only do one trick", which is false.

 

And goes on to say "Either wind tunnel tests or modelling show very insufficient changes in CL max.". But not that there is no increase.. thats all I was commenting on.

[Yo-Yo]

Sorry, I omitted the second obvious trick: the slats prevents tip stall that leads to abrupt wing drop... .

[Anatoli-Kagari9]

And, in DCS's K-4, just like in the real 109s, de deployment of the slats doesn't by itself create any buffeting, as I can observe in many of my flights, where they deploy and I can only be aware of it if looking at them while the aircraft continues to fly smoothly.

 

Only approaching critical AoAs creates the buffeting, as I guess makes sense...

[Hummingbird]

Another thing, saying wing washout and outboard slats do the same thing is not true.

 

Wing washout is applied to improve aileron control near stall as well as delay tip stall by twisting the outboard wing section downward, decreasing the AoA of the outboard section, thus there is no increase in lift or critical AoA - it is simply done to ensure the tip doesn't stall way before the root section.

 

Outboard mounted slats work differently whilst indeed achieving some of the same effects, but the difference is that they at the same time also increase the lift of the outboard section in the process. The slats keep the outboard wing section from stalling by actually increasing its critical AoA and thus CLmax. Since the wing isn't twisted the slats will deploy and increase the CLmax of the covered section before the inboard section stalls, and at the point where the inboard section is finally at its critical AoA & CLmax the outboard section will be matching it in lift, thus increasing the overall lift of the wing as a result.

 

To put it more simply:

 

Power off

 

Wing with washout (stalls at 15 AoA):

Root CLmax = 1.55 (freeflow)

Tip CLmax = 1.38 (freeflow)

Overall CLmax = ~1.45

 

Wing with outboard slats:

Root CLmax = 1.55 (freeflow)

Tip CLmax = 1.38+25% (freeflow, equalizes to ~1.55 as it matches the root before it stalls)

Overall CLmax = ~1.55

 

Power on

 

Wing with washout (stalls at 15 AoA):

Root CLmax = 1.70 (accelerated)

Tip CLmax = 1.38 (freeflow)

Overall CLmax = ~1.52

 

Wing with outboard slats:

Root CLmax = 1.70 (accelerated)

Tip CLmax = 1.38+25% (freeflow, equalizes to ~1.70 as it matches the root before it stalls)

Overall CLmax = ~1.70

Edited January 12, 2016 by Hummingbird
Corrected small typo

[Sporg]

I know V,

 

but indeed it appears slat deplyment is almost unnoticeable in the 109s, most of the time the pilot will only become aware of it if visually confirming the deployment, and pre-stall buffet almost inexistent too...

 

Hehe, jcomm, you really miss out on the FFB part of DCS here, as discussed in your other thread. ;)

 

As I experienced it, it is simply more difficult to register the visual cues than to feel the vibration.

When you have a FFB device, you get ample warning.

[Anatoli-Kagari9]

Hehe, jcomm, you really miss out on the FFB part of DCS here, as discussed in your other thread. ;)

 

As I experienced it, it is simply more difficult to register the visual cues than to feel the vibration.

When you have a FFB device, you get ample warning.

 

I'm trying to get one :-/ but it's difficult around here ....

[Yo-Yo]

Another thing, saying wing washout and outboard slats do the same thing is not true.

 

Wing washout is applied to improve aileron control near stall as well as delay tip stall by twisting the outboard wing section downward, decreasing the AoA of the outboard section, thus there is no increase in lift or critical AoA - it is simply done to ensure the tip doesn't stall way before the root section.

 

Outboard mounted slats work differently whilst indeed achieving some of the same effects, but the difference is that they at the same time also increase the lift of the outboard section in the process. The slats keep the outboard wing section from stalling by actually increasing its critical AoA and thus CLmax. Since the wing isn't twisted the slats will deploy and increase the CLmax of the covered section before the inboard section stalls, and at the point where the inboard section is finally at its critical AoA & CLmax the outboard section will be matching it in lift, thus increasing the overall lift of the wing as a result.

 

To put it more simply:

 

Power off

 

Wing with washout (stalls at 15 AoA):

Root CLmax = 1.55 (freeflow)

Tip CLmax = 1.38 (freeflow)

Overall CLmax = ~1.45

 

Wing with outboard slats:

Root CLmax = 1.55 (freeflow)

Tip CLmax = 1.38+25% (freeflow, equalizes to ~1.55 as it matches the root before it stalls)

Overall CLmax = ~1.55

 

Power on

 

Wing with washout (stalls at 15 AoA):

Root CLmax = 1.70 (accelerated)

Tip CLmax = 1.38 (freeflow)

Overall CLmax = ~1.52

 

Wing with outboard slats:

Root CLmax = 1.70 (accelerated)

Tip CLmax = 1.38+25% (freeflow, equalizes to ~1.70 as it matches the root before it stalls)

Overall CLmax = ~1.70

 

I like your elementary arithmetic wingspan circulation calculations... Prandtl, Glauert, Treftz and Fr. Lotz worked in vain. :)