PDA

View Full Version : Increased drag caused by sideslip



AndyJWest
06-07-2010, 02:49 PM
http://i958.photobucket.com/albums/ae65/ajv00987k/il2fb2010-06-0721-06-51-98.jpg

This subject came up in the recent thread on rudder usage (http://forums.ubi.com/eve/foru...3110283/m/4441005468 (http://forums.ubi.com/eve/forums/a/tpc/f/23110283/m/4441005468)), and I said I'd try to do some tests using my autopilot setup (Stock IL-2 4.09m).

I used a Fw 190 A8 on the Crimea map, at 1000m, on a heading of 270 degrees. Fuel was 50% - usage set to 'unlimited' Wind & Turbulence off, radiators fully open. I let the AP hold heading and height with aileron and elevator, while moving the rudder to its limits by hand.

110% throttle, no boost, clean:

No slip 501 Km/h
Holding full right rudder 318 Km/h
Holding full left rudder approx 280 Km/h, but unstable under AP control.

90% throttle in landing configuration - gear down, full flaps:

No slip 278 Km/h
Holding full right rudder 214 Km/h - the slip was approx 10 degrees in this configuration.
Holding full left rudder approx 180 Km/h, but unstable, and almost stalling.

My autopilot wasn't designed to handle a test like this, and struggled a little, but I think the figures give a good indication of just haw much extra drag an extreme sideslip will cause. As might be expected, the effect was greater at high speeds, where induced drag (which shouldn't change in a slip?) is less significant, so the increased parasitic drag becomes more of a factor.

Does anyone have any comments?

JtD
06-07-2010, 02:53 PM
Good for landing if you come in with a lot of speed. Which I usually do. It also helps you to keep an eye on the runway until touchdown. Thanks for putting numbers to that!

(If you were exact, induced drag changes because wing geometry is changing a little bit.)

TinyTim
06-07-2010, 03:06 PM
A question - you said besides altitude the AP maintained heading - did it do it by banking to the opposite direction of the one rudder was used, and using a bit of elevator? (that's an assumption from your scrshot) Since this might as well contribute some drag, I'd propose a very similar test to yours, with an exception - let the AP hold altitude only, and keep the bank angle at 0. This will result in aircraft turning a bit, but at least no banking and no elevator input will be used to compensate, so the drag measured could be attributed almost exclusively to the slip.

AndyJWest
06-07-2010, 03:12 PM
I see your point, TinyTim. Yes, the AP used opposite bank to hold heading, so you would get more drag than without the bank. However, holding the plane level (if the AP could - I'm not sure how well it would handle this) is still going to require aileron input, with increased drag - and since the plane will turn, there will be an increased G loading too...

TinyTim
06-07-2010, 03:18 PM
In this case I think the exact definition of the term "drag caused by sideslip" needs to be done. Are the secondary effects (like using an elevator in order to keep the heading) included into this drag or not? Besides - using the ailerons in my opinion would cause a lot less drag. Yes, g would increase a bit, but that comes with the sideslip, no?

Waldo.Pepper
06-07-2010, 03:26 PM
I have never thought that the game modeled this particularly well. Whenever I cross controls to cause more drag it never seems like it does much of anything to slow my aircraft down, no matter the airplane I am flying.

AndyJWest
06-07-2010, 03:28 PM
Ok TinyTim, let's call it 'increased drag caused by sideslipping while maintaining a constant heading'. However you measure it, there are going to be extraneous factors - P effect, the thrustline not being aligned with the track, etc. And since the airspeed is varying with sideslip, I'm not just measuring increased parasitic drag anyway. I'm not attaching any particular significance to the exact results, but I think it shows the scale of the effect, and the consequences of extreme sideslip.

TinyTim
06-07-2010, 03:59 PM
I see. Ok, did a quick test to determine the drag caused by sideslip without constant heading. I used F6F-3 (I picked a bird with all axis trim, so I could trim ailerons to keep bank at 0). Crimea, noon, 100% fuel, default loadout, ground level (I never climbed above 10m after 500kph was attained).

1. Max. speed at ground level (no sideslip):

http://www.shrani.si/f/1q/YY/1AGvETYb/1.jpg

2. Max. speed at ground level (max. right rudder trim):

http://www.shrani.si/f/8/US/4COMMUm/2.jpg

The reduction in speed is nearly 75kph, plus one has to consider that full rudder trim probably isn't equal to full rudder deflection (which Andy used). At any rate - it causes a lot of drag!

JtD
06-07-2010, 04:08 PM
Originally posted by Waldo.Pepper:
I have never thought that the game modeled this particularly well. Whenever I cross controls to cause more drag it never seems like it does much of anything to slow my aircraft down, no matter the airplane I am flying.

If that's in relation to real aircraft you fly, could it be down to aircraft weight and rudder layout (size, max. deflection etc.)?

AndyJWest
06-07-2010, 04:15 PM
one has to consider that full rudder trim probably isn't equal to full rudder deflection
I think it is equal to about 25% deflection, from memory. Thanks for the independent verification though, TinyTim.

p-11.cAce
06-07-2010, 04:57 PM
Great work guys http://forums.ubi.com/images/smilies/clap.gif I'm glad I was not just imagining this all these years!

danjama
06-07-2010, 05:52 PM
I can't think of a landing of mine in il2 within the last five years that I haven't used sideslip to bleed E on the turn-in. I've perfected the technique to the point where I can land most aircraft with a very short approach.

p-11.cAce
06-07-2010, 05:59 PM
I don't have the balls to fly turning approaches in real life - but for years they were the standard. I usually fly some form of turning approach in game - especially on the Slovakia map as those airfields are pretty small.

M2morris
06-07-2010, 10:51 PM
In real gliders I have been criticized for using slide slips on final alot because I tend to turn onto final too high. In the game I put the Corsair into a slip while using alot of power so I can see the deck on carrier landings. Of course it causes a bunch of drag, the planes flying sideways fro Chrissakes.

AndyJWest
06-08-2010, 05:44 AM
Yes, sideslipping on finals needs to be done with care, even in IL-2. Crossed controls at low speed are a recipe for a spin. Some planes are more sensitive than others, so I'd recommend trying it out at height first.

And yes, it causes 'a bunch of drag' at least in real life. The question was how much effect it had in IL-2. I've only actually measured it in the Fw 190, but I doubt if it would be modelled in some planes and not others.

I got the impression when flying MS FS9 that the drag increase there wasn't as great, if modelled at all, but then I've never been particularly impressed with FS physical modelling. What about other sims? RoF? The Lock On series (I think it's modelled there, but it's a long time since I flew anything other than the A-10). I've got a copy of CFS 3, but I think this uses the same flight model engine as MSFS.

In real life, sideslip can cause problems beyond the risk of asymmetric stall too. I believe the Handley Page Hampden had problems with a tailfin losing effectiveness in a sideslip, by being caught in turbulent air behind the fuselage. Once you started to sideslip, you couldn't straighten out...

M2morris
06-08-2010, 08:29 AM
This thread may have changed the way I fly because it has awakened my lazy procrastinating *** to the fact that in the game I should finally start using all three trim controls instead of just elevator.
Good topic.



Just a thought:
If you put Erich Hartmann on this sim and in his favorite 109 he would probably have some good input on the subject but then he would get shot down by some 14 year old kid in Sandusky Ohio.

Viper2005_
06-08-2010, 08:43 AM
Originally posted by AndyJWest:
...the effect was greater at high speeds, where induced drag (which shouldn't change in a slip?) is less significant, so the increased parasitic drag becomes more of a factor.

Does anyone have any comments?

Induced drag certainly changes in a sideslip; you are effectively sweeping the wings. This is one reason for the secondary effect of rudder rolling the aeroplane, and this highlights a glaring error with the He-162D (secondary effect of rudder should be reduced because of the forward swept wing).

Kettenhunde
06-08-2010, 09:11 AM
This is one reason for the secondary effect of rudder rolling the aeroplane,

I tried some hammerheads in IL2 once, the impression I got was the secondary effect was too powerful.

IRL, typically only it takes ~2-3 seconds to perform a hammerhead.

A few swags will confirm that as well.

TheGrunch
06-08-2010, 09:14 AM
I think you're right. It's a bit weird because occasionally it means that really quite heavy aircraft look rather too light.

p-11.cAce
06-08-2010, 09:20 AM
I'm unsure about hammerheads in the aircraft of IL2. I usually think they are wrong - but my only reference are aerobatic aircraft which will obviously handle much differently. Jim Pietz flies an aerobatic Bonanza and his hammerheads (first at 1:15) seem similar in curvature and duration to those I do in the Emil.

<span class="flash-video"><object classid="clsid:d27cdb6e-ae6d-11cf-96b8-444553540000"
codebase="http://fpdownload.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=8,0,0,0"
height="385"
width="640"
><param name="allowScriptAccess"
value="never"
></param><param name="wmode"
value="transparent"
></param><param value="http://www.youtube.com/v/hHTVhYgyyls&hl=en_US&fs=1&"
name="movie"
/><param value="true"
/><param value="always"
/><embed pluginspage="http://www.macromedia.com/go/getflashplayer"
type="application/x-shockwave-flash"
allowScriptAccess="never"
wmode="transparent"
height="385"
width="640"
src="http://www.youtube.com/v/hHTVhYgyyls&hl=en_US&fs=1&"
/></object></span>

M_Gunz
06-08-2010, 09:27 AM
Originally posted by Kettenhunde:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">This is one reason for the secondary effect of rudder rolling the aeroplane,

I tried some hammerheads in IL2 once, the impression I got was the secondary effect was too powerful.

IRL, typically only it takes ~2-3 seconds to perform a hammerhead.

A few swags will confirm that as well. </div></BLOCKQUOTE>

What version/patch?

TheGrunch
06-08-2010, 09:56 AM
I remember the earlier 4.0x ones being a bit wibbly wobbly in all axes.

M_Gunz
06-08-2010, 11:09 AM
Do you remember before 4.0 when you couldn't hold a stall or even edge of stall?
Do you remember before when you had to hold aileron to stay banked?

It's only because as years went by the newer computers were more capable that such things could change.
Try running 1946 or even Aces on a PC that could only run IL2 1.0 at low settings.

TheGrunch
06-08-2010, 11:26 AM
I'm sure it's still acceptable to point these things out.

Kettenhunde
06-08-2010, 07:37 PM
What version/patch?


It says V4.09m on start up.

AndyJWest
06-08-2010, 08:51 PM
Yes, that's the latest patch, Kettenhunde. http://forums.ubi.com/images/smilies/25.gif

I always find two problems with hammerheads, tailslides and the like in IL-2 - firstly getting a true vertical ascent, with limited peripheral vision - a consequence of display limitations, and secondly, judging the appropriate airspeed to apply rudder etc where applicable. I suspect this mostly comes down to practice though.

If I can risk the wrath of the moderators, I'll point out that SAS have a 'proper' Su-26 mod aircraft (unlike the odd specimen in the default UP install), and this seems more capable of doing the sort of aerobatics one would expect it to be capable off - or it would be in better hands than mine. This has sufficient grunt to actually climb in a knife-edge, though how stable and sustainable it is will need a better pilot to demonstrate. I think it's FM may be a tad too 'user-friendly', and if it was up to me I'd move the CoG back a little, making Lomcevaks and the like more of a possibility - maybe it can do these, but it seems marginal. Manoeuvres like this depend on accurate flight modelling physics in a sim, and I don't know how well IL-2 does at these extremes, but testing in sideslips etc at least indicates what is right, and what may be suspect.

And a reminder for those who may have forgotten - Oleg has promised a Su-26 as a 'bonus' aircraft in SoW:BoB!