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APenNameAndThatA

Calculating the radius of a skidding turn

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Hi all

 

In a previous post, I said that I did not think that the inside wing of a skidding turn would stall first because the difference in speed was not great enough. 

 

The maths were r(feet)= speed(kts)*speed/11.26*tan of bank angle (degrees)

 

foxbat turning onto final...

r = 32*32/11.26*tan20

= about 250 ft

 

speed is proportional to radius, and, at the most, the difference in radii of the wings would be 20 ft. So the difference in wing speed would be 20/250=8%

 

So, I take it back: because you go so slowly when you stall, if you are in a co-ordinated turn, the inside wing will be significantly slower. One wing at 32 kts, and the other at about 29.5 kts. 

 

But a skidding turn will have a sharper radius than a co-ordinated turn of the same bank, making the difference in speeds of the wing bigger. 

 

And, for those of you who say not to over think it: a) this is for fun, and b) I am not doing these calculations while flying! 

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I can’t check your maths but others have shown in published articles that the difference in wing speed is insignificant

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Lots of other factors in a descending, skidded turn.

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 Any sweepback will have a big effect on this as does wing shielding. and the position of the ailerons. . Nev

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and one effect of skidding is to effectively add sweepback on one wing, sweepforward on the other

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9 hours ago, APenNameAndThatA said:

But a skidding turn will have a sharper radius than a co-ordinated turn of the same bank, making the difference in speeds of the wing bigger.

I suspect this to be an erroneous assumption also. Skidding will only point you in the direction, you won't actually be going that direction. It's all about the relative airflow.

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Regardless of which is the principal factor, it is not a very smart idea to lead into a turn with rudder - a habit which I often see during BFRs.  It is a very, very dangerous habit if you are at low level, or turning final, because it's likely to kill you.

 

Right from lesson #1, instructors need to be guiding the student towards properly balanced turns at all a-o-b. This is seriously important, because when it comes to slipping the aircraft, the student should immediately, (via L & R 'cheek' feedback), be able to recognise the 'out-of-balance' situation. 

 

Slipping down to an upwind mainwheel touchdown in crosswinds is the worst demonstrated item of both the GA and RAAus syllabus. That probably explains why there are so many R-LOC accidents - most of which are reported as being due to a huge, unanticipated gust of wind!

skid-3d.jpg

blanketing.jpg

17-3.Crosswinds tw.jpg

17-1.crosswind wing dn in 172.jpg

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12 hours ago, pmccarthy said:

I can’t check your maths but others have shown in published articles that the difference in wing speed is insignificant

I read the articles, too. It is more likely that they are right than I am right. The faster the aircraft, the smaller the difference in wing speed, too. 

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 You need to be more  "careful" about those statements. Faster planes will have bigger radius turns for the same angle of bank. Like a motorcycle will have to lean over more on the same corner when going faster. At steep bank angles the radius of turn gets less different as at the (impossible) theoretical 90 degree banked turn the radii are equal. That's one extreme.

 Back to a more normal situation.. Say a reasonable turn radius is 2,000 feet (approx. 1/3rd of a mile the difference in distance travelled is Proportional to the effective wingspan say 20 feet, 20/2,000 is 1%. Not a lot and other factors would come into play like accuracy of rigging and more likely rudder balance. Ball in centre. OR whether you are climbing or descending where the effective variation of Angle of attack of each wing  (due to different diameter helix's being described by each wing) is a factor also. You might recollect your climbing and descending turns  briefings /lessons. Usually the theory s ok but the ability to actually demonstrate holding  bank on or off stick wise is less convincing. It 's still a consideration where right at  the point where the wing stalls, it will happen more to one wing than the other. One goes first. As this happens the downgoing wing compounds it's stall, because of the RELATIVE airflow and the upgoing one temporarily relieves itself. More so when the dynamic loads are higher  Ie in a turn or recovering in a dive.  You then have the plane rolling  uncommanded at stall speed and it quickly becomes a spin if you pull the stick further back  and /or try to oppose the roll with normal aileron application. Again relative airflow.  Nev

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4 hours ago, facthunter said:

 You need to be more  "careful" about those statements. Faster planes will have bigger radius turns for the same angle of bank. Like a motorcycle will have to lean over more on the same corner when going faster. At steep bank angles the radius of turn gets less different as at the (impossible) theoretical 90 degree banked turn the radii are equal. That's one extreme.

 Back to a more normal situation.. Say a reasonable turn radius is 2,000 feet (approx. 1/3rd of a mile the difference in distance travelled is Proportional to the effective wingspan say 20 feet, 20/2,000 is 1%. Not a lot and other factors would come into play like accuracy of rigging and more likely rudder balance. Ball in centre. OR whether you are climbing or descending where the effective variation of Angle of attack of each wing  (due to different diameter helix's being described by each wing) is a factor also. You might recollect your climbing and descending turns  briefings /lessons. Usually the theory s ok but the ability to actually demonstrate holding  bank on or off stick wise is less convincing. It 's still a consideration where right at  the point where the wing stalls, it will happen more to one wing than the other. One goes first. As this happens the downgoing wing compounds it's stall, because of the RELATIVE airflow and the upgoing one temporarily relieves itself. More so when the dynamic loads are higher  Ie in a turn or recovering in a dive.  You then have the plane rolling  uncommanded at stall speed and it quickly becomes a spin if you pull the stick further back  and /or try to oppose the roll with normal aileron application. Again relative airflow.  Nev

Just wow. I hadn’t thought that there will be a natural tendency for a plane to drop one wing when it stalls. Whichever wing stalls first increases its angle of attack and stalls more, and drops more. Whichever wing does not stall, rises, decreases its angle of attack, goes further from stalling and increaes its lift. No wonder some planes (esp with low, laminar-flow wings) can tend to drop one wing in a stall. 

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Not to mention high wing aircraft without laminar-flow aerofoils ..... I’ve experienced much less tendency for an uncommanded wing drop in low wings with laminar-flow aerofoils (but there are many types that I have not flown). Perhaps something to do with the wing taper, washout ... 

 

Edited by djpacro

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Washout gives more aileron control when close or at the stall, but you would be best advised to not count on it. A Cessna-172 can behave very benignly but bite when heavily loaded.  Resist relying on  ailerons UNTIL you get  a more normal flying speed on any plane . Stick relax or forward and power is the go, unless you are already pointing at the ground.  The conventional way of training for stalling is only a box ticking exercise and gives the impression the nose will  be at a crazy high angle when you stall.. People are dying with the turn onto final . Nothing to do with a high nose pitch attitude. .Nev

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Here’s a great video that talks about both skidding & slipping turns in the stall, explains the aerodynamics of why as well as an actual demonstration of which wing will stall first.

42E202B2-B08A-426E-AF37-262AEF012B24.jpeg

Edited by Bennyboy320
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On 1/4/2019 at 8:26 AM, facthunter said:

Washout gives more aileron control when close or at the stall, but you would be best advised to not count on it. A Cessna-172 can behave very benignly but bite when heavily loaded.  Resist relying on  ailerons UNTIL you get  a more normal flying speed on any plane . Stick relax or forward and power is the go, unless you are already pointing at the ground.  The conventional way of training for stalling is only a box ticking exercise and gives the impression the nose will  be at a crazy high angle when you stall.. People are dying with the turn onto final . Nothing to do with a high nose pitch attitude. .Nev

Silly high nose in stall training a tick box?  Maybe in 3axis - but never was when I trained or was training - Stall training in a flexwing will be a lot more than box ticking because you must stall in a turn and demonstrate recovery ... we do not spin but hell we can spiral dive like there is no tomorrow ... and if you don’t know and react quickly there may be no tomorrow for you because vne will be so far behind you SO quickly that recovery itself may break your wing up.  

 

 

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