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#151
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Robert Ehrlich wrote:
Colin wrote: ... 1. A slipping turn can be made at a high bank angle and low airspeed. 2. Rate of turn is dependent on angle of bank and airspeed, such that the highest rate of turn is achieved with a high bank angle and a low airspeed. Brian illustrated this by inviting us to compare the rate of turn achieved by a C150 and a jet fighter at the same angle of bank. It has to be a slipping turn or we would stall, so the maximum bank which can be used is dependent on the amount of top rudder available. ... Ok, if you use the lift provided by the fuselage in a slipping turn, you can devote more of the lift provided by the wings for generating the centripetal force, by increasing the bank angle, and so the horizontal component of the wing lift. But you pay this by an increase in drag, i.e. more height loss. It is not obvious if the gain in time to turn override the increased rate of sink, but I have some argument that should show that the answer is no. The analysis of Dr. Rogers as well as my own one can be used in the case of a slipping turn can be used if you consider as bank angle not the geometric bank angle, i.e. the angle between the wing plane and the horizontal plane, but the aerodynamic bank angle, i.e. the angle between the total lift vector (wings + fuselage) and the vertical (well as we consider a glide it is rather total aerodynamic force than lift, but the difference between both angles can be neglected). In this case our common conclusion is again that the optimal (aerodynamic) bank angle is 90 degrees, although the geometric bank angle is higher, and we have to maximize Cl/(Cd^2) in Dr. Rogers' analysis, minimize Vz*V in my analysis. It is difficult to determine if you may have a higher max Cl/(Cd^2) with slip, but I have a good argument that we can't lower Vz*V. Remember in this analysis Vz and V are the horizontal and vertical speed at zero (aerodynamic) bank angle and the same angle of attack, i.e. what you find in an usual glider polar. In the case of a slipped turn, we should use a polar showing Vz versus V for a straight flight with the same slip angle. I never saw such a polar, but I think it is obvioous that at any speed V the vertical speed Vz is higher with slip than without it. i.e. the polar with slip is entirely below the polar without slip. Now the minimum of Vz*V for the polar without slip is where this polar is tangent to one of the hyperbolas Vz*V = constant, and except for the point of contact, the polar is entirely below this hyperbola. So the polar with slip is also entirely below this hyperbola, and no point of it can provide a value for Vz*V that is lower or equal to the the value on the hyperbola, i.e. the minimum of Vz*V obtained without slip cannot be obtained with slip. Er, yes ... But how does the result compare with a large tear-drop flown at normal glide speed, or for that matter, a smaller tear-drop flown with higher airspeed and more bank ? The point is how to complete a rapid 180 without spinning-in, and a full sideslip with small angle of attack precludes a spin. The author also encourages us to go out and try it, high up to begin with of course. - Colin. |
#152
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Ian Forbes wrote:
On Thu, 06 Nov 2003 00:02:30 +0000, Colin wrote: - Spins occur when you stall and the glider is not "co-ordinated" ie either slipping or skidding. I used to think this, but I soon discovered our club Blanik would happily spin from a coordinated turn by using a shallow bank and simply reducing the airspeed. Since then, I've done this with other gliders. A coordinated turn doesn't prevent the inner wing from flying at a higher angle of attack than the outer wing, which is why it stalls first, and a spin can begin. I haven't experimented with it enough to be certain, but I suspect a slipping turn would reduce the tendency for the inner wing to stall first. |
#153
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Eric,
Point of interest: did you let the spin fully develop after the coordinated turning stall? There is an aerodynamic tipping point -- that is the self-righting tendency of the tail that would typically favor a spiral over a spin assuming that the only deflected control surface was the elevator. Of course a wing drops when in a turning stall, but without aileron deflection generating drag my guess would be that designed yaw stability would prevent spin development. There is a significant difference in the assymetric drag profile with and without aileron deflection. Remember that most modern aircraft begin their stall at the root. That means less torque and less disposition to overpower yaw stability and enter a spin. Slapping an aileron down to pick up the low wing adds significat drag at the tip. Add some rudder (cross-controls), and now you have a greater disposition to get the aircraft spinning rather than spiralling. I'll give this a try over the weekend -- that is, making no recovery to a coordinated turning stall to see how it develops. My Ventus spins happily if aggrevated. It should prove a good test bed. Eric Greenwell wrote in message ... Ian Forbes wrote: On Thu, 06 Nov 2003 00:02:30 +0000, Colin wrote: - Spins occur when you stall and the glider is not "co-ordinated" ie either slipping or skidding. I used to think this, but I soon discovered our club Blanik would happily spin from a coordinated turn by using a shallow bank and simply reducing the airspeed. Since then, I've done this with other gliders. A coordinated turn doesn't prevent the inner wing from flying at a higher angle of attack than the outer wing, which is why it stalls first, and a spin can begin. I haven't experimented with it enough to be certain, but I suspect a slipping turn would reduce the tendency for the inner wing to stall first. |
#154
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Most pilots instinctively recover long before they can
tell the difference between a stall -- recovery -- spiral dive scenario and a stall -- spin. This often causes confusion about which is which. I personally intentionally tried a spin entry once in a glass glider and got a surprise and made an immediate spin recovery. It seems the airspeed indicator rotates all the way around, so 80 knots indicated is the same as 20 knots indicated. Imagine my surprise when the glider stalls, the nose drops, and the ASI wobbles and then indicates ??? I tried it a few more times and by god could never tell the difference, so I was too scared to do anything but recover immediately (release the cross-controlled inputs). Whichever it was, the glider sure picked up speed like lightning when nose down. I still wonder if this killed the Nimbus4DM pilots in Reno. Imagine looking at the ASI and not knowing if you should be doing a spin recovery or a spiral recovery (two very different things). |
#156
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#157
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Chris OCallaghan wrote:
Eric, Point of interest: did you let the spin fully develop after the coordinated turning stall? No, but there didn't seem to be any need to, as the inner wing dropped and rotation began. There is an aerodynamic tipping point -- that is the self-righting tendency of the tail that would typically favor a spiral over a spin assuming that the only deflected control surface was the elevator. Of course a wing drops when in a turning stall, but without aileron deflection generating drag my guess would be that designed yaw stability would prevent spin development. As Mark points out, the ailerons on the Blanik are significantly deflected with "down" aileron on the inner wing, which is part of the reason the inner wing stalls first. They are also deflected this way on other gliders, but perhaps not as much. There is a significant difference in the assymetric drag profile with and without aileron deflection. Remember that most modern aircraft begin their stall at the root. At least, for a straight ahead stall. I don't think this is true for many gliders in a turn. That means less torque and less disposition to overpower yaw stability and enter a spin. Slapping an aileron down to pick up the low wing adds significat drag at the tip. Add some rudder (cross-controls), and now you have a greater disposition to get the aircraft spinning rather than spiralling. I'll give this a try over the weekend -- that is, making no recovery to a coordinated turning stall to see how it develops. My Ventus spins happily if aggrevated. It should prove a good test bed. We await your report with interest! |
#158
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In my first reply to Chris, I should've said "as Bruce Hoult points out"
instead of Mark Boyd. Chris OCallaghan wrote: Eric, Point of interest: did you let the spin fully develop after the coordinated turning stall? There is an aerodynamic tipping point -- that is the self-righting tendency of the tail that would typically favor a spiral over a spin assuming that the only deflected control surface was the elevator. Of course a wing drops when in a turning stall, but without aileron deflection generating drag my guess would be that designed yaw stability would prevent spin development. |
#159
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"Mark James Boyd" wrote in message news:3fac007b$1@darkstar... tell the difference, so I was too scared to do anything but recover immediately (release the cross-controlled inputs). With all due respect ...... That is not a spin recovery, never was and never will be although I'll admit that in the right circumstances it will occasionally work. If you are intentionally trying to spin not knowing how to recover properly then you either have balls the size of footballs or are seriously mentally challenged. A STANDARD recovery is CENTRALISE AILERONS FULL OPPOSITE RUDDER. SLIGHT PAUSE STICK PROGRESSIVELY FORWARD UNTIL THE SPINNING STOPS. CENTRALISE THE RUDDER. RECOVER FROM THE RESULTING DIVE. Very occasionally there may be a 'non standard' method in the POH but they are few and far between. To be certified a glider should recover reliably using the standard proceedure. There is an excellent post by Bill Dean ( and others ) about this in the archives ( a year ago ) http://groups.google.co.uk/groups?q=...ara.net&rnum=2 Ian |
#160
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I still wonder if this killed the Nimbus4DM pilots in Reno.
Imagine looking at the ASI and not knowing if you should be doing a spin recovery or a spiral recovery (two very different things). ************************************************** ****************************** What really killed them were wings which, by design, are only good for 3.5 g (+50% if the glue holds) when you get into a stall/spin situation. |
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