aerodynamics of pylon turns
02-17-2003, 02:45 AM
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aerodynamics of pylon turns
I’m interested in evaluating the performance of various airfoils during the ‘turn’ mode of powered pylon racing classes, say in the range of 75 - 175 mph. Ideally, I want to be able to crank out the ‘numbers’ that correspond to the graphical results shown on Martin Hepperle’s website: http://www.mh-aerotools.de/airfoils/index.htm. This was semi-discussed on rcu at
http://www.rcuniverse.com/showthread...9&pagenumber=1
The main variables I want to play with are the airfoil, the wing geometry, speed & turn radius, the remaining model-specific variables are defined & held constant. I visualized (hoped?) I could evaluate different combos along the lines of the following procedure (pardon the aerodynamic hackishness).
1. specify velocity & wing geometry & air properties, calculate Re
2. pick an airfoil, generate polars @ this Re
3. specify a turn radius ( & and bank angle?) & model weight, calculate resultant centrifugal force the model sees under g-turn (which equals minimum lift required?)
4. assuming this ‘g-lift’ value, calculate the associated Cl (lift coefficient) required. Assume this is Cl = L / (1/2 * rho * V2 * S) ?
5. take this Cl value to the airfoil polars at appropriare Re, determine the corresponding drag coefficient Cd
6. take this Cd value & calculate the resultant (induced) drag force
7. somehow relate this to velocity reduction through the turn, or heck, Id be happy to end it at #6
Is it procedure too simplistic? Any help or suggestions in this regard would be appreciated. I need more elaboration on #4 & #5; what are the applicable equations? I planned on using profili/xfoil for airfoil data & polars, any comments??
http://www.rcuniverse.com/showthread...9&pagenumber=1
The main variables I want to play with are the airfoil, the wing geometry, speed & turn radius, the remaining model-specific variables are defined & held constant. I visualized (hoped?) I could evaluate different combos along the lines of the following procedure (pardon the aerodynamic hackishness).
1. specify velocity & wing geometry & air properties, calculate Re
2. pick an airfoil, generate polars @ this Re
3. specify a turn radius ( & and bank angle?) & model weight, calculate resultant centrifugal force the model sees under g-turn (which equals minimum lift required?)
4. assuming this ‘g-lift’ value, calculate the associated Cl (lift coefficient) required. Assume this is Cl = L / (1/2 * rho * V2 * S) ?
5. take this Cl value to the airfoil polars at appropriare Re, determine the corresponding drag coefficient Cd
6. take this Cd value & calculate the resultant (induced) drag force
7. somehow relate this to velocity reduction through the turn, or heck, Id be happy to end it at #6
Is it procedure too simplistic? Any help or suggestions in this regard would be appreciated. I need more elaboration on #4 & #5; what are the applicable equations? I planned on using profili/xfoil for airfoil data & polars, any comments??
02-17-2003, 09:22 PM
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aerodynamics of pylon turns
ptxman,
No, your procedure is not too simplistic. I'd say it's about right on. It probably does make sense to stop at step 6. In fact, you pretty much want to work with Cl and Cd, rather than the values of lift and drag, so that your 'design goal' is the minimization of Cd in the turn. Once you calculate the Cl that you need for level flight, using the formula, you can simply work with multiples of that Cl, depending on how many g's you sustain in turns, which is entirely a function of bank angle. Then you use the polars to go from Cl to Cd. I don't think you can do much better than to read Hepperle's site, it is really good. In a perfect world, you want an airfoil with low drag at low Cl, for level flight, and a 'drag bucket' which is wide enough to give fairly low drag at the high Cl you have in turns. Airfoils with wide drag buckets make this easier, and it also implies that you have a tradeoff involving bank angle. This is because bank angle determines the Cl, so you might come out ahead by using a less extreme bank angle, if it keeps your Cl within the drag bucket. One way to widen your drag bucket is to use variable camber, which means flaps. Hepperle has a very nice writeup of this as well, so I won't give my amateurish explanation here. I will say that I predict variable camber to become a standard technique in pylon racing.
The component of drag that you can reduce through airfoil selection is 'profile' drag, not induced drag. Induced drag is more a function of the wing planform and aspect ratio. Depending on the racing class, those might be determined by the rules. If so, the only tools you have available to control induced drag are weight and bank angle, since these can be chosen to reduce Cl needed for turns, and induced drag increases as Cl increases. Your weight must be spot on the minimum allowed by the rules. I think that Hepperle's site has a nice analysis of the turning radius ( same as bank angle ) tradeoff.
Good luck,
banktoturn
No, your procedure is not too simplistic. I'd say it's about right on. It probably does make sense to stop at step 6. In fact, you pretty much want to work with Cl and Cd, rather than the values of lift and drag, so that your 'design goal' is the minimization of Cd in the turn. Once you calculate the Cl that you need for level flight, using the formula, you can simply work with multiples of that Cl, depending on how many g's you sustain in turns, which is entirely a function of bank angle. Then you use the polars to go from Cl to Cd. I don't think you can do much better than to read Hepperle's site, it is really good. In a perfect world, you want an airfoil with low drag at low Cl, for level flight, and a 'drag bucket' which is wide enough to give fairly low drag at the high Cl you have in turns. Airfoils with wide drag buckets make this easier, and it also implies that you have a tradeoff involving bank angle. This is because bank angle determines the Cl, so you might come out ahead by using a less extreme bank angle, if it keeps your Cl within the drag bucket. One way to widen your drag bucket is to use variable camber, which means flaps. Hepperle has a very nice writeup of this as well, so I won't give my amateurish explanation here. I will say that I predict variable camber to become a standard technique in pylon racing.
The component of drag that you can reduce through airfoil selection is 'profile' drag, not induced drag. Induced drag is more a function of the wing planform and aspect ratio. Depending on the racing class, those might be determined by the rules. If so, the only tools you have available to control induced drag are weight and bank angle, since these can be chosen to reduce Cl needed for turns, and induced drag increases as Cl increases. Your weight must be spot on the minimum allowed by the rules. I think that Hepperle's site has a nice analysis of the turning radius ( same as bank angle ) tradeoff.
Good luck,
banktoturn
02-19-2003, 10:35 PM
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aerodynamics of pylon turns
Here is something else to throw into your mix. A number of pylon racers used only the right aileron. The thought being that the increased drag would help hold the nose up in the turns.
Hal
Hal
02-21-2003, 05:33 PM
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aerodynamics of pylon turns
Getting from step 6 to 7 isn't too difficult. Once you know the drag increment, divide that by the mass and you will have the axial decelleration in the turn. Multiply this number by the amount of time spent in the turn and you have the change in velocity.
Tom
Tom
