Table of Contents
History of lap times
Engines for F3D pylon racing models
Propellers for F3D pylon racing models
Fixed or retractable landing gear ?
Challenging the wind
Flaps in pylon racing models ?
Landscape or portrait?
How it all works together
A Comparison of Pylon Racing Airfoils
Using flaps in pylon racing models may help to adapt the airfoil of the wing to the flight envelope. By deflecting the flap, the camber of the airfoil is changed, which makes it possible to move the drag polar Cl=f(Cd) up and down. When the low drag region of the polar (laminar bucket) can be adjusted to the flight condition, a thinner airfoil can be used, which may result in lower drag.
Two airfoils have been analyzed for different flap deflections, using the computer code XFOIL. The first airfoil, MH 18 is relatively thick (11.14%), whereas the second one, the MH 23 is much thinner (8.01%) and also features a more pronounced laminar bucket.
The drag polars of both airfoils were calculated for a 20% deep flap at a Reynolds number of 1'000'000. Looking at the results, it should be noted, that the lift coefficient Cl, necessary for pylon racing, mainly falls in the range between approximately 0.0 and 0.6; higher lift coefficients are usually not needed.
Drag polars for the 11.1% thick MH 18, equipped with a 20% chord length flap.
The polar of the MH 18 moves smoothly up and down when the flap angle is changed. Using the default configuration (delta=0º), the laminar bucket extends from Cl=0.1 to Cl=0.6, which is sufficient, but could be improved by applying -2.5º flap deflection. It remains questionable, whether the improvement justifies the additional work of installation and the problems of control.
Drag polars for the 8% thick MH 23, equipped with a 20% chord length flap.
The thinner MH 23 is a much more extreme design, which results in less drag, but also in a very narrow laminar bucket. Without a flap, which is adjusted according to the necessary lift coefficient, the low drag region ranges from Cl=0.1 to 0.3. For a successful usage of this airfoil, a flap, whose deflection may be linked to the elevator deflection is recommended. For tight turns, the maximum deflection may reach 7.5º; during the straight and level parts of the flight a negative flap setting of -2.0º should be used.
Thick airfoil sections, like the MH 18, usually cover a range of lift coefficients, which is wide enough without using flaps. On the other hand, modern radio control equipment reduces the effort to the installation of a second aileron servo.
When using thinner, low drag airfoil sections (like the MH 23...MH 27 series), a flap is well worth the effort, or even absolutely necessary, to get all the benefits out of these airfoils.
Because there is no time to manually adjust the flap setting, it has to be linked to the elevator, which is rather difficult to adjust.
Last modification of this page: 21.05.18
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