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The Zenith STOL CH-750 experimental aircraft uses a modified NACA 650-18 airfoil (having a 17% fatter-than-normal thickness) with a fixed leading-edge slat. In an effort to reduce drag and improve cruise performance, some of the builders remove the leading-edge slats. Instead of removing the slats, I am curious what the effect would be if one left the slats in place, skinned over the slots with a removable leading edge (shown in red dotted line), and added vortex generators.

Modified NACA 650-18 Airfoil

How would this modification affect the center of gravity for the aircraft?

Would this modification affect the chord length?

What are the pros and cons of such an experimental modification?

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  • $\begingroup$ It would destroy the beneficial effect of the slats. The gap in the slats channels high pressure air from the bottom of the airfoil to the top of the airfoil, where it adds energy to the flow across the top of the airfoil, and increases the lift at any positive angle of attack. At low angle of attack (high speed), this would reduce form drag with negligible effect on lift, but at low speed (high AOA) it would increase induced drag significantly. $\endgroup$ – Charles Bretana Mar 19 '18 at 15:55
  • $\begingroup$ @Charles Bretana Yes, but that can be compensated with vortex generators. There is an Italian-built clone of the Zenith 750 (the 'Savannah') that is supplied either with slats, or with no slats, but vortex generators in their place. I use to fly a Savannah with VGs, and it's close to impossible to stall (with some gas...). $\endgroup$ – xxavier Mar 19 '18 at 16:54
  • $\begingroup$ I don't know about VGs, but maybe you meant to say, "could be replaced with VGs"? ... VGs, or no VGs, covering the gap in the slats destroys the effect of the slats... $\endgroup$ – Charles Bretana Mar 19 '18 at 19:49
  • $\begingroup$ @xxavier: If it is impossible to stall you don't have enough elevator authority. The vortex generators cannot prevent a stall and delay it only a bit. $\endgroup$ – Peter Kämpf Mar 19 '18 at 22:17
  • $\begingroup$ @Peter Kämpf It has lots of elevator authority. With the engine at 60%, you start pulling the stick, till the anemometer shows zero and the body of the plane gets vertical, Or almost so... With the engine at idle, a maximum of 45º is possible. Beyond that, a wing drops... $\endgroup$ – xxavier Mar 19 '18 at 22:36
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To answer your question, I suggest to look at the effect of slats. NACA technical report 407 studied a fixed slat similar to what is used on the Zenith CH-750, and the result of the optimum geometry is plotted below. Dashed lines are for the airfoil with a fixed slat and solid lines for the airfoil with the slot between slat and wing filled:

Lift and drag coefficients for a fixed slat over angle of attack

Lift and drag coefficients for a wing with and without a fixed slat over angle of attack, from NACA TR 407.

Three conclusions are obvious:

  • Drag is higher with a slat for no benefit in lift (even a small reduction, actually) up to an angle of attack of 15°. Don't let the scale of the plot mislead you: Zero-lift drag for the slotted wing was 0.0229 compared to 0.015 for the plain wing, an increase of 52.5%.
  • The maximum angle of attack has been increased by 9° by the slotted wing, from 15° to 24°. This translates into an increase in the maximum lift coefficient of 34.6% (from 1.297 to 1.751)
  • The stall behavior has changed from benign to outright nasty by the addition of the slot.

While the airfoil studied in NACA TR 407 is quite close, it is not exactly what happens once the slat is removed: While the outer contour of the airfoil did not change in NACA TR 407, those builders removed part of the wing when removing the slat. Therefore, wing area would be reduced and the loss in lift will actually be even greater. NACA TR 400 hints at a lift loss of more than 40%, which would increase the minimum speed of the Zenith CH-750 by about 30%. Note that the take-off and landing distances will grow accordingly - the STOL in the aircraft's name will not apply any longer!

When putting a glove over the slot, the wing area will be maintained and the minimum speed will not go up quite so severely. Using the 34.6% from NACA TR 407, the minimum speed increase would only be 23.6%. The change in airfoil shape (with the glove the point of maximum thickness is further back and the relative thickness smaller than with the slat removed) will not alter the flight characteristics in a significant way - in both cases the airfoil is of the peaky upper surface type with a large nose radius, so separation around stall will start from the trailing edge and move forward with increasing angle of attack. Long laminar runs on the top surface cannot be expected in both cases.

Now I only wonder why you would add vortex generators. I thought the idea was to reduce drag and improve cruise performance?

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  • $\begingroup$ The idea IS to reduce drag and improve cruise performance...without sacrificing too much of the short field capabilities of the aircraft. Per Vortex Generators: Band-Aids or Magic?, an article in AVweb, by Mike Busch, "VGs do produce some drag, they also reduce drag by reducing the thickness of the boundary layer on the aft portion of the wing. The net result is about a "push" with no measurable degradation in cruise speed." The article indicates that properly placed VGs result in low-speed performance improvements. Best of both worlds, or Wishful thinking? $\endgroup$ – Mike C. Mar 20 '18 at 0:10
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    $\begingroup$ @MikeC. Wishful thinking. If they can give a "push", then only because the basic design was faulty. VGs can only reduce the boundary layer thickness if there was a separation before. A well designed airplane does not need them; they are a cheap and sometimes effective fix but no cure-all. In most cases, they raise drag for 95% of the time but do (sometimes) essential service in the last 5%. But they cannot do magic. $\endgroup$ – Peter Kämpf Mar 20 '18 at 19:57
  • $\begingroup$ Why does that graph have two greatly-different curves for each of the two airfoils? $\endgroup$ – Sean Jun 5 at 23:14
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    $\begingroup$ @Sean: Because one pair is for the lift coefficient ($c_L$) and the other for the drag coefficient ($c_D$). The round symbols are for the slotted version and the x and + symbols for the plain version. $\endgroup$ – Peter Kämpf Jun 5 at 23:46
  • $\begingroup$ @PeterKämpf: Ah, I see. Thanx! $\endgroup$ – Sean Jun 5 at 23:47

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