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Drag Reduction Edit

Smooth the airflow to reduce drag.

When you performed the drag analysis of a Kitfox/Avid, were there fundamental design features which caused the drag (which could not be fixed), or was it just due to unstreamlined struts, gaps above horizontal stabilizers, no wheel pants, no fairings over intersections such as strut/wing, strut/fuselage, landing gear/fuselage, etc?

Jonathon Brent

These were the estimates based on a typical Avid Speedwing (24' span):

Component Proportion of Equivalent Drag Area
Fuselage 48.5%
Wing 18.8%
Struts (faired) 10.1%
U/C legs (faired) 5.8%
Stab/Elev (gap sealed) 4.2%
Flaperons 2.9%
Spats (2) 2.7%
Elev struts 2.5%
Spinner 2.2%
Jury struts 2.0%

Each component was measured to find its wetted area, frontal area and wetted length and then a value of drag coefficient applied to account for fineness ratio, an estimate of interference, etc. etc.

It is reasonably easy to determine total drag from the amount of power required at various speeds and weights mainly from fuel burn. Estimates of wetted area, frontal area etc. area reasonably easy the complex bit is attaching to each a reasonable value for Cd. The above were reached after a lot of iteration and are based on my aircraft so other a/c may have different apportionments.

However it does point up by how much the fuselage is dominant. Where improvements can be made are in cowling and cooling drag. For instance the Mark IV has its radiator apertures either side of the prop spinner which in most are poorly faired to the radiators themselves allowing a lot of air to pass into the cowl. This causes a poor pressure drop across the radiator cores as well as a lot of excess high velocity air slowing down inside the engine bay all to be speeded up again to exit. By making up a pair of baffles which are stuck to the rads with silicon adhesive the gaps around the sides have now gone and the pressure drop over the radiators has been restored resulting in better cooling. Because less air is entering less energy is being wasted in slowing it down and speeding it up again at the cowl exit, which was also reduced in area.

The more noticeable result was a 4mph increase in cruise speed for the same power level.

In most aircraft cooling drag approaches 30% of total fuselage drag and if air is entering and leaving unnecessarily then you can effectively double the cowl wetted area as both sides are active.

Other places to look at are door shuts with wide gaps or overlaps as well as poor gap sealing into the cabin.

The wide gap formed between the gear legs and the fuselage lower longeron. The latter can be sealed by a flexible tape. The underwing transition to the butt rib and door and the transition area from the wing trailing edge to the turtledeck. There's a lot of little fixes around there which can make big differences.

At the back end gap sealing the rudder and elevators help as well as sealing the small triangle at the front of the fin and the fin/stabilizer joint.

Blanking off the rear part of the wheel pants with a glassed over styrene bulkhead stops them acting as airbrakes and also from collecting mud. Incidentally fitting spats on small diameter wheels may generate more drag than plain wheels. The interference effects can be predominant. Have a look at how much better the fit on an RV6 is for instance especially around the gear leg/back plate junction.

Sealing off any gaps formed in the lift strut fairings around the jury strut connection, fairing off the strut pickups on the wing underside etc.

There's a whole bunch of places to experiment with so long as the weight of the improvement doesn't net off the useful load. Given that the design is fixed the best that can be expected in flow improvements is only of the order 6-7% but that's still worth going for as it unloads the engine.

Useful trick #2. Don't wash the plane and let it get an even covering of fine dust then go fly it in light rain. By studying the streaking after landing you can bet some idea of what the air is trying to do. If you find little dark spots then the air has stopped moving, have a look upstream for a possible cause. The airflow may have separated from the surface by an ill fitting joint causing turbulence. The easiest one to see is by how much the protruding filler caps rob the upper wing surfaces of lift. The area spreads out about 20 degrees aft of the caps as a long plume of turbulence. It's worth about 3% of the wing area but because the inboard sections work harder probably reduces the lift by as much as 7%.

Least that's my excuse for rarely washing it.

Graham Laucht

Tape Edit

Clear SkiSaver or AllSaver tape can be applied to:

  • Horizontal stabilizer/elevator gap
  • Vertical stabilizer/rudder gap
  • Main gear/fuselage gap

Cut two strips of tape about 2 inches wide. Lay one piece down on the workbench, sticky side up. Hold the second piece sticky side down, overlap it 1 inch and apply it to the first piece.

Apply the strip to the //top// of the horizontal stabilizer and the //bottom// of the elevator through the gap, forming a loose S-curve. Make sure the elevator maintains its full range of travel with no binding.

Tape can also be applied to close off:

  • Tubing triangle at horizontal stabilizer strut
  • Flaperon openings at hinges

Weatherseal Edit

Foam weatherseal can be applied to:

  • Seal wing root rib or fuel tank to butt ribs
  • Seal door edges to fuselage

Wood Covered with Fabric Edit

Wood fairings for:

  • Lift struts (Avid supplied these with its kits)
  • Landing gear tubing (Avid supplied these)
  • Jury struts (model airplane balsa works well)
  • Horizontal stabilizer struts (balsa)

Round tubing has about 8 times the drag of streamlined tubing.

Fiberglass Edit

Fiberglass or other material to make fairings for:

  • Fuel caps
  • Lift strut brackets
  • Cowl openings to radiators

Metal Edit

  • Nosegear tubing
  • Exhaust tubing
  • Main gear bungee opening, or fair over the Grove gear (photos)(.cad file)

Accessories Edit

  • Wheelpants or wheel covers
  • Prop spinner

Windshield Edit

A more sloped windshield may look faster, but may actually have more drag as it is slightly larger and has more wetted area.

Other Edit

Seal the spar tube ends with foam.

Direct air into the radiators with baffles.

Add cuffs to the ends of the lift struts.

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