Drag reduction experiments
 

I've been experimenting a little with drag reduction ideas and thought I'd share my results.

I've been checking my airspeeds at 8000' DA using the 4-way GPS spreadsheet to determine TAS. I've got a number of data points charted from different flights and think my measurements are relatively accurate.

My -9A is still unpainted and I had shot high-build primer on all my fiberglass parts. This left a slightly rough texture to the surfaces. I never sanded them until recently. Once I sanded the cowl and wingtips to 240/320 grit testing showed I picked up around 1.5 kts in the 160 kt TAS range.

Later I also sanded the rest of the fiberglass surfaces (fairings, wheel pants, etc) but couldn't measure any additional improvements.

I decided to experiment a little with cooling drag. My CHT's and oil temps are very good, so I assume I have some margin to play with. Over the last few days I made up some inlet reducers. These reduced the height of my inlets from about 3.5" down to 3.0", reducing total inlet area from around 44 sq. in. down to about 38 sq. in. My exit area hasn't been modified from the standard RV-6 cowl exit (for my IO-360). They transition under the cowl to about mid-way along the length of the ramps.



I did a short test flight this morning. My CHTs were higher, between 340-360 degrees (vs. 310-330 normally). OAT was still pretty low - ground temps were only around 62 degrees. My oil temp was very slowly climbing at 2700 rpm, showing 202 degrees by the time I reduced power to repeat the 4-way GPS runs at slower speeds. My oil temps normally don't exceed 200 even in climb.

The results of the inlet area reduction (drum roll please) - no measurable improvement in airspeed.

I was thinking of playing with the exit area as well, but given no change with the inlet reduction I don't think I'd see any difference.

I do have a little rework to do with my wheel pants and fairings which I assume will provide a little more improvement. And of course, I fully expect the standard 10kt speed increase when I finally get it painted this winter.

Some interesting work has been done re. airflow thru and out of the cowl. You might want to contact Bob Axsom before you give up in this area. Some speed here is available. It's all a question of effort vs. results.
Terry, CFI
RV-9A N323TP

I'll place my bet that you will see an appreciable difference, especially if you can reduce the area without creating any flow seperation on the bottom side of the cowl. (i.e. just blanking it off won't do)

The trick is to reintroduce the air exiting in line with the airstream. Great improvements can be made if you can do this effectively. Nice looking Cardinal in the background, is it yours?

Easier ways to reduce drag !
 

I see you have not painted her yet. Than there are much easier ways to reduce drag!: Close off the sides of the hor. stab. and elevator and the vert. stab top. According to Bob Axsom, and some other people who have done this, you will gain 2-3 kts. I used PUR foam and fiber glas. The beauty about the elevator is, that you will ad no additional weight (because you will be removing the additional weight from the lead counterbalance when you balance the elevator)

Also you can install cover plates, a la Bob Axsom, on the rudder and elevator hinge holes. This should give you another 2-3 kts speed increase. (for about 42 g. weight increase)

I am planning on doing some of the same on the wings, ailerons and flaps (where possible) and hope to get 165 kts cruise, or better.

Kind regards, Tonny.

[quote=MTBehnke;355816]
The results of the inlet area reduction (drum roll please) - no measurable improvement in airspeed.
QUOTE]

Your the first honest builder on this site! The gains from small aerodynamic improvements are generally less than the margin of error for the flight testing. It will probably be difficult to detect anything smaller than +/-1.5 knots and impossible to accurately quantify it.

Keep experimenting and you will eventually see some improvement from a collection of mods. Some other easy fixes might come from sealing controls and a wing root fairing. Good luck! -David

Different approach same result
 

In my testing I reduced the cooling air inlet opening by reducing it with incremental 1/4" slices of shaped balsa plugs building out from the inboard side of the opening on each side of the spinner. It was very measurable in increasing CHT but provided no measurable increase in speed for me either. Tom Martin reported similar results on his EVO Rocket.

Bob Axsom

No...not a wing root fairing
 

[quote=David Shelton;355846]It has been proven before that the flat fairing Van provides is the faster. The curved fairing is around 7 MPH slower.

Regards,

Inlets "fix" themselves somewhat
 

The lack of gain from reducing inlets is probably because "excess" air not taken into the inlets simply splits to go around the cowl. I.e. imagine streamlines going towards the opening(s). At some point just forward of the cowl, a split occurs, sending the streamline either into the cowl or around it. The size of this perimeter is determined by how much the flow rate is into the inlets.

As others have said, the exit area is the key...

If you haven't already, search for cooling exit modification and you will find several fun threads.

Along the same lines what is the best exit for exhuast?