[AX-O]

I have been working on my gear leg intersection fairings and wanted to see if the work I did was a waste of time or if it would potentially work. I decided to do a quick test for FUN. I put some tuffs on my plane and then used a 6HP vacuum cleaner to blow air over the surface. I changed the angle of the airstream to simulate different flight path. On the last part of the video you can see some reverse flow. I may have to do this test again in a more scientific way with additional and better tuffs.

The reverse flow on the last few seconds of the video , I was pointing the nozzle along the longitudinal axis but at approx. 45 degrees to the fairing/fuse.

BTW, I have been pretending to be a little air molecule all week. We use this same method at work but we use 4 jet engines to generate localized airflow at high speeds.

Airflow video

 

[Bob Axsom]

What did you learn?

What I saw that surprised me was the tufts in front of the cuff indicating the air being deflect downward from the line of flight by the top to bottom taper of the leading edge of the cuff. I causes me to wonder if a straight vertical leading edge would give a better flow.

Bob Axsom

 

[Tom Martin]

I looked at the pictures you had posted on the other thread of this faring. My impression at the time was that there was too much frontal area and that this would cause drag. Your video seems to confirm this.
The frontal shape, that the air "sees", is quite a bit larger then it really needs to be. Clearly the air is not flowing well around this area. I would decrease the size of this area but still maintain the generous radius aft of the faring. If you had flown the plane, this is a location were you would likely see that bugs would hit directly and not smear. In general try to change locations where you see direct hits of bugs. You can see this around the front of your cowling, leading edges etc.
You have nice airflow aft of the faring at the current time. Most RVs do not have a large radius aft of the upper gear faring and this will cause drag. I have done in flight tuft tests of this area, using a camera to confirm the need for this radius.
The stock lower gear leg farings on my rocket had a frontal area much like you have on the upper faring. It was not difficult job to reduce area and although I have no before and after speed tests my overall improvement in speed came with some of these small changes.
We all know that at least 10 knots to 15 knots can be added to your airplane with gear legs and wheel pants. This is a huge increase in speed. Thus any subsequent improvements you can make in this area will likely yield greater gains then you can find in other locations on the aircraft. A lot of time is spent on improvements to cooling drag, and it is important, but from my experience greater gains can be made with the things that hang out in the wind.
I appreciate your posting this video of a quick test. I wish that I had done this, it would have saved a lot of time and fuel!!

 

[Ironflight]

Neat idea Axel! I think you need to get Steve Smith to have a look at it to give you a good evaluation of the results.

 

[flyboy1963]

more tuft testing!?!?!?!

I just love this stuff, it's what I wanted to do as a kid!

...then came computers, and a LOT of wind tunnels became software!

Has anyone (Van's, others?) extensively tuft tested the airframe?
for example, if the belly is generally a huge pile of turbulent cr@p due to engine exit, exhaust velocity, gear & antennae, would a few well placed fences or VG's improve the flow here? ( beyond the fine work on exit geometry by Bob and Dan).

I am still working on a good mount, and convincing my wife that the 'science' is worth borrowing her iPhone to video all these things!