NACA Vent Drag with internal closure

[Bob Axsom]

I am looking for speed any where I can find it and an area of concern was the NACA scoops for passenger cooling. I did a careful comparitive test at 6,000 Density altitude with the internal vents closed then I topped the tanks and immediately repeated the test with contact paper over the NACA inlet scoops. I used the USAR handicap procedure (5 consecutive ground speed recordings on headings of 360, 120 and 240, with no variation greater than 1 kt, WOT, leaned for speed, trimmed for hands off level flight, autopilot track & altitude hold, at 6,000 dalt) to collect the raw data then pluged the average speed of each track into the National Test Pilot School KTAS spreadsheet. The covered vent configuration was 0.9 KTAS faster than the uncovered NACA vent configuration.

Bob Axsom

[rocketbob]

Bob, what is your procedure to climb to the 6000ft density alititude?

[Kevin Horton]

Originally Posted by Bob Axsom

The covered vent configuration was 0.9 KTAS faster than the uncovered NACA vent configuration.

I've wondered about this myself. I expected that there had to be some speed increase with the vents closed. But It would be very interesting to see if the amount of difference is a repeatable result.

[Bob Axsom]

I believe the procedure was developed by Bob Cole. I am going by memory but it went something like this set the altimeter to 29.92 climb to 6,000 ft and record the temperature in C. If it is 3C conduct the test at that altitude. If it is above 3C subtract 3 from the temperature and multiply the remainder by 100 and subtract the product from 6,000 and fly the test at that altitude. If it is below 3C fly at the altitude determined by the difference times 100 above 6,000.

Oh, the results of my test were 181.4 uncovered and 182.3 covered. I would expect faster planes would have a greater impact and slower ones to have less.

Bob Axsom

[elippse]

It's a good idea, also, to keep track of the difference in rpm along with the change in speed since that will show the change in power. For instance, before 2009 Reno I modified the prop on Bob Bryson's biplane, and he picked up 10 mph, from 201 to 211, and 150 rpm. On the face of it the 5% change in speed seems to mean that he was converting his engine's HP into 15% more thrust, i.e. 15% more efficient. However the increased rpm allowed the engine to develop 5% more power, so the real efficiency increase was 10%.

[Bob Axsom]

I am starting the process of making removable covers. They will be 0.032" 2024-T3 aluminum mounted with one screw in the front and a tab & standoffs in the rear. I plan to have them in place by the TVAR3 race on October 30, 2010.

Bob Axsom

[Toobuilder]

How about speed tape? Light, easy, temporary...

[Bob Axsom]

I like the metal plate but a tape like cover does work. That is how I proved the value of the mod. When we arrived at Sherman, TX for the race on 10-2-10 Mike Thompson had his covered with tape and it was peeled back like a banana. I covered mine with very low profile contact paper and after the race it was still in place covering the holes but it was deformed over the cavity - no telling what was going on during the race. I have two separate envelopes of race cover plates in the pocket behind the seat now and adding a third will not be a problem.

Bob Axsom

[Bob Axsom]

I just got the right vent cover essentially completed and installed - some RTV seal cushion (cured before installation) work to be done yet on back side at edges and back of standoffs. I did use a better dimple die after the photos were taken so the front end mount point is cleaner.




Bob Axsom

[logansc]

Bob: You are a fanatic!! Keep it up!


Lee...