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Bob Axsom

Well Known Member
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
 
Bob Axsom said:
The covered vent configuration was 0.9 KTAS faster than the uncovered NACA vent configuration.
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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.
 
The procedure was described at www.us-airrace.org

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
 
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%.
 
I am starting the process of making removable covers

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
 
I like the metal plate

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
 
Near Final Configuration

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.

IMG_4639.jpg

IMG_4637.jpg


Bob Axsom
 
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vents closed, or covered?????

This reminds me of the classic 'driving with the windows down, or the a/c on" debate. If only we had a wind tunnel big enough, eh?
I'm sure covering the vents cleans up the airflow, but I wonder what effect open or closed vents produces? ( air blowing into the cabin, or just weaving in and out of the duct ramp.?)
I hear some of the Reno racers tape up every entry & exit, then pressurize the fuselage slightly, ( or was It depressurize?) to control what little leakage there was left.
 
Bob, what's that shiny stuff it's made of, and what are those 10 round, gold thingys with the dimple in the center?
 
I have run that test several times

flyboy1963 said:
This reminds me of the classic 'driving with the windows down, or the a/c on" debate. If only we had a wind tunnel big enough, eh?
I'm sure covering the vents cleans up the airflow, but I wonder what effect open or closed vents produces? ( air blowing into the cabin, or just weaving in and out of the duct ramp.?)
I hear some of the Reno racers tape up every entry & exit, then pressurize the fuselage slightly, ( or was It depressurize?) to control what little leakage there was left.
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It is an easy test to run and the results are impressive. Get every thing maxed out in level flight, autopilot and altitude hold on. Open a vent and make no other change. The speed bleeds off and observable amount. Re-close the vent and the speed slowly comes back to the previous high speed. No mystery here. I have seen as much as 3 kts difference.

Bob Axsom
 
Its miracle Stuff

elippse said:
Bob, what's that shiny stuff it's made of, and what are those 10 round, gold thingys with the dimple in the center?
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It's miracle stuff but they call it aluminum for short - it is amazing what you can do with it.

Bob Axsom
 
Bob,
By closing the vents and seeing as much as 3 knots, do you mean closing the eyeball from the inside to see this, or closing from the outside with you mod? If with your mod, do you see any improvement with just closing the eyeball?
 
Eyeballs

I meant the eyeballs where the full flow of air is entering the cockpit as opposed the the eyeballs closed. This mod was intended to pick up 0.9 kt above the eyeballs closed condition. The only time I opened an eyeball valve during a race was the 2005 AirVenture Cup when my wife was suffering in the heat. It was well over 100 F when we got to Oshkosh. With this latest mod for racing that option will not be available once airborne - the vent system is blocked off completely.

Bob Axsom
 
Eyeball delta not universal

I have tested my top speed at Dalts from 6,000 to 10,000 and I never get more than 1 kt difference. While I never made note of the open/close status of the vents, I know I have done this in both warm and sometimes very cold weather, so it's pretty likely that some of the runs were with vents open and some with vents closed.

Mine is a 7A and Bob's is a 6A and we run different props, different cowl, different fairings, etc., so it's not the same thing. But the status of the internal vent control is not universally worth 3kts. I wish it were!

Just data.
 
You are probably right

hevansrv7a said:
I have tested my top speed at Dalts from 6,000 to 10,000 and I never get more than 1 kt difference. While I never made note of the open/close status of the vents, I know I have done this in both warm and sometimes very cold weather, so it's pretty likely that some of the runs were with vents open and some with vents closed.

Mine is a 7A and Bob's is a 6A and we run different props, different cowl, different fairings, etc., so it's not the same thing. But the status of the internal vent control is not universally worth 3kts. I wish it were!

Just data.
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I never rigorously tested this since I don't have to make any decisions about it. The interesting thing is these vents are cooling drag and when they are opened you can see the speed drop off and when you close them the speed comes back. I went to the aluminum eyeball controls because the original plastic ones leaked and caused a little drag (We could hold hands over them when closed and see an increase in speed). I seem to recall testing and reporting the difference in this forum but maybe not. Anyway it was a mod that didn't require testing to decide whether to take it out or not so I may have let it slide.

Bob Axsom
 
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Bob Axsom said:
The interesting thing is these vents are cooling drag and when they are opened you can see the speed drop off and when you close them the speed comes back.

Bob Axsom
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This could also indicate a leak in the pitot/static system.
 
Done and done

Finished making the second cover and I 1/2 a$$ tested them. Since my OAT probe is mounted in the vent and has to be removed for the installation of the covers I could not use my normal procedure for density altitude determination. I didn't have time to work out another procedure. The speed was 182.0 kts using the NPTS spreadsheet for the three leg method. It was good enough to validate the plan to use them in this coming Saturday's race in Courtland, Alabama.

They seal really well and you would not want to have them installed for normal flying - very hot and stuffy. I have my canopy well sealed so the combination may not be as bad on another airplane.

Bob Axsom
 
OAT in NACA vent.

Bob Axsom said:
... Since my OAT probe is mounted in the vent...

Bob Axsom
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It has been my experience and, I think, that of others, that the OAT in the NACA vent will read higher than it should by a significant amount.

At a pressure altitude of 6000', the standard temp should be 37.6 degF. However, if your OAT is reading 2 deg. high, it will tell you that your Density Altitude is 6,131' and for 3 deg. high it will tell you 6,197'. Or said the other way around, a 3 degree error will tell you 6,000' when the actual density altitude is about 5,800'.

Of course, this has no effect on your GPS measured TAS except that it's not at the altitude you want it.

The CAS (191.2 mph) that gives you 209.48 mph (182 kts) at 6197' gives you 208.81 mph at 6000'.

I moved my OAT to the second inspection port on the right wing, not the innermost one, and finally got what I think are correct readings. I also, (thank you to Paul Lipps) recessed it slightly so as to avoid friction heating and I'm confident that I avoided that problem. I test that by running at low and high speeds at the same altitude and observing no temperature change.
 
I just want to determine if the modification works

My interest is more configuration relative than absolute. I try to conduct my testing the same way every time so I can tell if one configuration is better than another. You are right that my reported test results are not what I claim them to be.

Bob Axsom
 
I don't recommend this mod for normal operations

Just to reinforce the comments earlier I thought I would add a few words about the experience in the test flight yesterday. The temperature was in the 70s which is not hot - what you would expect in late October in northwest Arkansas. When I was cleared for takeoff, I closed the canopy which I said before is well sealed at every point (even the skirt opening around the center track). As I climbed out the air felt stuffy inside the cockpit space and it really hit home "I cannot relieve this uncomfortable environment by opening the vents since they are sealed from the outside." I flew the test for 36 minutes which is close to the time required to fly the 100+ miles races sanctioned by the SARL. I will be flying in the TVAIII race at Courtland, Alabama this Saturday with the cover plates on but I will install them (one screw each) after I arrive and take them off before flying home. If I use them in a long race I think I will bring my oxygen bottle along. It is probably an imaginary problem but it sure got my attention.

Bob Axsom
 
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taping over vents etc. for winter

I considered this too.....just not for drag............another data point?
I don't want snow & water entering the vents, so why not cover for the winter?
Hmmm, what if I WANT cool fresh air on demand?....like fumes in the cabin, or to clear fire extinguisher powder, or someone hurls etc. etc.

thus, the brilliance of the flush door.!

If only I had the time, and a hangar, and spare time, and no lawn to mow, and, and,......etc. :)
 
If instead of the curved-divergent NACA duct you were to use the parallel-wall NACA duct with a square aperture of about 1", you could make a very simple door hinged at the forward end of the ramp which would fold down onto the ramp when open and then move out to where it's flush with the skin when closed. A 1" square parallel-wall NACA duct, to have a 7 degree floor, would be 8" long.
 
Another option

Remove the standoffs and add a 2-position control to pull broad rear of the cover plate back into the inlet molding for normal operations and push them out flush with the fuselage skin for racing.

Bob Axsom
 
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Another option for those still building is a flush door. Dick Martin put one on his fuselage vent and kindly shared the sketch. Just a simple sliding door of sheet aluminum, in a pocket, that slides between the skin and NACA vent controlled by a steel wire or in my case a Napa control cable.
 
RV8RIVETER said:
Another option for those still building is a flush door. Dick Martin put one on his fuselage vent and kindly shared the sketch. Just a simple sliding door of sheet aluminum, in a pocket, that slides between the skin and NACA vent controlled by a steel wire or in my case a Napa control cable.
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Pictures please. I know they are worth 1,000 words for me.
 
Sorry, I don't have a picture handy. I won't be off until Thursday so I will try and take one for you then.

In the meantime it is dirt simple. The sliding plate simply has the back edge bent up 90 degrees and the push wire is clamped to that. The cable runs straight back along the side and the T-handle mounts to one of the fuselage ribs. Push to close, pull to open.
 
RV8RIVETER said:
Sorry, I don't have a picture handy. I won't be off until Thursday so I will try and take one for you then.

In the meantime it is dirt simple. The sliding plate simply has the back edge bent up 90 degrees and the push wire is clamped to that. The cable runs straight back along the side and the T-handle mounts to one of the fuselage ribs. Push to close, pull to open.
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Thanks, I would love to see pictures of the mechanisms from the external opening up to the panel and how the cable is run. Does the cable penetrate the plastic formed NACA air vent, or the tubing, and then run to a mounting point somewhere? This is what I cannot picture in my mind. I am trying to visualize this since the flexible tubing runs from the NACA vent opening to the eye ball vent on the panel and the plate is inside that vent airspace but the cable is outside that space. How does the cable get to the inside of the vent area in order to manipulate the plate?
 
No, the vent is not modified in any way. If you look closely at the picture you will see the rivets that go around the vent on three sides. You simply mount the vent to a rectangle of aluminum the same way you would to the skin. You make a "U" shaped spacer the same thickness of the sliding door plate and has an opening width slightly larger than the door and length equal to the vent attached plate. You rivet the vent plate and spacer over the opening in the skin. You have now have the vent mounted with a pocket that the door slides in and out of.
 
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RV8RIVETER said:
No, the vent is not modified in any way. If you look closely at the picture you will see the rivets that go around the vent on three sides. You simply mount the vent to a rectangle of aluminum the same way you would to the skin. You make a "U" shaped spacer the same thickness of the sliding door plate and has an opening width slightly larger than the door and length equal to the vent attached plate. You rivet the vent plate and spacer over the opening in the skin. You have now have the vent mounted with a pocket that the door slides in and out of.
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I see. I was thinking the door would swing inboard and outboard to open and close. Now, with your explanation I see that the door slides forward and back to close and open. This explanation helps a great deal. Thanks.
 
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