[Bob Axsom]

When I did all of the baffle work in the lower cowl to isolate the flow path from the areas above and outboard of the cylinders and the area between the rear of the engine and the firewall there was a apparent gain in speed of approximately 4 kts over the baseline I had established of 170 kts. I was using the U.S. Air Race handicap procedure for testing and it is a little flawed. Since then I have recomputed using the NTPS spreadsheet and I believe the gain still holds up. Sometime after that my vacuum pump failed and when I replaced it I added another blast tube hole in the baffle behind cylinder #3 to supply cooling air theough a hose and shroud bought from Airgraft Spruce. All of the holes in the rear baffle Heater/defroster air, mag blast tubes, Vacuum pump blast tube and the oil cooler dump the air into the cavity behind the rear baffle and must find its way to an narrow vent at the top of the cowl outlet. There was an apparent reduction in speed after the new cooling system was added for the vacuum pump and it has been a long slow process of minor mods getting the speed back to the 172-173 kt range. Here is the new thought: Make a sealing plate to cover all of the holes except the oil cooler for races only. Any thoughts.

Bob Axsom

 

[mannanj]

Baffle holes

Bob:

I don't think it should be a problem for short term racing. Give it a try.

The only holes in my baffle/plenum set up is a 1in. tube in front to the alt. rectifier diodes and the 2in. scat to the cabin heat located in in the rear baffle. I block that off in the summer. Of course there's the oil cooler just aft of #4 cyl.

So far after 90 hours, there's been no problem with the mags nor any vapor lock signs. (Forward facing fuel injection.) I don't have a vacuum pump to worry about.

Cht's have never been over 350F. Usually runs 325 in cruise. Oil temp. in winter is lower than I'd like. In the summer I've seen 205-210 in a steep climb out after a fuel stop. Usually runs 180 in cruise.

 

[Bob Axsom]

Potential Nuget

Mannan,

My instincts are the same as yours. Now for the priceless piece of free information. When you block the heater port did you observe a speed gain? If so, did you note what it was? I am going to have to make this simple change and test it anyway but it sure has feel good value if you observed a speed improvement.

Bob Axsom

 

[Bob Axsom]

Ok its ready to test

Below are the before and after photos of the rear baffle relative to baffle hole cover instalation. Weather and home chores permitting, will test tomorrow - I love retirement.

Bob Axsom

 

[mannanj]

Speed changes

Bob:

Sorry, I haven't done any real testing on changes in speed with the cabin heater hole covered.

My pea brain scientific instinct makes me think that there's probably no discernable difference. Reason: The air that's not going thru the heater hose is now being dammed up at the cowl inlet-plenum area causing some drag. reducing the inlet area might help. That air now isn't being heated-expanded and exiting the lower cowl at a rate that would be accelerated because of same, creating a miniscule amount of thrust. This shade tree reasoning could lead you to believe that leaving a hole that gets added heating open, might be better.

Good thing I'm not allowed to think often!!!!

 

[Bob Axsom]

I would agree with you but...

In my plane I go through a very complex path to positively dump the hot air overboard and because of lower cowl baffling I have added to divide the cowl into three semi-isolated cavities that routes the cooling air to the cowl outlet in a smoother flow that avoids the firewall and all of the hardware behind the engine, there are special restrictions to all of the air that exits the rear baffle of the plenum. Except for the heater/defroster air all of this air impinges on the mags, and vacuum pump or comes through the oil cooler and has to bang around in what I imagine is turbulent flow until it finds its way to the 1/4" by 7" vent for this cavity that I have provided at the top of the cowl outlet. When I added the Vacuum pump cooling hole the airplane speed dropped significantly. It could be an artifact of the test method and conditions but I kind of think the loss was real. Anyway, I should find out tomorrow. If the speed does not increase the CHTs should drop but I doubt that I can measure that objectively. However, I have been thinking along the same lines as you and I have flown several tests with 1/4" increments in inlet restriction. My old tests indicated that 1.75" reduction if each cooling air inlet by blocking off the inboard end was best. Those tests were with the rear baffle holes open so the there is going to have to be some work done yet to get this near optimum.

I bought some modeling clay at Hobby Lobby yesterday and I am going to build a mold on each balsawood plug that I created by gluing my test plates together. The inboard edge of the plugs have been extended forward to just about even with the forward edge of the spinner backplate. I test flew these with the leading edge fairly straight across just keeping clear of the trailing edge of the prop blades in high pitch and tapered back into the inlet. The ~straight across leading edge was faster. I rationalize that this is due to the fact that the air hitting the tapered plug is being dumped into the plenum (no it is not a hard plenum with sealed inlet tubes and a top separate from the cowl) so even though the crossection of the opening is smaller, the air is accelerated throught the "pinched" passage and there is no reduction in cooling air mass. I plan to come back from the inboard edge and then come forward to near the trailing edge of the prop blades at high pitch then have a protruding rounded edge that defines the inner edge of the cooling air inlet. I bought some clay workers tools so I hope to come up with some nice smooth curves that blend in well with the top and bottom of the inlet opening. I will layup one layer of fiberglass on the clay then dig the clay out and build up successive layers of fiberglass on the original layer and the balsawood which will remain my plug base. I will attach the completed removable plugs with long screws in place of the six short screws that normally hold the upper and lower cowl together around the prop shaft as I did in the earlier test series.

There are two air races next month and I would very much like to average 200 mph or better - kind of a private race within the races.

I will tell you how the next test with these new baffle plates comes out.

Bob Axsom

 

[mannanj]

Speed

Bob:

I commend you on your quest for speed. This is in the same spirit as the aeronautical pioneers that brought us to where we are today.

Keep us posted.

 

[Bob Axsom]

Test Complete - Success

We had clear skies and light and variable winds in Fayetteville, Arkansas today so I flew the test to determine the true air speed impact on my RV-6A whth the new baffle hole cover plates installed. The flight was conducted in accordance with the Handicap Procedure defined in the U. S. Air Race web site http://www.us-airrace.com to get the data then it was input the the NTPS Excel spreadsheet to mathmatically eliminate the wind component.

The Pressure altitude (29.92) 6,000ft temp was 19C so the test was flown at 4,400 ft for a 6,000ft density altitude. MAP=25.5, RPM=2710, Oil temp=190, Oil Pressure=80. Many GPS ground speed recordings taken at 20 second intervals with no exclusions after stabilization, then averaged.

000 track speed=178.523
120 track speed=173.5
240 track speed=170.733

NTPS computed TAS 174.3 kts
NTSP computed wind 4.5 kts @ 200.6 degrees

CHT/EGT at end of test: 1=321/1286, 2=372/1355, 3=367/1316, 4=328/1283

The speed increased 2.367 kts over the speed determined by the previous test to evaluate the tail tiedown ring fairing. This may sound trivial to the casual observer but it is a major jump in speed for my airplane. Also observed was a lowering of CHT and a change in the relative CHT/EGT measurements. For cylinder #3, which has three holes in the rear baffle behind it, it was for the first time not the hottest cylinder and cylinder #4 did not have the highest EGT for the first time. The Cylinder #3 CHT reduction relative to the other cylinders is exactly what I would have expected but I do not know why the EGT of Cylinder #4 dropped relative to the other cylinders - perhaps the heat transfer for the oil cooler behind cylinder #4 was more efficient because it did not have to compete with the other baffle ports for the small cowl chamber #3 vent (1/4" x 7") but I do not see a connection with the EGT. I was leaning based on the history of cylinder #4 as having the hottest EGT so I will have to scan them all in the future.

Anyway, the test results of this change on my particular airplane were very good! 200.58 mph. It may not be a big deal to anyone else but it is a significant achievement for me. In last year's AirVenture Cup race I was beaten by an RV-8 by less the 1 mph over the ~500 mile course. I'm working it and this is big.

Bob Axsom

 

[kentb]

Great work Bob.

Now what I would like to know is for normal operation and long service life, how much cooling air is needed through the blast tubs. I am currently cooling my fuel pump and alternator and one mag with blast tubes. Plus I have the cabin heater. If these could be fine tuned for every day flying, I could save some money.

Kent