Upper cowl inlet ramp plugs

I carved blue foam from Aircraft Spruce yesterday to plug the side openings of the upper inlet ramps after two years of flying with them open. The TAS has been just over 171 -173 kts all that time. I epoxy resin (EZ-Poxy 24 hr cure) "glued"them in this afternoon. Over thhe next few days I will be glassing them in and finish sanding. Hopefully it will add a little bit of speedby eliminating those two drag buckets.

Bob Axsom

Anxiously Awaiting Results!

I hope you get a speed increase Bob, 'cause that sounds like a pretty easy thing to do! (Easy even for someone like me that doesn't enjoy glass....)

How about some pictures when it's done?

Paul

Will do

I use the slow cure epoxy resin so it will be a few days coming but we plan to fly to Ashville, NC on the 17th so it will be done by then. I am photographing it as I go.

Bob Axsom

Bob Axsom said:

I carved blue foam from Aircraft Spruce yesterday to plug the side openings of the upper inlet ramps after two years of flying with them open. The TAS has been just over 171 -173 kts all that time. I epoxy resin (EZ-Poxy 24 hr cure) "glued"them in this afternoon. Over thhe next few days I will be glassing them in and finish sanding. Hopefully it will add a little bit of speed eliminating those two drag buckets.

Bob Axsom

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BOB please update with speed if you don't mind, it will be fun to see what diff it made. If anything when some one looks in the cowl at the airshow, they will go WOW! Darn this thing is plugged!

No really record your oil temp and cht, also. It may improve cooling?

Cheers, George.

The speed difference will be reported

I will honestly report the speed difference. The plane isn't instrumented to provide CHT. The oil temp is around 180 (under 200 anyway) all the time. The only time it went high was when the original LASAR timing from Lycoming was about 60 degrees off (85 BTDC). I will report that as well.

No one will see the plugs. They are up inside the upper cowl where the there is a very slick airfoil shaped ramp from the trailing edge of the upper part of the inlet opening to the inside surface of the cowl itself. I believe that the air comes in and is accelerated across the surface and drawn up into the plenum where it decelerates and follows the normal plenum outlet flow from there. The sides of the ramps are wide open so I am pretty sure that the air spills over the edges of the ramps and curls in and runs into the inside of the junction between the ramp and the inside of the upper cowl. My thought is, if I block off the access to the backside of the ramp in a manner that provides a smooth transition, the spillage will curl and rejoin the inflowing air to the plenum.

The only inputs I have received on this were from Mark Manda and the fellow from Colorado that finished 1st or 2nd in the RV Blue class (360 cu. in.) in the AirVenture Cup race last year. Mark had done his from the start so there is no comparative data there. The other fellow's achievement speaks infavor of the mod but there again, I have no before/after comparative data.

The mod is a little tricky because it should be done in a way the eliminates sharp corners and does nor interfere with the inner or outer baffle seals. At this point my plug shapes do not do a good edge rounding job but I hope to progressively improve it if the current configuration shows promise. One subtle thing that needs to be considered is some vent for pressure change with altitude.

Bob Axsom

I thought everybody sealed the sides of the ramps. I did it a few days ago right after putting in the ramps:
http://www.dualrudder.com/rv7/2006/04/02/912

I didn't use a plug or anything. I just draped some fiberglass cloth from the ramp down to the cowl making it curve gently to avoid sharp corners as Bob mentioned.

Dave

Finished fairing in the plugs

The plugs are in and everything is filled and ready to sand but no glass yet. all of the baffle seal areas are clear but I'm not happy with the break off of the sides of the ramps. Dave not everyone sealed them obviously but certainly a lot of people did. I thought about it but decided not to at the time since it was not called for in the drawings (at least as far as I saw). I will be able to tell if it makes any difference in a week from now at the most.

Bob Axsom

Here's a current photo

Here is an early photoof one of the plugs. The edge radius is only about 1/8". A lot of sanding to do before the fiberglass goes on. You can see the markings on the cowl from the front baffle seal so that prollem area is clear.

Bob Axsom

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Why would plugging these ramps improve the speed?

I'm having a hard time understanding why we should expect a speed increase from plugging these ramps. I can understand why the cooling might be improved, if air was leaking from the upper cowl area to the lower side through these ramps. Stopping that would force more air to go through the fins and oil cooler, and improve cooling. But it won't affect the amount of air coming into the cowl inlets, which is what affects the amount of cooling drag.

If the cooling is improved, then we could reduce the size of the cowl inlets. Reducing the amount of air coming in would reduce drag.

Enlighten me, please.

Note: my cowling work is not yet complete, so maybe I don't understand what we are talking about with these ramps.

I will in a few days

Kevin,

I can't right now but in a few days I will get the mod finished and test it. I've been thinking about this for a couple of years and "thinking" just doesn't provide knowledge like testing does. S-o-o-o-o I decided to modify it and test it. I have an airspeed established under rigorous requirements for the Marion Jayne Air Race put on by U.S. Air Race, Inc. last year so I can repeat that if it is necessary (so close that a comparision standard is needed) but I want to know whether it makes any difference or not. Any time you have an air trap in the system there certainly is the potential for parasitic drag. Those open sided ramps just seem like a problem to me unless the pressure under (above actually) the ramps is such that no air coming through the inlets rolls off of the sides and impinges upon the inner wedge shaped junction of the ramp and the inside of the cowl. I put on the first layer of glass this afternoon so I have a few days of finish and cleanup before I fly it again. When I'm done I know I will be able to tell you if it is faster but I can only theorize about why. Inquiring minds want to know as they say - it's an easy mod and test with little risk.

Bob Axsom

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Bob,

I'm certainly looking forward to your results.

How repeatable was the airspeed you established for the Marion Jayne Air Race? How many different flights did you test out your speed on, and how did the results vary?

Copy of the actual handicap form

Here is the actual form used for the race handicapping with my numbers. If you open it up you should be able to see all clearly. At 75% power I almost always see a TAS in the low 170 kt range. I have enough time in it on coast to coast flights that it will immediately be obvious to me if there is any performance change at full throttle, leaned 100 deg rich of peak EGT, 2450 rpm (the way I fly it regularly). If I don't see 175kts reliably I'll call it no performance improvement. If I can't get 170kts reliably I'll call it a degradation in performance and will be thinking of ways to reverse the mod.

Bob Axsom

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Do they do this race every year? If so, I'll contact them and explain how wind triangles work.

If you do multiple runs in different directions, and average the ground speeds from those runs, the average is only equal to the TAS if the wind speed is zero.

The wind looks like it was only about 15 kt when you took your data. The error from that wind have been around a third of a knot, which isn't that significant (your TAS was about a third of a kt less than the average GS). But the error increases significantly as the wind speed increases. If the wind had been 30 kt (which is quite possible at 6,000 ft), the error would have been about 1.4 kt, which is significant when you are trying to determine the effect of minor mods.

An earlier thread on average GS vs TAS.

It is very hard to measure the performance change due to small mods. You need to have a very good test technique in order to get an accurate measurement. The only way to know whether you have a repeatable test technique is to do the same performance test on many different flights, in different weather systems, and see how repeatable the results are. The results from any given flight can be bogus, due to rising or descending air. If you have results from multiple flights, in different weather patterns, then you can start to see whether you have a repeatable number or not.

I do not doubt that

I followed the threads on the subject before. I know the test method is not adequate for detecting small changes but I feel confident that I will be able to tell if there is an improvement for my personal knowledge.

The contact for the race is Pat Purcell at [email protected].

The race is being run again this year out of Hutchinson, Kansas to with the following stops with remain over night options but none mandatory:

Akron, Colorado
Rapid City, South Dakota
Wolf's Point, Montana
Devil's Lake, North Dakota
Orr, Minnisota
Waupaca, Wisconsin

The plan includes a block of rooms for racers in Wisconsin and shuttles to and from AirVenture Oshkosh. I am torn between this race
(http://www.us-airrace.org) and the AirVenture Cup Race out of Dayton, Ohio (http://www.airventurecup.com). I have flown them both I lean toward the AirVenture Cup Race because it is not handicapped but they are so screwed up with preparations due to EAA "Risk Management", etc. that they still don't have entry forms available. In 19 more days if they don't have it worked out I'm giving up on them and entering the Marion Jayne Air Race put on by U. S. Air Race, Inc.

Bob Axsom

Test Results Inconclusive

I tried to comply with the handicap determination form used before the mod but I was unable to get five consecutive ground speeds in two of the three directions. Just taking the averages of the numbers I did get, showed less than 1/2 kt speed increase. My immediate conclusion is that it is a waste of time to close the ends of these ramps. I did notice in my 75% power return to the airport, that the speed was higher than I saw before the mod but it was not a credible test and therefore I still believe the mod was not worth the trouble.

Bob Axsom

But, if the cooling is improved, that would allow you to reduce the size of the inlets a bit, and that would reduce the cooling drag. So this might not be a complete waste of time.

Lower CHT's

I remembered reading about this awhile back and just got around to doing the mod. I was having higher CHT reading on #3 than I like to see on climb out so I thought this may help. I had to keep the speed =>130mph to keep the CHT below 450 deg on #3 after take off which was manageable. Today, after the mod, it just would break 400degs on a 100-110mph climb out and ran cooler and more consistent temps between cylinders in cruise. I have not checked for increased speed but will keep yall posted

My expectations have changed

I agree that it is a simple and worthwhile mod. Back when I made it I was thinking in terms of multiple knot gains. Now 1/2 a knot on a clean airplane is something I consider a significant improvement. I only tested it once but my gain was on the order of 0.4 kts.

Bob Axsom

Absolutely

Kevin Horton said:

But, if the cooling is improved, that would allow you to reduce the size of the inlets a bit, and that would reduce the cooling drag. So this might not be a complete waste of time.

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correct. I've spent many hours of trying to find the best solution to cool these dang airplanes and I'm a firm believer that the cowl inlet shape and design is very critical for the first few inches. I found this out by testing many different shapes and profiles. I only wish that Bob would install CHT guages to see how it effects the temps. There is much room for improvement even on a stock cowl. Some people will look at my very small 2 3/4" round inlets and wonder how the engine cools. It's all about air management and designing the complete cooling package. Even with the small round inlets I'm still running in the lower range on CHTs....325F range on all 4 cylinders on a 70F degree day, but I'm not using cowl inlet air for the oil cooler so that needs to be considered. Good luck with your testing Bob and let us know how it works out. It would be nice to know what Bob's CHTs are doing though.

Have 4-probe CHT

I have the temperatures for many tests and configurations but not back before the ramp plugs were installed. Today I made a test flight checking for performance difference between minimum and maximum spark plug gaps for my O-360-A1A and found the speeds were within one tenth of a knot of identical using the NTPS spread sheet method (the USAR handicap procedure indicated a nonexistent 2 knot difference). With the 0.016" gap the the CHTs were lower than with the 0.021" gap as shown below:

1-318 328
2-354 377
3-362 371
4-321 342

I talked to a Lycoming customer service rep and confirmed that they want the engine operated in the 300 - 400 F CHT range. That is where I attempt to keep it. I have incrementally reduced the size of the cooling air inlets with test plugs using the CHT as the control limit parameter. I found it very easy to increase the the CHTs in a directly related manner but I was completely unsuccessful in increasing the aircraft TAS at the same time. Previously I had used the U. S. Air Race Inc. Handicap procedure to measure TAS. I have gone back and calculated the TAS using the NTPS (Naval Test Pilot School - I think that's what the letters mean but the important thing is it eliminates the wind impact by direct calculation not three direction averaging) spread sheet - Xcell. Based on this new information there may be a slight increase in speed associated with a 3/4" plug at the inboard edge of the stock inlet. From there the speed decays with each additional 1/4" of plug thickness. There may be a small speed increase in there somewhere with the stock cowl but it is very elusive and may in fact not exist. I have 27 personally recorded test flights with many associated data points (date, temperature at 6000 ft pressure altitude, altitude actually flown to maintain 6,000 ft density altitude, oil temp, oil pressure, manifold pressure, RPM, all four EGTs and CHTs the USAR TAS and NTPS TAS. I believe the plugged ramps of this thread and the lower cowl baffling at the rear of the engine combined with the horizontal baffles crossing the valve covers have given me some increase in speed but I am very skeptical about any further gains without significant cowl changes, more power and a better prop.

Bob Axsom

NTPS = National Test Pilot School. It is a civilian organization in Mojave, CA, with most of the instructors being ex-USAF test pilots.

Their GPS to TAS Excel spreadsheet uses a mathematical approach derived by Australian RV-6 builder Doug Gray. I was at NTPS for a two week refresher course several years ago, and told them about Doug Gray's 3 leg method of using GPS track and ground speed to calculate TAS, as the GPS method that NTPS was using at the time was not very well optimized. The bright guys at NTPS came up with a clever 4 leg adaptation of Doug Gray's method, and published their spreadsheet with both the 3 leg and 4 leg methods. The spreadsheet also has the ability to calculate airspeed system and static system error.