Engine Baffles

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The engine baffles are critical to proper operation of the engine. They provide the sides for the box that holds the cooling air, and they also direct the air around the cylinder fins to minimize hot spots.

Its very common to see the metal portion of the baffles cracked and fretted from vibration, and to see the rubber seals hardened from heat and age, missing sections and replaced with the wrong types of material. The orange silicone rubber is the most popular replacement material, and its not really suitable for the job.

The rubber baffle seal needs to be thin and flexible, and it should have some fiber reinforcement to prevent tearing. The orange silicone rubber is too thick (1/8"), has no reinforcement, and tears easily.

Baffle Seals for Sky-Tec Starters

When installing a lightweight starter such as the Sky-Tec, the front metal baffles on the 75 Traveler and all Cheetahs and Tigers must be trimmed to clear the different contour of the starter motor and solenoid.

Here are some examples of the trimming:

Center-trim.jpg


LH-Skytec.jpg


This is a seal I added to decrease the air leakage around the starter.

Skytec-seal.jpg

This is seal for the pilot's side of the starter, that allows one to clear out the baffle to a larger rectangle, and then rivet on a cover plate that can be trimmed to match the starter contour without multiple removals of the baffling. This is very similar to the design used by later American General and the Tiger LLC plane's baffles. It is also workable to install the add-on plate with sheet metal screws, rather than permanently attach it with rivets.

Skytec-seal pilot-side.jpg


[Gil Alexander] The seal is a little harder to form on the outboard side of the starter due to the starter leads. One example of sealing is here. The baffle material is extended as much as possible over the top of the starter to get into the gaps created above the starter.

Sky-tec-gaps.jpg

Sealing Around Skytec 149-HT Starter

Here's an example of the seal around the Skytec 149-12HT, which is a little different than the PM/LS models. Here's a view of the rear of the starter. You can see a bead of RTV applied to the gap between the motor and the solenoid housing:

Skytec-149-HT-rear.jpg


Note that the body of the starter is on the outboard side of the unit, with the solenoid and electrical connections on the inboard side.
The outboard side is fairly easy to seal, with the body of the motor a 3-inch diameter cylinder. A 3 1/8 inch instrument panel punch would be perfect to get the desired cutout in the baffle. Otherwise, chop it out with tin snips as shown here:
Skytec-149-HT-front-outboard.jpg
Add a seal that wraps the outside of the body of the motor. The piece at the upper left helps seal the gap between the starter and the engine case:
Skytec-149-HT-front-outboard-seal.jpg
Sealing the inboard side of the 149-HT is trickier, due to the hexagonal shape of the solenoid body and dealing with the starter cable. Start with leaving a large enough gap to work with:
Skytec-149-HT-front-inboard.jpg Skytec-149-HT-front-inboard-gap.jpg
Here's a template of the odd shape around the solenoid. It's a mirror of the cutout, so you have to flip it about the vertical axis to match the cutout:

Skytec-149-HT-rear-inboard-shape.jpg


This is what the resulting seal looks like. The piece on the bottom is notched to pass the starter cable through, and the piece on the top seals the gap between the solenoid and the engine case:
Skytec-149-HT-front-inboard-seal.jpg
The installed seals look like this:
Skytec-149-HT-front-inboard-finished.jpg


Baffle Attach Points

Also check all of the baffle attach points. This AA5B broken bracket was hard to see, but was obviously not doing it's job. It is attached to the baffles by the oil cooler and held by the upper left engine mount bolt. A previous repair was less than effective. New brackets are available at Fletchair.

Broken-baffle-bracket.JPG

Cheetah Baffles - Part of my Hunt for High Oil Temperatures

I have a Cheetah that suffered from higher oil temperatures than I'm comfortable with (CHT's look OK) during the warmest summer months. Everything has been done to this engine to make it run warm (High Compression mod, LASAR system) so this is not a big surprise. But hearing that people fly these machines in Arizona in the summer and the fact that I'm a bit of a perfectionist...I wanted to see if there is anything I could do to lower the temps.

In the course of chasing down the issue, (besides checking the oil cooler, vernatherm valve, and other system components) the old and ratty baffle seals were replaced under the supervision of one of the most Grumman knowledgeable shops around. After the seals were replaced, the following summer my temperatures were actually worse than the previous year. And thus began my education in the proper fit of baffle seals...

Finding Problems

I started by simply taking a flashlight and looking into the cooling air intakes in the nose bowl. I found several places where the seals did not sit flush against the cowl. In one location the seal bowed enough to create a quarter sized hole. This occurred because in the replacement of the seals, the multiple separate pieces of seal material were replaced with a few single longer strips of seal material that would buckle when pressed against the cowl where they were supposed to seal. Where the seals along the back and sides overlapped, they were attached together in such a way as to pull them away from the cowl. So, I removed these attachments and determined where to cut slits in the seal material so it would sit flush.

I then started looking around the nose bowl seals and found a number of locations where the seal fit was as poor as the seals against the cowl doors. I had to pull the cowl and hold the nose bowl in position as best I could (by holding it relative to the spinner/prop just as it looked when the cowl was attached) and inspect the seal fit. I used a marker to mark the locations where the seal did not fit well and made a number of additional slits in the ribbons of seal material. Often these slits were made where the contours of the cowl or nose bowl change. The seal near the landing light needed to be trimmed to fit the contour of the landing light recess in the nose bowl. There was also a location near the alternator where a longer strip of seal material was needed to close a hole caused by the additional distance between the baffle and the nose bowl in that area.

After 20 or more new slits in the baffle seal material and a new piece near the alternator, I put things back together and used a combination of drop lights and flashlights to check how well the seals were seating. Everything looked good, so I reassembled the cowl, checked the baffles one more time, and waited for the warm summer days to return for the tests.

Test Flight

Well, summer returned and I went on a test flight. The temps were lower...but only back to the point they were prior to replacing the seals. So, I continued my search. Off came the cowl and nose bowl and I found the seal around the starter (a location Bob mentioned above) was not that tight in spots and completely missing along the top of the starter (between the starter and the engine case) in what would equate to about a 1" x 3" hole. So, I constructed and installed a seal to close the hole. I also replaced the seal around the pipe that provides air to the muffler shroud as I determined the seal material there was the original and in bad shape. I then put everything back together and and performed a couple test flights. Things were looking better, but when the high temps of July and August hit, the oil temps wee still a bit high.

Gathering Data

Manometer
Stumped, I decided to try collecting some figures on the cooling. I constructed a simple water manometer to determine the pressure differential between the high pressure and low pressure side of the baffles. After some searching around the internet, I found that for effective cooling you need a pressure difference of at least 6" of water. We ran the tubing ends of the manometer from the cockpit out the canopy and in thru the oil door with one tube on each side of the baffle in the cowl. In flight we were able to determine that in cruise around 120 mph IAS the pressure difference was right around 6" of water. In a 100 mph IAS climb, the pressure dropped to around 5". So, the numbers indicated that I still had some issues as I was just barely coming into an acceptable cooling pressure range in cruise.

The latest (and hopefully last) find

So, my A&P (who had been overseeing my work all along) and I tried to find any other possible problems. After a bunch of snooping around with flashlights and inspection mirrors, my A&P was able to find another hole hiding in the seals. This hole was located just above the airbox and was caused by the fact that the standard width of the seal material sold in rolls is not wide enough to close a gap in that area. So, I obtained a larger piece of seal material (around 4.5" wide, if memory serves), off came the cowl and nose bowl again, and replaced the pieces that were too short.

Pictures of the Results

Below are several pictures of my baffle seals after the changes. Most of the slits you see were added as part of my attempts to improve the fit/seal. Sorry, I didn't take any before pictures, maybe I'll remember next time.

UpperLeftSeals11u.JPG
UpperRightSeals11u.JPG
LowerSeals11u.JPG
LowerRightSeals11u.JPG

Conclusion (sort of)

Of course, by the time we found and fixed this last issue, the hot summer temperatures had given way to cooler ones of fall. The oil temperatures have looked good since the last fix, but I guess I won't know for sure if the problem is resolved until summer returns again. I'll probably try the manometer again to see what it says...when my mechanic and I get a chance. I'll keep you posted. In the meantime, I hope this information helps if you have a Cheetah experiencing similar issues.

One last note. I was supplied the silicone type baffle seal material by the shop where the seals were initially replaced. I've found that this material isn't necessarily the best for seals as it is rather thick and does not "set" (retain a curved form) so you have to wrestle with it and be careful of the fit anytime you need to close the cowl or install the nose bowl.

Cooling Outlet Ducts in Lower Cowl

Tiger-sagging-outlet-duct.jpg

After the air has passed over the cylinders, it still needs to leave the cowl area. The pre-78 fibreglas ducts are somewhat heat sensitive, and will deform over time, reducing the area of the air outlet, as seen in this photo from Gil Alexander.