Pictures of Bottom Cowl Tunnel Opening

[whee]

I'm hoping some of you guys will share a pic of your tunnel opening and how you trimmed the bottom cowl in that area. I know some guys trimmed quite a bit off your bottom cowl and created a big opening while some guys kept it pretty tight. I'm just curious what you guys have done.

The Newton Finish manual shows a fairly large opening with an exit lip to aid in airflow. I was unable to find a representative picture on the internet. Edit: found one


Battson's is pretty tight but with cowl flaps:
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[Glenn Patterson]

We sealed our engine tight to the cowl. The skin ends at the tunnel opening along the same line as the cowl over lap on the fuselage. The exit is as per the drawings and the lower is simple the cowl skin wrap following the firewall shape. I added ducting to the inlets to project the air up over the flat front of engine baffles to best to create a pressure in the cowl so that the air over the engine goes evenly through all the cylinders. The nice thing is that with your Continental IO-360 the under side is free open space so air flow is clean through and under the engine. We used the MC3 bowl for our Continental IO-360 and I created a modified lower half to fit our engine better and give a nice clean slope flowing into the lower cowl. My partners wanted to get the underside cowl sloped to look similar to a Mountain Goat. I narrowed the air inlets to cover the engine fuel pump and fuel lines & made both inlets identical. We have run the airplane on the ground for good lengths of time with out any heat issues. The panel has CHT & EGT on all 6 cylinders to see all the engine temperatures are comfortable with the front cylinders a couple degrees cooler.

I did some research on pressure cowls used by the racers, Zavaston, Schmidtbauer, LoPresti,& the James cowl. The James cowl on an RV can give a 10 -12 mph speed improvement. Schmidtbauer put a pressure cowl on his Mustang II and with progressive improvements to eliminate drag gets over 250mph with his original engine. The common principle to a pressure cowl is that the engine needs to a have a good sealed plenum over the engine from the the air inlets. and the prop opening sealed to the engine. The nose bowl around the prop opening should be sealed between the engine & nose bowl otherwise up to 20% of the air going in the air inlets is sucked back out by the propeller hub and spinner. That is a lot of cooling air lost and may be the difference between running hot and cold. Our gap between the nose bowl and back of the prop is a hair under a 1/4". The Continental engine bed mount does not allow engine to move like a Lycoming on a dynafocal mount.

The other advantage of the pressure cowl is that it has smaller inlets and clean air exits. The ideal situation is the cooling air exiting as the same speed pushed by the cowl pressure so there is minimal cooling air drag. A true pressure cowl has a smaller exit area than inlet area. The air is rammed in and expands with the heat so the velocity of the exit air is very high. We did what we could do to try to avoid a lip or some device that creates back suction to draw air. The air passing over our cowl would help to induce air flow. Our friend has a Murphy Rebel and claims he got 6mph improvement by taking the lip off of his cooling exit. We have our fingers crossed that we accomplished good cooling.

[svyolo]

Cooling drag, drag, weight. What is the mission? If you are building a family hauler and cross country machine, reducing drag will allow you to cruise faster, or the same speed for the less FF. On a BH, a pressure cowl will add weight, and make routine maintenance harder. But if cross country is your goal, it might be worth it. On an RV or Lancair it is probably definitely worth it. On a BH, I am not sure.

If you are building a BH as a bush plane, you are probably flying on big tires, or floats, or have stuff strapped to the outside. Adding weight to reduce drag will be mostly masked by the extra drag of 26-31 inch wheels, or floats. You are not going fast, period. Adding weight to reduce drag is probably not a good trade off in this case.

Depends on your mission.

[Glenn Patterson]

We built a flying truck. The point is to get good cooling. Our cooling is pressure cowl "like" so we have not done much more than anyone else as far as weight. We took great care to seal it to the cowl and nose bowl. The continental engine has its fuel system on top of the engine. We ran the baffles and baffle seals tight to the upper cowl and nose bowl to make a sealed plenum. Maintenance is easy as soon as we open the doors the engine is right there or the whole cowl comes off plane quick. The engine doors have Skybolt fasteners that are amazing simple 1/4 turn fasteners for quick access. If the cooling is efficient and the drag is reduced then the airplane is burning less fuel to cruise with the big tires on. My partners listened to Schmidtbauer at Oshkosh telling how he cleaned up his Mustang II to go 250+ when he started with180mph. He said now he can throttle way way back to cruise 180 to save a lot of fuel. The point is less drag is less fuel.

We were conscious of the cooling struggles that some builders go though. Jon (whee) has the same engine as we do and as far as I know there are 3 BH with the Continental IO-360. The engine has given both of us some interesting twists that are not seen with the Lycoming engines. If we are fortunate enough to have nailed the cooling then oh happy day. The inlet ramps are aluminum that simply gets the air over the blunt engine face. The inlet ramps are aluminum flushed in & screwed in. We made a set of winter closures that share the same screws. Clean flow is good cooling pressure. May be 8 oz for ducting top & bottom. The upper duct is 2 layers of glass that I added while doing all the crazy mods to make our nose bowl work. Baffle seals to cowl are unavoidable so we made the best seal as possible and practical for our set up. The underside of a Contintenal IO is clean so the cooling air has a clean flow to exit clean. We added the half tear drop shape to round the edge of the firewall cooling exit with .020 aluminum that makes a big improvement to clean flow over a square edge. The taper of the cowl to the engine is perfect as the plenum expands as it passes each cylinder so the back cylinders look to have the same air flow. The temperature readings seem to verify that as all the cylinders read the same with the front pair a couple degrees cooler which is normal. 2 oz of weight added for good exit flow. The engine air intake filter is right on the throttle body so no carb ducting or carb heat. The lower cowl does not have the Lycoming air intake so the total cowl weight should be the same or lower than a Lycoming.

My partners wanted the underside to slope like a Mountain Goat that is a good STOL. I had to widen the nose bowl, raise the lower lip slightly and flatten the bottom slightly to to fit the engine the bed mount. This gives a wider sloped belly surface going into the wind which may help lift. Who knows. It is the way the underside of the cowl naturally turned out and the aerodynamics of that is well beyond what little I know.

We made skin extensions that we slid between the rear spar and the skins to make our version of upper & lower gap seals, put on the vortex generators and did the flying tail mods to make the truck better. If the airplane flies better with the flying tail mod and a degree or less down angle reduces drag then the little weight is acceptable. My partner read that angling the wing fairing at the back of the wing where it transitions into the fuselage reduces drag so a simple step during construction. Each minor change for each drag improvement has an accumulative effect so if a person is building their own then it is all simple mods that make a difference by looking after the details.

You are right that it is mission. We live in the middle of nowhere so it is all bush and lakes. We have the big tires on, a set of skis and the intent is floats one day. A ton of drag so if we can take reasonable measures to shave some drag out of the picture during construction then why not. If it will take off a little quicker then that is less beating on the fuselage and the gear. If it lands slower then less beating on the gear. The little weight for gap seals, flying tail, VG's. cooling are minor if it gives returns. We installed the stinger tail wheel for less weight & drag so the weight savings on the flat spring pretty much cancels out the weight of all the mods.

I think the new Bravo has the tighter gaps, stinger tail wheel, flying tail & stronger strut shock leg. We slid a tube in the streamline tube to stiffen the shock legs. The logic is that we would rather bend the fuselage then have a prop strike. We are relatively current to the new version except the improved Bravo wing which would be a nice touch.