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  Bonanza Cowl Airflow Mod


One of the most important areas of speed improvement is the reduction of drag. (DUH!) Not surprisingly, you see lots of the recent plastic plane designs with very small cowl openings and quite a lot of emphasis on the airflow through the cowling for engine cooling. LoPresti is famous for their cowling mods and STC's for a number of aircraft.


Here is a mod by Beechcraft Debonair owner and Beech parts guru, Kevin O. that essentially smoothes out the buffeting and turbulence created in the bottom of the engine bay. This increases the airflow (a good thing for cooling your engine) and reduced the drag created by the air essentially being "stuck" in the cowling and moving slowly while it bumps into all those ribs and walls in the bottom of the Bonanza engine bay.


BTW, Kevin reports about a ~5 KTAS increase in speed with his engine bay floor pans and curved firewall lip.


Here is additional background info provided by Kevin which was the inspiration for his mod:


Charlie Nogle ( of T34 fame) told me he had a STC to put cowl flaps on T34s---but they noticed that when they replaced the large augmenters ( big round tubes sticking out into the airstream about a foot) with the cowl flaps. It slowed them down by 5 knots. They figured out that the amount of air taken in at the air inlets was greater than could be extracted by the outlets----the excess air burbled out of the front of the air intakes causing disruption of the air flow around the front of the airplane--thus slowing it down. This is why a small oil leak in your engine puts a mist of oil on your windshield---the excess air ( along with the oil) burbles out the front of the air intakes.


I have a cousin with a PHD in Hydrology who helped me design the changes in the bottom of the cowling---and yes--the rolled piece of metal at the aft bottom is a important piece of the project. I gained a little speed--but more important--The CHTs on my IO550 deb never get above 340 in the climb or run above 300 in cruise( LOP)--and thats flying in HOT Oklahoma summers--I also have DeShannon baffles---and I do NOT have cowl flaps.


Look at Cirrus airplanes and the fast home builts--they all have small air intakes and larger fixed outlets


Gami experimented with making the air intakes smaller as did Mike Smith. I think they both decided that the gain was not worth the cost


I remove the panels at every annual to inspect the area under them----takes about 10 minutes.




Here are Kevin's pics and narrative:








No cowl flaps on my bird--no silicone--no funky looking extra vents on the side.



On a 100 degree day on climb out--no cylinder gets over 350 degrees--at cruise--no cylinder is above 300 ( most around the 280 range) My take on this--you want the same amount of air exiting the cowl from the cowl flap area as you have going in through the cowl intakes--if you have a imbalance--the air burbles out through the air intakes causing drag--thus lower speed..


SO--if you increase the air removal from the lower deck, lowering its pressure--you draw more air past the cylinders--cooling them--and as a added benefit, less air burbles out the front causing drag.


You assume there is only a limited amount of air coming in through the air intakes, and you want all of it to go by the fins on the cylinders--by filling every small hole with silicone.--BUT , My thought--the real problem is the lower deck--the upper deck has more than enough air pressure to cool ( remember the excess coming out of the air intakes)--the problem is getting it out of the lower deck. If you lower the pressure on the lower deck by making the air easier to leave the cowling--more air will go by the fins. Problem solved without the use of silicone.


A simple way to look at this--think of a dam with a hydroelectric generator. Even if the dam leaks a lot--the difference in pressure will still spin the generator as water flows past. BUT if the river gets dammed up down stream and starts backing up and the water level is almost the same on both sides of the dam--little water will move past the generator--THE PROBLEM WITH BONANZA COWLINGS---the air down stream can't get out fast enough! Cowl flaps were a Band-Aid fix---almost none of the new plastic planes have cowl flaps.


Have a cousin with a PHD in Hydraulics ---told me that if I smoothed out the flow of air leaving the cowling---could get like a 30 % increase of flow.--if you look at the pictures below--you can see the plates in the bottom of the cowling that smoothes the flow of air---also notice the rounded plate on the lower part of the fire wall --by making that point a rounded corner instead of a square corner--you reduce burbling--thus flow of air.


BWTFDIK---I'm only a Dentist.


Phil: "So, a nice radius on the outlet from the back wall of the engine compartment, if it didn't reduce the area of the outlet in the direction of flow, would have the effect of increasing the outlet flow area, helping both the inlet-outlet ratio and the total throughput."


Kevin: My thoughts exactly---you can see the radius on the bottom pan curving down directing the flow out--and the curve coming off the back wall doing the same thing --would love to make the intake smaller.


Kevin O.  10/05/2009



Professor Dave Rogers offers the following Beech List thoughts on Bonanza engine cooling and cowl drag:


I don't consider the cowl flaps on the Bonanza a poor design but rather a simple weight saving, parts saving design. They were more than adequate for the original design with the "E185" engines and are also adequate for the IO-520 and IO-550 engines.


The issue with cooling is not the cowl flaps but rather the baffling and the nose bowl opening.


Considering that on a typical light single engine reciprocating powered aircraft cooling drag is estimated to be on the order of 30% of the total drag, improving the airflow in the engine compartment will improve the aerodynamic performance.


Atlantic Aero has done this with their conversion. That conversion uses a different engine with a tuned intake manifold above the engine case, completely redesigned nose inlets which utilize NACA inlets, and new carbon fiber cowl and cowl doors and a tuned exhaust. You can see it here:


I did a level flight performance flight test on the original A-36 modification. That flight test showed a 10 KTAS increase over book values (IO-550B). However, not all of the increase was attributable to a decrease in cooling drag. You can read the flight test article here:  (Scroll down to the Flight Test of Atlantic Aero A-36). Figure 1 shows that the original cowl flaps were still in use.


If I ever do a 550 conversion, it will be this one. Beech should buy this design from Atlantic Aero.


The nose bowl design on the Bonanza brings way too much air into the engine compartment and poorly manages it once it is there. Much of the air that enters the engine compartment actually exits back OUT the FRONT of the opening behind and in the vicinity of the propeller spinner. (I have tuffed in flight studies that show this.) On the other hand, did the designers know about this in the mid 1940s? Probably not.


Good cooling design brings just enough air into the engine compartment to provide adequate cooling for the worst case and then efficiently directs that air to cooling the engine.


Having said that, aside from buying either the D'Shannon or the Liquid Air baffles, what can you do to improve the cooling?


The fundamental objective is to make sure that the air goes through the cylinder fins (and oil cooler for the aircraft with rear mounted oil coolers). Keep this fundamental objective in mind.


The flexible material on the baffles of most of the Bonanzas, and other aircraft as well, that I see is in terrible shape. Spend the money and time to replace the flexible material. Shape it with the above fundamental objective in mind.


Fill every single hole through which air can leak past the cylinders without actually going through the fins with GE Silicone II. Be anal about this.


Two of the largest areas for leakage are around the inner cylinder baffles and between the #2 cylinder and the rear mounted oil cooler.


To get to the inner cylinder baffles you will have to remove the intake and exhaust manifolds. Once that is done, get a large mirror and a good light and look at the cylinder bases. You will see that the inner cylinder baffle leaves a good size hole(s) between the baffles and the engine case. A lot of air leaks through those holes.


Fill the holes with silicone.


Fill every piece of metal baffle that has a cut out for a nut or bolt with silicone.


Look closely at that area between the #2 cylinder and the rear mounted oil cooler. Again there is a hole between the metal baffle and the engine case. Fill it with silicone. There is also a hole on the outboard side that needs attention.


Continue looking for "air" holes and filling them with silicone.


A good way to do this is to stick a trouble light under the engine in a dark hangar, get a step ladder and look straight down at the engine. If you can see clear light, then you have identified yet another hole and need to fill that hole with silicone. Keep looking.


Now turn your attention to the top of the engine. Typically it is a mess with wires, hoses, fuel lines etc. running everywhere.


The fundamental objective here is to get the magnetos, the wires, etc. organized and as high in the engine compartment as possible without interfering with the cowl.

Typically the magnetos are positioned down very close to the top of the engine case. These things need attention. Get an A&P to rotate the magnetos up as far as they will go and still meet the timing specifications.


Sort out the spark plug wires and elevate them on standoffs. Mine are at least 3" or more above the cylinder fins. You can see what I mean here in the engine images:  (click on the image for a larger image.)

Notice, elevating the magnetos and the wires allows the incoming air to cleanly, well relatively so, pass under the wires, hit the rear engine baffle and be directed downward through the cylinders rather than having to "fight" its way through a tangle of wires or past the magnetos before passing through the cylinder fins. The high mounted magnetos actually help deflect the air downward of the #2 cylinder and through the rear mounted oil cooler.


All this will help but it takes effort and attention to detail.


Most A&Ps won't do this or the customer does not want to pay for them to do it.


'Nuff said,


Dave Rogers

E33A, A-36, W29



So, get to work with your A&P/IA to discuss this potential cooling mod to your Bonanza. It could be especially helpful to you folks with IO-550s.


BTW, excellent aftermarket baffle set up options are:











great improvement options for your engine cooling and airflow management!


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