Cooling drag control

[Bob Axsom]

Cooling drag has been identified as something that has to be addressed if you are going to increase speed without boosting power or changing the prop. It is fairly obvious that you have to minimize the air flowing through the cooling system if you are going to minimize cooling drag. Under "Traditional Engines" a lot of good information has been exchanged on the cooling drag question. I have cleaned up the flow downstream of the cylinders and reduced the volume in this zone of the cowl before the outlet. The net result was a 4 kt increase in speed. Now, there is still room for cleanup in the post cylinder fin flow path but the dominant effect is a greater mass of air is allowed to flow through the system and the cooling drag is actually increased and the CHTs continue to go down. It has been pointed out that the air mass flowing through the system is now much more than is needed to cool the engine and that and further internal flow cleanups without controlling the air mass in the system will probably slow the plane down. The results of personal experiments with my RV-6A support this view. It has been said by several people that at this point you have to control the the air mass flowing through the cooling system by reducing the inlet and/or the outlet if you want to increase the speed. OK, that sounds right and I have thought of some approaches I can take to do that but it seems to me that if you could fine tune the opening in flight based on CHT observations you could optimize the flow for the current configuration and continue the internal flow cleanup without having to "cut 'n' try" the inlet/outlet to re-optimize for the new configuration. This would certainly be more motivational for making internal cleanups and ultimately get the cooling drag minimized. Cowl flaps reflect this kind of approach but it seems to me that that approach is a compromize suitable for a fixed system configutation. If you want to develop the best configuration and you are not worried about the demands of product liability etc. you would want to be able to be able to reduce the port(s) beyond some arbitrary nominal position and deal with it like a trim control.

The question is, what is a good configuration to achieve the desired control? One posibility might be a cone in orifice.



Bob Axsom

[rv6ejguy]

I've studied both air and liquid cooled configurations extensively and carried out both flight and ground based experimentation on controllable exits (cowl flaps).

Requirement one is sufficient mass flow available in the climb regime to keep engine temps below redline.

Requirement two is to be able to control mass flow in cruise to keep engine temps at the desired level.

On naturally aspirated engines, power drops off in proportion to ambient pressure (excluding temperature variation). Therefore as power drops off with altitude, required cooling mass flow drops as well. In this case, assuming WOT climb cooling is sufficient at SL and a given IAS, so will be the case at any altitude.

Once cruising altitude is reached, if we remain at WOT, mass flow through the ductwork will increase as velocity builds. Temperatures will drop as a result. By using a controllable exit, we can throttle the mass flow to the minimum required to maintain desired engine temps. The movable exit flap has the advantage of constricting the exit area, thus increasing cooling air exit velocity. The higher the exit velocity, if done correctly, the lower the drag.

Reducing mass flow through the ductwork and across the baffles and fins reduces the pressure loss hence drag. Cowl flaps attack the drag problem on two fronts.

A simple calculation might clarify the concept. We'll assume the aircraft climbs best at 100 knots and requires 382 lbs. of air per minute at SL to cool the engine at 200hp. At 8000 feet, the engine is now producing only 75% (150 hp). True airspeed in level flight is now 200 knots, mass flow required to cool 150 hp is 287 lbs./min. Total mass flow available under these conditions is 574 lbs./min. with a fixed duct (probably slightly less in reality). The cowl flap would allow us to throttle total mass flow down to 287 lbs./min. and reduce drag if we were willing to accept the same temps as we had in the climb.

Turbocharged engines present a much bigger problem as power does not drop off until critical altitude is reached yet air density, thus mass flow does. Compounding this is the higher inlet charge temperatures with increasing altitude as compressor pressure ratio increases (assuming constant MP) with increasing altitude while intercooler effectiveness drops. Oil temperatures get higher as well due to constant power being maintained in the climb plus increased thermal loads being applied to the oil from the turbocharger as it is worked harder via increased exhaust mass flow and the constantly closing wastegate.

Indeed, some turbocharged engines may reach a thermal limit on CHTs, oil temp, EGTs or intercooler discharge temperature before the wastegate is fully closed. These are thus thermally limited rather than by the turbomachinery limits.

My research has revealed that some liquid cooled WWII fighters were 15-25 knots faster with the radiator flaps closed as opposed to open in level flight. I could not find references to closed cowl flaps on air cooled engines but logically these gains should be similar.

In summary, a variable cooling air exit should provide measurable gains in level speed. The amount of gain is dependent to a large degree on how hot you want to run the engine.

[Bob Axsom]

Very good information that expands what I have learned so far from others and my own experiments. The thing that is stewing in my brain right now is the best way to implement a variable control, such as a flap, that is continuously variable over a fairly broad range. I'm at the stage where my mind is exhaustively going over possibilities looking for that near perfect way to do the job. I'm still catching up with work around the house after returning from Oshkosh but soon I will get back to the hangar and a view of the hardware will help me focus on the best approach.

Bob Axsom

[chuck]

Originally Posted by rv6ejguy

A simple calculation might clarify the concept. We'll assume the aircraft climbs best at 100 knots and requires 382 lbs. of air per minute at SL to cool the engine at 200hp. At 8000 feet, the engine is now producing only 75% (150 hp). True airspeed in level flight is now 200 knots, mass flow required to cool 150 hp is 287 lbs./min. Total mass flow available under these conditions is 574 lbs./min. with a fixed duct (probably slightly less in reality). The cowl flap would allow us to throttle total mass flow down to 287 lbs./min. and reduce drag if we were willing to accept the same temps as we had in the climb.

Thanks for the insight on outlet effects on cooling. You (and just about every airplane design I've seen) seem to gravitate towards the cowl flap solution for the reasons you mentioned.

Inlet geometry control seems ignored. A few questions. During the climb (worst case cooling load) the inlets see a different angle of attack than they do in cruise, however the inlet AOA seems optimzed for cruise AOA. Second when you close the cowl flap I assume that less air gets through the inlet. Won't this cause plume drag at the inlet.

It seems like a more optimal solution needs to be able to adust inlet and exit cross section (and inlet shape) to minimize drag and perhaps adjust inlet AOA geometry as function of aircraft AOA.

Are these inlet effects real but not significant or not real?

[rv6ejguy]

Bob's mods and flight testing have been very interesting to follow. Thanks for sharing all the details here with us.

Inlet control has not been popular probably for 2 reasons:

As we restrict the inlet area with a fixed exit, velocity at the exit decreases which is not desireable.

Building a variable geometry inlet which does not cause serious airflow separation in typical cheek type inlet cowlings is difficult due to the short distances involved and transition to the plenum.

We must think of the duct inlets and outlets as the pumps for the system, both are equally important for the lowest possible losses.

I've run some CFA plots on the 6A at different Alphas and the pressure distribution at the inlets does not change appreciably.

Plume drag is mainly evident when there is leakage between a high and low pressure zone at obtuse angles to the relative airflow. A non- gap sealed aileron or flap is a good example where high pressure air from the wing bottom seeks the low pressure area on top, funneling through the gaps to exit on top . The high velocity plume coming through the gap causes drag by forming a wall of air nearly perpendicular to the relative airflow.

Air cooled engines present a big problem packaging a proper duct for efficient airflow due to all the other things in the way like exhaust pipes, engine mounts etc. Ideally we'd like to encase the entire engine in a sealed duct with round inlets feeding a smoothly transitioning plenum on top and a similar plenum on the bottom of the engine to smoothly turn the airflow aft and out the exits. This is hard to do with all the junk in the way and all the junk causes drag just as if it was on the outside of the aircraft. The duct needs to be treated as an internal aerodynamic structure for lowest drag.

Of historical interest is the use of streamlined tubing on the Westland Whirlwind to form a open spar truss for the cooling air to pass through to the wing root mounted radiators for lower drag. The same idea could be applied to an RV with lightweight teardrop shaped fairings being applied to any round tubes in the cooling airstream.

On most planes, we rely on the cowling bottom to turn the air and focus it out the belly exit. It works ok, it is simple, light and makes the engine easy to work on. A sealed duct... well you see the compromises.

If we could make a nice duct on our race plane, it would be beneficial as well to change the exit duct shape and length, encorporate our cowl flap and use wasted exhaust energy to pump cooling air through our duct. Unfortunately on an existing design like the RV, this is difficult to do again properly. DG's Reno winning Lancair uses the exhaust pipes submerged in the exit duct to pull air through the system, heat it, expand it and accelerate it to good effect.

While we might not be able to do all of this, we should be able to make improvements on the existing design.

Now Bob, what will you be trying next?

[Bob Axsom]

I need to reinstall the airbox cover that I took off for the AirVenture Cup Race first. That is just too obvious a good thing to do to pass up and it will get me into the hands on mode again. That provides a smooth flat surface instead of the big 1/2" deep frying pan. It not only eliminates the air trap (which ironically makes the plane go faster with the current inlet/outlet configuration) but it provides a very good base structure for some other improvements like a streamlined carburetor fairing and/or a deflector off of the flat surface. All of my engine mount tubes are behind the lower cowl air flow fairing but the NLG strut and four round braces and the center cowl support will be streamlined. All of this should increase the airflow mass and cool the engine more and slow the plane down more if experience is interpreted correctly.

Then I need to have the cooling air exhaust throttle worked out to restrict the airflow mass, take the CHTs back up to an acceptable level and get an increase in aircraft speed to a higher level than the 174+ kts seen in the current configuration. That is my benchmark with all of the current lower cowl baffling and nothing on the airbox. The benchmark for the stock configuration of my airplane is just barely over 170kts.

Bob Axsom

[Bob Axsom]

After washing the cars and truck and cleaning out the garage I had time to look at my builder's album (~8in thick) and study the profile of the firewall forward with the cowling off. I have a large remnent of that .025 2024T3 sheet left from the lower cowl baffle mod. The exhaust pipe below the #2 cylinder comes very close to the inside of the cowl where the crossover makes the bend to the rear so I can visualize the contours in the area. It is possible to make a baffle I think that would go from cowl side to cowl side with soft red rubber seals and attach to the engine just forward of the #1 & #2 cylinders come down and curve back just below the exhaust pipes pick up some hard mount points on brackets attached to the air box cover I made earlier then continue back and dump off into the outlet. The nose gear structure seems like a pain but it is surprising how often these "problems" have a benefit. Conceptually it seems that the post cylinder air flow cavity would be reduced once again, the flow path would be smoothed more than before and the outlet cross sectional area would be reduced at a point forward of the actual end of the cowl. If it weren't for the nose gear structure you could mount a sliding flap instead of a hinged flap to adjust the outlet cross sectional area. The only way to know the results of such a configuration is to build it and test it. This may take a while but I think it is worth trying once the honey do backlog is worked off here at home.

Bob Axsom

[rvbuilder2002]

Bob,
This is a very interesting topic... I hope you continue your experimenting.

I don't know anything about the book or what value any of the information is, but I just noticed an add for a (newly released?) book titled Principals of Engine Cooling authored by Jim LoPresti.
The add is on page 144 of the latest (August) Sport Aviation mag.

I may order one myself if it isn't too high priced.

[rv6ejguy]

Originally Posted by Bob Axsom

If it weren't for the nose gear structure you could mount a sliding flap instead of a hinged flap to adjust the outlet cross sectional area. The only way to know the results of such a configuration is to build it and test it. This may take a while but I think it is worth trying once the honey do backlog is worked off here at home.

Bob Axsom

Yeah the 6A nose gear structure is ugly aerodynamically and hard to clean up. I agree the best way to expand knowledge in this area is to build it and fly it. Keep up with the good ideas and keep us posted on the results.

[Bob Axsom]

I went to the hangar today to wash the plane but I took some file folders and did some preliminary pattern work. It looks like the forward lower baffle is going to come down inline with the middle of the alternator then curve back just under the fowrard crossover member of the exhaust. There are plenty of potential mount points. The baffle will have to be made of at least three sections. Fitting into all of the hardware is fairly straight forward. I'm thinking red baffle seal material for hole closures. The tough item is going to be getting a good cowl fit.

Bob Axsom