So Youd Like to Go Faster

Secrets for reducing drag, part 2.


In Part 1 of this article, we covered basic principles for drag reduction, then looked at rigging, gear leg fairings, wheelpants, hinge fairings, and empennage gap seals. Taking a peek under the cowl, we saw small upper baffles on the cylinders and discussed cooling air drag and exhaust air convergence zones.

That’s a good start, but there’s much more work to be done. Let’s begin with the upper plenum and inlets, followed by the augmenter outlet and cooling tunnel. I made these modifications in 1995, and they reduced the drag another 6.0%. Remember, it gets harder and harder to find large areas for additional drag reduction as you go faster because all the low-hanging fruit is gone.

I read Stan Miley’s data about plenum design and talked with aero engineer Dave Lednicer of Computational Fluid Dynamics at the time regarding efficient engine cooling.

Mass flow = inlet area x a/c velocity x plenum efficiency

The Lycoming installation manual for a 360-cubic-inch engine indicates 6-1/2 inches of H2O delta p across their cylinders will adequately cool. In a max climb, below 4 inches may produce heating. I did the mass flow calculations years ago and decided a pair of 4-5/8-inch inner-diameter inlets should cool my engine. In fact, they overcool except at wide-open-throttle (WOT) and in maximum-rate climbs. I originally estimated the inlet size to be 10% smaller in volume, and knowing I’m not an engineer, I increased the estimated size. So, they work in the most adverse conditions and overcool in cruise.

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The plenum seals to the engine and essentially eliminates all leaks. By achieving maximum pressure recovery from free stream velocity, it produces more potential for cooling.

Plenum Considerations

1. Inlets should be positioned away from the spinner to prevent or decrease possible reverse flow out of the inlet on the side nearest the spinner. This is due to the pressure differential across the inlet and the root region of the prop, which suffers a momentum deficit and thick boundary layer, especially with the cowl shapes necessary for compact hubs.

2. Using a diffuser to slow the incoming air and increase pressure (Bernoulli’s principle) decreases cooling drag losses. Up to 10 degrees appears to work, however, I use 8 degrees because if the flow separates, it’s equivalent to choking the inlet. A nicely sealed smooth transition into the plenum results in lower losses.

3. The plenum can be sealed directly to the engine with soft foam weather stripping on the plenum edge and essentially eliminate all leaks. This achieves the maximum pressure recovery from free stream velocity, which will increase the delta p across the cylinders producing more potential for cooling.

4. The inlet area in the throat of the inlet is called the A1 area. The diffuser starts here and proceeds toward the cylinders. At some point, you can no longer maintain the conical expansion, which is effectively the end of the diffuser. This is called the A2 area. The area over the cylinders and heads above the fins is the A3 area. The general formula for a good plenum is that the A1 area conically expands at 8-10 degrees to the A2 area, then the A3 area above the heads and cylinders is equal to the area of the A2 location. The A3 area includes only the area above the cylinders and heads—not the case. This is a very oversimplified explanation of Stan Miley’s work.

5. Of course, all the principles are subject to best application for your installation.

Oval-shaped inlets like those on this Grumman F8F Bearcat produce lower losses but are harder to build and seal than round inlets. (Photo: Spartan7W [CC BY-SA 3.0,])

Inlet Considerations

1. Generally, the inlets or inlet cowl interface should have round-lipped shapes about the size of a nickel or a Kuchemann A30 shape. Both will work well.

2. Oval-shaped inlets will produce lower losses, as the shape of the plenum over the cylinders is more rectangular and requires less energy to change flow shape. Look at the wing root inlets on the Grumman F8F Bearcat and notice they are slightly below the leading edge of the wing. Obviously, they had something in mind. Tight corners are bad. Oval inlets are just harder to build and harder to seal than round ones.

3. A rounder lipped inlet may also reduce the possibility of inlet stall.

4. Oversized inlets do not produce more cooling; they only increase drag around the inlets.

5. Inlets are easily sealed to the diffuser area of the plenum using 1/4-inch bi-directional stretchy wetsuit material. Just make the seal about inch tighter, so it will stretch over the diffuser without any leakage. The gap between the plenum and inlet doesn’t need to be greater than 1/4 inch.

Inlets are easily sealed to the diffuser area of the plenum using quarter-inch bi-directional stretchy wetsuit material.

Augmenter Outlet Considerations

1. Exhaust velocities are higher than our IAS, so we should attempt to recover as much as we can for thrust.

2. Some have advocated crushing the exhaust pipe to increase the velocities, and this may work. However, I decided to get the most I could without potentially causing some possible adverse condition for the engine. I selected a 4-into-1 exhaust system, which produces the smallest cross sectional area for four exhausts.

3. Direct the exhaust flow as close to the free stream direction as possible to recover maximum thrust. Considerations for increased noise and heat can cause some to abandon this avenue. However, by proper cowl exit design, you can use the high exhaust velocities as a type of augmenter.

4. Augmentation can be accomplished with multiple exhaust pipes as well; it’s just a little tougher.

Cooling Tunnel Considerations

1. NACA studies indicate cooling air should emerge with free stream air as close as possible to parallel at the highest velocity achievable.

2. A shallow-angled, straight-sided ramp is very effective. For cooling air, a width-to-depth ramp ratio of 7 to 1 is recommended.

3. For augmenters the distance around all sides is best at 1 to 1.

My first attempt at an improved outlet is still in use today. The ramp is attached with POP rivets tangent to the cowl. The ratio of the ramp is about 7 to 1, shallow angled, and it diminishes to zero at the wing spar. The area around the exhaust pipe is 1 to 1. The exhaust collector is cut off inside the cowl to aid the extraction of the cooling air. From examination of the exhaust flow on the bottom of the plane, you can see the straight flow.

A shallow-angled, straight-sided ramp can be very effective. For cooling air, a width-to-depth ramp ratio of 7 to 1 is recommended.

The area of inlet on my RV-4 is 34 square inches, and the exit area is about 26 square inches, for a ratio of 1 to 0.76. With the combination of systems I have on my RV, I have the following:

Upper plenum pressure @ 200 knots: 24.5 inches of H2O (using a Magenhelic inches of H2O gauge).

Plenum efficiency: 24.5/26.63 (standard day, H2O at 200 knots) = 92% recovery

Delta p across the cylinders:

87 knots = 5.0 inches
104 knots = 6.8 inches
174 knots = 14.8 inches

From 104 knots to 200+ knots, the delta p is almost a straight line, and the delta p is greater in the power-on condition then the power-off condition at all speeds, indicating that exhaust augmentation is helping to extract the cooling air from the lower plenum. This aids in cooling the engine during sustained full-power climbs at lower airspeeds.

Controlling Airflow

Air exiting or entering places it isn’t supposed to causes drag. The airlines recycle air in the cabins for various reasons but one is to reduce drag, so only use what you need. It takes energy to slow the air down and then reaccelerate it as it enters the free stream.

Half-inch machine wiping felt is used to seal the area between the spinner and cowling. The felt rubs against the propeller hub.

Seal all possible gaps. The area around the propeller is often neglected. The flow at the back of the spinner is poor, and the boundary layer is thick. If air were colored, you’d see an arching spray coming from all around between the spinner and front edge of the cowling. This is because the pressure in the lower plenum is exiting into this area and causing drag.

I sealed this area with a 1/2-inch machine wiping felt from McMaster-Carr that rubs against the propeller hub. It is beveled at the inside rubbing edge (it cuts well with an angled dye grinder with a coarse abrasive wheel), so it’s a little tighter inside the cowl, and any pressure will help it seal. The felt was saturated with silicone and it has worked well for over 1500 hours. The felt is sandwiched between two pieces of aluminum and screwed and sealed to the cowl with a bead of RTV. If your cowl doesn’t permit sealing to the prop hub, you can easily seal to the angled front surface of the starter ring. Be creative.

Another area often overlooked is the fuselage air entering or exiting through the empennage. This can be sealed easily—it just requires taking your plane apart, so maybe do it when you’re already working in that area. Simply get out a sewing machine and sew a cone. You do have to figure out where the control tube goes through the bulkhead because you want the panels on the cone to produce the least amount of load as the control tube goes through its motion.

The canopy rails in my plane are sealed with “P” molding to prevent leaks. The small hole to the right of the hinge is sealed with a small piece of foam rubber bonded to the canopy.

You don’t have to secure the cone to the tube; as a matter of fact, that may be a bad idea in case it acts like a diaphragm with the changing air pressure. I used synthetic leather because it’s soft and air tight. The seal to the tube is just 1-inch-wide Velcro (the fuzzy side snug to the tube) sewn in snugly so it prevents air from flowing through too easily. The cone is bonded to two aluminum rings that are bonded in place to the bulkhead. The rings may need to fold to get them in place, so they might have to be split. The bulkhead is sealed with a bead of RTV to the skin, and SunMate pressure sensitive foam is used to seal the rudder cables. SunMate is not very thermally sensitive foam. You’ll never know the tube seal is there.

If air exits anywhere it’s not supposed to, it produces drag. You can use a decibel meter to check for leaks around your canopy while you’re flying along, and any area producing drag will be quite noisy.

The canopy rails in my plane are sealed with “P” molding to prevent leaks. Although it’s difficult to see in the photo, at the rear of the canopy hinge is a small hole that is easy to seal with a small piece of foam rubber bonded to the canopy.

I sealed flow-through air on control surfaces by contouring a piece of SunMate pressure-sensitive foam. Simply cut it to fit the void it goes into, and make the fit a little snug. It holds in place by conforming around the rivet heads. Do not use temperature-sensitive foam here. The high-pressure air from free stream flow normally enters the gap between the stabilizer and counterweight arm and flows toward the hinge line. It then turns 90 degrees around the corner and flows out of the gap between the stabilizer and control surface, which produces drag. The simple addition of the foam prevents this. Also, a small piece of self-adhesive soft weather stripping can prevent flow through the vertical stabilizer and the rudder counterweight.

A small piece of self-adhesive soft weather stripping prevents flow through the vertical stabilizer and the rudder counterweight.

I’ve flown with these changes for thousands of hours with no problems. But like any modification, you need to evaluate its safe application before using it on your own plane.

These mods are also small and might not seem to make much difference. However, if the changes are scientifically sound, they all add up and will help reduce drag.

If you’re flying an RV with Van’s older Hoerner wingtips, you might consider changing to the new wingtips with a higher aspect ratio that produces smaller wing vortices. These are common now, but when I changed mine in March 1997, the new tips were marginally faster, perhaps 2 mph.

I also increased my ram air pressure a little with my first attempt at a Bernoulli-principle ram airbox with corresponding sigmoidal shapes that were calculated to smoothly increase volume. One was used as a diffuser cone over the end of the air filter, and by this method I was able to get a little ram boost through the filter. The other shape was the airbox having a 5-degree divergence until it got to the K&N filter, but this was not optimal.

During this time period, I added 3.4% more drag reduction from all the modifications. By March of 1997 the total drag reduction was about 17%.

These wingtips are common now, but when I added them in March 1997, they were perhaps 2 mph faster than the original Hoerner wingtips.

Fastback Modification

By the year 2000 I did the last major drag change, a lower canopy and fastback deck. It’s very important to make certain that the canopy totally seals. Remember, air entering and exiting is bad, so it could be that you go through the entire exercise and not add anything.

The last major drag change was a lower canopy and fastback deck, which added an estimated 4-5 mph. At this point, it was getting very hard to find additional meaningful changes.

My canopy is completely sealed all around. You can place a business card anywhere on the back edge of the canopy skirt, close it, and it will still be there after a high-speed run. Then I painted the plane. It started out polished and had remained that way for eight years, but there was strong evidence out there that painted planes were faster, maybe 1 mph or so. That was worth $5000—not hardly. But I was tired of polishing, and I had melted three canopies during that time. Low-wing planes should not be polished due to risk of melting your canopy. It took me a long time to learn that. The last Sun100 race was in 2000, and the last modifications added another 9% of drag reduction or 6 mph at top speed. I believe that the fastback added about 4-5 mph of that, which is amazing since it was the last modification. It was getting very hard to find additional meaningful changes.

The rear canopy seal is 1/32-inch-thick self-adhesive felt. It not only seals, but also prevents chaffing the paint.

The rear canopy seal is part of Dave Howe’s entire fantastic technique for building the fastback. The seal is from McMaster-Carr and is 1/32-inch-thick self-adhesive felt. It not only seals, but also prevents chaffing the paint. I’m still using the original felt, placed perhaps 1800 hours ago. Careful work can produce a no-leak seal.

I also improved the elevator inboard fairings to help decrease the turbulence off the empennage (see “Putting the Experiment in Experimental,” KITPLANES, March 2017).

During all this time, I documented what I changed and what it produced, and I use the same formulas and procedures today—but now with autopilots and downloadable data you can analyze on Savvy Analysis, it is much better and easier.

I am still working on the plane, but my efforts have been redirected toward efficiency. It now has dual Light Speed Plasma III electronic ignitions by Klaus Savier, driving an electronic fuel injection SDS EFI by Ross Farnham with a much better induction system. This has worked really well (see “Putting the Experiment in Experimental,” KITPLANES, March 2017).

The business card closed into the rear canopy overlap is still there after a wide-open-throttle test run.

Current Efforts

Since I first tried to increase ram pressure, worthy scientific articles have become available, and more empirical evidence from good experimenters like Tom McNerney has come to light. It’s worth trying to improve your induction because on a 360-cubic-inch Lycoming, every 1-inch increase in manifold pressure (MAP) is good for about two hp per cylinder (rule of thumb). This has been borne out by the performance formulas of predicted speed increases from increased hp. So when you can go up another 1000 feet in altitude, run the same MAP and engine settings as you did at the lower altitude, you will be going about 2% faster. Remember, I’m not an engineer, so most things are rules of thumb and trial and error. Isn’t experimenting fun!

A very easily read article, “Best Bell” by Gordon P. Blair & W. Melvin Caboon , describes the elliptical profile velocity stack. It even includes the formula for the best bellmouth ELL-23-23-49-3 with a picture. So, in order to build the best ram intake you can for your application, first design the optimum elliptical velocity stack, then see how much room you have. Next, design the largest 8-degree divergence Bernoulli-principle airbox that will fit. This will decelerate the airflow and increase the pressure as much as possible in each case.

I made molds with a cheap Harbor Freight woodworking lathe, using aluminum templates to guide the final shapes for the molds. The result was a ram induction airbox that works much better than my earlier attempts.

A simple way to attach the new airbox parts to the throttle body.

This airbox, coupled with the improved, very impressive induction tubes and cold induction plenum from Kevin Murray, president of Sky Dynamics, is the induction system I am currently running. Sky Dynamics makes incredible products that when you look at them you just say, “How did they do that?” Just look at the formed lightweight stainless steel intake plenum (it’s one piece) and tapered curved stainless steel induction tubes. Amazing! They also made the exhaust tubes that are 4 into 1 with cyclonic firing order and nearly equal lengths.

With its 8-degree divergence, the new Bernoulli-principle airbox works much better than earlier attempts.

With the modifications described, and having an engine that now produces about 195 maximum hp at 2825 rpm, I can maintain 16.4 inches of MAP at 17,500 feet while getting 37.6 mpg at 209 mph TAS turning 2230 rpm. Not too bad for a fat-winged RV that is now producing about stock hp for a 360-cubic-inch Lycoming when running 2700 rpm. So what’s next? Only your imagination can stop you.

I remember when I was out there trying things. I really appreciated all the help everyone gave me, and it really helped to see what they did or how they did something. I hope this article can get your creative juices flowing. What’s really going to be neat is to see what the next generation of ideas will be.


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