Winging It

I was at a fly-in and had one of the builder-pilots showing his Long EZ to me, and it was beautiful. He pointed out the smoothly finished fillets and fairings and the tight gaps for the control surfaces. It was a fine example of good craftsmanship. Then the fellow showed me his buddy’s adjacent Long EZ and started pointing out all of the less-than-stellar craftsmanship. The gaps were uneven, the fillets were lumpy, and the winglets were laid back at the wrong angle, just to name a few items that I can recall. This pilot then finishes up with: “And the worst part of it is that his airplane is just as fast as mine is!” To say that he was both mystified and peeved is an understatement. I nodded in sympathetic understanding, but inside my head was a loud, “Duh!”.

What I chose not to mention to him is this: Sometimes the air doesn’t care. In fact, “The air doesn’t care” is a phrase that I’ve used fairly frequently over the last 30-plus years. For what it’s worth, I think I invented the phrase; if so, I’m going to claim trademark. “The Air Doesn’t CareTM”. And copyright too, just to be sure. If I didn’t invent it, disregard. 

What I mean by this somewhat snarky-sounding phrase is this: sometimes a feature matters in terms of drag (the air “cares”), and sometimes not. In the case of a gap, whether it is longways to the flow of the freestream air or transverse makes a big difference. When we were designing the 777 at Boeing, every transverse gap or butt joint in the skin had a drag penalty assigned to it. Given that medium and long-haul airliners spend so much of their flight at cruise, the drag of those small gaps adds up and require that much more fuel for a given flight. I seem to recall being told the pounds of drag per linear inch of transverse gap of a given width, and its equivalent fuel penalty. After seeing the drag and fuel cost of those gaps, I began to appreciate the attention to them. Given that airliners are preferably flying most of the day, those annual fuel amounts add up a lot over the course of the years.

What makes me chuckle when I fly these days is how we were sweating tiny bits of drag all over the airplane, and now nearly every bit of that has been more than offset by the drag of the satellite antenna fairing that has been grafted onto the top of nearly every fuselage. I think of it as the wi-fi drag penalty.

In the initial design work on the 777, we did a lot of studying of and scaling up of the 767. In looking at the 767 drawings, it appeared that the forward skin lap joints protruded outside of the radome mold line. I was given a “field trip” pass to the Everette factory to go investigate. After some wide-eyed walking around in that cavernous facility, I finally found where the radome was installed and saw that, sure enough, there was a forward facing step of the skin on the outside of the skin laps. It was only for four or five inches at five locations (if I’m recalling correctly) around the radome, but remember how much we were sweating the details. 

I contacted the aero group and they looked at it and came back with the following explanation: that while the forward-facing step looks bad, the air at that point is still getting pushed aside by the blunt nose of the radome, still hasn’t accelerated much, and wasn’t/isn’t enough of a drag penalty to warrant trying to redesign (which would also mean re-certifying) the radome. Again if I am recalling correctly, the windshield wipers aren’t as drag impacting as one might think due to their location. The fellow went on to say that were we talking about drag back behind the cockpit windows, the air at cruise would be getting close to high sub-sonic as it finished transitioning from the nose to the constant cross-section part of the fuselage, and in that case the drag would be significant. For that fuselage station the air would indeed care, but as it turned out, the air didn’t care for the skin laps common to the radome.  

I was responsible for forward fuselage structure around the 777 nose wheel well, and encountered some odd items appearing in the overall CATIA assembly model. The anomalies turned out to be a pair of blade antennas for the microwave landing system (MLS), should an airline want to add that option. The antennas were falling right on top of an intersection of primary structure and would need to be moved an inch or two in order to be installed. When I first contacted the antenna Design Build Team (DBT), they weren’t keen on having to move their antennas, but since primary structure is, well, primary, they weren’t going to win that battle. 

In the meantime, young me had also pinged the aerodynamics team about orienting the blades to the local airflow, and as it turned out, canting the blades inward (like toe-in for tires) a couple of degrees would align them with the local airflow and reduce drag. The antenna guys didn’t like any of it, but after running the numbers, confirmed that moving the blades and canting them inward a bit meant only a few percent degradation of the signal versus ideal, versus a non-trivial amount of drag reduction. Based on that, the new antenna locations were approved. Later I found out the aero guys were quite pleased that anyone had come back to them with any drag related questions after the initial loft lines were established. To any of the airlines that installed the MLS antennas, you’re welcome. 

Somewhat related comment: next time you board an airliner through the forward door, notice that there is a drip rail over the door to prevent rain runoff from coming in while boarding. And the angle of that strip of metal? Dictated by the local airflow to minimize drag. 

For our own homebuilt aircraft, it is important to know where the air cares and where it doesn’t, or doesn’t care much. For instance, the circular tube of a nose gear: already draggy enough in the free-stream, and more-so for being so close to the propeller; it can have a drag coefficient around 1.0 according to the interwebs (as I’m writing this, I’m also watching football, and not going to look for my old textbooks…). Now lets put a tapered fairing on the back of that round shape so that it looks more like a teardrop, and its drag coefficient drops closer to 0.10 or less. That is a non-trivial drag improvement, and kudos to the enterprising individual who started making a fairing to attach to the back of RV nose gears. 

The amount of dragginess of a round object isn’t intuitively obvious to most of us, and came as a bit of a surprise way back in my fluids classes. There is a reason that vintage aircraft with round flying wires have so much drag. And that big round light on the top of so many Cessna fins? Its drag can nearly equal that of the whole fin and rudder. Those gas caps and filler necks protruding from the top of a typical Cessna 152 or 172 wing? Besides adding drag, they blank out a fair amount of the wing behind them with very turbulent flow. 

Ideally, all aircraft rivets would be flush rivets to minimize drag, so why are button-head rivets seen so widely? Without the additional steps required for dimpling or countersinking, the protruding rivets are less expensive for manufacturing. Additionally, most of them are within turbulent flow or the boundary layer, and their additional drag is minimal. But one location where the drag benefit is enough to warrant flush riveting is on the first 25% of the wing chord, whereas after quarter-chord the flow is turbulent enough that the button-head rivets become a don’t-care in terms of drag vs production costs. 

In a similar vein, gaps for control surfaces on legacy production aircraft tend to be rather large and draggy. Your Wichita or Vero Beach spam can would have significantly less drag just with tighter gaps and/or gap seals and fairings on the previously mentioned lights and fuel necks. For this reason, guys running at Reno apply a lot of tape to seal gaps as much as possible to eke out every last bit of speed. On our homebuilts, control surfaces tend to be more of a compromise of a tight gap versus ensuring no possibility of binding between the control surface and mounting structure. In another area, wheel pants can be relatively heavy while adding only four to five mph in cruise so maybe they aren’t worth the weight penalty, especially if you don’t spend much time at your cruise speed. For me, looks alone will make wheel pants worth it, plus the extra cruise speed. d. 

In another discussion, a guy was very surprised that cooling drag was a thing. In his mind, drag was associated only with the air flowing around the outside of the airplane. But the air flowing through the cowling also incurs a significant amount of drag as it enters the cowl, slows, flows around the cylinders, through oil-coolers, and out the exit. Cleaning up drag, particularly cooling drag, was the principal business of Roy LoPresti’s Speed Merchants and its 35 different STCs for drag reduction. 

In stark contrast are the STOL planes being flown without cowlings. At first I was aghast at the thought of all the drag of the firewall plus the forward facing lip around the firewall where the cowl would attach. And then after some thought nodded to myself that all that drag was beneficial for a short landing and a don’t-care for take-off and while in the pattern. Then when the competition ends, put the cowling back on for the cross-county back to the home airport. Smart. 

One bit of cooling drag that engineer-me doesn’t care for is the oil cooler at the front of the engine, as shown in one of the photos accompanying this column. The air coming into the cowling will pile up in front of the air dam and form a high pressure area that will then flow down through the oil cooler; in that respect, mission accomplished.  However, that same air dam will create fluidic chaos with turbulence behind it before flowing to cylinder numbers 2 and 4.  The other photo shows an air duct in the exact same location, but without the air dam. In this case the air will flow into the duct, but with far less turbulence for the air flowing back to cylinders number 1 and 3.  

My preference would be taking the air from the back of cowling after it has flowed past the tops of the cylinders and before being forced down through the jugs; we saw a good example of this layout in the August 2025 issue of Kitplanes. Based on articles I’ve read, the inlet air is going to hit the back of the cowling or plenum still pretty much at ambient temperature, so any air bled off for the oil cooler will still be cool air. From there it can flow through the oil cooler and out the bottom of the cowling, leaving plenty of less turbulent air remaining to flow down through the cylinders.  

There are plenty of other drag examples to be given, but such discussion is well beyond the available space of this one column. The point to all of this is that one can usually do a lot of drag clean-up on a given airframe, and the beauty of reducing drag is that it is usually easier and cheaper to decrease drag than to add horsepower, given that drag increases with the square of the speed.  If you start down the route of drag reduction, it will definitely help to give your airplane a long look with a critical eye for potential sources of drag, and do your homework to know where the air will care and where it won’t.