January 18, 1997 High Wing or Low Wing?

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by

Ed Kolano

About the Author ... Ed Kolano's 20-year aviation career has encompassed military, general aviation, test flying, and writing. His special expertise is in evaluation of aircraft flying qualities.

"So what's all this debate about high wing versus low wing? When I get on an airplane, I have just one question — 'Is THIS wing good enough?' If it isn't, then I say let them get another one. So far, the flight attendants have always said, 'Yes,' and that's good enough for me."

"Wait a minute, Jerry. How do you know that plane wouldn't fly better if that low wing were up high? I'm telling you, the wing is an important piece of an airplane. Do you really want something that important in your basement, or do you want it safely tucked away in the attic with the heirlooms and family treasures?"

"There's no basement. No attic. It's a wing! I just want it to work. I don't care where it is."

"Oh, you'd care. You'd care a lot if that low wing was actually a high wing — just in the wrong place."

"Ah, that's why I always check. First I ask if this particular wing is good enough. If it passes that test, I next look inside the plane. If all the seats are on the ceiling, I'm out of there."

High wing or low wing? A century-old question which has found a recurring place in the rainy day hangar schedule of topic du jour. Man's first leaps off hills were in high wing gliders, mostly because foot-launching a low wing glider required an aileron roll on takeoff. Since ailerons hadn't been invented yet, most aviators opted to just hang below.

Today, however, there is a choice. Which configuration a pilot selects might be based on something as simple as what the local FBO has to offer, or it might be the result of a careful analysis of the pros and cons of each. So, let's see which is better.

First Things First

Pre-flighting the high wing airplane (HWA) has several advantages. Damn, bumped my head! Okay, that's not one. I can inspect the landing gear without getting my knees dirty. On the low wing airplane (LWA), to get a really good look at the tire and brake probably means doing it on all fours. Humility is generally a good thing, but physically humbling oneself before an oleo deity is a religion I'm not anxious to learn about.

Removing the tie down ropes is an eye level affair in the HWA. This is a significant factor if the previous renter allowed his enthusiastic young passenger to apply the theory of the-bigger-the-knot-ball-the-stronger-the-security. Untying my LWA, I have the option of quadrapeding my way around the gear or displaying a brief Darwinian insight and temporarily becoming bipedal for my journey to the other side of the landing gear.

Likewise, taking a fuel sample in an HWA is a fairly gentlemanly event. With the fuel tank low points being several feet above the ground, this can be accomplished while merely hunching over rather than using the grass stain preventing squat technique of the LWA.

Turn on the master, check the fuel gauges, and both needles are pointing left. Of course Mr Previous didn't refuel; he was too busy supervising the tie down. Let's push it over to the pump. These angled things between the wing lower surface and the fuselage are called struts. Must be because it allows the crew to push on them while strutting with dignity over to the fuel island. Note: Symmetry is important here; a single pilot pushing a strut is susceptible to deja vu. Since this LWA has no tow bar, I have to pull the thing by the prop like a stubborn mule. Of course, if I have a traveling companion, we could each haul a wing tip. Note: Symmetry is important here; a single pilot hauling a wing tip is susceptible to deja vu.

Find the step stool, and place it in front of the HWA's leading edge. Climb, remove the fuel filler cap and look in. Can't see in. Damn the short step stool. Damn the short-legs DNA. Finger check - gauge was right, it needs gas. Now is when I usually smile at my traveling companion, hoping to convey the fact that everyone, not just I, looks this awkward refueling an HWA. Attach the grounding strap, and lug the hose over. Climb the stool with the hose, aim, and squeeze. No point pausing to visually check the fuel level; I can't see it anyway. With the LWA, it's a look-down maneuver. No 100LL finger cologne, and Ms Companion can sit on the step stool.

Okay, let's get aboard. Step over the LWA's "NO STEP" flap onto the black stuff. Ouch - shin! There has got to be a way to make those trailing edges thicker or at least softer. Next time we'll take the HWA where we can just open the door and get in. Well, we might have to step ahead of the main gear, but remain behind the strut or the other way around.

Who Cares? Let's Fly!

I point my HWA into the wind at the hold short. Run-up looks good. Things have been going smoothly now for almost five minutes. Now, taxi around some more to get that wing out of the way to see if anyone's on final. The LWA allows a more suave post run-up technique: I lean over to take a peek up final, shoot my companion my best lounge singer smile, and call for takeoff.

Airborne, that field of view issue is there still. In fact, this is probably the major determinant of HWA/LWA preference among general aviation pilots. In the HWA I can fly over my companion's house at whatever altitude the FAA says I must have plus 50 feet. She can look virtually straight down at it. Points. Isn't this half the fun of flying - to look at things on the ground? The LWA lets its occupants look at the ground, but they either have to look in front of the wing or enjoy steep turns.

Of course, HWA pilots can enjoy steep turns also. That high wing, however, pretty much hides what's ahead during turns. Obviously, no pilot should begin a turn without first ensuring a clear sky in that direction. Still, being prevented from watching where I'm going makes me nervous. This is particularly true in the landing pattern, especially at uncontrolled fields. Some HWA have skylights which help, but they're usually too small or located such that the geometry works only during turns in one direction.

It seems more people fly during the summer. The LWA's field of view advantage can be a thermal disadvantage. HWA pilots enjoy the shade of the high wing. LWA have more of a greenhouse effect, especially those with bubble canopies.

Gravity is a friend of the engine-fuel relationship in the HWA. It may obviate the requirement for a fuel pump in some planes, and may provide sufficient pressure to get home should the fuel pump fail in others. To make gravity work in the same manner in the LWA, the pilot must roll upside down. Then the gas is higher than the (no-longer-running) engine, but then the little hole it has to go through to get to the engine is higher than the gas . . . scratch that idea.

OK, But What About Dihedral?

The dihedral effect is the airplane's rolling response to a sideslip. "Slipping it in" to steepen a landing approach usually requires cross-controlling the plane (left pedal, right stick). Right stick is needed to prevent the plane from rolling left. It wants to roll left because left pedal deflects the rudder trailing edge left, which causes a left yaw. Holding the pedal maintains the left yaw, and this means the relative wind is coming from somewhere right of the plane's nose. How far right is called the sideslip angle. Airplanes with positive dihedral effect roll away from the sideslip. In this case, the plane has a right sideslip, wants to roll left, so right stick is needed to prevent the roll.

HWA have an inherent positive dihedral effect contribution from the wing-fuselage arrangement. In balanced flight (which means there's no sideslip on the airplane), the air either travels over the wing upper surface or under its lower surface, parallel to the fuselage in either case. Picture the HWA in a left yaw, so the sideslip is to the right. What happens when all those air molecules, which would slide right under the wing lower surface, encounter the fuselage? They pile up in the inside corner formed by the fuselage and lower wing surface and cause a couple of things to happen. First, that "piling up" forces a bunch of air over the wing, increasing the inboard wing section's angle of attack. That means more lift on the right wing, causing the plane to roll left The downwind wing-fuselage interface experiences the opposite effect. Instead of piling up, the air molecules are forced to spread out. Air passing across the airplane's nose dives under the wing toward the low pressure on the downwind side of the plane. Since the air is moving downward, the angle of attack of the downwind wing's inboard section is reduced and so is its lift. Less lift on the left wing creates a tendency to roll left.

All right, you bleeding shin, grass stained knees LWA pilots, here's how it works for you - not good. The same piling up of air molecules that the HWA experienced also occurs during a sideslip in a LWA, but it occurs on the upper wing surface. In a right sideslip, air dives under the right (upwind) inboard wing section, reducing its lift. Air passing under the nose is swept up and over the inboard section of the downwind (left) wing, increasing its lift. In both cases the change in air direction causes a right rolling tendency in a right sideslip or a negative dihedral effect.

"Horsepucky! My LWA rolls away from sideslip." This may be a true, albeit sanitized, statement. Most LWA have a couple of degrees of geometric dihedral (wing tip higher than wing root) designed into the airplane. HWA may have some geometric dihedral as well, although usually not as much as their LWA counterparts. Remember, it's the overall effect that the pilot experiences. He cares which way the plane rolls when in a sideslip, not the individual contributions of geometric dihedral, wing-fuselage interface, vertical tail effect, propwash, etc.

What A Drag!

There are some aerodynamic drag differences between HWA and LWA. Interference drag arises whenever two or more objects are close to each other in an airflow. The total drag can be more or less than the sum of the drag of the individual objects. The less-drag case was used by a close, personal friend to get better gas mileage driving to Florida. By remaining one car length behind 18 wheelers, the drag on his Vega may have been reduced sufficiently to raise the miles per gallon to what he would have gotten had all four cylinders been functioning. (Disclaimer: Don't try this; he was a professional bad decision maker.)

Regarding airplanes, wherever objects meet, there's likely to be a drag increase. Such is the case at the wing - fuselage junction even in balanced flight. This is because the boundary layers (air close to the surface) of the wing and fuselage interact and thicken. This interference drag is worst when the two surfaces meet at acute (less than 90°) angles. Fillets, or rounded inside corners, are often used to minimize the effect.

In the HWA, the interference drag arises from the interaction between the boundary layers of the fuselage and lower wing surface. Since the boundary layer on the lower surface is quite thin for most normal flight conditions, the effect is less than for the LWA case. It's the upper wing surface and fuselage boundary layers that interact on LWA. The upper surface boundary layer is appreciably thicker, and therefore usually has more interference drag associated with its interaction with the fuselage boundary layer.

Add struts to the HWA, and you've created four more interference drag possibilities. Of course struts themselves are rarely aerodynamically invisible, except for that pair I seemed to have misplaced.

These comparative arguments assume identical wings and fuselages with only attach point changes. Clearly, or maybe not so, we're in the theoretical here. So, attempting to compare boundary layer effects between a Cessna 150 and Piper 140 at the airport next weekend, assuming it could be done, is really an apples and not-apples situation.

Tradeoffs are abundant. Perhaps the struts enable a weight saving in the wing structure of the HWA which is more beneficial than the drag penalty. LWA might have shorter main landing gear, because they can protrude straight down from the wing. HWA gear usually extend to either side of the fuselage to achieve a reasonable wheel track - that's extra stuff hanging in the breeze.

The relative position of the wing and horizontal tail is significant. Should it be above the wake of the wing as the airplane approaches stall? Is it better below? Do you want a stall warning that consists of natural airframe buffet caused by disturbed air from the wing impinging on the tail. Perhaps you are content to rely on that little tab in the wing's leading edge and the light or horn it's supposed to trigger. If the wing position dictates a horizontal tail at the top of the vertical stabilizer, how much stronger, and heavier, must the vertical stabilizer be?

Enough! Take Me Home!

Let's take one more look at salvaging a high final approach by slipping both machines. If the HWA requires a lot of bank angle for the slip, that high wing could obscure the runway. While visual impediments may not be an issue in the slipping LWA, bank angle tolerance may. The LWA which is upset by a gust in roll just prior to touchdown is likely to strike a wing tip at a lesser bank angle than its HWA counterpart.

In order to touch down, both types of airplane must get pretty close to the ground, and that raises the question of ground effect. Away from the ground a lift-producing wing alters the airflow. The tips generate vortices as higher pressure air under the wing rushes around the tip toward the lower pressure upper surface. There is also an upwash of air approaching the wing's leading edge and a downwash or downward flow component imparted to the air leaving the trailing edge. Near the runway the ground inhibits the vortices and reduces the downwash angle. The result is less induced drag and less AOA required to produce the same lift compared with the same flight condition out of ground effect. There are other ground effect consequences, but we'll stick with these two for this HWA/LWA comparison.

Ground effect generally comes into play about one wingspan above the ground. The closer the wing gets to the ground, the more dramatic the effect. Since LWA wings generally get closer to the runway than HWA wings, the ground effect is more evident in LWA. So, the LWA will experience a larger drag reduction then its HWA counterpart under the same conditions. This is why LWA have the reputation for "floating" in the landing flare. The increase in lift for the same AOA can have the effect of making the flight path angle more shallow as the wing gets closer to the ground. This may be one source of the "self-flaring" reputation of some LWA.

Made it down. The only thing standing between the end of the runway and the tie down spot is taxiing through this high crosswind. The HWA probably feels more susceptible to the crosswind than the LWA. The dihedral and ground effects are still working although with much reduced consequence. It's doubtful that the height of my HWA wing is sufficient to allow a vertical gust to develop under it. Nor is there likely to be a noticeable suction effect under my LWA wing. Maybe there's a more persistent roll activity in the HWA because the center of gravity is higher. Maybe it just seems that way because the wing is always in view, and I notice vertical wing tip motion more. I tend to look over the LWA wing, so my perception is not influenced visually. And maybe a dozen other things, but as long as the wind is within the operator's handbook limit I should be okay.

So...Which Is Better?

It depends. That's not the cop-out answer it may seem. While some pilots will always opt for a particular arrangement, others see it as mission dependent. Pipeline patrol might be a good HWA candidate. The pilot's job is to look at the pipeline, which is generally located below his cruising altitude. The HWA a permits maximum eyeball-on-the-pipe time. Pilots operating from short fields may want to minimize the LWA float. Formation flying might be the kind of situation where an LWA is preferred. Where a typical formation has the wingman looking up, left, and forward at his lead's airplane, a wing located down, left, and aft seems to be in the correct place.

Die hard HWA advocates offer a couple of additional pieces of substantiation to support their cause. A forced landing on a road with guard rails or field with scrub bushes might be less damaging in an HWA since the wings can pass over these obstructions. Finally, at least one HWA proponent has adamantly stated he'll not change his mind until he's shown a successful low-wing bird - real bird . . . with feathers.

Whether the HWA or LWA is better to learn in or teach in remains a good question. Every instructor and student must weigh the pros and cons. If field of view is a bigger factor for the student than ease of cockpit entry, an LWA might provide a more conducive learning environment. If an instructor has 500 HWA hours and much fewer LWA hours, he may be likely to provide better instruction in the HWA.

So, having debated an arbitrary number of contrasts between the HWA and LWA, the following can be concluded. It comes down to personal preference, and it's okay if that preference is driven by emotion, airplane availability, or mission requirements. After all, the Wright brothers couldn't see eye to eye on this issue either, and look at how they solved it.