Very often, I see people write that low-wing planes like my Piper Warrior have a longer flare (i.e. they float longer) than high-wing planes like the Cessna 172, usually based on the argument that lower wings benefit more from ground effect.
In fact, that does not seem to be the case: the numbers in the POH all indicate that the Warrior actually has a slightly shorter flare and shorter landing distance than the 172. My own personal experience is more dramatic: I find that the Warrior’s flare decays very rapidly at the end compared to that of the 172, and it took me a while to learn to land the plane smoothly without dropping a foot or two at the end. Other Piper owners have reported similar experiences, though, obviously, the differences are much smaller than going from either the Warrior or the 172 to a more heavily wing-loaded plane like the 182. The condition and rigging of any individual plane will also make a huge difference — if someone is flying battered, badly-rigged 172s and then switches to a clean, well-rigged Warrior, the Warrior will certainly flare better.
So what’s going on? Why would a plane with lower wings and about the same gross weight float less, when the wings are closer to the ground and should benefit more from ground effect? Here are two possibilities:
- The Warrior has a wing loading of 14.4 lb/ft^2, vs 13.8 lb/ft^2 for the 172P. That’s not a huge difference, but it will affect the plane’s floating ability. By comparison, the Cessna 182P has a wing loading of 16.9 lb/ft^2 (note that all of these apply at maximum gross weight, not with just one or two people on board).
- The Warrior’s wings have a lot more dihedral than the 172’s wings. Low-wing planes need more dihedral to get the same roll stability as high-wing planes, and the dihedral creates a lot of drag, as well as putting the wing tips (though not the roots) fairly high off the ground. Both the dihedral on the Warrior and the wing struts on the 172 cause drag, but I suspect that the struts produce only parasite drag, which is fairly constant, while the Warrior’s dihedral affects induced drag, which can increase dramatically near the stall (hence the abrupt end to a Warrior’s landing flare).
Basically, these two factors overwhelm any benefit gained from ground effect, increasing the Warrior’s stall speed and decreasing its flare and landing distance compared to the 172, as can be verified by the numbers in the POH. With no flaps, the 172P stalls at 51 kcas (44 kias), while the Warrior stalls at 56 kcas (50 kias); with full flaps, the 172P (with its huge fowler flaps) stalls at 40 kcas (33 kias), while the Warrior stalls at 50 kcas (44 kias) — since both planes have the same approach speed, a higher stall speed means a shorter flare.
As a final confirmation, the published landing distance over a 50 ft obstacle at sea level/ISA/maximum gross weight is about 50 ft longer for a 172P than a Warrior.
Interesting theory, but the 4% wing loading difference seems intuitively too small. Plus I would not expect any more induced drag due to dihedral – that’s a function of angle of attack, which operates in a perpendicular axis. I suspect the responsible party is still on the loose.
Maybe wing washout.
Maybe general airframe drag.
Maybe airfoil shape.
Airfoil shape is certainly in play — presumably (though I haven’t flown one) the Hershey-bar wing Cherokees have a much shorter flare still than the Warrior wing Cherokees. I’m still trying to think through all the implications of the dihedral.
Here’s another thing to consider. Landing distances are often declared for short-field landings, ie., trying to minimize them. In such cases, one may not flare much or at all, just get the thing on the ground and brake. This would mean that comparing POH values for landing distance may have little to do with flare endurance.
The Transport Canada Flight Training Manual states that for a short-field landing “the aircraft should touch down on the main wheels at its lowest possible airspeed.” The 172P POH is silent on touchdown speed, but the PA-28-161 POH agrees with the FTM: “reduce the speed during the flareout and contact the ground close to the stalling speed”.
Unless you’re already at stall speed on short final over the fence (a scary thought), you’re going to need some kind of a flare to transition to a slower touchdown speed. With a short-field approach, you have a slower approach speed (and thus, a shorter flare), but it’s still there, and the flare still ends when the plane nears stall speed — if a plane floats longer in ground effect, it will take longer to decelerate to its minimum speed.
Cherokees – particularly hershey-bar PA28s but also tapered-wing Warriers, Achers, etc – have a tendancy to get a good sink rate going when power-off and at low speed. Cessnas seem to produce more lift closer to the stall. I believe this to be a difference between the Clark Y type airfoil found on the Cessnas and the semi-laminar flow airfoil on the Pipers. In hersey-bar Pipers, it is exaggerated by high induced drag due to the wing planform. When I was a young pilot – 120 hrs – I dropped in my first few Piper landings pretty good.
On the other hand, if the pilot is carrying in *too much* airspeed on final, the Pipers may well float more than Cessnas, because then parasite drag plays a larger part than induced drag, and you’re not quite so close to stall on the L/D chart…
Good blog, I’ll put you on my blogroll. If you’re interested, read Blogging at FL250, wherein I chronical my life of ultra-junior regional pilot and sometime freight dog. It’s at fl250.blogspot.com.
Great comment, Sam — thanks. I’ve never been one to approach fast (at least not since I finished my PPL and bought my Warrior), so I haven’t experienced what would happen in ground effect at 80 or 90+ knots.
We’re starting to get a good community of aviation blogs. I had trouble finding any at first (and most were student-pilot blogs), but now I can barely keep up. I’ll look forward to reading yours.
One other twist regarding short field landings, related to touchdown point. If one flares in such a way that the touchdown point is some way down the runway, then the bets are off. Airplanes decelerate much better on the ground (due to brakes) than in the air (due to flaps or whatever). So, unless you can flare ahead of the threshold and touch down on it, it may be better to avoid flaring, touch down at the earliest possible spot, and brake hard.
I’m not fully convinced about the braking. If the plane is still going fast enough to keep flying, braking won’t be as effective, and it also runs a serious risk of loss of control during the rollout. I’ve had success with short landings in my Warrior (obstructed and unobstructed) holding the plane off until the stall — it decelerates *very* quickly that way and has almost no ground roll afterwards — but that requires approaching at an appropriately slow speed with full flaps (so that there is very little flare), and that scares a lot of pilots.
My biggest problem with a short-field landing on an obstructed field is giving too much clearance over the trees — I think you have to almost set up for a landing on the treetops so that you will clear them as tightly as possible.
DOES GROUND REALLY EFFECT?
Flight Instructor Duncan MacDougall hollered from the front seat of the E2 Cub, “Do a 360 degree turn to the right.” I added full power, smoothly banked into the turn and watched the horizon pass around in front of us until the entry point coming up alerted me to level the wings and ease stick back pressure. Just then I felt a shake and a gentle rocking of the wings. To my question, Duncan answered, “You hit your prop wash.”
Back on the ground my instructor explained that by hitting our own prop wash it meant that the turn was perfectly level. I appreciated the compliment but could not believe that the backwash of that little prop passing by the fuselage after over a minute could still be strong enough to rock the airplane with two people in it.
Years later I found out that it was wing tip vortices we had hit and it did not mean I had made a level turn; it meant we had descended slightly.
Ever since those early $2-a-lesson days, I keep hearing about “ground effect” and how it causes an airplane to float over the runway or how the airplane gets “horsed” off the ground but can’t climb out of ground effect. Accidents are blamed on it. Even the astronauts reported entering ground effect on their first return in the space shuttle.
Like that “prop wash”, I think the prevailing spin on ground effect is “hog wash.”
Granted that air closest to the ground is most conductive to flight because it is denser than air one wingspan higher and air near the ground can’t be a downdraft because the ground is in the way. Maybe Piper should advertise better takeoff performance than Cessna because their wings are closer to the ground.
If ground effect were all it’s cracked up to be, wouldn’t beginning flight students be scattered all over the ends of runways where they ran out of ground effect?
The Federal Aviation Administration refers to ground effect in certifying aircraft and they claim, “The airplane is considered to be out of ground effect when it reaches a height equal to its wing span.”
In FAA’s Basic Helicopter Handbook there is an illustration with the caption “Ground effect results from the cushion of denser air built up between the ground and helicopter by the air displaced downward by the rotor.”
Why not prove if the above is true with a simple test? Mount a pressure instrument such as a spare altimeter on a helicopter skid and watch the reading on it while the helicopter takes off. Will it show a higher pressure at the instant before liftoff? It will not if what I learned in high school physics still holds true– that air cannot be compressed unless it is confined.
Ground cushion vehicles do confine air under them by use of “skirts” that cause the machine to rise a bit off the surface so air can escape under the skirt. Any airplane or helicopter flown by us men does not have a skirt.
What causes airplanes and rotorcraft to fly is to have their lifting surfaces push down masses of air equal to or greater than their own weights. This holds true at any altitude or closeness to the ground. Besides, no wing or rotor blade knows how far down below the ground is.
As an old bold pilot still enjoying flight adventures, I have never experienced floating over a runway except when I held too much airspeed and I have never lifted off and found it difficult to climb unless I was holding too high an angle of attack for the available thrust.
How about you?
Glenn Gibbons ATP 790358 1725 SW 52nd Street, Cape Coral, FL 33914 239-945-7075 firstname.lastname@example.org AOPA #00026550
Thanks for the detailed comment, Glenn. I think you’re right that the “compressed air” thing is an old wive’s/pilots’ tale.
When I was coding for the open source FlightGear flight simulator, I had a chance to discuss this issue with some hard-core aeronautical engineers, and I also did a bit of reading, trying to get ground effect right. While nobody is absolutely certain yet, the best explanation seems to be that ground effect actually cuts drag (by interfering with wingtip vortices) rather than increasing lift. You can find more information at Wikipedia.