You’ll find our 20 Aviation Myths listed below. You can read the whole article in the October 2007 issue of Plane & Pilot.
Myth 1: If you make a sudden turn from upwind to downwind,
the airplane can stall.
Jessica Ambats
Myth 2: You can buy a fixer-upper airplane and save money by restoring it yourself.
Myth 3: Tailwheel airplanes require much more skill and are inherently dangerous.
Myth 4: Extending flaps while turning base or final can cause the airplane to stall.
Myth 5: A few hours of aerobatic training will save you if flipped upside down on final.
Jessica Ambats
Myth 6: Short-field approaches require hanging the airplane on the prop from a mile out.
Myth 7: Flying approaches at higher approach speeds is safer.
Myth 8: 2,000 feet is a short runway.
Myth 9: Pumping brakes is more effective and easier on brakes than steady pressure.
Myth 10: Wear lighter-than-normal shoes for increased rudder sensitivity.
Myth 11: A calm day is safer/easier than a crosswind day.
Myth 12: Power-off landings shock cool engines.
Myth 13: GPS is all that’s needed for cross-country flying.
Myth 14: Ice only occurs in clouds.
Myth 15: Stall-spin accidents always start with a nose-high altitude.
Myth 16: Running up your engine on the ground once a month prevents rust.
Myth 17: On takeoff, it’s safer to leave it on the ground until fast, and then rotate off.
Myth 18: Power-off landings are unnecessarily difficult.
Myth 19: Only licensed mechanics can do mechanical work on an airplane.
Myth 20: Once you fall off the “step,” you must increase power or lose altitude to regain it.
10 comments:
Re myth #1. I can't believe this one is still being argued. The canoe in the stream is completely bogus. The canoe doesn't derive it's bouyancy (lift) from it's speed over the water. The wind gradient is a new one on me. It might have some validity, but not to this situation.
What everyone seems to miss is that, in a downwind turn, the MASS of the aircraft has to accelerate to maintain airspeed. To use an extreme example, if I'm climbing my piper cub into a 40 KT headwind with a 40 KT airspeed, my groundspeed is ZERO. If I now make a rapid 180 degree turn, I need to accelerate the MASS of my aircraft from 0 KTS to 80 Kts in the time it takes to turn 180 degrees (and my cub can turn pretty quick) just to maintain my 40 KT airspeed. Obviously, I'm going to have to lower the nose to do this or my airspeed will decay. If done low enough, the onrushing terrain may cause the inept to keep the nose up and induce a stall. The faster and more powerful the aircraft, the less apparent is the phenomenon. Nonetheless, the physics still apply. Sincerely, John Roberti, P.E., CFI, A&P.
Myth 99: With the exception of 90-degree crosswinds, there’s always a component down the nose that’s making your groundspeed slower.
Busted! Even a 90-degree crosswind reduces your groundspeed. Try it on your E6B.
John, the airplane doesn't care how fast it's moving relative to the ground. Instead, it cares how fast it's moving relative to the air--and that isn't affected by the speed of the air relative to the ground. As far as the airplane is concerned, doing a fast 180 degree turn at 40 knots in a 40 knot wind is *exactly* the same as a quick 180 degree turn with zero wind.
You care because you can see the ground, and your GPS cares because it knows where the ground is, but your airplane does not care.
-Rob, Ph.D. Physics
I confess to being a GPS junkie. I won't fly without mine. In flight I use the VORs as backup and even look down once in a while to make sure the ground is where I think it is. Next to my GPS, my radio is a fail-safe for getting vectors and radar services. I keep a sectional in my flight bag to separate the headset from the GPS gear and once in a while look at it to make sure it's still in the flight bag.
I used to be amazed when I arrived at an airport I had planned for. Now, when the controller asks if I have the airport in sight, my first glance is at the screen of the GPS, then off the nose.
I'll probably get the deserved contempt of a bunch of orthodox pilots, but for those of you who rely on satellites to navigate--I'll keep your dirty little secret.
No. 20: by "off the step" I presume you mean back to pushing the whole float through the water instead of hydroplaning on the bow portion.
At low speed, the AC is a boat with all its weight opposed by the buoyancy of the floats. When it hydroplanes, weight is reduced by lift (ground effect) and by the reduced drag of the water. The floats aren't hulls any longer, they are sponsons.
If you slow down out of partial ground effect and are no longer planing on top of the water, you must add power to increase lift to break free of the water. This takes energy, both to get up to flying speed and to break the suction of the water beneath the current waterline. Once out of the water and on top of it, you can reduce power to stay on the water without flying. On some AC, if you raise the nose at this point, the increased angle of attack will lower the airspeed, return the stern portion of the floats to the water and kill the hydroplaning effect.
I'll confess that I don't have any certificates or a PH.D. in Physics, and as such I may be an amateur next to most of the posters here. However, drawing from what I have learned in a physics class I am currently taking, I can see how Myth #1 can be disproven (except for the part about the crop dusters) to the satisfaction of those who insist on using the ground as a reference frame: The wind has no effect because when you make a 180-degree turn in your Piper Cub that is moving at 40 knots airspeed (and groundspeed) in absence of wind, you are going to have to go from moving 40 knots South (talking about groundspeed here) to 40 knots North-an acceleration of 80 knots, just as if you were turning out of a 40-knot headwind and accelerating from 0 knots groundspeed to 80.
The only difference is whether you accelerate from 0 to +80 or from -40 to +40.
Rob, Maybe if you think of it as self-induced windshear you'll get my point. John
Budd Davisson knows his stuff!
#20 "on the step myth" is named after an aspect of seaplane operations, but the myth is not about seaplanes. Seaplanes do actually get "on the step" on water, no plane gets "on the step" at altitude. The myth is about trying to apply this seaplane concept to aerodynamics. It probably started from pilots overshooting their altitude a little on climb. When they pitched down to get down to altitude and powered back to cruise their cruise speed ended up higher than normal and tended to stay higher for a short while. It seemed like they were somehow in some special low drag aerodynamic mode, analogous to a seaplane "on the step" where there is less drag and more speed. Of course thrust and drag would soon find equilibrium and speed would quickly return to normal cruise. When the pilot eventually noticed that cruise speed had slowed to normal, he concluded that he had somehow "fallen off the step", so he would have to climb and descend again to reestablish this mythical low drag aerodynamic "on the step" condition. It is all about perception, not physics.
#1 As far as turning down wind, John is correct that there is mass involved, and that redirecting that mass 180 degrees does consume some energy (Newton #1). But as Dr. Rob points out, the energy consumed, and any resultant speed loss, is exactly the same in still air as it is in (non-turbulent, non-wind-shear) strong wind. The airplane is flying in an airmass. As long as the wind is steady, the airplane doesn't care which way the wind is blowing. (We care because it affects our ETA, our landings, etc, but the airplane's aerodynamics are not affected).
A good illustration, I think, is a goldfish in a bowl on the seat of your car. You are going 70 MPH down the highway, and the fish is going 70 MPH down the highway. But Mr. Goldfish does not have to swim 70 MPH to keep up with the car. It calmly swims 1 MPH toward the front of the car (ground speed 71 MPH) then calmly swims rearward at 1 MPH (ground speed 69 MPH, backwards). The fish is swimming in the mass of water, and while he is making great progress toward his destination (average 70 MPH ground speed), this has no effect on his "aerodynamics" (or "aquadynamics"). He continues to swim 1 MPH with the same effort regardless of direction. Lift vs. buoyancy is not the issue either. His speed through the water is not affected, nor is the thrust required to maintain that speed. If he was concerned about lift, drag, angle of attack, etc, these would not be affected.
BUT, hit a bump in the road and that is a different story. This can definitely affect Mr. Goldfish. Likewise, wind-shear can have a tremendous effect on the airplane because it suddenly changes the relationship between the air and the plane. Here mass and inertia come into play.
In the myth, the wind does not change, only the plane's direction within the mass of air changes. Therefore the relationship between the plane and the wind remains the same as in still air. But with wind shear the air changes, its relationship to the plane changes, and it will take some moments for the mass of the airplane to accelerate or decelerate to match the new wind direction. An aircraft descending on final with a 20 kt headwind which suddenly descends into a 10 kt tail wind will instantly lose 30 kts airspeed. Also, ground speed will not change immediately. As the plane accelerates back to normal approach airspeed, ground speed will increase 30 kts.
#3 So true! Tail wheel airplanes are not really "harder", they just won't let you get away with being sloppy like tri-geared planes will. Getting your tailwheel endorsement will make you a better pilot. The skills you learn on and near the ground in a tail-dragger, picking up on small deviations and correcting them early, apply to all aspects of flying in all types of planes. Only about 1 in 400 Americans can pilot an airplane, that makes you special. Even fewer can handle a taildragger. That makes you extra special! Try it!
#11 And finally, yes, a 90 degree crosswind does slow you down enroute. That is one of the great curses of physics. On a round trip, whether it is headwind then tailwind, or direct crosswind both ways, you still lose overall if the wind remains constant. However, I find that when I am dealing with a 30 kt, 90 degree X-wind on final, by the time I add 5 kts to my approach speed because of turbulence, then have to give a shot of power in the flare because I am running out of rudder authority, I don't think a 90 degree x-wind actually helps me land shorter than in calm conditions. X-wind does tend to make my landings smoother because I roll the mains on one at a time. So perhaps everyone is right on this one!
Thanks, JT Rairigh
CFII, MEI, Heli, IA, GFG (general flunky and go-fer)
Anyone who's ever done Ground Reference maneuvers (that should be any pilot), and done them correctly (that might be slightly fewer pilots), should know that the myth of turning downwind and stalling is completely false.
While circling a fixed point, your air speed remains constant while your ground speed is variable. You adapt your bank angle to compensate for your relation to the wind. If you perform it correctly, you can maintain height, airspeed and distance from the object you're circling.
As for the Cub in the 40 knt headwind, you should remember that your ground speed (the 80 knts you so desperately wish to obtain) has absolutely nothing to do with lift. As you turn (however abrupt it may be) your air speed should remain the same. Your ground speed will change drastically as, in the turn, the wind will begin to blow you sideways increasing your speed in the 180 degree direction you're shooting for. By the time you're pointed completely in the opposite direction, your ground speed will have reached the magical 80knts you were looking for. However, your airspeed will be 40knts (which is really what you should be concerned about).
I defy anyone who holds on to the downwind turn myth to climb to altitude with a good stiff wind, and with no ground reference, tell us the speed and/or direction of the wind.
Trim your plane to fly as slow as possible while circling, at some point during the 360 maneuver, your plane should stall if the myth holds any truth.
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