• Vne, IAS, TAS and BVDs

Never Exceed Speed (Vne), Indicated Air Speed (IAS) and True Air Speed (TAS) are closely linked, and if you don't understand how and why, they could have an effect on your BVDs.




Studying the Pilot Operating Handbook of a Grob 103 I was surprised to see a chart that showed never-exceed speed decreased with altitude. The prop powered aircraft I've flown for over 40 years all have a redline at Vne and none varied with altitude. Vne was Vne. Never exceed that speed or bad things will happen.

But now, the Grob manual told me, Vne was not a fixed speed on the airspeed indicator. Vne, it insisted, was related to true air speed (TAS) not indicated air speed (IAS). To keep from exceeding the 135 knot TAS Vne, the never exceed indicated airspeed had to go down as you climbed higher.



Now hold on, common sense said IAS should be the limiting factor not TAS. What was I missing? How could the aircraft be exceeding Vne if the needle was way down in the green arc, which the sailplane's POH and a placard demanded?

TAS is the true speed an aircraft moves through an air mass, and the lying air speed indicator doesn't show that, at altitude, because it's confused by the lower air density. To get the true story we have to pull out our prayer wheel and correct IAS for altitude and temperature--in other words correct IAS for air density--to get TAS. No new news there, every student pilot learns how and knows why.



So what's the problem, why does Vne go down as you go higher? The answer has do with something that almost sounds sweet and friendly but is actually very nasty: flutter. Ribbons and butterflies flutter in a pretty way, but airfoil flutter can ruin your whole day life.

Flutter is a so-called aeroelastic effect that results from the springiness (elasticity of the airfoil and the surrounding air), aerodynamic forces, and inertia.

A wing has mass and when it's tweaked--when a force is applied, say by turbulence or a control input--it overcomes the inertia of the mass, and the wing responds by flexing (even a metal wing). Usually, that acceleration is absorbed or damped by the wing, the structure it's attached too, and the surrounding air and it returns to its original position. But if that tweak is suddenly imposed or is energetic enough as the result of high speed, you can make the structure resonate like a tuning fork.






It doesn't have to be a plastic ship, either. Here's a NASA film of a Twin Comanche's empennage (note how the fuselage skin flexes just ahead of the horizontal stab).



And a Lockheed C-141 (note the tail is resonating too)



What happens if you go too fast, push the aircraft past the speed where it flutters? Here's one of Grumman Iron Work's A6 Intruders.




Why does Vne go down with increasing altitude? Because there's less air to act as a shock absorber. Because there's less resistance to the wing (or other structure) flexing, you have to reduce the forces on that structure. The only way to avoid flutter (without redesigning the wing) is to go slower.

Aircraft manufacturers determine Vne by using numerical methods to compute Vd, design diving speed. (FAR 23, by the way, requires that Vd be not less than 1.4 design cruise speed, so the days of aircraft that climb, cruise and glide at 80 are over.) Flight testing is used to establish Vdf, flight test dive speed, which must be lower than or equal to Vd. Then Vne is set at 90% of the Vd or Vdf, whichever is lower, to add a margin for error--theirs and yours.

What does all this mean to you as a glider (or power) pilot? The faster your True Air Speed the more likely you are to encounter flutter. Push your slick glass ship up to redline and you're flirting with danger.

Remember, your airspeed indicator lies. It won't tell you what your TAS or never exceed speed is. In fact, it may be a compulsive liar and even lie to you down low (click to enlarge).









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• Taming a Taildragger

A friend, the late CAPT Carroll ' Lex,' Lefon USN, former XO of Top Gun, wrote a nice piece about his experience taming a taildragger. He was flying a Citabria, but they come in all sizes and shapes, even in our little air force.



In it Lex writes,  ". . . and [after landing] then it’s 'stick aft, stick aft!' until you slow to a stop . . ." And that's true enough, as far as it goes. But it should read ". . . and then it’s 'stick aft, stick aft!  and aileron full into the wind' until you slow to a stop . . ." or else you may find yourself in this predicament:


If you have full stick into the wind, if you get a little gust, the downwind wing will lift, your lift vector will have a component into the wind, and you won't crow hop sideways.


Also the increased induced drag on the downwind side, produced by the downgoing aileron, will use the long lever arm of the wing to pull the nose away from the wind, countering any weathervane tendency. But sideways motion is the beginning of a groundloop, not weather-vaning--although it's a co-conspirator. The lateral motion in the raised tail isn't impeded in the same way that the pavement stops the main gear, and that starts the tail moving sideways when the mains aren't, and that rotational couple around the CG is the start of a ground loop.

Why do taildragger pilots prefer grass? Not just because of the sweet smell of clover after a perfect landing, but because it will let the mains slip sideways some if you're not tracking straight, thus reducing the tendency to pivot around the CG if you botch the landing. Ergo, easier. But not entirely forgiving.



Full aileron into the crosswind at the start of takeoff too. Leave it there until the downwind tire lifts and then start reducing aileron as necessary. A one wheel takeoff in a crosswind (and a one wheel landing) proves you know what you're doing, and have a deft touch. There doesn't have to be much wind to cause problems, either. This guy lied to us about why he wanted some dual (future competitor), and when we called him on it in a subsequent email he wrote, "Oh well, it's only an airplane," then went without any dual, and did this to a somewhat dowdy but perfectly innocent Travel Air.




Speaking of the right touch, wheel landings are ever so much easier if you add some nose down trim on short final. When the wheels touch, don't try to find just the right pressure to stick 'er on and risk a PIO, just relax! She'll stick herself to the runway. And if you have to go around, the trim is all set.

Some will disagree, and some aircraft are not happy with the attitude, but I advocate always using three pointers in a crosswind (a Twin Beech is a case in point where it's dicey at best because the flaps blanket the rudders, and in heavy iron such as B-17s where you could break something if you drop it in). I have two reasons:

1.- If you have the wing down to counter the crosswind and it appears the wingtip will touch before you're able to stop the drift, you know it's too windy to land in that direction. Far better to find that out with the airplane under control in the flare before touchdown than at the point when the tail comes down after a wheel touchdown (which you can do in any gale).

2.- There's a point in a wheel landing where you have to transition to a three point attitude, and at that point what have you done? Increased the angle of attack, made the airplane at least get light, maybe fly again with a gust, and what happens? It starts to go sideways unless you have enough aileron in and enough wing down. That's a recipe for a ground loop. Anyway, at that point your right back to a three-pointer so why not plan on that from the beginning? But what is this Brit doing with the wrong wing down?



Incidentally, if in #1 you discover there's too much crosswind (more of an issue in a bird with a low wing like a biplane than a Cub or Citabria) there's no law that sez you have to land on the centerline, parallel to the runway. Approach over the downwind edge of the runway, aiming for the upwind edge a thousand feet down. You can even land part way down instead of at the threshold and aim for a high speed angled taxiway to use as a runway extension. (There were windy days at CRQ where we'd take a midfield intersection departure and be airborne before we actually reached the runway--but that angled taxiway meant we had no crosswind on takeoff).


Most little taildraggers, even Travel Airs and Stearmans, only use a few hundred feet of runway in a good breeze. If another runway isn't available, you could, in extremis, land 90º to the runway across the approach end using the runup area--or even on a ramp. But watch out for wind gradient. If it's strong enough to demand this emergency procedure it will surely be less windy at 10' than it is at pattern altitude, and that 300 foot long and 8000 foot wide runway will suddenly look very short. (The voice of experience speaks.)


That said, if you have no other alternative, no landing alternate and the wind is stronger than your wing low touchdown will control, then you have to make a wheel landing on one wheel to ensure you don't have any sideways motion. You'll need lots of forward stick to hold the aircraft down, and then you need lots of brake to help keep it going straight when you run out of rudder and aileron. All in all, a landing you want to avoid if you can. But sometimes you can't as described in this C-46 story. (If you're not sure what a C-46 is, here's a picture).


BTW, those big muscles in your brawny arm aren't very adept at precise control, use the fine muscles keep you from over controlling, if you're doing wheel landings.

And while I'm thinking about it, the way winds shift around here when the Santa Ana winds blow, don't be bashful about asking for a runway change. Most of the groundloops I've watched have occurred during a downwind landing. 

Another tip: assume you don't have any brakes when you land a taildragger in a crosswind. If you can do that, you've done everything right, but if you really screw up brakes might save your butt. Keep in mind, though, brakes often cause a ground loop away from the wind thanks to over enthusiastic and overcompensated use of them when things get scary. And, anyway, if you oveer use brakes other bad things happen.


We took the biplanes up to Corona for an airshow 10 years ago and there was a guy there who would land his Twin Beech and consistently make the first turn off, where we were turning off in the biplanes with some effort. I finally cornered him and he admitted to coming over the fence well below Vmc, and then standing on the brakes in the ensuing wheel landing. He said he could apply far more brake with the weight of the aircraft on the mains--and the tail has to get very very high before you'll get a prop strike.

That said, a friend put some new Redline brakes on his Stearman, and then promptly put it on it's back in the run-up area when he decided to see how well they work. Just proves there is too much of a good thing, I guess.

Which also reminds me of a Waco that ended up on its back at CRQ because he landed with the brakes locked. Seems there is a peculiarity where, if the parking brake lever is out just a bit, every time you push the brakes in flight, maneuvering, you set them just a bit tighter. Happened to my dad and I in a Cessna 140 in Guatemala City when I was kid too, for the same reason. Caught the prop, but that was bad enough.


Use of proper controls on the ground isn't just a good idea they teach you, BTW. The airline pilot we sold our Cub to put it on it's back taxiing downwind after landing. Second stop on the way home to TX with it. But he restored it and it turned out just beautiful. One of my favorite aircraft.



Also, don't be in a hurry to turn off the runway. Wait until you're going slow enough to control the centrifugal force the turn will create. Almost ground-looped a Twin Beech one night trying to make an early turnoff to help a Commuter landing the opposite direction, on a dead calm spring evening. Once she started to rotate I knew I was in trouble. Full inside engine, and full opposite brake--and a wonderfully forgiving airplane--saved my bacon. Here's the one we fly now landing, at Flabob.



I know a biplane pilot that suffered a more unhappy outcome when he allowed the tower to hurry him.

The B-52 (30 seconds into video), The Cessna 190/195 and some light planes had crosswind gear so you could actually point into the wind and land sideways. I flown a 195 with the option and it works up to a point, but it sure is spooky. Here's a Stinson with the Goodyear gear.


If things aren't going well GO AROUND! Don't get so mentally committed to landing that you forget to cock yourself in go-around mode. But make the decision early. If you wait too long all you'll do by adding power is add energy to the crash. This is what happened to a guy at CRQ* who at least tried to go around, but when he was 90º to the runway dragged the poor bird into the air only to stall it into the tree. He'd have been better off to chop the power and take his licks, me thinks. Mighta caught a wingtip and/or the prop, but at least he wouldn't have had to rebuild the whole airplane.



Such pontificating as this usually ends with a statement to the effect that flying a taildragger is no more difficult than flying one with a nose wheel, just different. Bull. Yes, it is different, but it requires that you fly the aircraft, not just drive it. And that's a valuable ability even when you fly a nosedragger and it's something worth your time and money to learn if you want to be an accomplished pilot. Right up there with some glider and aerobatic experience--don't leave home without it.

*From the NTSB (http://tinyurl.com/yj4ovx2) 
On June 25, 2006, about 1930 Pacific daylight time, a Bellanca 7KCAB, N1098E, impacted a tree during an aborted landing at McClellan-Palomar Airport, Carlsbad, California. The pilot/owner operated the airplane under the provisions of 14 CFR Part 91. The private pilot, the sole occupant, was not injured; the airplane was destroyed. The cross-country personal flight departed French Valley Airport, Murrieta/Temecula, California, about 1900, with a planned destination of Palomar. Visual meteorological conditions prevailed, and no flight plan had been filed.

The pilot submitted a written report. Prior to requesting a clearance to land, the pilot obtained the current automatic terminal information service (ATIS) information. He reported a 2-mile base and was cleared to land runway 24. The controller informed him that the winds were from 170 degrees at 8 knots.

On final approach, the pilot maintained the runway centerline using the crosswind correction technique. He performed a wheel landing to keep his speed up for the crosswind conditions. The airplane touched down on the left main wheel first, then the right main wheel touched down. After touchdown, the tail wheel started to vibrate and the pilot released backpressure on the flight controls to stop the vibration.

As the left wing started to move in an upward direction, the pilot felt the airplane "weathervane" to the left. Fearing that it would tilt or ground loop, he elected to abort the landing and applied full power. The airplane became airborne about 30 degrees left of the runway centerline heading towards buildings. He turned it to the right to proceed back towards the runway. He stated that the flight controls felt slow to respond to his input making the airplane difficult to maneuver back towards the runway. It crossed over the runway and was now facing buildings on the north side of the airport. He initiated a "gentle" left turn back to the runway. Shortly after, the airplane collided with a pine tree on the north side departure end of the runway in a straight and level attitude.

The pilot communicated his location to the tower controller, turned off all the switches, and released the emergency door latch to exit. He climbed down the tree, where emergency personnel met him.

The pilot stated that the airplane and engine had no mechanical failures or malfunctions during the flight.