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Feature Article 2

FEATURE ARTICLE

Max Trescott, 2008 National CFI of the Year mtrescott@comcast.net

Nailing Airspeed on Final

Learning to fly the Cirrus SR20 and SR22 and the Diamond DA40 can be rewarding in many ways. Both are fun to fly, and the extra speed helps on longer trips. For example, last weekend I brought back a SR22T from Richmond, VA to Palo Alto in 17 hours over two days.

Transitions into these airplanes take longer than stepping up from a C172 to a C182, mostly because there are more avionics and systems to learn. But the airplanes are also a little slipperier, and I sometimes notice clients I’m working with having a little challenge nailing the airspeed on final. Here are a few tips that may help you manage airspeed in these faster airplanes.

You might think, “a wing is a wing,” so why would flying one of these airplanes be any different from flying a Cessna 172. The general principles are indeed the same, but the nuances are more important in the faster planes. Why that should be the case had me wondering, until I ran across an article written by my friend Alan Brown, chief designer of the F-117 Stealth Fighter.

Alan makes the case in his article that if he were flying a trainer “which was reasonably clean, and didn’t have a very high lift wing,” then he “could quite safely cut the power on approach down to idle and just let it float in.” He goes on to say however, that if he were flying a “very high lift airplane… then my total drag is rising rapidly as the speed drops, and I need substantial power to maintain speed.” He argues that the higher lift airplane has a landing speed that’s further up the backside of the power curve, so as speed decreases, more power is required to maintain altitude than would be required in the trainer.

The Basics

Regardless of whether a client is flying a trainer or a Cirrus, I’ve always taught them to use small adjustments in pitch to make airspeed adjustments on final. Then, if they’re either too high or low, they should adjust the throttle until they have the right amount of power to make it to the runway.

You could probably get away using throttle to control airspeed on final in a trainer simply because these aircraft have so much drag and the airspeed won’t change rapidly with small changes in throttle. But try this technique in a Cirrus or Diamond, and you’ll likely see larger variations in airspeed on final. Generally, when I see someone having trouble nailing the airspeed on final in these planes, it’s because they’re using power as their primary means of controlling airspeed.

The Nuances

The challenge with maintaining airspeed in these faster airplanes is that pitch and power interact with each other. A good example is when one of these aircraft is low on final and you add power so that the aircraft doesn’t land short of the runway. When you add power, invariably these planes will also speed up. If you’re at your target airspeed of 77 knots in a SR22 or the DA40NG and you add power because you’re low, the airspeed will likely increase well above 80 knots, unless you also simultaneously pitch up as you add power. If you do, the slight increase in pitch mitigates the speed increase that would have occurred when you added throttle because you’re low. By making these slight pitch changes, up as you add power, and down as you decrease power, it’s much easier to nail the target airspeed.

This effect is particularly dramatic if you try pulling power to idle in one of these planes when you’re 50-75 feet AGL over the pond, as you might do in a Cessna 172. As Alan indicated in his article, the C172 will float in. But pull the throttle to idle in the DA40s, and the airspeed will VERY rapidly decay, unless you also pitch down substantially when you pull the power. That’s one of the reasons I like leaving some power on in these planes until after I’ve started the roundout, and am almost in the flare parallel to the runway.

By the way, Alan Brown’s article didn’t appear in a scientific journal. He’s an active RC modeler, and you can find his article in the RC newsletter he edits here. http://www.rcbees.org/newsletters/RCBN1609.pdf

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