Saturday, April 02, 2005

 

Holy Wars in Aviation

In the computer programming world, there are lots of endless debates about the best way to do things: C++ vs. Java, Windows vs. UNIX, object-oriented programming vs functional programming, Lisp vs. anything-not-Lisp, and so on. These debates are called "holy wars" in recognition of the fact that they are tied to peoples' beliefs and not to any objective facts or reproducible experiments.

I was surprised to find that aviation has its own holy wars. I assumed that because there were laws of physics involved, the "truth" would be easy to demonstrate. Even if that's not so, due to the fact that peoples' lives are on the line I figured the FAA and insurance companies would decide what was "true," and everybody would have to accept those decisions. But no, there are still debates about basic principles of flight and aircraft design. Aviation is like computer science in that it is only a few decades old, so the industry is still learning, each each new generation finds new ways to challenge conventional thinking. And as with programmers, what makes sense to one pilot will seem completely wrong to another.

Pitch Vs. Power

An airplane's speed and altitude is controlled by pitch, the height of the nose in relation to the horizon, and power, the thrust of the propeller. Learning how to balance pitch and power is a basic part of learning to fly. For reasons I don't understand, aviation experts are divided into two camps about The Right Way to learn how to do it.

On one side are the "pitch for altitude, power for airspeed" crowd. They say that if you want to climb, pull the nose up, and if you want to descend, push the nose down. Then use the throttle to increase or decrease airspeed as desired.

On the other side are the "pitch for airspeed, power for altitude" people. They say that if you want to climb, increase the throttle, and if you want to descend, decrease the throttle. Pull the nose up to slow down, or push the nose down to speed up.

"Pitch for altitude" is the easier one to understand for new pilots (and non-pilots). Pull the nose up and you go up; push the nose down and you go down. Increase the throttle, and the plane goes faster, just like a car. It seems obvious that this is how to do it.

So why would anyone advocate this weird "pitch for airspeed" idea? It is because of issues of aerodynamics, balance, and trim. When you pull the nose up, the airplane will start climbing, but will also slow down because some of the engine's power is now being used to increase the altitude instead of maintaining airspeed. Similarly, pushing the nose down causes both a descent and an increase in airspeed. Thus, changing pitch to climb or descend will also always require a change to power if you want to maintain a particular airspeed.

In contrast, changes to power have a different effect. When you increase the throttle, the nose will "automatically" (that is, without pilot input) rise, due to increased airflow over the tail. You end up in a climb, but the airspeed remains more-or-less as it was. If you decrease throttle, the nose will drop, so you descend but again the airspeed doesn't change. So, you can use the throttle to go up and down at a constant airspeed. You can use pitch to control airspeed, pushing the nose down to go faster or pulling it up to go slower. The airplane has a mechanism called trim that makes it easy to set the controls to lock-in a particular airspeed with this approach.

Another wrinkle is that the "pitch for altitude" approach only works well at higher airspeeds. At low airspeeds, raising the nose without increasing power will actually cause the airplane to descend, because the increase in drag at high angles of attack negates the increase in lift. This is known as the "region of reversed command," and in this environment the "power for altitude" style works much better than "pitch for altitude."

So which style is The Right Way? They both are, and I find each style useful in different situations. When trying to maintain a particular altitude while cruising, it's easy to just pitch up or down as I notice the altimeter straying away from the place I want. However, when approaching for a landing, trimming the plane for the right airspeed and then using the throttle for small adjustments to the glide path provides a nice stable approach. In the execution of the landing flare, where throttle is at idle and the pilot uses pitch control to both halt the descent and slow down to minimum airspeed, it is clear that a combination of styles is useful.

So why do people argue about it? I don't know, but I think instructors believe they are helping their students by focusing on one style or the other. Unfortunately, many pilots believe whichever style they were taught is The Right Way, and the other style is wrong.

Effects of Relative Motion to the Ground

A popular topic in aviation circles is the Downwind Turn Myth. Some pilots believe that if they are flying directly into the wind, and then turn so that they are flying away from the wind, they will lose airspeed because they suddenly have a tailwind. A drop in airspeed could cause a stall or spin, so according to this theory, turning downwind is potentially dangerous. This belief is false: the airplane's maneuvers are made relative to the air around it, so airspeed will remain constant regardless of how the air is moving in relation to the ground. The concepts of "upwind" and "downwind" only make sense in relation to the ground; they have no effect on the airplane's performance in relation to the air.

So, the Downwind Turn Myth is easily debunked, but there are some related debates that are not so clear.

In these arguments, everyone starts with an idealized physical model: the airplane is flying through a uniform airmass that is traveling at a constant velocity in relation to perfectly smooth ground. All the forces on the aircraft are caused by action/reaction with the airmass. It doesn't matter whether the airmass is stationary in relation to the ground, or moving at 50 knots, the airplane motions will always be made in the inertial reference frame of the airmass, and the ground has no effect. If the ground was not visible, the pilot of an aircraft would have no way to tell how fast the plane or the airmass wer moving in relation to the ground. Thus, wind direction and speed have no effect on an airplane in flight, except in relation to ground speed and ground track.

The problem with the idealized model, say some, is that it is not realistic. In the real world, the ground is not smooth, so as the airmass flows over it, it will be disrupted by updrafts, downdrafts, currents, and eddies. Uneven heating and cooling of the ground lead to thermal updrafts that move downwind as they ascend. Airmasses are not uniform, they contain turbulence, gusts, and other irregularities. So some argue that that a pilot with sufficient knowledge of the local topography could detect the wind direction without seeing the ground, and therefore airplane performance must be noticeably affected by wind direction. The effects of the real-world model would be most noticeable at low altitudes, such as when taking off, landing, and flying in the traffic pattern, so the idealized model is not relevant when talking about those situations, they say.

So who's right? The idealized model is useful in predicting what the airplane will do, but not sufficient to explain everything. The real-world model is correct, but it is not very useful because it doesn't provide a way for a pilot to predict exactly what is going to happen in that chaotic environment. So it seems that the appropriate position to take is to rely on the idealized model, but be aware of its limitations.

High-Wing vs. Low-Wing

Small single-engine aircraft can be classified as high-wing, meaning that the wings are attached to the top of the fuselage, or low-wing, meaning that the wings are attached to the bottom of the fuselage. High-wing aircraft include many popular Cessna models, and older Piper Cub-style tailwheel aircraft. Low-wing models include the newer Piper models (if 40- and 50-year-old designs can be considered "newer"), Mooneys, and many others.

Each design has is advantages and disadvantages. High-wing aircraft tend to be more stable, as the wings are above the center-of-gravity, but low-wing aircraft can compensate with higher wing dihedral angles. High-wing aircraft can rely upon gravity to feed fuel from the tanks in the wings down to the engine, whereas low-wing aircraft need fuel pumps to draw the fuel up from the tanks to the engine. High-wing planes make it easier to observe the ground; low-wing planes make it easier to see the sky and to watch where you are going while turning. Low-wing planes have longer, smoother landings than high-wing planes. You don't have to worry about bumping your head when walking around a low-wing plane, and you don't need to climb a ladder to fill the tanks.

So which design is better? I'm not going to even try to answer that.


Comments:
Airspeed is controlled by angle of attack, not pitch or power. It's a critically important distinction, especially during takeoff and landing in different density altitudes and at different gross weights. I'm going to post about this in my own blog.

I agree that there are interactions with topography during flying -- for example, you'll usually get rising air on the windward side of a hill or ridge, and subsiding air on the leeward site -- but your stall speed, Vx, Vy, etc. (i.e. the aircraft performance in the airmass) is still the same; it's just that the airmass happens to be moving vertically as well as horizontally.

High wing planes have better natural roll damping than low wing planes, but it's not because the weight is underneath the wings -- that's irrelevant aerodynamically. The way it was explained to me, it has something to do with the way the fuselage interacts with the airflow, but I cannot remember the details. You are right that high-wing planes get the same effect through extra dihedral -- that adds a bit of drag, but then, so do wing struts on (most) high-wing planes.
 
I was going to post a long comment here, but I'll take David's tactic and make it a posting in my own blog, if he doesn't cover everything I want to say.

Preference of high wing versus low wing is a holy war, but I've never actually seen the others pitched as two equal but opposite camps before, only misunderstandings or folk aerodynamics. If your instructor isn't giving you a conclusive answer on these, hit an aerodynmaics textbook.
 
My training was of the "pitch for airpspeed, power for altitude" variety. Then on my private checkride the DE was all "no, no, no" and endorsed the opposite. Not exactly the kind of thing you want to hear during that high stress situation! :)
 
I never asked the question, but during my training I found that for *minor corrections* on altitude, power would be used. For example to slow the descent or even to climb a bit.

For a *serieus climb*, let's say more than 100 ft, pitch would be used (of course an increase in power is needed also).

Your posting helps me a lot though!

David v L
 
During hang gliding training I was warned about turning downwind whilst low.
Two factors at play: One is the perception of speed based on ground speed. Folks would typically slow down (push out) when they perceived their "speed" increased when turning downwind and this would cause a nasty low stall.
The other effect is due to wind gradient and would occur close to the ground when turning either way. Wind gradient causes a positive feedback in a turn that can lead to over banking and excess slippage. Add this to pilot overload and it is a cause of accidents.
Of course both situations are outside the "ideal model".
 
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