Saturday, November 13, 2004

Oh, What a Lucky Man I Am

This is the first of a multi-part post, in which I talk about flying airplanes! I love airplanes, and I like explaining how the things work, and what it's like to pilot one. Hopefully this will make sense as well as being fun to read...


It’s been more than a month since I’ve hopped into the pilot seat of an airplane and taken to the skies. Normally I fly every two or three weeks, but Northern California has been hit by an early winter (the earliest I’ve ever seen; we haven’t had rain this heavy in October since 1964 around here), which robbed us of Indian Summer flying weather. Moreover, I’ve been suffering for the last week and a half from a pretty severe cold/cough, and there was no way I was going to get in a plane in that condition!

Yesterday, I finally felt well enough to fly, and good weather was in prospect. My somewhat urgent mission was to fly the club’s TB-20 Trinidad, in order to keep my currency in that model. The Trinidad is a French-built airplane, and it has the looks to prove it (click to zoom):

Click to Zoom

Elegant, and yet strange, non? Why such a big-assed tail? Why such short landing gear for such a big plane? Ze wing, she is a little inelegant, no? Zo blunt and thick. Ahhh, but in zees thick wing we have enough room for almost 6 hours of fuel! And ze wing, she is docile close to the stall. Unlike a Beech Bonanza, it is not so easy to enjoy a last moment watching the ground spin and spin while the earth comes up to dash you!

However, the blunt wing has a lot more drag than the Bonanza’s wing, so something must be done to allow the plane to fly almost as fast as a Bonanza. Compared to the Bonanza, the Trinidad has a very low cockpit height in order to decrease the overall drag profile of the fuselage. In the Bonanza, you sit upright and have about 6 inches between the top of your head and the cockpit ceiling. In the Trinidad, the seats are slung back, so you’re laying back a little in a reclining position, with your head still almost touching the ceiling. But the Trinidad has a pretty cool Delorean-like gull wing canopy, with a door on each side. Taxiing around in a Bonanza, you feel like you’re on a Harley. Taxiing around in a Trinidad, leaning way back, riding low to the ground on those short little landing gear struts, you feel like you’re in some sort of French pimpmobile. But it’s great to have a choice!

The Trinidad has a 250 hp engine and cruises through the air at about 185 mph. This was the kind of plane I did my high performance training in after learning to fly in a Cessna Skyhawk, and getting my instrument training in a Piper Archer. Both of those planes are docile, fixed landing gear, 180 hp machines, which cruise at 140 mph. The Trinidad has retractable gear and a constant-speed propeller, both of which add to pilot workload. When I first started training in the Trinidad, it was quite a handful! Compared to the simple trainer airplanes I’d been flying, this thing was a comet, and you definitely had to think more steps ahead than you do in a trainer. I had about 50 hours of training (including lots of instrument flying and emergency practice) in this plane before I even wanted to think of going up alone in it. I finished my training in the Trinidad back in the summer of 2003, and have done some fun trips in it, including Santa Barbara, California, Eugene, Oregon, Sunriver, Oregon, Mammoth, California, and Seattle, Washington.

The plane I actually fly is not as pretty as the one above, and looks more like this one (click to zoom):

Click to Zoom

A drab gray, but with a three bladed propeller. A three bladed prop is quieter and has better ground clearance, so it suffers less nicks and abrasion from grit on the ground.

Here’s what the cockpit looks like (click to zoom):

Click to Zoom

It’s really an outstanding ergonomic setup in there, much better than in the 1959 Bonanza which I fly. The three levers in the middle from left to right are the throttle (black), the prop control (blue), and the mixture control (red). The throttle determines how much air goes into the engine to be mixed with fuel at the cylinders. The mixture control determines how much fuel gets mixed with the air. The fuel injection system simply squirts fuel continuously at a particular pressure. As the air gets thinner at higher altitude, it is necessary to decrease the fuel pressure so that the chemical ratio of fuel to air for proper combustion is maintained, and this is done with the mixture control.

The propeller control takes some explaining.

In a trainer airplane like a Skyhawk, the propeller is fixed pitch, meaning the geometric angle of the propeller blades doesn’t change. While this makes for an inexpensive, light, and uncomplicated arrangement, there are some big drawbacks. Firstly, there is one perfect angle for the propeller to be meeting the air in order to create the maximum thrust to drag ratio. If the propeller is biting the air at any angle other than this, then horsepower is being wasted. There are different kinds of fixed pitch propeller. A fixed pitch propeller can be optimized for cruise efficiency, which means it has a pretty steep geometric angle. At cruise speeds, the air is flowing over the propeller at the most efficient angle, but during takeoff, the angle is far too sharp, and in fact, parts of the propeller are in an aerodynamic stall condition which generates lots of drag, but very little thrust. Moreover the situation is like trying to ride a bike up a hill in too high a gear. The engine just cannot spin fast enough to generate full horsepower (horsepower is determined by how wide open the throttle is, which governs how big a bang you’re going to get in the cylinder, as well as by RPM, which determines how often the bangs happen). This all results in crappy takeoff and climb performance as well as underutilization of the available horsepower of the engine.

A climb prop has a much shallower geometric angle which yields an optimal thrust to drag ratio at takeoff and climb speeds, as well as allowing the engine to spin faster and develop full rated horsepower. It is like riding a bike uphill in a low gear, which is what you want! I would far rather have a climb prop than a cruise prop. It’s generally much more important to make the ground drop away as fast as possible, than it is to shave a few minutes off the trip! This is especially true for mountain flying and bad weather flying. The trouble with the climb prop is that at cruise airspeeds the blade angle isn’t steep enough, so the propeller doesn’t take a good solid bite out of the air and wants to spin too fast. In practical terms, this means that the pilot must throttle back in order to keep the engine RPM below redline. Once again the engine is restrained from being able to generate its full available horsepower, thereby limiting cruising speed. This is akin to trying to ride a bike at high speed in too low a gear. You just can’t get enough purchase on the pedals to put power into each stroke.

Either kind of fixed-pitch propeller suffers from RPM changes with differing airspeed. In a Skyhawk, if I’m doing a trip and it is time to descend, I pitch the nose of the plane down, so that I begin to lose altitude. Just like a car rolling down a hill, the plane would like to accelerate. But once again, the faster airspeed causes the prop to spin up too fast, requiring the pilot to throttle back. For all practical purposes, a Skyhawk cannot descend at a much faster airspeed than the cruise airspeed for this reason. This is definitely not the case for the Trinidad or other constant speed prop airplanes! The linkage between airspeed and RPM also causes an annoying problem of the RPM (and therefore the power) never quite settling down. A slight increase in airspeed results in an increase in RPM, which means there is more power being generated, which means the plane will accelerate, which means the RPM will increase, etc. The process also acts in reverse. Over the span of five or ten minutes, the pilot will find himself having to tweak the throttle several times to keep the power where he wants it.

The constant speed prop solves all of these problems. With these propellers, the blades are able to pivot about their long axis, thereby changing the angle they make with the oncoming air. The pivoting is controlled by a governor which seeks to keep the propeller turning at a constant RPM no matter what the airspeed. If the prop starts to spin too fast, the governor steepens the blade angle in order to create more resistance, thereby slowing the propeller back down. If the prop starts to spin too slow, the governor decreases the blade angle to allow the blades to slice through the air more easily, thereby speeding the propeller back up. So there are none of the airspeed/RPM/power problems experienced by the fixed pitch prop. If you put the Trinidad into a descent, the airspeed can increase without causing the engine to overspeed. This results in greatly increased performance when flying a long trip. For example, I might be at 11,000 feet needing to descend to 2,000 feet in the vicinity of the destination airport. A comfortable descent rate (comfortable for the eardrums) is about 500 feet per minute. Therefore I’ll be descending for 18 minutes. The plane will be doing better than 3 miles a minute, and going 20 mph faster than it otherwise would for almost 60 miles of the trip. That makes a difference!

Finally we come to the blue lever. The governor will hold the prop at a constant RPM, and the blue lever (in conjunction with a tachometer) allows the pilot to determine what that RPM will be. Push the lever forward and the prop speeds up (assuming there is enough throttle to allow it) to up to 2700 RPM. Pull it back and the engine can be slowed down to 2200 RPM or less. If the throttle is closed enough, the governor will have the blades as flat as it can make them and eventually the RPM will be determined by throttle alone. This is the case when idling and taxing on the ground, and when landing. Takeoffs are done with the lever all the way forward, so the engine can go to maximum RPM at full throttle thereby generating maximum power for takeoff, just like a climb prop. At cruise, the RPM is pulled back to maybe 2300, in order to pivot the blades to the efficient angle of a cruise prop. Everybody’s happy.

Here’s an aerial picture of Palo Alto airport (click to enlarge):

Click to Zoom

The picture is probably taken from about 2000 feet altitude, and the picture is oriented facing a little west of north, in other words, up the Bay towards San Francisco. At the top right of the picture is the Dumbarton Bridge, and the town of Palo Alto and highway 101 would be about 6 inches off the left side of the picture. The airport is adjacent to tidal marshes and a bird sanctuary/recreation area to the south (called the Baylands). It is a very scenic location. The Baylands is my go-to place for doing walks or bike riding, and when you’re out there, in addition to looking at birds (herons, egrets, pelicans, gulls, sandpipers, terns, and marsh hawks), you can watch the student pilots going around and around, following the “staying in the pattern” pathway shown in the picture. The word “pattern” refers to the flow of traffic in the air around the airport. The pattern is a rectangle made up of four legs. Taking off and heading straight (or as in this picture with a 10 degree right turn to reduce noise over the town), you are on the “upwind” leg. A 90 degree right turn puts you on the “crosswind leg”. Another 90 degree turn from here puts you on the “downwind” leg flying the opposite of the takeoff direction at 800 feet with the runway out at the wingtip. Downwind is flown until the landing end of the runway is about 45 degrees behind you. Then you turn 90 degrees to fly the “base” leg, with a final 90 degree turn to have you lined up with, and heading toward the runway on “final”.

When you take off, you’re either intending to “stay in the pattern” in order to practice takeoffs and landings, or you are leaving the airport to go somewhere else. You can leave the area using the “Right Dumbarton” path (heading across the Bay and inland), or the “Left Dumbarton” path (heading over Palo Alto and out over the coast).

As it happens, when I flew yesterday, I did 3 times through the pattern to practice takeoffs and landings, then did a Left Dumbarton departure to dodge clouds and see if I could get out over the coast!

To be continued…

Note: In the Palo Alto airport picture above, you can see a dock in the extreme lower right hand corner. You can take a look at a quicktime panorama of the view from this very dock here. Press the mouse button and drag to scroll around the 360 degree panorama! The starting view of the panorama shows the final approach course. Planes come in from the left and exit on the right heading toward the runway.

Update: Part 2 of the series is here.

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