Two nice dual flights

Today the air was crystal clear, providing wonderful views of the snow on the mountains to the north and east. But due to about 50% cirrus cloud cover, it never got warm enough for good soaring. I was determined to fly regardless of lift! (Also, I got a tiny new camera recently and wanted to try it out.)

N, one of the students in the club, has passed his written test for the Private Pilot license and wanted to start to practice for the Practical test. He asked me to fly with him and tell him what maneuvers to demonstrate. I have a list of them, so after going over the list in some detail, up we went. (You can just see the towplane and the mountains in this shot.)

We found just a little lift on the first flight, keeping us up for 30 minutes from a 4000' tow. The second flight was not so lucky, essentially no lift and a 23 minute flight.

The list of tasks for the Practical is pretty long - it's quite a bit to accomplish in a single flight. N did nearly all of the tasks on both flights.

1. Aero tow
a. Box the wake when above 300' AGL
b. Slack line control
c. Signal the tow plane for turn, speed up, or slow down
2. Release at 5500' MSL (4000' AGL)
3. Steep turns
a. One 360 degrees
b. One 720 degrees
c. Complete turn on a specified heading
d. Final turn serves as one clearing turn
4. Stalls
a. Straight ahead with and without dive brakes
b. Turning (left and right) with and without dive brakes
5. Slow flight - 35 knots
a. Straight ahead flight
b. Left and right 90 degree turns
6. Straight ahead flight
a. Minimum sink speed
b. Best L/D speed
c. Speed to fly if in sink
7. Thermal soaring
8. Approach to glider area of runway 22
a. Slip on base or final without dive brakes
b. Use of radio
9. Land in designated box

So I got to fly for nearly an hour with a friend, practice some instructor skills, and split the tow fees. Not bad! The only downside to dual flights with students is that I can only log as PIC the time that I am actually flying the plane, which in today's case was only about 5 minutes per flight.

Thinking about lift

I've been thinking a lot about lift lately. Not the kind of lift that glider pilots seek out (upward moving air), but the kind of lift that both airplane and glider pilots care about: the lift generated by the wing.

All pilots, and most passengers, at some level want to know "What makes an airplane fly?" The wing, obviously! Since I want to be an instructor, I should be able to explain what enables a wing to fly. I've read a number of sources, and am still unsatisfied with the explanations.

Most sources talk about Bernoulli's principle (why the top of the wing causes lift) and/or Newton's law of reaction (why the bottom of the wing causes lift). Some synthesize the two, and speculate on what proportion of each effect causes the most lift. Some introduce "circulation". Eventually they all start getting vague and say that the reason a wing accelerates air that flows over the top surface can't be easily explained and mumble something about Euler's equations, as if that will help.

Stick and Rudder, otherwise a fantastic book, simply says "It shoves the air down with its bottom surface, and it pulls the air down with its top surface; the latter action is the more important." Langeweische says a lot about how lift varies with Angle of Attack - he is much more concerned with how to fly the wing than with why the wing flies.

See How It Flies Section 3, Airfoils and Airflow, is filled with wonderful diagrams of stream lines:

and talks a lot about the timing of the parcels of air. Like other books, it tries to correct the false notion that the air flowing over the top of the wing "must" move faster in order to meet up with corresponding parcels flowing below. That makes sense - there's nothing tying the two flows together.

Later, Denker talks about circulation but never explains it. He starts with the idea of air circulating around a stationary wing on the ground (artificially stirred by a paddle), then expects us to accept that circulation continues during flight. I'm sorry, but I don't think there's any air making its way around the trailing edge from the top to the bottom and flowing forward - it's all flowing past the trailing edge. In fact he makes that point himself by discussing the stagnation point at the trailing edge. He and other sources seem to say that circulation is a mathematical construct to explain pressure differences, not a real circulation around the wing. Huh?

Aerofoils and Wings explains a lot about how different wings behave, but as far as lift generation goes, Brandon just refers us to Denker. Later, "As the air accelerates away from the stagnation line, the local airflow over the upper surface gains a greater speed than the lower." We're supposed to take this on faith? What makes it accelerate so much? OK, it accelerates upward because the leading edge forces the air up over the top surface. But why would air flowing below the wing not also accelerate? It's being pushed down, just as the air over the top is being pushed up (due to the Angle of Attack), but I suppose not as much.

In the January 2009 issue of AOPA Flight Training, "The Magic of Lift" can't seem to decide whether or not air meets up at the trailing edge at the same time. Christensen spends some time trying to explain why air traveling faster has lower pressure, using a car-traffic analogy. That's a useful analogy, but only if you assume that the air above must move faster because it has to get to the trailing edge at the same time as the air below. He starts to debunk that idea and then reverts back to it.

But at least Christensen considers the wing from what I think is a better point of view. Most texts describe a stationary wing, and a moving flow of air. I think that's because wind tunnels have been used for years to study airfoils (it's easier than observing airflow by looking down the tip of a moving wing!). But in flight, the wing moves and the air is still. Most texts would claim that the two situations can be described by the same laws. But I'm not so sure... I think the behavior of the air may be different, primarily because of inertia and ambient pressure.

The air has stationary inertia (it must be shoved out of the way by the wing), it doesn't have inertia of motion. As the air is shoved out of the way, wouldn't its pressure be increased, rather than decreased as is assumed in the air-moving-faster-over-the-wing model? He states that as the air comes back down after the wing has passed, it "now possesses momentum, causing the air to overshoot ... resulting in the downwash." I get that. But what started it moving downward - isn't it the ambient pressure of the air above it that is being shoved against? I think of the air as squishy, and once the wing gets out of the way, the air is squished back downward.

According to Bernoulli, moving air has lower pressure. But it seems to me that it may not have lower pressure in all directions. A parcel of air moving upward, being shoved out of the way by the top surface of the leading edge, may have lower pressure horizontally, but wouldn't it have higher pressure vertically? After all, it's being squeezed between the wing and the air above it. Once it gets moving at a constant rate, maybe... but as it's accelerating, it seems to me the pressure in the vertical direction would be higher. (I remember reading a description of pitot tubes and venturis that made me think of it this way... I'll have to look for it.)

NASA cops out and fails to provide a good explanation, too. They spend a lot of time explaining flow patterns and have some great interactive illustrations, but in the end they just say "The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. ... To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations."

But equations merely quantify physical phenomena. Mass is moved around by energy, not by numbers. There must be a way to explain what the air is doing, without getting into the details of how much. Langeweische did such a tremendous job of explaining phenomena and behaviors without invoking mathematics - I wish he had tackled this aspect as well.

Some texts also get into bound vortices (the full extension of what we pilots usually call "wingtip vortices"), and I think this aspect is far more important in explaining lift than most authors do. To put it as simply as I can, if some air is shoved downward (whether by Bernoulli and suction on the top, or Newton and deflection on the bottom), that displaced air must be replaced by air from above. You can't leave a hole in the air - that's a vacuum, and as we all know, Nature hates vacuuming. The most natural way for the ambient air to fill it is from a circular region above and around the hole. Then that gap is filled by air from below the circular region (all around the original hole). Then that circular gap is filled by the original shoved-down air spreading out. Viola - a donut-shaped flow of air - like a smoke ring. And due to inertia, it keeps on vortexing until it's diluted.

I have to think about this a little more, and find some good diagrams of it. It's almost got me believing in circulation again. The BGA Gliding: Theory of Flight has a good diagram of the donut shape of the bound vortex (Chapter 3 Figure 40), but again I think the bound vortex is partly a mathematical abstraction. I really don't think air flows forward under the wing!

So, writing this out, I think I've arrived at a conclusion. Why does the air accelerate over the top of the wing? Because it's been given energy by the leading edge, shoving it out of the way. Where did it get that energy? In the case of an airplane, it comes from the engine shoving the wing forward. In the case of a glider, it comes from gravity pulling the glider and wing downward-forward. The ultimate purpose of the wing is to transfer energy from the (engine or falling fuselage) into downward-moving air at the trailing edge, which shoves the wing upward. Most texts talk about the flow of the air over a stationary wing, as if the air has the energy. I think the wing has the energy, the leading edge forces the air up, then the compressed air above forces the displaced air back down past the trailing edge. The displaced air takes the energy with it, causing a vortex. Somehow Newton's action-reaction law gets invoked to cause the wing to go up - still not sure why. I think air moves the wing up, not old Newton. I'll have to think on this some more.

I have no aerodynamic training, just what I've read along the way to becoming a pilot and an instructor. If you have some insight into this topic, please comment! If I'm all wet, I'd like to know where I've got it wrong. But I really care about what mass and energy do. Analogies don't hold wings up. Equations quantify but don't explain. It has to make sense.

Some work and a sled ride

Today the weather forecast was really iffy. Light winds, and lift could go to 7000', but incoming cirrus clouds could spoil it.

The PW5 needed a little work, so I tackled these in the morning:

• The battery wires were fraying. A and I put new lugs on the ends.
• The gust locks for the ailerons needed new foam and bungee cord. We'd been using rags as padding for a few months!
• Replaced the long rear bungee cord on the canopy cover.
• The O2 system had been left turned on, so the battery was dead. A kindly went to town for a battery.
  

Some of the student pilots are coming to me with questions about procedures and resources. Without actually getting into ground school, I explain what I can. As one pointed out (and as I've blogged about), many of the books tell you what but they really don't explain the why. For example, one wanted to know about radio procedures on the UNICOM frequency in the landing pattern. What do you need to say, and in what sequence? Why in that sequence?

1. First, say who you're addressing, e.g. "Hemet traffic:" Why first? To get the attention of people who may not be fully listening. You're saying, "Hemet traffic, listen up!"

2. Next, who you are. "Glider two papa delta..." If you said this first, then who you're addressing, people would miss your call sign because they didn't start listening until they heard "Hemet traffic." Listeners aren't fully listening all the time.

3. Where you are. "Entering 45 for..."

4. What you are going to do. I get really specific because we gliders share the airport with power traffic, and they are most of the radio traffic. They're on the other runway (23) and they do a left pattern (which they don't specify). I want them to understand where I am going and that I'm not conflicting with them. "... right-hand pattern to runway two two." I emphasize the second two because they're normally listening for "two three".

5. Finish with who you're addressing. "Hemet". In case #1 got cut off, or was garbled, or they really weren't listnening at the beginning.
Once you understand the why of the sequence, it's easy to remember!

A had not flown the PW5 for a while so I went over the controls and features with him. He took off before noon and had a nice hour-long flight. Unfortunately, he used up all the lift. ;-)

I let off in lift but could not get back into it. The cloud cover had gotten thicker and there was very little sunlight hitting the ground. All I found was about 3 knots of sinking air. Near the Initial Point at about 1300' AGL I found a little weak lift but it was not even big enough to complete a circle in. I ended up with an 18-minute ride. Bummer! After about 2:00, most people were not staying up any more.

At least my landing was good. There was maybe a 4-knot headwind at most. I think I touched down right on the line, and stopped well within the first box. I think what made the difference was that I picked out my aiming point as soon as I turned base, and kept checking my angle to it all the way on the base leg. I think I've been looking elsewhere on base leg, and then not being at the right altitude when turning final. By establishing my aim point on base, the base and final legs are all part of the same glide slope.

Landings

One of the requirements for the Commercial practical test is landings twice as precise as for the Private certificate. The Practical Test Standard says "stopping short of and within 100 feet of a designated point". I'm told that at our field, that translates to touching down and stopping within the first of our two landing boxes, which I think is about 500 feet long. I've had some issues with touching down too soon, i.e. short of the box, and I've been planning to work on this for some time. But the weather conditions have been pretty good lately, so I've tended to go soaring rather than spend time (and money) on landing practice.

I really need to practice this in a Blanik, since that's what I'll take the test in, but one was busy with students all day and the other had its control surfaces removed for replacement of the fabric, so I decided to practice in the PW5. It's different from the Blanik: lighter, doesn't have flaps, and is different in many ways, but my problem hasn't been with those aspects. My problem has simply been the glidepath to the aiming point. For some reason I tend to pick an aiming point too far downwind from the box border, making it so I have to float too far. In other words, I have tended to come down too steeply during the base and final legs.

Flight #1: I let off at the Initial Point at 1000' AGL. In lift! By the time I got to the point of turning base, I was STILL at nearly 1000'! I extended my downwind leg much further than usual. Full spoilers and forward slip and turning slip brought me down steeply, but again my roundout point was too far downwind. The standard advice is to not adjust spoilers after rounding out, but sometimes I do close them slightly to slow the sink rate. At least with the PW5, that's hard to get right, and it's easy to balloon up a bit. I touched down about 15' short of the line. Braking on the ground was very good (there was about a 6-knot headwind component), and stopping smoothly within the first box was easy. (Speed control is crucial to touching down with little energy, making for a short rollout. This I seem to have no problem with.)

Flight #2: This time I didn't have a bunch of lift on downwind, and had a more normal pattern and approach. Speed was right on, didn't have to mess with spoilers, just kept easing the stick back and floating... floating... floating... touched down about 1-2 feet short of the line. Good rollout again.

Flight #3: I decided to do a normal tow and do some soaring. Gotta have some fun, right? Uneventful flight: let off in lift, went up to about 5800' MSL, didn't find any other major lift. Nice clear day! A bit of lift on downwind, but not nearly as much as the first flight. Speed was a little high but I fixed it. Touchdown was... hmm... I don't really remember, I think still about a foot short of the line. Rollout again was nice and short. The wind was just right to "ground fly" balanced on the wheels for about 30 seconds or so.

On all three flights, I think I only used about 2/3 of the first box - the PW5 really stops short and smooth. So... I should definitely move my aiming point even more upwind on the field, to float further into the box, at least when there's a light headwind. I think that's part of the problem: trying to estimate how much the wind will help kill the float - maybe I'm underestimating that aspect. And flying all three different aircraft doesn't help. They're all different weights and different glide ratios, so adjusting for multiple variables is tricky. When I can nail landings in the first box in both Blanik and PW5, in varying wind conditions, then I'll know I'm ready for the practical. (Not a chance in the Grob - it's way too heavy and its wheel brake is weak. I was lucky to stop it in about 1200' a few weeks ago.)

First Mountain Wave - well, hill wave anyway

Very interesting weather at Hemet today. A low pressure system to the east was to bring rain in by evening, and there were lots of thin cumulus clouds hanging around. The thermal forecast looked good, though no CU were forecast. The temperature profile looked good - no inversion at all. Driving in, I could see occasional little lenticular clouds, indicating wave activity. Huh? There were big "lennies" on the west side of Mt. San Jacinto and smaller ones to the north and west of the valley, but only CU and clear areas over the valley. The tricky part for me has been to figure out from the lennies which direction the wind is actually blowing, and therefore where the upwind side should be. The sounding map showed the wind at about 240, and driving in I was able to observe the trailing edge of a lennie dissipating, and that was the northeast side, which confirmed for me that the wind aloft was from the southwest.

The thermals were only supposed to go up to 7300' MSL, not high enough to reach the big lennies... but maybe the little ones?

The temperature was about 90F, higher than forecast, and way over trigger temperature. The wind at ground level was 13 to 16 knots, and was forecast to be about 20 at 5,000'. By the time I took off in the PW5 about 2:00, the sky was 90% obscured, and I was afraid thermal activity would shut down. But I let off tow in lift at 4500, and worked up to about 5500, and headed southwest toward the closest cloud that looked lenticular. To get there I had to head directly into the wind (approaching what I thought would be the lift area by flying under the cloud - watch out for rotor!). That's usually a killer for altitude, but I was in zero sink much of the way. Groundspeed was way slow! (I didn't bring my PDA, so I didn't have any true wind or groundspeed info.)

The cloud was further away than I thought, beyond the hills beyond the little town of Winchester. Thinking there might be rotor directly under the cloud, I skirted the edge between it and the next cloud. Wrong idea: there was no lift, even a little sink between the clouds, and the zero-sink or 1-knot lift had been under the cloud. I turned around and headed back toward the airport. I was fairly low, but I knew I'd be flying downwind to get back, so I would not lose too much altitude.

Close to the airport, I found lift again and worked it up to 5800'. My drift in the thermal confirmed the wind direction. I headed off toward the lennie again. This time, I was starting higher and closer, so I had a better chance of making it all the way. The minor lift was there again, all the way over to Winchester and beyond. I stayed under the cloud and drifted up, though I never reached cloudbase (which AWOS reported as 7,500'). But the cloud was still further southwest of the airport than I was comfortable going, so I did not go all the way to the leading edge. I eventually turned back and headed home, planning for about a one-hour flight duration.

Under a cloud, it's hard to see the actual shape, but earlier I had been able to see it was a combination of lenticular and CU: smooth and curved at the upwind side, then broken and puffy and going upward on the downwind side. And the next cloud over was definitely smooth and lens-shaped, clearly a wave cloud. So I conclude that I was flying in wave lift, not "cloud suck". It was not as smooth as most wave is described, but then I was not at the leading edge. And these clouds did not quite fit the classic lennies caused by wind blowing across a ridge. The hills upwind that created these waves are small and isolated, so the waves were not wide areas, more like small points. Since I was only up at between 5 and 6 thousand feet, I also conclude that the wind was probably bouncing off the ground, not off a stable air layer. So when I was flying under the cloud, I was essentially flying upwind in a huge updraft between the ground and the condensing moisture. Really a strange flight: I flew about 7 miles directly into a 20+ knot headwind, and actually went UP!

Coming back to the airport, I hit some fairly strong turbulence and some 8- to 10-knot sink. It was either rotor, or all that air that went UP coming back DOWN.

The thermal forecast certainly got the condensation level wrong - it forecast 18,000' and the cloudbase was actually 7,500.