Saturday, November 17, 2007

Final approaches

Some of us were talking about landings today, and one pilot mentioned something that I had just thought about a few days ago. I had been working with the polar (L/D curve) for the Grob 103, calculating appropriate speeds for flying it through sink, when this occurred to me. Each glider has a "normal" approach speed (assuming no wind). For the Blanik L13, it's 50 knots, for the Grob 103, it's 55. Each glider also has a "best L/D" speed (best glide speed). For the Blanik, it's 46 kts, for the Grob it's 60. So what?

  • For the Blanik, the approach speed is faster than best L/D. If you pick up speed on final, your drag INcreases and you will sink faster, but then after the flare you will float farther in ground effect. If you slow down on final (down as far as best L/D), your drag DEcreases and you will glide farther until the flare.
  • For the Grob, the approach speed is slower than best L/D. If you pick up speed on final (up as far as best L/D), your drag DEcreases and you will glide farther, and then after the flare you will float farther in ground effect. If you slow down on final, your drag INncreases and you will sink faster until the flare.

If this is not clear to you, get out the polar for your glider(s) and see where the "normal" approach speed lies in relation to the best L/D speed. If you are not clear on how the Angle of Attack affects the sink rate in a glide, read the excellent discussions in the book Stick and Rudder. It may seem counterintuitive, but in some aircraft (such as the Grob), pointing your nose down (up to best L/D speed) will make you land farther down the field. If you want to get down faster (ignoring the spoilers), pointing your nose up will cause the glider to "mush" and sink faster.

I'm not yet sure what to do with this information. The airbrakes surely have more effect on the sink rate than the speed does, and we try to keep the airspeed dead-on the recommended value (plus a wind adjustment). It does tell me that it's doubly important to not let a glider such as the Grob gain more speed on final, because both the glide and the float will be longer.

Two things:
  1. I'm not an instructor. Please make sure to discuss this topic with your CFI-G! I'm interested in readers' (especially instructors') comments on this.
  2. I can't say enough about Stick and Rudder. Although it was written for power planes a long time ago, it is still a tremendous resource for learning about how an aircraft really works in the air. It is non-technical - no formulas, charts, or math, but you can tell that the author knows the technical background and he does an excellent job of explaining things in plain English. It really explains why so many things are counterintuitive in flying - and why it is so important to learn those "backward" concepts thoroughly, to overcome our instincts and ground-sense. Get yourself a copy and read it carefully. Then read it again!


Today I shared a flight in the Grob 103 with L. Now that the days and therefore the lift periods are shorter, it makes more sense to do a couple of 2-person flights in the middle of the afternoon than to try for several individual flights and have some people miss the soaring times. Though it does mean that we can only log part of the time as PIC, the part when we are actually at the controls. This time L did the takeoff and all the soaring, because he was not happy with his previous takeoff and because he doesn't fly as often as I do. I did the landing and flew from the rear seat because that's what I want to work on in preparing for my Commercial rating. (I'll start writing about the Commercial soon.)

The forecast was for just about 250 fpm of lift, and the day just reached the calculated trigger temperature. There was a lot of haze in the air, clearly showing an inversion. The previous flight reported no lift, so we didn't have high hopes. We let off at 3100' AGL, above the inversion, and we never got that high again. I worked hard at spotting sources of lift and keeping a general lookout. Twice I did find crows nearby, higher than us, and we were able to find some lift under them. I also spotted swallows very close - we nearly hit one - and they can be helpful because they are often hunting bugs that have been brought up by a thermal.

We found some weak lift and worked it extensively. All we got out of it was about 300 feet, and combined with some zero sink, we accomplished a 42 minute flight. I think for a while we were at the junction of two air masses, because to the west the ground was far less visible due to brown haze, and to the east it was much clearer. But there was no obvious shear line lift.

The Grob's wheel brake is kind of weak (we'll be working on it next week) so we wanted to have plenty of space to land in. So another pilot recommended I use the little dirt strip that the tow plane lands in, which I've never done before. It worked out fine... I could perhaps have floated a little further (eased the stick back to a higher angle of attack and landed with less speed) but it landed smoothly and straight. Actually, landing in that spot helped avoid sun glare which can be a problem in the late afternoon. I get to log a whopping 5 minutes of PIC time.

Flying like this with another pilot is fun. We can share ideas and learn from each other, but it's more relaxed than flying with an instructor. And we split the tow fee.

Saturday, November 03, 2007

Test flight

The last couple of weekends we were unable to fly due to the high winds and some restrictions at the airport due to heavy firefighting activity. So I was eager to fly any of the aircraft, for any length of time. My preference was to fly the Grob 103 because I plan to take some passengers up in it in a couple of weeks.

So today I took a short flight in the Grob to check it out after a repair. Although an A&P mechanic actually approves the return to service, our club practice is to have a private pilot take the glider on a test flight before it is flown solo by a student pilot or by an instructor and student. So I planned to take a 2000' tow in the morning (before lift started working) and then land. I actually let off at 1800' AGL because there seemed to be some lift. I never found anything useful, and landed with an 11-minute flight.

I had to leave fairly early in the afternoon because of family plans in the evening. The way the tows and the glider scheduling worked out, I didn't get a chance to fly later. Others seemed to be staying up, reporting 2 to 5 knots of lift.

I now have 77.5 total hours logged.