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!

2 comments:

smithcorp said...

Hi Roger - you're a thoughtful sailplane pilot, and your blog has been a bit of an inspiration to me as I start my training.

I got through my solo in a week program in Taupo NZ, going solo in a PW-6 on Sunday after 7 hours of flying time. I managed to do three safe solos in a row and qualified for my A Certificate.

Now I'm back home in Oz, I'll be driving down to my local club to get signed up to make my way towards the B.

Interesting to read your thoughts on best glide speed versus approach speed. I struggled mightily with final approach (my problem was judging rate of descent and my error was taking my eyes off the aim point and looking nervously at the boundary fence to check how quickly I was descending. I'd then end up with the nose down, the speed creeping up and messing up the landing) and it took me a day and a half to get sorted out.

Cheers

smith

Roger said...

Congratulations on your solo!! It's really a thrill to realize that you are now qualified (and trusted by at least one CFI) to take an aircraft up and bring it safely back down again. And isn't it the good soaring season for you now? So you can go have some fun!

After my first solos I don't think my feet really touched the ground for several days. But then the next week, the CFI on duty was a different one than soloed me. He insisted on flying with me before he would let me go up by myself. That kind of popped my balloon. But it also made it clear that training isn't over at that point. I think I flew with an instructor for one out of every 8 flights over the next few months... then a lot more when I got ready for my practical test.