Flaps and V-Tails of Death

Hi all,

I'm here to report that I cheated death and survived a flight in a glider with the dreaded 'Landing Flaps and V-Tails of Death'. Jim Harper graciously allowed me to fly his beautiful HP-16. What's an HP-16 you say? Well, looks to be just like the HP-18 (or I should say the HP-18 is just like the -16) except that the -16 has a large and comfortable all metal cockpit instead of the narrow composite cockpit of the -18.

Jim's -16 is equipped with winglets and, as far as I know, the ailerons are standard (not with the J.D. Colling mod, correct me if I'm wrong Jim). Center stick. Tow was behind our 180hp Cessna 175 on a 275 foot long rope. Started out in -2 flap position, went to + 5 at 40 knots. Had no trouble keeping the wings level. The bird lifted off level and was easy to fly on tow. Released and tried slow flight (no stalls), flight up to 90 knots. Steep turns, etc. Well sealed and quiet. Good rudder response and easy to coordinate compared to my 301 Libelle. More stable in a thermal than my Libelle. Aileron response was a little slower than my Libelle at thermaling speeds, but was positive and more than adequate for centering thermals. Climbed a few hundred feet in a very week thermal. Very nice thermalling glider.

Landing was the best part. I stayed high and close in the pattern. I rolled on some flap on downwind, maybe 30 degrees or so. I was way high on final so I started rolling in more flaps. I never quite got to the full 90 degrees mark as I had the nose down at a truly obscene angle just to maintain 50 knots. Steeper than my 301 with the tailchute and full divebrakes. Steeper than a Mosquito with everything hanging out. This was more like parachuting than flying. Nothing but dirt out the front of the canopy. The horizon was visible more towards the top of the canopy. I aimed at a target and just kept the nose pointed there. I pulled back on the stick when I ran out of nerve. The bird settled on with a slight bump of the tailwheel and a little bounce as the main dropped on (I flared just a bit too much). I have no doubt that I could easily put this bird in a tiny field much easier than I could my Libelle (and that is not difficult at all).

So, I'd have to say that all the bad things I've been told about landing flaps on gliders and V-tails are definitely untrue with respect to the HP-16. It is a comfortable and easy to fly glider and compares very well to my Libelle and also to other glass birds, including my favorites for handling, the Mosquito and LS-4.

OK, It's winter time and I'm bored, so let me throw my 2 cents in here. The only flight I ever had in a ship with flaps only was in a PIK-20B. It was a test flight after a broken fuselage. Things went well until it got time to land. I rolled in 45 degrees of flaps and everything looked just about right. Came over the fence at 50 knots and waited for her to settle down. I waited and waited and waited. By now I had floated down most of the 4000 foot runway and I'm still floating about 1 foot off the ground. What do I do? Musn't dump the flaps or she will drop like a stone, right? Finally got the wheel on the ground and jumped on the brake. That night I read the flight manual and it said to slowely crank off the flaps in this situation. I think the flaps only ships are OK, but require a different set of skills that must be mastered. Probably not for the low time pilot or those who don't read the flight manual before flight. My real problem was being about 5 knots too fast. Proper speed control is critical. JJ Sinclair

Flaps only ships are very rare in France and kowledge about their handling is probably even more rare. I wonder how one can handle in such a ship what is described in our flight Bible, the 'blue book' (Manuel du pilote Vol a Voile, i.e. glider pilot's manual) as the 3 most common mistakes when landing: 1) flare to high; 2) flare with excessive back stick action; 3) bounce. In this 3 cases the glider comes a few feet above the ground at a speed just marginally above stall speed and quickly decaying due to the drag of open airbrakes. The immediate action to avoid that the glider falls on the ground like a stone in the following seconds is to retract the air-brakes, so that the drag stops killing your speed and you regain some lift, then try to land better ahead. But what can you do with no air-brakes?

Flaps, once deflected beyond about 10 degrees, produce mainly drag. In steady-state glides, drag is drag whether produced by flaps or spoilers. For a given amount of drag, a flapped glider will have the nose much further down than a glider with spoilers, however. This gives a much better view of the runway.

The main difference is in the transients as the drag devices are extended or retracted. The trick with flaps is to manage the transients. Yanking on flaps from the retracted position will produce a upward transient before the drag steepens the glide path. Yanking on spoilers will produce a downward transient before the glider settles into a steeper glide. This generally means that the pilot should not make large, sudden changes in flap settings near the ground - but this is good advice for spoilers as well.

Large flap deflections change the relationship between pitch and airspeed. Large changes in pitch result in small changes in airspeed so that the nose can be steeply down and the airspeed will not increase much. This invites the technique popular with HP glider pilots of just aiming the glider steeply down at the flair point with 90 degrees of flap deployed. If the glide appears to be overshooting the aim point, pushing the nose further down steepens the glide without much increase in airspeed. If it appears that you are undershooting, just raise the nose a little and shallow the glide. Extremely accurate landings are possible with no changes made in the flap setting.

The height to start the flare is a bit tricky to judge at first and the flare is greatly exaggerated compared to spoiler only gliders. If you flare too high, the recovery is to just relax some of the back stick and let the glider settle before completing the flare. 'Leave 'em where they are' is good advice when landing with flaps.

I really like flaps. They eliminate the discontinuity in the upper wing surface created by the spoiler box. Once mastered, they permit much lower energy landings than with spoilers.

A for as a trainer, the IS28b2 Lark can be landed with flaps only although the glide is quite shallow. I looked at the flap drive geometry on my Lark with the thought that it might be modified to allow 45 degrees or more of positive flap just for training scenarios. It would be possible but the paperwork would be a nightmare.

Bill Daniels

The landing is executed in a way as not to use full 90 degree flap in the final stages of the final. Rather manage the final approach in a way that allows the use of less flap. You will have some flap action in reserve to arrest any excessive sink, similar to using only partial spoiler.

In Fact, when looking at my flying style with the flap only glider, I use the flap, like you would use the spoiler. I constantly make use of them during the pattern to place my self at any point I would like to be. When making misjudgements it is no different to correct them, as with spoilered gliders.

It is best for a newcomer to place the glider relative high on final crank in full flaps and aim for the beginning of the run way. That way you will have enough speed for corrections if you make a misjudgement and if you are on glide path you burn of all the energy very quickly. It really does not matter if you use and extra 200 feet. In the end it the touch down speed that counts.

As for out landings into small fields with high obstacle, it requires different approaches and only practice at your home field will make you proficient.

Boy, you picked up on that one quick!

I'm not qualified to address the actual engineering aspects of this issue. But speaking from the perspective of a sailplane development pundit:

I think that, absolutely, implementing 90-degree flaps on a composite wing has complications that you wouldn't find on a more rigid metal wing. However, the lessons of the PIK-20B and the Zuni suggest that it is doable.

As you point out, the big problem is bending of the wing with fully deployed flaps, which tries to bend the flaps in the plane in which they are most rigid. I suspect that overcoming this issue requires the right layup type and fiber orientation. I'd have to do test sections to be sure, but I think that either aramids or possibly newer polyethelyne fibers on some sort of bias orientation would be required. That might give reasonable torsional stiffness without undue bending stiffness. It seems to work for the LS-6, which uses Kevlar (tm) laminates in the flaperons.

Of course, a stiffer wing than the old ASW-20 would help, too. That, and more hinge points and more drive points.

Before I tried it for sure, what I'd want to do would be to test a candidate flap section, and see how close I can get it to the predicted wing curvature at the ultimate loading limit. It might turn out to be necessary to either make the wing stiffer, or to limit loading to a lower G value under landing flap deployment. Or perhaps something else entirely. That's what testing is for.

Thanks, and best regards to all

Bob K.

lots of good stuff snipped...

Geeze, Wally. Thanks! Just a couple of responses through the entire thread.

Floating down the runway? More flaps. With 90 degrees and the nose level...she stops pretty close to right now. Might want to make sure you aren't more than that foot or so above the ground, or you test Schreder's wonderful shock absorbtion.

Clouds? Full flaps at whatever speed you are going aren't going to put you up much...and you will come down quite quickly...and out of the cloud. If you go 90 degrees in my 16, you can pretty much let go of the stick (at altitude) and it descends quite docilly...well...it bobs and weaves but it don't scare me much. There is a bunch of drag there. I should note that I am at the forward limit of the CG.

My biggest concern about the -16 is that it is gonna ruin me for other gliders. No way will I ever find similar glidepath control. I can modulate out of pretty much any amount of trouble.

At any rate, Wally, thanks again. I love to share! And I had to fly tow anyway!!

Bob K wrote>>>>>>>>..>

If one were to bounce, or land in a rough field, in one's ASW-20, with 45 degrees of flap on, the downward motion of the wings would force the deployed flaps to move up, thus bending the internal flap push-rods. I would think some sort of sliding push-rod arrangement might prevent this situation. The Ventus has a gas strut that will allow the deployed flaps to come back up. Do I have the Ventus gas strut figured out correctly, you Ventus drivers? JJ Sinclair