Stall Testing

Stall Testing

February 19, 2006, 32F, 30.37 inches of mercury, Sterba 52 x 60? prop, 2700cc engine

Although the day started out snowing and with every church service in the county cancelled due to "icy roads", it got pretty nice this afternoon, with very little wind and about 50% blue I had to fly again. I thought I'd continue where I left off on Friday.

It was 32F degrees, and since I'd left the cowling off while adjusting idle mixture on Friday, I preheated the engine for 20 minutes with a "torpedo" heater before I started it. It fired up immediately, which was nice, because the 2700cc engine has always been slow to start when it's cold. I attribute this to the fact that I put the primer nozzles in a different place than the 3100cc engine. On the 3100 I put both nozzles in the top of the intake log, up between the forward-most and middle cylinders. My thinking was that the forward cylinders would get some fuel, and the aft cylinders would get it when the liquid fuel ran down that far. It always started within a few seconds, but admittedly that was a warmer time of year.

The 2700 has the nozzle in the same location on the pilot's side, but I hid it underneath the manifold on the passenger side for aesthetics (hard to believe, considering the rest of the engine), so it's midway between #1 and #3. This pretty much starves #5 altogether, and it really takes a long time to get it going. It still "starts" almost immediately, but it takes as much as 10-20 seconds to get it running on all 6, and even then I have to give it a shot of primer every two seconds or so until it catches. Next engine, they go back up closer to the front! This is a taildragger phenomenon, I'm guessing. I checked both nozzles for passage last weekend, so I know they're both working.

First I timed the climb on takeoff from liftoff....3000 feet AGL took 3:20, 4000' took 4:40, and 5000' took 6:10 (although I lost my concentration on that last thousand feet). All of this was at 80 mph. This tranlsates to 900 fpm average for the first three thousand feet, 860 between 3000' and 4000', and 807 fpm between 4000 and 5000 feet. Not too bad for a full load, but then it was only 32 degrees too!

I did 8 more stalls today, 4 each with flaps and without, and at 1000 rpm and at 1600 rpm. I forgot my kneeboard, so I duct taped some paper to my leg, and it worked just as well! The plane was full of fuel, and probably weighed 1000 pounds during the stalls. Average stall speed for 1000 rpm with full flaps was 58 mph again (like Friday), and 55 mph at 1600 rpm. "Clean was 63 mph at 1000 rpm, and 59mph at 1600 rpm. The stall was pretty gentle with a slight nose drop...nothing that a little power and right rudder didn't take care of in a second or two while losing only a tad of altitude.

I did one pretty nasty one with 1600 rpm and full flaps, because I was holding the wing up with the aileron instead of the rudder (I figured that out quick) and I lost about 200' recovering. I've done this twice before in the last year, and the result is a zero g-manuever best described as "falling out of the sky". My left wing is heavy anyway, so when the aileron influence went away, that wing dropped like a rock. Now that I know what it takes to keep that from happening, I'll practice that particular manuever some more next time around. I do all this at 8000' feet or so, so don't worry that I won't have time to recover. The only clue that it's about to stall is a little instability about the longitudinal axis, with an almost imperceptible buffeting on the tail (I assume).

I talked to Troy Petteway today, and my flapped numbers are almost exactly what he sees on his KR2 clean (with the same wing), but then his plane is 100 pounds lighter, and a whole lot cleaner (gap seals, for example). Gap seals on the new airfoil reportedly make a huge difference in climb speed, so I would think when I clean up the bottom of my wings, my stall speed will drop some more. That'll have to wait for warmer weather though. Besides that, I want some "before and after" numbers. Also, my wings are really crappy, not only with imperfections in the surface, there's dry primer overspray all over them. That's got to make a difference as well.

All of the numbers above are my best calulation at actual speed, using GPS runs down the runway at 60 mph or so (in the midst of several touch-n-gos) while comparing to the airspeed indicator to find the differential. Bottom line is a 16 mph difference between actual and indicated in that range. I averaged several runs, but they were all within 1 mph of 16. This is at the low end. The numbers are much closer together at the high end at low altitude.

So now that I knew exactly what ASI error I had at stall speed, I had to try to fix it. After all, it's the bottom end where I want the accuracy. It's almost always off at higher speeds anyway, because I'm usually at high altitude if I'm not near an airport.

I added some dams behind the static ports (I have one on each side) to raise the pressure at the port to try bring the pitot and static pressures closer together, narrowing the difference, and lowering the ASI's indication. I forget to take any welding rod with me, so I clipped off two number 6 screws and used aluminum tape to make dams 1/4" behind the ports. The result was, well, dramatic! The ASI had barely moved off the peg when I lifted off, and it read low all the way around the pattern. It was too late and too dark to climb up and do four stalls in four directions, and since it was definitely off, I just landed again. Fortunately I've gotten to the point where I don't really need an ASI anyway, so I landed it just a tad fast and vowed to try something different for the next flight. The sad thing is that I had finally figured out exactly what my indicated stall speed is, have gotten used to flying patterns with the current indications, and now I'm going to change it all in the name of accuracy! This is the third location for the static ports, so rather than drill another set of holes, I'm going to try to make this one work.

I'll start my new port tuning efforts with some self adhesive gasket material that's best described as 1/16" electrical tape. I can easily move these back and forth til I get it right, and then I'll "finalize" it with aluminum tape over the top.

Oh, and here's the obligatory sunset picture...

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