I’m sorry I missed the initial discussion. I was asleep. Anyway, here are my thoughts and observations, after putting 2,200 hours in my Sting S3, the last 1,100 of which were with the DUC Swirl Inconnel propeller:
1. Yes, Warmi, I would agree there’s something amiss with your S4 and it's most likely the propeller pitch. You should have better speed than that! How much better depends on your goals in consideration of the associated trade-offs.
2. In my Sting, I am sorry to say, I lack the ideal value for comparisons: TAS. I did buy a temperature gauge years ago, so my Garmin could talk TAS with me, but I’ve never installed it. The only values I can share are KIAS and KGS (ground speed; from two GPSs). On a typical windless day aloft, I easily pull 115 KGS, with 115 KIAS (or more) straight-and-level (SNL) at 5200 RPM. At 5400 RPM SNL, I usually see 120 kts or more for both of these values. During take-off climb-out at 60 kts with WOT, I see 5100-5180 RPM. At SNL WOT I always turn the obligatory 5600-5650 RPM. On days with significant winds aloft in my favor, I enjoy the “rush” of flying up to 140 KGS, usually turning just 5300 RPM, or less. Another performance marker I check regularly is this: At between 1,000 and 2,000 ft MSL, I go to WOT and enter a 95-KIAS climb. I focus on holding that 95 KIAS climb. This normally produces a climb rate of 700-900 FPM and about 5200 RPM. Try that–-just remember to hold 95 KIAS at WOT. All of these mentioned values are most often with a 300-lb load (i.e., me, fuel, & “stuff”) at about 2,000-2,500 ft MSL; and most of my take-offs are at 50-200 ft MSL (except for mountain flying, discussed later), at least around my home airport. Naturally, I experience a wide range of air temperatures here (30-112 F), but I fly mostly in mornings, so I’m usually up when it is 40-80 F.
3. Roger said it well. Your first priority should be to get that SNL WOT RPM up to where it needs to be–-at 5600-5650.
4. I would not focus too much on what you think are universal relationships between coarseness of prop pitch and either speed or take-off performance with this particular prop. Some of that thinking goes out the window. That’s because of the way it flexes, thereby mimicing a constant-speed propeller. And that’s why the DUC manual specifically says (paraphrased): “Set it to the recommended 24 degrees, then go fly it and observe the resulting performance; it is quite normal to have to do this–-and adjust pitch-–several times until you find your sweet spot.” It is also why the DUC manual cautions that this prop is subject to cavitation in various situations, including when static and during initial take-off roll. Furthermore, DUC cautions that you shouldn’t even try your ingrained “short-field take-off procedures,” where you throttle all the way up, then release your brakes. It’s counterproductive! You must instead throttle up gradually and smoothly while rolling, to get the blades flexing and “grabbing” the most possible air without cavitating.
5. That (#4) said, don’t assume that your prop is actually currently set at 24 degrees, as your logbook entry by Sportair, or whoever, may indicate. My friend here with a 2015 Sting S4 had the same 24 degree entry (from Sportair) for his DUC Swirl Inconnel, but after he experienced the same problems you have, we discovered it was actually set at 24.5 degrees (refer to my pitching method below).
6. I see no logical reasons why an S4 should be any slower that the earlier S3 model I have (at least my friend’s S4 isn’t–-i.e., if his pitch was the same as mine). In fact, I would think the newer technology might be faster, all else being equal. We lowered the pitch of my friend’s S4 closer (but not all the way) to my pitch of 23.3 degrees and got him the right combination of improved speed and climb-out performance he is comfortable with. Myself, I’m sticking with the setting (23.3 degrees) that focuses on speed, and I know I am definitely sacrificing take-off and climb-out performance for it! When we travel to the mountains together, he gets the jump on me on take-offs, but I usually edge slowly past him during cruise. I prefer to mitigate my poorer take-off performance in the mountains, especially when density altitudes are high (>7,000 ft), by foregoing the weight of a passenger. Or, if I do take a passenger along to the mountains, it’s when low air temperatures drive DAs way down!
7. You said that we have the “exact same prop.” I’m not so sure. There is a “thin” DUC Swirl Inconnel model and a “fatter” one. I have the thinner one and it really bends and reveals its stretch marks; the fatter one doesn’t show much in the way of such marks and I suspect it provides less ‘constant-speed’ effect–-and quite possibly, maybe less potential speed.
8. If you do actually have the thinner model my friend and I both have, consider that it may get faster over time. I found the critical break-in period to be from about 100-500 hours, when I continually noticed incremental speed improvement. It seemed to stabilize after about 500 hours.
9. If and when you do try to: (a) determine where your pitch is set now; and (b) begin flight-testing various adjustments to it, in search of your preferred sweet spot, I recommend you use my pitch-setting method described below. It’s specific to the DUC Swirl Inconnel prop with its very unique blade design, including that very aggressive blade “twist.” You’ll achieve more standardization, accuracy and hopefully repeatability, than if you try to follow DUC’s abbreviated and vague (in my opinion) procedures. Note that TL Ultralight allows us Sting owners to set the pitch of our props, but we cannot remove or install a prop unless we become an A&P or LSRM.
A. Use a digital protractor, such as available from Aircraft Spruce for about $100;
B. Place a small square block of wood over the bolt-heads on the propeller hub to provide a level surface, then place the protractor vertically
against the board and “zero” it;
C. Move the first blade into position for pitching on the pilot-side of the aircraft and level it horizontally, using a carpenter’s level always
placed on the same place on the stainless steel leading edge;
D. Mark the stainless steel leading edge 20 cm (7+7/8-in) inboard from the blade tip;
E. Extend a line (again, using a carpenter’s level) from that 20-cm mark vertically (i.e., perpendicular to the stainless steel edge) down the
back side of the bade and reference it with the outboard edge of a 2-inch-wide piece of masking or painter’s tape;
F. Adequately “shim up” the digital protractor (to nullify the uneven, slightly convex surface of the back side of the blade) using small strips of
Gorilla tape or other appropriate material;
G. Place the shimmed protractor on the masking or painter’s tape, with outboard edges of both tape and protractor in perfect vertical
alignment;
H. Clamp the shimmed protractor in place on the blade using a small ratcheting clamp with rubber feet, such as the ones Harbor Freight Tools
sells;
I. Measure the existing blade angle (ensuring the angle-–and not its reciprocal-–is measured) or, loosen hub bolts, apply a similar, but larger,
ratcheting clamp to the root of the blade for incrementally moving it–-and begin setting a new blade angle;
J. Repeat the process for the other two blades;
K. Once all blades are set to the same angle, tighten and re-torque the hub bolts per DUC specifications. This can be a tricky, time-consuming
process to get them all just right (i.e., within 0.1-degree, or so, of each other) after final torque-down. Take your time and don’t rush it. The
smoothness of all your subsequent flight depends on it; and finally...
L. Once everything is perfect, go flying and check the performance.
(Note: After I set my pitch this way, to 23.3 degrees 800 hours ago, I took it in for a dynamic balance. The technician told me there was “nothing more he could do-–it was already as perfectly balanced as it could possibly be!” That may speak more to the quality of this prop than to my method of setting pitch, however.)
10. So, in summary, If your Sting S4 was mine, and sitting in my hangar, this would be my “treatment” regime: First, I’d measure and record the existing pitch of each of those three blades. While I was at it, I’d go ahead and check their tracking, too. Then, I’d pick a time of day and temperature that’s somewhat “standard” (that I could count on to be about the same every day) for the area, and go flying, to once again measure all the parameters discussed above in #2. Next, I’d re-set the pitch to either 23.7 or 23.5 (or, 23.3--what the heck!, as starting points), then go flying again under the exact same conditions (or as near as feasible), and record the resulting performance. And I’d keep at it until I found my particular sweet spot that also met the ‘required’ parameters. I’d do all of this myself; however, if I wasn’t already a “wrencher” and pretty well-versed on the subject, I would certainly involve a trained mechanic in the process. Good luck, and please post your findings for the benefit of the next Sting owner with the same issues!
I’m out. There’s a nice, smooth wind-flow aloft, here today. I’m going to go see if I can set a new speed-over-ground record in my Sting!
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