912S Tandem Dominator

[Doug Riley]

Chris Spadafora asked a couple questions about my Dominator tandem trainer with 912S. Prop = 70" 3-blade Warp (Ernie's standard for the tandem). Pitch = about 10 deg. measured at the tips.

Max RPM: static, I have no idea (it's irrelevant except as an approximate starting point); I've pitched it as a no-compromise climb prop, redlining at 5800 RPM at 70 mph. Best rate of climb two-up seems closer to 75, so I may add a touch more pitch. Rotax specifies 5800 as redline.

Redrive ratio: the standard one for 912's, 2.4-something. They don't give you a choice of ratios as far as I know. Unlike the 2-strokes, in the 912-914 series the gearbox is a part of the crankcase, is open to the interior of the crankcase and shares the same oil, motorcycle style. This affects the choice of oil.

[CLS447]

Thanks Doug, those are good numbers. Are you very happy with it's performance? Did you see Dick DeGraws' EJ-22 Install on his Dominator? Very nice. BTW Did Carol & Dick show up at Bensen Days?

So.....You could add pitch but it would probably slow your rate of climb?

[GyroRon]

Chris they were there, the Degraws, but they didn't bring any flying machines to fly. Bummer huh? they have the coolest aircraft and they show up empty handed.... Carol and me bummed flight time in Ernies ultrawhite Dominator

[Doug Riley]

Chris, I live in northern Vermont. It's winter most of the time, and even the summers are cool. (It snowed earlier this week.) My airport's elevation is only 200 ASL. Given these conditions, the 100 hp engine is plenty. I weigh 180-185 and fly 220 lb. students. The climb when solo is scary. Still, it probably could use more power if I lived where the density altitude was frequently higher than it is around here.

I don't know anything about Subarus, so even if Dick had brought his machine, I wouldn't really know what I was looking at. My general impression is that they are brutally heavy.

My idea about adding pitch was actually to improve climb. For no-compromise climb performance, you'd like the engine just to hit redline at your best rate of climb speed. Mine currently redlines at 70 mph, which appears to be about 5 mph below best rate of climb. At 75, the engine will exceed redline unless I back off throttle. I may add a touch more pitch to better match the engine-prop unit to this machine.

[Brent_Brown]

Doug what would the rpm be at max angle climb this is what I set my prop at slower but steeper climb out. This is how I would need to climb if in trouble and headed for trees at the end of a low flight.

[Doug Riley]

Brent, sorry, I didn't quite catch the question. The 912S redlines at 5800. Set the prop so that you hit this redline at whatever airspeed you feel is most critical. I don't go out hedge-hopping with students aboard, so my particular interest is to get as high as possible as soon as possible. This gives you the most choice of landing spots WHEN the engine quits.

People who are interested in long cruises often set the prop at coarser pitch to keep engine RPM's down in high-speed flight. Naturally, you give away some climb performance when you do this; your engine won't hit redline on climb.

You're right, best ANGLE of climb is typically a little slower than best rate, but the difference isn't very great in a small open gyro. The best insurance when hedge-hopping is to have lots of excess airspeed that you can convert into a quick zoom climb when necessary. IOW, don't go chugging along at 50 feet with no more than best-angle or best-rate airspeed. Have an extra 10-15 mph or more on hand.

(And remember that several of our brethren have died from encounters with power lines. Jack Cohee, Bob Lindsay and Russ Jansen are three of the famous ones.)

[CLS447]

Doug , I was under the impression that a faster spinning prop with less pitch would give best climb but poor cruise(always keeping redline in mind). Slower spin, more pitch, = better cruise. The answer = in-flight adjustable prop.

Isn't that pretty much the way that one would use it?

[Doug Riley]

Chris: Yes.