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 Previous Item - Lake of the Woods Gaggle
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GPS speed check, Jemima |
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Chris Vernon-Jarvis - Apr 26,2004
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Got my Airmap mounted in Jemima today and checked out my ASI. Well, I thought it would be aout 5 to 10 knots shy but seems it is 2 or 3 knots over. <br /><br />At 5,000 revs I had 63/64 knots going and 72 /73 knots coming all at an indicated 70 knots.<br /><br />Unfortunately I can't do the three direction thing, long enough to get settled would put me in the rocks.<br /><br />What does every one else get? I can't see why Jemima would be the slowect Searey around, she's not quite the heaviest.
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Don Maxwell - Apr 26,2004
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Chris, 70 knots is about 80 mph--so that's not terribly slow. (I've forgotten which engine you have--a 912S?)<br /><br />Couldn't you go higher to do the three direction thing, then convert from TAS to IAS?<br /><br />Is your prop pitched right?
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Chris Vernon-Jarvis - Apr 26,2004
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Frank,<br />I'm guessing the RPM is pretty accurate, it's an EIS digital Engine monitor. The air was very smooth, went most of the way hands off, 15 mins out, 15 mins back! I was surprised how much wind there was at 2,000 ft (1300 AGL) as on the ground it was dead still. Wind seemed pretty constant, maybe only 1 to 2 knot variation in GS over the distance each way. I have a 912S.<br /><br />Jemima stalls at 41/42 knots flaps up. guess that's a straight 40 corrected but given a decent tolerance for the ASI I would say reading within 2 to 3 knots at 68 knots isn't worth spending a whole lot of time on. She weighs 926lbs empty. I have tried increasing prop pitch, could not decide which I preferred. She will easily pull 5,800 in level flight, 5,650 to climb at 65 knots. (Originally I set her up to pull 5,400 static.) This Summer I'll try setting it coarser again.
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Jeff Arnold - Apr 26,2004
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Compare the following:<br /><br />1. Pitch prop so maximum rpm in cruise is 5800 and then throttle back to 5000 rpm.<br /><br />2. Pitch prop so full throttle at curise is 5000 rpm.<br /><br />Same power? I think not. Rotax manual suggests YES. Any comments?
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Chris Vernon-Jarvis - Apr 26,2004
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Jeff<br />If the power curve shows max power at 5,800 revs then an engine constrained below that will not be developing max power.<br /><br />If you cruise at (say for instance) 5000 revs and you fly at Airspeed 70 knots, then you wind out the prop a turn and get 72 knots obviously you are using more power (at a higher throttle setting) as you are overcoming more wind resistance. However you can not develop more power than the engine is capable of at any particular rev setting .<br /><br />If the max the engine will produce at 5000 revs is, say, 75hp, then even if you wind the prop out so that you are full throttle at 5,000 revs you will still only get 75hp, the problem is then that you can not get 5,800 revs and 100hp for climb out!<br /><br />As usual there are several options and compromises. <br />You can set the prop so you get 5,800 maxed out in level flight. You won't over rev in level but then you won't get max power in climb.<br />You can set for best speed at 5,000 revs. You might be a little power shy climbing unless you lower the angle slightly, not a huge deal<br />You can set for max power in climb, either at best climb angle, probably about 55 knots or best climb speed at about 60 knots but then you may give up two or three knots at cruise.<br /><br />Here is another interesting point. I have heard some pilots talk about a sweet spot at about 90mph, a point at which some combination of aerodynamics and lift drag curve meet to give an extra few mph. Technichally I suppose that is possible but of course the form drag is also going up so it is a bit unlikely. I certainly have not found it yet.<br /><br />
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Don Maxwell - Apr 26,2004
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Chris, the 912S has a torque peak at about 5500 rpm, which is also the maximum continuous rpm. The horsepower rises slightly above 5500 rpm and peaks at 5800 rpm (because that's the redline speed), but you can run at that rpm for no more than 5 minutes.<br /><br />(Note that for the 912S, the 5 minute limit is related to rpm, not throttle setting.)<br /><br />Also, the chart for a 912S with a variable speed prop shows that the maximum cruising power at full throttle is at 5500 rpm. <br /><br />In other words, this suggests that for maximum cruising efficiency with a GROUND-adjustable prop, you'd want 5500 rpm at full-throttle cruise. For maximum takeoff performance, you'd want 5800 rpm at full throttle during takeoff, but then you'd have to throttle back to 5500 (max) for cruise. Because the manifold pressure will then be less than if you were at full throttle (with a greater prop pitch), you'll be getting less than maxumum cruise power, even though the rpm is the same, 5500.<br /><br />So you can vary the pitch according to your needs--greater pitch for cruising efficiency and flatter pitch for quicker takeoffs and faster climb.<br /><br />(We might want to move this discussion to STS.)
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Chris Vernon-Jarvis - Apr 26,2004
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More or less what I said Don, but my book says 5,400. max continuous.<br />Does anyone use 5,400 continuous?<br /><br />I know that you might turn out some more power technically if you could run the revs up a bit more but usually it falls off due to other considerations, eg, valve bounce, poor combustion chamber design, breathing difficulty etc. Went through this whole thing with racing cars about 35 years ago. Of course then we could run them till they broke. Would not want to try that in a plane.
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Don Maxwell - Apr 27,2004
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Chris, I referred to the online Operator's Manual at Kodiak Research: <a href="http://www.rotax-aircraft-engines.com/pdf/dokus/d00118.pdf">http://www.rotax-aircraft-engines.com/pdf/dokus/d00118.pdf</a> , page 10.1.2, ff.<br /><br />The 914 manual, by the way, gives the same max continuous rpm of 5500, but also specifies 35' max continuous manifold pressure (because of the turbocharger). So the 914 max cont is either / or.
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Paul Friddell - Apr 27,2004
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Chris, your indicated airspeeds are approximately in line with my SeaRey with the 912UL engine. I have the old two-blade Ivoprop pitched to 5400 RPM on the ground. After considerable experimentation, this setting seems to provide the best overall performance for my bird. <br />
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Roy Pinner - Apr 27,2004
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Chris---I have done a lot of cross country and played with prop pitch and power settings---I get best all around preformance from my 912S with prop adjusted to 5300 take off rpm on take off run( 75 degrees 'F'). This RPM will allow me to cruise at 82 knots or about 94 MPH @ 7500 to 8500' burning 4.5 GPH. It took about 300 hrs break in on engine before it attained full power. the wate take off preformance is very good at this rpm setting below 90 degrees 'F' over that with 2 people you may want to flatten the pitch a small amount aprox-5500 take off RPM. My SeaRey empty wt is 960#--also have long range tank and carry a lot of fuel---My prop is 3 blade Ivo ground adj.---hope this imformation will help---I have almost 600 hrs at this time---Roy
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Chris Vernon-Jarvis - Apr 28,2004
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Thanks Roy<br />Yup, that's a point, my engine only has 67 hrs and probably has a way to go to be broken in yet. Not sure I'll ever get another 8 knots though.
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Roy Pinner - Apr 30,2004
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Chris---forgot to tell cruise rpm----5200 for 94mph at 7500 ft aprox 21.5 in mp----you will be amazed at the diference prop pitch makes and engine break in--give it a try---Roy
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