GlennM Posted February 27, 2013 Report Share Posted February 27, 2013 I disagree with the statement that other engine experience does not apply to Rotax engines. I think the physics are the same no matter where you are or what you are working on. I become very skeptical when the only reasoning behind statements are that someone says this is so. I tend to want to have some proof or some scientific principles for reasons. For one, I like the discussions and the chance to learn. You are correct, I don't have much Rotax experience besides operating the one in my CT. However, I am a mechanical engineer and have worked as a test engineer at Pratt and Whitney on military aircraft engines. Over the last ten years, I have worked on failure analyses of gas turbine engines for Siemens Energy, which is the former Westinghouse Electric Corporation. Currently, I am the manager of the engineering testing group at Siemens Energy. In addition, I have also consulted on bearing designs for top fuel motorcycle dragster engines and reliability changes for OX5 engine restorations, so I have an interest in the reasoning behind explanations given for the statements that are made here. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted March 9, 2013 Report Share Posted March 9, 2013 For those not believing lower RPM can be worse for an engine, I have one word: harmonics. Link to comment Share on other sites More sharing options...
Doug G. Posted March 10, 2013 Report Share Posted March 10, 2013 Doesn't that also apply to higher RPMs also. Harmonics can occur at any frequency. Link to comment Share on other sites More sharing options...
ctfarmer Posted September 1, 2013 Report Share Posted September 1, 2013 FWIW and to back track a little. ie Drag is directly proportional to air density (rho). peter Link to comment Share on other sites More sharing options...
ctfarmer Posted September 1, 2013 Report Share Posted September 1, 2013 As well - temperature is inversely proportional density according to the ideal gas law. So it temp (K) goes up, pressure goes down proportionately and so does drag (and vice versa)! Link to comment Share on other sites More sharing options...
FlyingMonkey Posted September 1, 2013 Report Share Posted September 1, 2013 Took this shot a couple of days ago, straight and level at 5300rpm...125 knots TAS...I'm happy! https://www.facebook.com/photo.php?fbid=10151801108243286&l=8116ce2532 Link to comment Share on other sites More sharing options...
FastEddieB Posted September 1, 2013 Report Share Posted September 1, 2013 Andy, TAS is calculated using the input OAT. Was it really 89°F at 5,400'? If not, the computation is off. Still, I'd kill to get numbers anywhere near that in my Sky Arrow! Link to comment Share on other sites More sharing options...
FlyingMonkey Posted September 1, 2013 Report Share Posted September 1, 2013 It was a hot day, I believe 92-94 degrees on the ground. But I would not be surprised if it was off a little, not sure how I can verify the accuracy of my OAT. It's on the bottom of the plane though, so it's definitely not getting sun heated. Could get heat-soaked on the ground from hot pavement though. I had been in the air about fifteen minutes when I took the picture. My max RPM is about 5700 at 3000ft, so maybe 5600rpm at 5400. At WOT I saw 130kt TAS at 3000ft on a slightly cooler day, but again that may be inaccurate. Link to comment Share on other sites More sharing options...
FastEddieB Posted September 1, 2013 Report Share Posted September 1, 2013 Oh, you have an OAT probe. That's different - I don't, so I thought maybe you had just failed to input it correctly. That's great speed, nonetheless! Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted September 1, 2013 Report Share Posted September 1, 2013 The indicated might be a bit high resulting in the true being a little high. I can match that speed only with more RPM or winter conditions. Link to comment Share on other sites More sharing options...
Roger Lee Posted September 1, 2013 Report Share Posted September 1, 2013 Hi Andy, I'm about the same. I hate to say it, okay no I want to say it: I told you so. You and I are about the same altitude for test. Ed's could be a little different just because he lives and flies pretty high and looses some of his HP compared to us so his numbers could easily be slightly different. We all live, fly and have things on our plane setup a little different (i.e. props) not to mention our gauges can be different and usually are. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted September 1, 2013 Report Share Posted September 1, 2013 I was taking altitude into account. The reason so many have the same numbers is they are optimistic. If we had low numbers we would get our pito-static systems checked. Edit: Maybe you are right I was assuming an optimized prop, without flattening it I would believe that speed at that RPM. Link to comment Share on other sites More sharing options...
Roger Lee Posted September 1, 2013 Report Share Posted September 1, 2013 It would not take much to alter the numbers. You and I both know that being closer to 5600 up to 5700 at WOT is far better than being at 5400 rpm at WOT at altitude. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted September 2, 2013 Report Share Posted September 2, 2013 I'm about the same. I hate to say it, okay no I want to say it: I told you so. I have no problem at all with you being completely correct on this. Link to comment Share on other sites More sharing options...
S3flyer Posted September 2, 2013 Report Share Posted September 2, 2013 It was a hot day, I believe 92-94 degrees on the ground. But I would not be surprised if it was off a little, not sure how I can verify the accuracy of my OAT. It's on the bottom of the plane though, so it's definitely not getting sun heated. Could get heat-soaked on the ground from hot pavement though. I had been in the air about fifteen minutes when I took the picture. My max RPM is about 5700 at 3000ft, so maybe 5600rpm at 5400. At WOT I saw 130kt TAS at 3000ft on a slightly cooler day, but again that may be inaccurate. Andy, It is certainly possible OAT was 89F at (what I think is) 5000 AGL but not probable given the ground temp. Average temperature lapse is about 3.5F per 1000ft up to 36K feet. At 5K AGL, this would be about 18F lapse giving you a theoretical OAT of 76F. As a comparison, it was 104F in Dallas a couple days back and OAT at 5K was in the seventies. A couple ways to check: 1) Simply look at the forecast winds aloft/temp and compare your actual. I find this to be within 5 degrees depending on the age of the forecast. 2) Calculate TAS using your GPS then back into OAT. For those comfortable in Excel, use this for a mathematically sound method: Link to comment Share on other sites More sharing options...
FlyingMonkey Posted September 2, 2013 Report Share Posted September 2, 2013 If the OAT probe is off, is there anything I can do about it? Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted September 2, 2013 Report Share Posted September 2, 2013 replace it or recalibrate if your EFIS permits. if you want accurate TAS you would be concerned about your IAS's accuracy as well as OAT. Link to comment Share on other sites More sharing options...
Roger Lee Posted September 2, 2013 Report Share Posted September 2, 2013 Go into the D100 menu and turn the OAT on or it has a loose wire up where wings attach. Link to comment Share on other sites More sharing options...
Karl Posted September 30, 2014 Report Share Posted September 30, 2014 I've been flying my new CTLS for nearly 3 months now and during one of my many local flights I noticed that on a nice calm morning flight the Dynon skyview always showed a head wind component regardless of what direction the aircraft was flying to So I did some testing, flying opposite headings at same alititude and air speed and comparing TAS with GPS ground speed and found the skyview indicating a 6knt higher reading than the GPS at 105 IAS and at 1500ASL I've repeated this test many times now with the same outcome. Static vent position is mostly blamed for incorrect airspeed readings, have any of you ever tested your airspeed indicators and found them to be out by that much ? Karl Link to comment Share on other sites More sharing options...
Dan Kent Posted September 30, 2014 Report Share Posted September 30, 2014 Karl, I have a low wing composite with dual SV's and i have done the same testing many times at different engine rpm. What I have done is select a DA (usually 3500 to 4000'), set the autopilot and rpm (I've tested from 4,000 to 5500), use the SV wind vector to either nose into or have it on my tail, let everything stabilize and record ground speed and TAS. I then hit the 180 degree button and once turned let everything stabilize and record the GS and TAS. Average the GS and the TAS (which should be the same or within 1-2 kts) and I have found my TAS is 8-9 kts low at all rpm's. I have an appointment this Thursday to have my system tested for leaks. Link to comment Share on other sites More sharing options...
Karl Posted October 1, 2014 Report Share Posted October 1, 2014 I did some more research in to static vent position to see if that is causing the high IAS reading on my CTLS If there would be a problem with the static vent this would also affect altitude and vertical speed indicator, so I kept a close eye on my altitude and VSI as I commenced a take off and noticed an instand increase in altitude of 20 ft and a vertical climb of 300ft/ min before the landing gear had left the runway. This lets me believe that the static vent is located in a low pressure aerea on the aircraft, it's position is just back and between the main landing gear. Is this the same position on all other CTLS and CTSW models ? Karl Link to comment Share on other sites More sharing options...
Anticept Posted October 1, 2014 Report Share Posted October 1, 2014 The prop wash has an effect on the static air pressure. They put an air dam on it to negate some of the effects, but no static port position is perfect (especially on a tractor). That's why we have something called "installation error" that is corrected with charts, and using those charts, you change from IAS to CAS. Add in density altitude, and now you have gone from CAS to TAS. Link to comment Share on other sites More sharing options...
Doug G. Posted October 1, 2014 Report Share Posted October 1, 2014 Only, as far as I know, no such document exists for the CTLS. Link to comment Share on other sites More sharing options...
Anticept Posted October 1, 2014 Report Share Posted October 1, 2014 Doug: do you mean the charts for IAS > CAS? I find a LOT of aircraft lack such charts in GA. It's more common in large and corporate aircraft. Generally, the installation error is only a couple knots off. By the way, Karl: The test you did is not a good test. Interference on the airstream from the ground during rotation causes odd effects. A good test is with secondary instrumentation that performs a reading outside of the aircraft airstream, or comparing airspeed in still air with that of a GPS device. If you want to check for your instruments accuracy, take your plane to an instrument shop. There, they can do a pitot static test, which involves sticking a "sucker" to the static port and a condom looking thingy over the pitot tube. The pitot-static testbed has it's own calibrated instruments and generate a vacuum on the static port, and the readings in the cockpit are compared with the calibrated readings on the testbed. Also, you can recalibrate your static port on D series dynon units too. You put the aircraft in a hangar on a still-air day and cover the pitot and static lines with a cloth to make sure the air is as still as possible, and then instruct the EFIS to reset reference to 0. It is important that this be done with air as still as possible, as it's very sensitive instrumentation and a windy day can lower the static pressure inside the hangar. Link to comment Share on other sites More sharing options...
Doug G. Posted October 1, 2014 Report Share Posted October 1, 2014 Yeah, but then they would want money! Link to comment Share on other sites More sharing options...
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