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#21
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Kevin Horton wrote in message ...
Please detail how you determined the cruise speed and climb rate. If you used the IAS and converted to TAS there are a lot of ways that errors can be introduced. For example, I've seen reports of ASIs with over 10 mph of instrument error, and I've communicated with builders who found that flush static ports causing the airspeed to read over 10 mph too low. A local RV-8 builder had that problem recently. And some of the methods that people quote for using GPS to get TAS aren't mathematically sound, although the error here would only be a few mph, unless you've done something really creative. Lots of info on how to figure out TAS from GPS data, how to calibrate ASIs and how to measure static source position error at my web site: http://go.phpwebhosting.com/~khorton...ex.php?&PID=48 http://go.phpwebhosting.com/~khorton...ex.php?&PID=49 If you used the VSI to determine climb rate, be aware that VSIs can have very large errors. You really need to use an altimeter and a stop watch. And you need to understand that the rate of climb that Van quotes is almost certain an extrapolation down to sea level at standard temperature. You are almost certainly at a higher altitude when you do your testing, and the temperature may very well be warmer than standard, so your rate of climb would be a bit less than 2500 even if everything else is sorted out. First to Jerry: Yep, am a member of the RV-8 list. Got a couple of great ideas from them. Now to Kevin: Yer right. I am currently using TAS from a Blue Mountain Efis compared to GPS ground speed. The two seem to match up pretty well. I am fully cognizant that the the GPS ground speed is affected by wind, but over a hundred hours or so I am recognizing that the two track quite well. This ain't science, yet. :-) Climb rate is absolutely based on VSI and again, agree that there is substantial error present...possibly. What started this quest was more of a global sense that we were way off from the standards...followed by a flight with an acquaintence's RV-8 with ostensibly the same set-up as mine (well, different prop, and no nose wheel)...and he made me look like I pretty much have been making everyone ELSE look. As I said, we went through everything on Tuesday and are replacing the gas filter for one that is less restrictive and tuning the prop governor for FULL take-off power. I am quite a lot more confident that there are no subtle engine problems, and it is likely going to be a fix each aerodynamic detail at a time to get it to where I want it to be. We have also fixed the fuel-flow indicator, and between that and the BMA EFIS's ability to record flights, I am going to be able to actually generate some data. My first two projects will be actual time-to-climb vs fuel usage and the leaning exercise needful to work with the GAMI folks so I can further tune my fuel system. I will be (after I have some better data (as opposed to impressions...hey, impressions are good enough for newsgroups, I need data to dicker with manufacturers)) talking with the aerocomposites folks to make sure I have the right prop. They are advertising an RV 8 with fairly spectacular performance with one of their props (but a 74 inch prop, mine is considerably shorter, but with (I think) a considerably wider chord). We'll compare and contrast and see if we can gain from the prop losses that I suspect are occuring. Then on to the nickle-and-dimers...off with the tie-down rings, better seals at the various interfaces, maybe have a look at a different cowl/induction system. I have an outstanding airplane. It is fast, climbs very well, and is more airplane than I have ever flown. And a great deal of fun. Now I just want more! :-) Jim |
#23
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Jim,
Just to eliminate all errors, I guess you don't misstake knots with mph, but 210 mph is 182,45 knots, what units is your ASI and GPS reading? Jan Carlsson www.jcpropellerdesign.com "Jim Harper" skrev i meddelandet om... Hi, group. I've come to this august assembelage of persons to ask for ideas, and to discuss a few of mine. I am the proud owner and flyer of a lovely, new, RV 8A. Alas, I am not the builder, but as I am sure you know, owning one begins the process of working with one... At any rate, we have around 135 hours on this machine. Engine is an IO-360-A1B6 with one Plasma ignition, one Bendix mag, Gami injectors, Aero Composites CS prop, stock Van's intake with K&N filter on the left front baffle shelf. Since the airplane started its flying life, itsbeen power down. The engine is one of those Bulldog engines imported from England. It has around 900 hours total, and is fresh (just before installation) off of an overhaul. Compressions are fine, plugs look good and I am taking good care of it. Van's says a 200 horsepower RV8A will cruise at 210mph at 75% power at 8000 feet. Book says it will climb at 2500 fpm at 1500 pounds. Mine cruises at 182mph and climbs at 1800 fpm. Now, nothing wrong with that performance, but still and all...The airplane is straight and slick. No unusual drag makers. It would seem that I am missing around 25 or so horsepower. Oh yeah, the engine SOUNDS like it is running great, starts great. It is somewhat less great starting hot, and idles badly, when hot...not something that one can't work around, but not as good as it should. I have written that off as typical fuel-injected lyc problems, but perhaps not. Oh yeah, the performance hasn't deteriorated...pretty much what it was from the start. You might ask why I have tolerated it up to now, but I really didn't consider that I was limited until I flew with another RV 8A...which just walks away from me. Makes me look like I make purt' much everybody ELSE look! :-) The two aircraft are nearly identical in appearance, so I became concerned. The most common cause for missing hp is clogged fuel injection/injectors. Now, this is a real possibility, given that the fuel pressure misbehaves (I thought it was just a monitor problem, but I am starting to wonder. I am also dealing with the afore mentioned idle problem. The airplane also really doesn't run well without the boost pump on. I was originally told that the boost pump thingy was "the way that one works" but in the face of my power loss, I wonder, now. Mitigating against that is that the airplane still burns 10-11 gallons per hour, and I would think that if I was fuel limited I would burn less...but that is my speculation. Final point is that the cylinders all run reasonably close both in CHT and EGT...so again, speculating, I think that the injectors per se are OK. Cause number 2 (or so) is a 1 tooth misalignment on the valve timing. It would seem that such a phenomena occurs more regularly than one would think. It is pretty easy to check...you just put the engine at #1TDC and then, with the #2 valve cover off, rock the prop. If the lifters of #2 don't move within 20 or so degrees...that's the diagnosis. Certainly this one is possible, and easily checked. Another possiblility is retarded timing. I have little speculation on that. Tuesday, I am going to go to an A&P who is a bud and we are going to go through things. We'll start with the valve timing, given that it is a trivial exercise. We'll clean out the injection system, 'cause it likely needs it anyway. Of course we'll check the timing. Any of y'all have any OTHER suggestions? The intake manifold would look to be OK, no holes or the like. Thanks for sharing my quandry. Jim |
#24
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"Jan Carlsson" wrote in message ...
Jim, Just to eliminate all errors, I guess you don't misstake knots with mph, but 210 mph is 182,45 knots, what units is your ASI and GPS reading? Good thinking, good question. Knots. :-) Jim |
#25
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I was typing in the figures for the RV8 in my program when notis that 210mph
was 182 knots. by the way, with all settings at standard it gave an optimum diameter of 72" but there are many but's. Jan Carlsson www.jcpropellerdesign.com "Jim Harper" skrev i meddelandet om... "Jan Carlsson" wrote in message ... Jim, Just to eliminate all errors, I guess you don't misstake knots with mph, but 210 mph is 182,45 knots, what units is your ASI and GPS reading? Good thinking, good question. Knots. :-) Jim |
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