201er Posted October 18, 2011 Report Share Posted October 18, 2011 I've been making some recent efforts to look for ways to optimize max speed in my 201. Still can't get close to book numbers. Then while talking to someone about a speed test flight I did today, he caught on to something. I pointed out that today was a very close to standard sort of day and yet the max MP I was getting came shy of POH power table values. I explained how I was getting 22" MP at 7,500 while the book was giving figures based on 23.6". I originally figured it was just the conditions or I wasn't developing max available MP (even with ram air open). Well based on the 1" per 1000ft rule of thumb and more specifically looking up a table, I discovered that the POH inflate available MP substantially for the max power settings and that has a bit to do with why I'm not attaining book value speeds. So while standard pressure at 8,000ft should be 22.225 inHG, the Mooney POH gives a 75% power setting based on 23.6" at the same standard temperature of -1C. Where the heck are they getting this number from? How is this supposed to be useful to me when it is inflated over 1.3"? I checked for other altitudes and the error was not constant so I don't think it is expected boost as the result of ram air. I also looked at power tables for other airplanes and they tend to have a more standard MP value relative to altitude. Anyone know what this is about? Is the error strictly limited to the MP value or are the %power and speeds derived based on that exact MP? Does this mean that 75% is actually impossible above 6,500ft under standard conditions? Quote Link to comment Share on other sites More sharing options...
Hank Posted October 19, 2011 Report Share Posted October 19, 2011 My Pre-J book shows 24.5" at 5000 msl [41º = 5C] and 22.5" at 7500 msl [32º = 0C]. I can confirm that at 10,000 msl I show a smidge above 20", and "the book" shows 20.2" so I have no reason to think it's off at lower elevations. Then again, I don't have Ram Air, either. My Book gives performance in 2500' intervals, so I cannot comment on 8000' with any exactness. If you are well off on MP, I would look for proper Ram Air Door operation and obstructions. How closely does your MP approximate atmospheric prior to engine start? Could be a gage problem, or the transducer, or a leak in the line to your gage. Quote Link to comment Share on other sites More sharing options...
201er Posted October 19, 2011 Author Report Share Posted October 19, 2011 No, I think my MP and ram air are just fine and the POH is in error. 22.5" sounds about right for 7,500MSL whereas my POH suggests 23.6" for 8,000!!! The MP gauge reads normal pressure when the engine is off on the ground and the ram air gives me the expected .25inHG boost in MP. But to be over 1" off on the book makes a pretty big difference. So what I'd like to know is if everything else is thrown off or just the MP values. Is the plane only producing 70% power at 8000ft as the result of actual MP being 22" rather than 23.6" or do you really get 75% at 8000ft but their MP listings are off? Anyone have additional data to verify this? Quote Link to comment Share on other sites More sharing options...
jetdriven Posted October 19, 2011 Report Share Posted October 19, 2011 No way we can get 22.5" of MP at 8000'. Maybe 21" at most Quote Link to comment Share on other sites More sharing options...
fantom Posted October 19, 2011 Report Share Posted October 19, 2011 Quote: jetdriven No way we can get 22.5" of MP at 8000'. Maybe 21" at most Quote Link to comment Share on other sites More sharing options...
KSMooniac Posted October 19, 2011 Report Share Posted October 19, 2011 Marketing used to write the POH at many companies...not engineering. 21" is about all I get in that neighborhood as well. I calculate HP using my fuel flow times 15.1 while flying LOP...very simple, no charts needed. Quote Link to comment Share on other sites More sharing options...
jetdriven Posted October 19, 2011 Report Share Posted October 19, 2011 yep. set power with fuel flow. Thats easier than MP, RPM, and EGT all combined. Quote Link to comment Share on other sites More sharing options...
carusoam Posted October 19, 2011 Report Share Posted October 19, 2011 Marketing was brilliant that day. If you can't achieve the book MP, then you can't complain when you don't reach the corresponding book AS. Best regards, -a- Quote Link to comment Share on other sites More sharing options...
jetdriven Posted October 19, 2011 Report Share Posted October 19, 2011 I'm asking Bill wheat just how they got 201 MPH from a M20J back in 1976 at Kerrville. Quote Link to comment Share on other sites More sharing options...
fantom Posted October 20, 2011 Report Share Posted October 20, 2011 Quote: jetdriven I'm asking Bill wheat just how they got 201 MPH from a M20J back in 1976 at Kerrville. Quote Link to comment Share on other sites More sharing options...
Shadrach Posted October 20, 2011 Report Share Posted October 20, 2011 Quote: jetdriven I'm asking Bill wheat just how they got 201 MPH from a M20J back in 1976 at Kerrville. Quote Link to comment Share on other sites More sharing options...
201er Posted October 20, 2011 Author Report Share Posted October 20, 2011 No idea how they did the original test but the POH seems to imply that the stock 201 should get 201mph at gross weight and even more near empty. However, it again assumes you'll be doing it below sea level or in the dead of winter! Can someone explain to me how a Lycoming IO-360 was designed to require 30.2" MP to develop 100% power!? Or is it because of pressure loss in the system that you need 30.2" at the manifold to have 29.92" in the combustion chamber? Quote Link to comment Share on other sites More sharing options...
jetdriven Posted October 20, 2011 Report Share Posted October 20, 2011 Thats probably ram air open. Quote Link to comment Share on other sites More sharing options...
201er Posted October 20, 2011 Author Report Share Posted October 20, 2011 I thought ram air just helps to compensate some of the natural loss of the induction system and filter rather than giving you a boost over ambient pressure? Quote Link to comment Share on other sites More sharing options...
Hank Posted October 20, 2011 Report Share Posted October 20, 2011 Oh, the benefits of modern engineering! You can get 100% power at sea level and 2700, if you can find the elusive 30.2" MP. My tables show only 28.0", and at 2700 that yields 99.5% power and 169 MPH at gross. No ram air, either. You now have an official excuse to visit the beach. Take off and have someone watch the MP gage while you open & close the ram air door in level flight over the water, as low as you feel comfortable and safe. Report back your findings. Enjoy fresh seafood and ocean breezes for the rest of us. Quote Link to comment Share on other sites More sharing options...
201er Posted October 20, 2011 Author Report Share Posted October 20, 2011 Did they really improve the induction system by THAT much going from C to J or did they just merely doctor the MP numbers? Quote Link to comment Share on other sites More sharing options...
KSMooniac Posted October 20, 2011 Report Share Posted October 20, 2011 The J induction system is VASTLY superior to that of the E/F (C is carb....apples and elephants). The ram air on the E/F offers a legitimate 1" of MP boost while the J is perhaps a 1/4" or less since the basic system is so well done. Quote Link to comment Share on other sites More sharing options...
201er Posted October 20, 2011 Author Report Share Posted October 20, 2011 Well I've heard that. But what is the deal with 28" max MP for C and 30.2" for J. Is it just fiddling with the numbers or does the J actually develop higher MP at same altitude/conditions as its predecesors? Does E/F with ram air = J with ram air? Quote Link to comment Share on other sites More sharing options...
KSMooniac Posted October 20, 2011 Report Share Posted October 20, 2011 Good question... I think short of testing one example of each with calibrated flight test instruments on the same day I can't answer the question accurately. This would be a great question for Bill Wheat if anyone can ask him during the homecoming. It would also be interesting to see several different versions of the C or E/F POH thru the years and see if that max MP number varied. Quote Link to comment Share on other sites More sharing options...
201er Posted October 20, 2011 Author Report Share Posted October 20, 2011 Don't you guys have Mooney flying groups on this list? I don't know any Mooney fliers in my area so I can't try this, but I'm sure some of you here have flown to the same place in various letters of Mooney? Next time someone is flying a C/E alongside a J for a hamburger, can you guys compare what MP you get with and without ram air at the same altitude? Or is that a contradiction in itself to have a C flying "alongside" a J? Quote Link to comment Share on other sites More sharing options...
KSMooniac Posted October 20, 2011 Report Share Posted October 20, 2011 That could be done, but I question the accuracy of 30-45 year old manifold pressure gauges and the integrity of the lines that connect them. Perhaps Mooney didn't have an accurate instrument when they ran the tests for the J. If flying in a group, no need to fly alongside since you're looking for max MP. Just run WOT and report back, but I don't think that will be of much use with dubious instrumentation. Perhaps if everyone had a modern electronic instrument and a good installation it might have some merit. Quote Link to comment Share on other sites More sharing options...
201er Posted October 20, 2011 Author Report Share Posted October 20, 2011 Hmm, good point. But can't the amount of error be determined and corrected by noting each plane's engine off MP on the ground (ex plane A has 29"MP on the ground and 22" in cruise and plane B has 28"MP on the ground and 21" in cruise, would really indicate they are equal but off scale slightly)? Or do you think the gage may be inconsistent in a non-linear fashion? Quote Link to comment Share on other sites More sharing options...
jetdriven Posted October 20, 2011 Report Share Posted October 20, 2011 I know a good running clean wing E can give a J a run for the money. 155 TAS all day Quote Link to comment Share on other sites More sharing options...
KSMooniac Posted October 21, 2011 Report Share Posted October 21, 2011 Quote: 201er Hmm, good point. But can't the amount of error be determined and corrected by noting each plane's engine off MP on the ground (ex plane A has 29"MP on the ground and 22" in cruise and plane B has 28"MP on the ground and 21" in cruise, would really indicate they are equal but off scale slightly)? Or do you think the gage may be inconsistent in a non-linear fashion? Quote Link to comment Share on other sites More sharing options...
Parker_Woodruff Posted October 21, 2011 Report Share Posted October 21, 2011 Quote: KSMooniac It could be non-linear, I believe. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.