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How to troubleshoot for missing knots?

UnPossible

Well Known Member
Hey -I am struggling a bit to figure out where my missing ~10 knots are. I now have around 60 hours on my RV-7A. All of the wheel pants/fairings are in place.

My plane has an Aerosport IO-375 with the 8:1 pistons and is rated at 195Hp. It has a P-mag and one slick Mag.
The prop is a "blended airfoil" Hartzell constant speed prop
As far as I can tell, the engine appears to be running well / smooth with no issues.

Below are Van's numbers for 180 and 200 HP RV-7A's at gross. I extrapolated out the expected cruise speed for 195 hp to be in the neighborhood of 175 kts

Cruise [75% @ 8000 ft]
180HP = 197mph / 171 kts
200HP = 204 mph / 177kts
195HP = 202mph / 175 kts


Last night I went flying to check my speeds. My weight was somewhere around 1600 lbs. With the density altitude around 1500 feet, I climbed to 6500 feet, left the throttle wide open and leaned it out a bit(but stopped leaning well before any power reduction was felt).


The G3X calculated the % power to be ~75-76%. The prop was set at 2600 rpm. Fuel burn was around 14 gallons per hour.

I used the autopilot to fly a box pattern at the 4 primary compass headings.
My calculated true airspeed was indicated at 166 kts. When I averaged the GPS derived ground speeds of the four headings, I came up with 167.5 kts.


Now 167 knots is not way out of the range of expected speeds, but I am a bit suprised that I am not at least making the speed expected of a 180HP plane.

Does anyone have suggestions/a check list I could use to try and determine why I seem to be a bit slow? I normally cruise at 145 -150 kts at around 8gph, so the missing 10 kts on the top end aren't that big of a deal, but I'd still like to figure it out.


Thanks for any help,
Jason
 
me too!

yeah, I feel a bit slow also.
....even closing the canopy, and waxing only brought me up a couple! :)

( next time I'll do BOTH legs and see if that helps!)
 
I used the autopilot to hold ground track and altitude. I did not adjust the throttle, mixture or prop settings once I started the tests.

Below are the results of my tests.

166 kts TAS @ 270 = 177kts GPS ground speed
165 kts TAS @ 360 = 147 kts GPS ground speed
166 kts TAS @ 180 = 188 kts GPS ground speed
166 kts TAS @ 90 = 158 kts GPS ground speed

As I mentioned earlier, when I averaged the four ground speeds, I came up with 167.5 kts.

I used one of the spreadsheets that Dan listed above, and the spread sheet calculated TAS of 168 kts - which is really close to the 166 indicated by the G3X and the 167.5 calculated by the average of the 4 ground speeds.

Unless I'm looking at this wrong, it looks like my G3X is providing pretty accurate air speeds.

Thanks,
Jason
 
UnPossible,
I have the same engine-prop combination than you on my straight 7. 7497 Blended Airfoil blades, one Slick and one P-mag.
I will be flying within a few weeks and will see what speeds I will come up with.
I am very interested what restrictions you have on your engine-prop combination in case I am missing something. Do you have any information here??
 
I believe your error is that you flew ground track instead of heading. To maintain a constant ground track you will always be turning some into the wind; thus the lower GS.

The way I read the test you should fly at least three headings 120 degrees apart then fill out the spreadsheet. But what do I know?
 
Van's specs are for 8000 feet altitude. That will make up the difference in speed compared to 1500 feet.
 
I suspect that Van's settings are taken at 8000' pressure altitude, 2700rpm and full throttle, leaned to best power. The extra altitude will get you a bit as will the 2700rpm. It is quite possible that the power chart in the G3 is not right on.
After that I would look at the fit and finish of gear farings and wheel pants. Next on the list would be to check all rigging, wing flaps, ailerons etc.
Last on the list would be to check your cooling system.
 
Max speed at sea level

Max speed in a normally aspirated engine will be at sea level because you are at max horsepower. The higher you go the less horsepower you will make. The ratio of true airspeed/indicated airspeed is higher, but true airspeed decreases the higher you climb due to reduced horsepower. (Non-turbo of course)

IE. at sea level max power you may be 180 knots true, but by the time you are at 12,000 feet max power you are down to 160 knots true. At sea level you may indicate 180 knots and also be going 180 KTAS but at 12,000 you may be indicating 140 knots and going 160 KTAS.
 
max speed

Max horsepower but not max speed. If you want to go faster it usually occurs somewhere above sea level. There is a trade off between thinner air to be penetrated and loss of hp. In my Mooney it is about 7,500. From what I can discern from it is about the same for the RV.
 
At 7500 ft the max power available is around 75 percent. So there you have the max speed for that power setting. At lower altitudes the max speed will be higher but the power setting is over 75 percent then. This relationship can be seen on performance charts like the Piper Archer if I recall it correctly.
 
Start by being sure of your speed.

Confirm you flew 4 headings at one stable power setting and altitude, recording GPS track and GPS groundspeed for each, then plugged them into this spreadsheet (third block from the top)....

http://www.ntps.edu/information/downloads

or this one:

www.kilohotel.com/rv8/rvlinks/doug_gray/TASCALC.ZIP

I believe your error is that you flew ground track instead of heading. To maintain a constant ground track you will always be turning some into the wind; thus the lower GS.

The way I read the test you should fly at least three headings 120 degrees apart then fill out the spreadsheet. But what do I know?

So should ground track or heading be used? Dan's links suggest ground track is what you should be recording though my (often flawed) intuition would have said to use headings, not tracks.
 
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Power = speed

Max horsepower but not max speed. If you want to go faster it usually occurs somewhere above sea level. There is a trade off between thinner air to be penetrated and loss of hp. In my Mooney it is about 7,500. From what I can discern from it is about the same for the RV.

http://www.mooneypilots.com/mapalog/M20C%20Evaluation/M20C_Evaluation_Report.html

Level Flight Cruise Performance 1966M20C N3411X, Full
throttle, 2500 rpm, 50 degrees rich of peak, cowl flaps closed.

Altitude OAT
F MP RPM Direction IAS
KTS/MPH GPS Ground-Speed KTS
10000 59 20.1 2500 W 118/137 144
10000 59 20.1 2500 N 118/137 144
10000 59 20.1 2500 E 118/137 136
10000 59 20.1 2500 S 118/137 133
Average GS/TAS 139 KTS

7000 72 22.5 2500 E 125/144 144
7000 72 22.5 2500 N 125/144 147
7000 72 22.5 2500 W 125/144 139
7000 72 22.5 2500 S 125/144 144
Average GS/TAS 143.5 KTS

4500 80 24.8 2500 W 131/151 148
4500 80 24.8 2500 N 131/151 158
4500 80 24.8 2500 S 131/151 144
4500 80 24.8 2500 E 131/151 137
Average GS/TAS 146.75 KTS
 
To get a real world feel for the speed you could ...

There is a SARL cross country air race at Waupaca this Saturday (see www.sportairrace.org calendar of events). You could run that course and look at the results when they are published Alan Carroll has an RV-8 with an O-360-A1A and Hartzell BA with F7496 blades. He is FAST. Also Jeff Barnes has a RV-6 with an upgraded engine. Both will be well over 200 mph - you may be faster than your instruments say you are.

Bob Axsom
 
So should ground track or heading be used? Dan's links suggest ground track is what you should be recording though my (often flawed) intuition would have said to use headings, not tracks.

Fly constant track or heading, doesn't matter. But record track, and enter track in the spreadsheet, since track is the direction you are actually going over the ground. The ground speed and corresponding track define a vector (direction and speed). Three of them with tails at a common point define a circle, the radius of which is the actual true airspeed.

Dan's link to the ZIP file on Kevin Horton's site includes a pdf that explains the math.

Note that the three directions of flight could differ by only a few degrees, and you'd still get an estimate of true airspeed, but a small angular difference would make the result much more sensitive to measurement errors. 90 to 120° apart will give the most accurate results.

Oh yeah: in the singular case of no wind, the heading and track are the same, and then the ground speed will be the same in every direction. I got close to this when doing some testing last week after making some static port changes.
 
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Since I flew four different ground tracks, and the program only required 3, I played around entering various combinations, and each of them gave the same result within 1/2 a knot.

As far as rigging, all my control surfaces appear to line up in trail in cruise in level flight.

As far as gear leg fairings, I very carefully alighned them... the plane flys with the ball nearly centered (within 1/8 a ball diameter) in all phases of flight.
 
Since I flew four different ground tracks, and the program only required 3, I played around entering various combinations, and each of them gave the same result within 1/2 a knot.

Excellent...that is the cross check.

FWIW, the lazy way to fly the test (assuming an autopilot) is to set power, stabilize, trim, then set altitude hold and a heading bug mode. When stable, record GPS track and GS, spin the bug 120 degrees, wait for the turn plus time to stabilize again, record again, repeat. You're done, go home and crunch the numbers. Take along a spotter since you spend a lot of time head down.

A top speed run would eliminate a few comparison variables. Pick a nice smooth early morning with near standard atmosphere, set full power and 1000 AGL. And it's fun....
 
Quick question headed in another direction.
Does the indicated speed seem correct on the low end? Does it stall more or less at the indicated speeds where it is supposed to? Just wondering if there is an indication (pitot or static) error.
 
Quick question headed in another direction.
Does the indicated speed seem correct on the low end? Does it stall more or less at the indicated speeds where it is supposed to? Just wondering if there is an indication (pitot or static) error.


She stalls pretty much at the same speeds that Vans lists in their perfomance charts.... solo around 45-46kts and 51-52kts at gross.

At this point I don't think I have a significnat airspeed indication error as the stall speeds are right at the expected numbers, and my indicated 166 kts TAS calculated out to around 167kts true air speed when I did the GPS based calculations.

Thanks,
Jason
 
She stalls pretty much at the same speeds that Vans lists in their perfomance charts.... solo around 45-46kts and 51-52kts at gross.

At this point I don't think I have a significnat airspeed indication error as the stall speeds are right at the expected numbers, and my indicated 166 kts TAS calculated out to around 167kts true air speed when I did the GPS based calculations.

Thanks,
Jason

Perhaps your engine is not producing rated HP.

I had the same sinking feeling for 4 years flying with the Subby H6 that was supposed to produce 200+ HP and was convinced I had built a dog of an airplane. Its 75% top speed was about 15 knots below Vans numbers.

Fast forward, same airplane but with a Barrett IO360 parallel valve Lycoming and all the lost speed was recovered and a bit more.

These airplanes can be tweaked to pick up a knot or two, but 10 knots? Its my guess the engine is not producing rated HP. Nothing makes speed like HP.
 
Thanks David... I am starting to wonder about the engine HP as well... not much else make sense.

Think I will call the friendly folks at Aerosport Power in the morning and see if they have any ideas, or what else I can check.

Jason
 
.....
Cruise [75% @ 8000 ft]
195HP = 202mph / 175 kts


Last night I went flying to check my speeds. My weight was somewhere around 1600 lbs. With the density altitude around 1500 feet, I climbed to 6500 feet, left the throttle wide open and leaned it out a bit(but stopped leaning well before any power reduction was felt).


The G3X calculated the % power to be ~75-76%. The prop was set at 2600 rpm. Fuel burn was around 14 gallons per hour.

.Jason

I think you may have an error in power calculation. My Dynon shows 89-90% at similar numbers. I would lean a bit more to pick up some power.
Also, I think 75%power at 8000' is closer to 10gph....which in my -7 is 180ktas+.

aviation2012015.jpg
 
The G3X calculated the % power to be ~75-76%. The prop was set at 2600 rpm. Fuel burn was around 14 gallons per hour.

How does the g3x compute power? I would be more than a little dubious about the figure and it would be worth exploring their assumptions.

Does anyone have comprehensive performance data (ie power and fuel consumption versus man pressure, rpm and da) for this engine?
 
Hey -I am struggling a bit to figure out where my missing ~10 knots are. I now have around 60 hours on my RV-7A. All of the wheel pants/fairings are in place.

My plane has an Aerosport IO-375 with the 8:1 pistons and is rated at 195Hp. It has a P-mag and one slick Mag.
The prop is a "blended airfoil" Hartzell constant speed prop
As far as I can tell, the engine appears to be running well / smooth with no issues.

Below are Van's numbers for 180 and 200 HP RV-7A's at gross. I extrapolated out the expected cruise speed for 195 hp to be in the neighborhood of 175 kts

Cruise [75% @ 8000 ft]
180HP = 197mph / 171 kts
200HP = 204 mph / 177kts
195HP = 202mph / 175 kts


Last night I went flying to check my speeds. My weight was somewhere around 1600 lbs. With the density altitude around 1500 feet, I climbed to 6500 feet, left the throttle wide open and leaned it out a bit(but stopped leaning well before any power reduction was felt).


The G3X calculated the % power to be ~75-76%. The prop was set at 2600 rpm. Fuel burn was around 14 gallons per hour.

I used the autopilot to fly a box pattern at the 4 primary compass headings.
My calculated true airspeed was indicated at 166 kts. When I averaged the GPS derived ground speeds of the four headings, I came up with 167.5 kts.


Now 167 knots is not way out of the range of expected speeds, but I am a bit surprised that I am not at least making the speed expected of a 180HP plane.

Does anyone have suggestions/a check list I could use to try and determine why I seem to be a bit slow? I normally cruise at 145 -150 kts at around 8gph, so the missing 10 kts on the top end aren't that big of a deal, but I'd still like to figure it out.


Thanks for any help,
Jason

At 2600 rpm, you would have needed 22" of MP to produce 75% power.
 
All of the RV-7/7A race data

I went to the records I maintain for the Sport Air Racing league and extracted the records of every RV-7/7A that has ever competed and pasted them here. RV Golds have greater than 360 cu. in. displacement, RV Blues have 360 cu. in. engines and RV Reds have 320 cu. in. or less. There may be an isolated case here and there where a pilot throttles back a little but I don't personally know of any. These speeds are generally with wide open throttles, leaned for best power as well as can be determined. I do not have any propeller data.If you go to www.sportairrace.org then Records and Stats you can down load the full 1453 race flight records sorted by race, speed, class, aircraft type and pilot.

Bob Axsom

Race Record # Race Date Class Aircraft Type Pilot & Crew KTS MPH
1356 AirVenture Cup 2012 7/22/12 RV Gold RV-7 Christopher, Stephen & Melissia 202 232.46
1312 BSAR II, Three Forks, MT 7/7/12 RV Gold RV-7 Christopher, Stephen 194.80 224.17
781 Rocket 100, 2010 11/20/10 RV Blue RV-7 Wilcox, Gary 193.72 222.92
1398 Pappy Boyington Memorial Cup 8/18/12 RV Gold RV-7 Christopher, Stephen 192.74 221.8
1287 GNAR III, Ephrata, WA 6/16/12 RV Gold RV-7 Christopher, Stephen 192.4 221.41
221 AirVenture Cup 2008 7/27/08 RV Blue RV-7 Mullaney, John 188.17 216.54
665 Great Canadian Air Rally 2010 8/22/10 RV Blue RV-7 Wilcox, Gary 187.70 216.00
482 Rocket 100 2009 11/22/09 RV Blue RV-7 Wilcox, Gary 186.30 214.39
1316 BSAR II, Three Forks, MT 7/7/12 RV Blue RV-7 Carroll, Brian 185.66 213.66
1043 Tennessee 150 (Tullahoma) 9/10/11 RV Blue RV-7 Cunningham, Ben 184.93 212.82
1317 BSAR II, Three Forks, MT 7/7/12 RV Gold RV-7 Babler, Bob 184.35 212.15
970 Big Sky Air Race 7/9/11 RV Blue RV-7 Carroll, Brian 184.00 211.74
62 AirVenture Cup 2007 7/23/07 RV Blue RV-7 Klusmier, James 183.84 211.56
1027 Indy Air Race 8/13/11 RV Blue RV-7 Cunningham, Ben 183.55 211.23
1215 Hill Country 150, 2012 4/21/12 RV Blue RV-7 Carroll, Brian 183.27 210.90
1146 Rocket 100, 2011 11/19/11 RV Gold RV-7 Edmiston, Ray 182.19 209.66
568 Top of the Chesapeake 7/11/10 RV Blue RV-7 Reed, Gary** 181.83 209.25
1403 Pappy Boyington Memorial Cup 8/18/12 RV Gold RV-7 Babler, Bob 180.85 208.12
1418 MERFI 150, Urbana, Ohio 8/25/12 RV Blue RV-7 Furey, John 180.65 207.89
1419 MERFI 150, Urbana, Ohio 8/25/12 RV Blue RV-7 Hempy, Dan 180.19 207.35
1149 Rocket 100, 2011 11/19/11 RV Blue RV-7 Sundstrom, Steve 180.14 207.3
934 Great Northwest Air Race II 6/18/11 RV Gold RV-7 Babler, Bob 179.30 206.34
1076 Thunderbird 150 10/8/11 RV Red RV-7 Baker, Seth 178.53 205.45
789 Rocket 100, 2010 11/20/10 RV Red RV-7 Baker, Seth 178.47 205.38
1292 GNAR III, Ephrata, WA 6/16/12 RV Blue RV-7 Owens, Don 178.04 204.89
935 Great Northwest Air Race II 6/18/11 RV Blue RV-7 Carroll, Brian 177.38 204.12
1152 Rocket 100, 2011 11/19/11 RV Red RV-7 Baker, Seth 175.78 202.28
643 AirVenture Cup 2010 7/26/10 RV Blue RV-7 Mullaney, John 175.69 202.18
182 Colorado 150, 2008 6/29/08 RV Blue RV-7 Haley, Gary 173.00 199.09
728 Grace Flight 2010 10/2/10 RV Red RV-7 Baker, Seth 171.00 196.77
974 Big Sky Air Race 7/9/11 RV Blue RV-7 Haynes, Joel 170.38 196.07
703 Pagosa Springs 2010 9/26/10 RV Red RV-7 Baker, Seth 170.45 196.02
1368 AirVenture Cup 2012 7/22/12 RV Blue RV-7 Mullaney, John & David Henning 164.28 189.05
705 Pagosa Springs 2010 9/26/10 RV Blue RV-7 Rovey, Daniel 164.12 188.74
1277 Big Muddy, Carbondale, IL 6/9/12 RV Blue RV-7 Deacon, Clarke 161.92 186.33
549 West Texas 100 6/6/10 RV Red RV-7 Baker, Seth 161.58 185.94
669 Great Canadian Air Rally 2010 8/22/10 RV Blue RV-7 Wester, Cor 159.92 184.03
956 Top of the Chesapeake II 7/9/11 RV Blue RV-7 Katz, Paul 147.84 170.13
927 Big Muddy, Carbondale, IL 6/4/11 RV Blue RV-7 Deacon, Clark 145.37 167.29
417 AirVenture Cup 2009 7/27/09 RV Blue RV-7 Haley, Gary & Stephen Stephenson 138.73 159.65
220 AirVenture Cup 2008 7/27/08 RV Blue RV-7A Moore, Tom 189.41 217.97
57 AirVenture Cup 2007 7/23/07 RV Blue RV-7A Moore, Tom 186.91 215.09
227 AirVenture Cup 2008 7/27/08 RV Blue RV-7A Verdier, Jean-Pierre 184.87 212.74
336 Texoma 100 2009 5/31/09 RV Blue RV-7A Williams, Rusty 181.12 208.43
640 AirVenture Cup 2010 7/26/10 RV Blue RV-7A Moore, Tom 179.97 207.11
8 Taylor 100 2007 5/20/07 RV Blue RV-7A Moore, Tom 178.27 205.15
877 Lone Star Air Rally 5/14/11 RV Blue RV-7A Pauley, Bruce 177.68 204.48
878 Lone Star Air Rally 5/14/11 RV Blue RV-7A Crawford, Rusty 177.55 204.32
1095 Cleveland Texas Air Race 10/15/11 RV Blue RV-7A Pauley, Bruce 176.25 202.83
1178 Texoma 100, 2012 3/31/12 RV Blue RV-7A Pauley, Bruce 175.90 202.41
814 Taylor 150, 2011 4/2/11 RV Blue RV-7A Pauley, Bruce 175.58 202.06
843 Texoma 100, 2011 4/16/11 RV Blue RV-7A Pauley, Bruce 175.24 201.58
487 Rocket 100 2009 11/22/09 RV Red RV-7A Reiley, Darrell 172.11 198.06
410 AirVenture Cup 2009 7/27/09 RV Blue RV-7A Moore, Tom 171.31 197.14
342 Texoma 100 2009 5/31/09 RV Blue RV-7A Jett, Thomas 169.93 195.55
649 AirVenture Cup 2010 7/26/10 RV Blue RV-7A Vedier, Jean Pierre 168.83 194.29
379 Colorado 150 2009 7/23/09 RV Blue RV-7A Reak, Brad 168.39 193.78
906 Memorial 130, Terrell, TX 5/28/11 RV Blue RV-7A Pauley, Bruce 167.99 193.32
504 Taylor 150 4/11/10 RV Red RV-7A Reiley, Darrell 165.94 190.96
344 Texoma 100 2009 5/31/09 RV Blue RV-7A Moore, Tom 164.78 189.63
413 AirVenture Cup 2009 7/27/09 RV Blue RV-7A Verdier Jean-Pierre 162.28 186.75
794 Rocket 100, 2010 11/20/10 RV Blue RV-7A Hard, Jim 161.50 185.85
75 AirVenture Cup 2007 7/23/07 RV Blue RV-7A Verdier, Jean-Pierre 161.02 185.30
1279 Big Muddy, Carbondale, IL 6/9/12 RV Blue RV-7A Polete, Pete 157.20 180.90
925 Big Muddy, Carbondale, IL 6/4/11 RV Blue RV-7A Polete, Pete 149.43 171.97
509 Taylor 150 4/11/10 RV Blue RV-7A Hard, Jim 133.88 154.06
 
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Check Timing

It has a P-mag and one slick Mag.

Jason,

Anecdotal evidence from one local builder is that the out of the box timing curve with a PMAG may reduce power available. Try either the A curve or reducing timing towards 25 BTDC.

Also, what is your CG position? Many 7's are nose heavy. More balast in the tail and paint will help.

Cheers
__________________
Richard Talbot
RV-7A
Sydney, Australia
 
How does the g3x compute power? I would be more than a little dubious about the figure and it would be worth exploring their assumptions.

Best I know, none of the popular EFIS/EIS systems computes true power. They work from manifold pressure and RPM, without an input for mixture. Within the typical EGT operating range used by many here (full rich off the runway to 50 lean cruise) some documents suggest a power reduction of 3% to 8% depending on mixture:

2hwg2o1.jpg


Every standard Lycoming power chart includes a note stating "Maximum Power Mixture". If you're not leaned to roughly 125-150 ROP you're not at the rated power indicated by an RPM/MP combination.
 
I think Dan is on to something here. With a CS prop, it is hard to know that you are losing power, since the prop compensates and keeps the same rpm, sacrificing pitch and therefore speed. With a fixed pitch (as I have) it is easier to spot.

While I believe Van's tests are with straight mags, I don't think you could expect any increase in speed with one P-Mag. I have had all three combinations, 2 mags, one each, and two P-Mags. I didn't gain any speed until I went to both P-Mags, although I did gain economy and smoothness with just one. I know this isn't your concern, but something to think about.

Timing (and which curve you are using on the P-Mag) would be the first thing I would look at. Also, leaning to various configs would be something I would explore. I just don't think you are getting the hp you think you are.

Bob
 
Best I know, none of the popular EFIS/EIS systems computes true power. They work from manifold pressure and RPM, without an input for mixture. Within the typical EGT operating range used by many here (full rich off the runway to 50 lean cruise) some documents suggest a power reduction of 3% to 8% depending on mixture:

I haven't been able to get the equations from Garmin (or anyone else) for how they do it, but I am pretty sure they use more variables than some of the other companies use. I have found the G3X % power calcualtions to pretty much match book values. GRT is next best, and the classic Dynon's (D180) are probably the least accurate in my experience. I haven't got enough time behind the AFS to comment on theirs.
 
For my GRT Sport EFIS ...

... you build the % power tables yourself with data from the Lyco bible. The only input is MP for 5 to 10 specific RPMs ... at a given altitude of course. The EFIS then couples that with altitude to produce the % power display. As others said, there is no input for mixture setting.
 
May need more data

If you have not already read thru my "5kts slow" thread, you might take a look. Bob was instrumental in me getting accurate data. Turns out my first GPS data set was far from enough to make any valid conclusions.
 
One other flight testing cross check is if you use the auto pilot to hold altitude and you get indicated airspeed changes you are in an airmass that is moving vertically and you won't get accurate test results. When testing the low end of the airspeed you can get some really whacky results:eek:
 
I agree with Dan, the Lycoming book values correspond to max power mixture settings. If the EFIS calculations only include the map, rpm and da then the percentage power would represent this max power book figure.

A cross check is the fuel flow from the Lycoming book, again this is only for the max power setting.

I suppose that at other mixture settings an EFIS algorithm could assume the same volumetric efficiency and proportionally reduce the power accordingly. Is this an accurate assumption? I doubt Lop and rop would share the same volumetric efficiency.

My own EFIS employs an algorithm that effectively interpolates the Lycoming power charts so assumes the max power mixture setting. But I know this, and am happy to work with it. See Kevin Horton's engine power spreadsheet(s) to see the mechanics of this method.

As Paul states the manufacturers do not disclose the algorithms they use, but I do know the GRT system makes some assumptions, in fact the critical power data points must be entered into the configuration pages, the percentage power is computed from this and adjusted for da. I don't know if the max power mixture assumption is required or not. In view of this Paul's comment is very interesting re the GRT.

So what about the IO-375? What are the book values for that engine? So far as I am aware no one has undertaken the extensive testing that is necessary to provide the book values like Lycoming have provided for their stock engines.

How do you tell the g3x that you have an IO-375 engine and how does it manage the computation without the benefit of the Lycoming power plots?

I think the waters are pretty muddy on what is popularly displayed as percent power and the figures have to be treated with skepticism, even the g3x, especially since they are not open about their algorithms. This is a great pity since I find setting the engine against percent power far more intuative than by rpm and map. But then I know exactly what my EFIS percent power means.
 
All the fuzzy stuff about indicated percent power is why I suggest a 3-leg WOT run, leaned for best power, at low altitude on a near standard atmosphere day. That's all your installed HP, and a speed easily compared.
 
Hi, I don't know anything about building airplanes, but I do know something about physics.

This technique (four cardinal headings/tracks) doesn't really work.

You can prove this by assuming that you know your true airspeed, and working backwards. For example:

Say you're flying four ground tracks (0, 90, 180, 270) at a true airspeed of 170. Also, let's say there is a 50-knot wind blowing from heading 090.

Your north ground speed will be 170 * cos (arcsin 50/170) = 162.5 knots
Your south ground speed will be the same: 162.5
Your east ground speed will be 170 - 50 = 120 (dead upwind)
Your west ground speed will be 170 +50 = 220 (dead downwind)

Add 'em all up, divide by four, and you get a calculated true airspeed of 166.25. Even though we said initially that our true airspeed was 170.

If you do the same calculations with compass headings, the result is too high, around 174 knots "true" airspeed.

It just isn't accurate.
 
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Hi, I don't know anything about building airplanes, but I do know something about physics.

This technique (four cardinal headings/tracks) doesn't really work.

You can prove this by assuming that you know your true airspeed, and working backwards. For example:

Say you're flying four ground tracks (0, 90, 180, 270) at a true airspeed of 170. Also, let's say there is a 50-knot wind blowing from heading 090.

Your north ground speed will be 170 * cos (arcsin 50/170) = 162.5 knots
Your south ground speed will be the same: 162.5
Your east ground speed will be 170 - 50 = 120 (dead upwind)
Your west ground speed will be 170 +50 = 220 (dead downwind)

Add 'em all up, divide by four, and you get a calculated true airspeed of 166.25. Even though we said initially that our true airspeed was 170.

If you do the same calculations with compass headings, the result is too high, around 174 knots "true" airspeed.

It just isn't accurate.

Which is exactly why you should use an online calculator like this to correct for winds loft...

http://www.csgnetwork.com/tasgpscalc.html
 
Best I know, none of the popular EFIS/EIS systems computes true power. They work from manifold pressure and RPM, without an input for mixture.

I did a test for you today.

Here is a pic ROP followed by a pic LOP with the Skyview.

The first one is 16 gph
The second is at 11 gph


IMAG0975.jpg

IMAG0977.jpg


Both were taken about 1 min apart from the other with the mixture being the only change made.

The %power is between the MAP and RPM.
 
Good check. The percent power display does seem to reflect the sort of reduction predicted by the Lycoming graph in post #30. Perhaps Dynon is doing something with fuel flow. How about asking them for us?
 
This from Dynon

Percent power is calculated using everything you would find on a standard Lycoming or Continental power chart. MAP, RPM, OAT, Altitude, and fuel flow are all used. It looks it up just like you would on the chart. These complex look ups all assume you are ROP. It's also critical that your EMS use things like altitude and OAT since they can change the %power by over 20%. A lot of EMS systems seem to think you can do it with just an RPM vs. MAP table, but look at any power chart and it's clear you can't ignore the air density effects.

The charts also have data for lean of peak operation, which is actually much easier. If you know you are LOP, % power is just fuel flow multiplied by a constant.

The real trick is figuring out if you are ROP or LOP automatically, since the number you calculate will be totally wrong if you use the ROP chart when you are LOP. How we do that is not something we can share since we're the only ones that do it.
 
Dan, the Dynon algorithm is seemingly correct for the ROP side of most Lycoming engines. Good data tables!

I have used Kevin Hortons data to check this.

On the lean side of peak, the simple USG/hr x 14.9 s spot on.

Dynon seem to have it right unlike many f the others, certified or not.
 
I've been beta testing for Dynon for years and they calculate % power very well. Here is an excerpt from their forum with an answer from Dynon:

Percent power is calculated using everything you would find on a standard Lycoming or Continental power chart. MAP, RPM, OAT, Altitude, and fuel flow are all used. It looks it up just like you would on the chart. These complex look ups all assume you are ROP. It's also critical that your EMS use things like altitude and OAT since they can change the % power by over 20%. A lot of EMS systems seem to think you can do it with just an RPM vs. MAP table, but look at any power chart and it's clear you can't ignore the air density effects.

The charts also have data for lean of peak operation, which is actually much easier. If you know you are LOP, % power is just fuel flow multiplied by a constant.

The real trick is figuring out if you are ROP or LOP automatically, since the number you calculate will be totally wrong if you use the ROP chart when you are LOP. How we do that is not something we can share since we're the only ones that do it.
 
The real trick is figuring out if you are ROP or LOP automatically, since the number you calculate will be totally wrong if you use the ROP chart when you are LOP. How we do that is not something we can share since we're the only ones that do it.

And that is the one area that it is a bit unstable, often it shows you are around peak when you are not yet. Not hat it matters much because the zone of uncertainty is not great and it is precisely where you don't want to be anyway.

Great work!:)
 
Checked my GRT EIS on the way to Thomasville today, and it does not reflect mixture changes in the percent power display. Nice to know Dynon units do work mixture into the display. Anybody know about Advanced?

Keep in mind more inputs offer better accuracy only if the inputs themselves are accurate.
 
Keep in mind more inputs offer better accuracy only if the inputs themselves are accurate.

Yep - turn on the electric fuel boost pump and see what happens to Fuel Flow and %power (depending upon FF sensor used such as the Flow Scan). Not sure about the red-cube.
 
Keep in mind more inputs offer better accuracy only if the inputs themselves are accurate.

True. I have a GRT and an IO-360-M1B. Because I was lazy, I used a power chart from the GRT website for an O-360-A thinking it should be pretty close.

I became suspicious after a few months of flying (missing a few knots) and started digging into the lycoming charts and coming up with my OWN values to put in the GRT table.

I have 2 screens. At the moment, I have one programmed with the O-360-A and one with my -M1B numbers so I can see both %s. Looks like a disparity of 5 to 6 % but I have more flying to do at differing altitudes to see if that sticks. Also, I am now carrying the lycoming altitude performance chart so I can check both tables for power settings vs % and altitude etc.

If my M1B chart is correct, I have been flying around at 69 to 70% rather than the desired 75%. The past flight showed a 2 knot boost in IAS and TAS.

The last bit is to get my upper intersection fairings on (in process). Hopefully that will get the final knot.
 
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