Request - M20J Cruise Perf Numbers for Benchmark!

[testwest]

OK guys!! I think I have a pretty darn good model of the M20J in Benchmark. I could use some help validating the model. If anyone has some good cruise data numbers I could throw 'em into Benchmark, and compare.

Attached is a Benchmark screenshot....fudged for a 25 degree LOP cruise at 8000'. So, for example, at 2600# gross weight, standard day, WOT/2500RPM/25degLOP, you are actually making about 67% power at the engine, and should have a fuel flow of 8.8 gph, IAS of 143 knots, TAS of 159 knots. The equivalent Best Economy per the POH under the same conditions would be a MAP of ~22.5 inches instead of WOT, and a FF of 9.5 gph. Same speed, 18 nmpg versus 16.5. I am using a combination of Benchmark and the iPad app called "Power Wheel" to come up with LOP data.

If there is enough interest, I can make a little test card to fill in the data. The program inputs are

RPM

MAP

Airplane Gross Weight

Pressure Altitude

OAT

Best Power, Best Economy, or LOP. I can estimate other than 25 deg LOP with some work.....

Any nonstandard configuration? I can handle most of the props now except an MT, but don't have an IO-390 or a Missile modeled yet.

Looking forward to some interesting discoveries with this.

[Jeff_S]

Norman, that's interesting. I'm finally learning how to interpret your chart. I have a 1990 J with the Hartzell 3-blade top prop that was put on 10 years ago.  I was in fact flying in just your configuration yesterday, 8000' WOT at 2500 RPM, although leaned 25 LOP.  I don't know exactly what my weight was, and it was warmer than standard, but I was only getting about 150 KTAS and ~135 KIAS.  I would love to see your chart with the Hartzell 3-blade Top Prop to see if there is that much of a performance penalty for the prop or if I need to look for other drag sources.

Jeff

[GeorgePerry]

Quote: testwest

 for example, at 2600# gross weight, standard day, WOT/2500RPM/25degLOP, you are actually making about 67% power at the engine, and should have a fuel flow of 8.8 gph, IAS of 143 knots, TAS of 159 knots.

[201er]

Quote: GeorgePerry

If you can find a bone stock J that'll do 159 KTAS on 8.8 GPH, then somehow it's defying the laws of physics.  I don't have any "cruise" numbers collected on a data card handy, but my 700+ hours in Mooney's tells me your model’s underlying assumptions  must be inaccurate.  No M20J is that fast on that little gas.  A more realistic number, and what I’ve seen, is 67% power would yield 156-162 KTAS at 10.2-10.8 GPH ROP.  With EGT set 30 deg LOP that should yield about 147-152 KTAS at 8.6-8.9.  And those performance figures assume, proper control rigging, a fresh wax job, and no bugs on the leading edges.  

[jetdriven]

Here you go Norman.  This is a 1977 J, SN 0162.   Completely stock, 5000 AFTT, 1400 SMOH, fresh painted prop.  There is 2 KTAS loss from improper gear door rigging.  But here you go.

 

Edit:   Rpm is 2350.  Tach is off 

[Cruiser]

My flight on Sunday was in smooth air, 2450 RPM, WOT, 2740#, 7000', 30.15 Baro, 10°C OAT, 15° LOP, 150 KTAS, 9.3 gph. What does Benchmark say the performance should be?

Sorry I didn't pay attention to MAP or IAS, I will start noting that in the future. 1984 M20J, McCauley prop, Lycoming FREM 275 hrs. w/roller tappets.

I too believe the POH numbers quoted above are unrealistic. 22.5 at 8000' is a bit high and 8.8 gph is much too low. Are you setting best economy at peak?

[testwest]

Wow! Such great response!! OK here we go, one at a time. See the attached for Jeff S, this is modeled with the Hartzell three blade. IAS is ~138 or so, but I am doing a wild ass guess on your conditions.

[testwest]

And for George, you said

"A more realistic number, and what I’ve seen, is 67% power would yield 156-162 KTAS at 10.2-10.8 GPH ROP."

Benchmark gives me 157.8 KTAS at 67% power, box stock airplane. So I think the drag, prop and power models are good. I can't make it do Best Power at 67%, it defaults to Best Economy for fuel flow. And LOP is really a WAG at this point. This tells me I need to focus on the fuel part of the thing.

And thanks! I am gonna have to defend this thesis some day soon in front of a bunch of professors, without a teleprompter. So keep me on my toes.

[testwest]

Here is 201ers model, it is about 5 knots optimistic, assuming max gross weight. It is a work in progress, and I really can't model LOP well at all at this point.

[testwest]

JimR, thank for the kind words!

[testwest]

So, for Jetdriven, I get 159 KTAS at 70% power under those condtions, where you "should" be getting 154 KTAS. If I drag the Cdo down (so to speak) to .1875 instead of .165 or so I get your 152KTAS...I think maybe those doors are more costly than 2 knots?

Roy Loresti always said the J had a Cdo of .1599, but the aggregate data tell me a higher number is more likely.

[testwest]

And for Cruiser, hoo boy, I am not sure what to say. Your FF should be getting you 70% power if you are LOP. Benchmark says 161 KTAS...and you are saying 150 KTAS? Hmm. I still have a bit of work to do!

The model does use Best Economy at peak. It is still a work in progress.

I may go get a much later version of the POH and model it. Maybe the drag and Oswald's E will be closer. The thought of hand entering another couple hundred data points is a bit daunting though.

[jetdriven]

Norman:  here is another.  155 TAS at 11,500 Ft.   check the FF !

[Cabanaboy]

How do you calculate your percent HP number? Can you show your formula titles and with #'s filled in?

[carusoam]

Byron,

I really like the nmpg #.  Getting that number above 20, while flying an airplane, means everything. At least until the price of avgas descends to about $2.  LOP at high altitudes is a wonderful thing.

Can you attribute this greater efficiency to the 25 deg. timing?

Best regards,

-a-

[Cruiser]

Here is another flight.

2350 RPM, 25MAP, 2275#, 5500', 30.02 Baro, 13°C OAT, 45° LOP, 144 KTAS, 8.6 gph.

[M016576]

Quote: carusoam

Byron,

I really like the nmpg #.  Getting that number above 20, while flying an airplane, means everything. At least until the price of avgas descends to about $2.  LOP at high altitudes is a wonderful thing.

Can you attribute this greater efficiency to the 25 deg. timing?

Best regards,

-a-

[testwest]

So, for Jetdriven, Benchmark nailed your performance, under those same conditions it shows 155.2 KTAS. Within a knot using 55% power. At the POH "Economy Cruise" you would have been using a little over 8 gph and around 19 NMPG. LOP is good.

Curious how you calculated TAS, under your atmospheric conditions you have 123 KIAS.

Assuming no instrument correction, the position correction is -2 knots, so you get 121 KCAS.

Running the density corrections to TAS on the "MyE6B" iPad app gives 146 KTAS.

Assuming your reported TAS is accurate (GPS Speed Course?) I'll bet you dollars to donuts your airspeed indicator is reading about 6 knots lower than it should.

Also, your MP seems to be reading a mite low, it should be a little closer to 20 inches to make the power.

For Cruiser, 8.6 gph LOP is 130 hp from the Power Wheel app on iPad. That is 65% power. Benchmark says you should be indicating 146 knots, less 2 for position error correction, giving 144 KCAS, and assuming zero instrument correction.

The density correction to TAS from 144 KCAS gives 158 KTAS. Again, how are you calculating TAS? If your numbers are accurate, there is something slowing you down, like cowl flaps open or landing gear not fully retracting.

[jetdriven]

Quote: CABANABOY

How do you calculate your percent HP number? Can you show your formula titles and with #'s filled in?

[jetdriven]

Quote: testwest

So, for Jetdriven, Benchmark nailed your performance, under those same conditions it shows 155.2 KTAS. Within a knot using 55% power. At the POH "Economy Cruise" you would have been using a little over 8 gph and around 19 NMPG. LOP is good.

YES SIR IT IS. That was at peak.  GASP, wing is going to fall off.

Curious how you calculated TAS, under your atmospheric conditions you have 123 KIAS.

I calculated Density altitude from an iphone app called DenAlt. Then I plugged density altitude into an iphone app called iE6B. It came to 154.3 KTAS.  I could be not doing something right. Such as the 2 KT loss for KCAS.

Assuming no instrument correction, the position correction is -2 knots, so you get 121 KCAS.

So I should show 152.3 KTAS?  Do I use density altitude or straight pressure altitude for TAS calculations? iE6B has no temp variable for TAS.

Running the density corrections to TAS on the "MyE6B" iPad app gives 146 KTAS.

I should look at that app. I am sure my method is not optimal.

Assuming your reported TAS is accurate (GPS Speed Course?) I'll bet you dollars to donuts your airspeed indicator is reading about 6 knots lower than it should.

Nope, all I have is IAS for that leg. I didnt fly it, but a NASA engineer did.

Also, your MP seems to be reading a mite low, it should be a little closer to 20 inches to make the power.

That was full throttle at that altitude. It also has a Donaldson paper filter which should give more than the Brackett.  Today with a 52 degree OAT and 1200 feet and alt setting of 30.20 we were showing 29.7" of MP on the guage.  And 160 KIAS on the other.

 

For Cruiser, 8.6 gph LOP is 130 hp from the Power Wheel app on iPad. That is 65% power. Benchmark says you should be indicating 146 knots, less 2 for position error correction, giving 144 KCAS, and assuming zero instrument correction.

The density correction to TAS from 144 KCAS gives 158 KTAS. Again, how are you calculating TAS? If your numbers are accurate, there is something slowing you down, like cowl flaps open or landing gear not fully retracting.

[testwest]

So, for Jetdriven, Benchmark nailed your performance, under those same conditions it shows 155.2 KTAS. Within a knot using 55% power. At the POH "Economy Cruise" you would have been using a little over 8 gph and around 19 NMPG. LOP is good.

[testwest]

[testwest]

[jetdriven]

Here are two sets of trend sheets. The first one (from left) taken a few months ago with the timing at 20 degrees.  The second one (from left) was taken last week, at 25 degrees.  Pretty much identical but fuel flow, the later one is ~8% lower.

 

3 and 4 are the same thing except at low altitude.  Again, only difference was timing.  Difference here is a higher airspeed with about the same FF.

 

20 NMPG seems high, perhaps there are errors in my calculations. As Norm pointed out, I failed to calculate the loss of 2 KTS in the conversion to KCAS.  I also have been plugging in density altitude to the iE6B app to figure TAS.

[Cabanaboy]

Quote: jetdriven

sure.  You will get more from my next post, but here is the formula.  (FF * 15.13) / 200

 

density altitude and PA are calculated with a iphone program called DenAlt

TAS is calculated from IAS by iphone app called iE6B