At what cost speed?

[0TreeLemur]

The recent thread on Carson's speed got me interested in performance.   At 1.3 times the glide speed, Carson's speed is pretty slow.  But what about at the higher end?  Today I went out in our C and collected some data that some of you might find interesting.

Here is a screencap of the data sheet, and a plot showing what I'd call the Carson's parameter (gal/h per knot) vs airspeed for 16 different combinations of RPM and MP.   I leaned prior to the first data point by leaning the mixture until roughness developed, then richened it until it smoothed out.   I didn't touch the mixture the rest of the flight,  so the FF values at higher power are high.  The numbers on the plot replace symbols, and represent the number shown in the right-most column of the data sheet.

Test done at a DA of 6500'.   Interesting stuff.  The data are a little noisy.  Points 2 and 10 are out of place compared with the rest of the data.  Anybody else done this?  Someday I'd like to re-do at higher DA.

The second reason I wanted to fly today was to pass 1000 hours!   Success!!!

 

[Ragsf15e]

13 hours ago, 0TreeLemur said:

The recent thread on Carson's speed got me interested in performance.   At 1.3 times the glide speed, Carson's speed is pretty slow.  But what about at the higher end?  Today I went out in our C and collected some data that some of you might find interesting.

Here is a screencap of the data sheet, and a plot showing what I'd call the Carson's parameter (gal/h per knot) vs airspeed for 16 different combinations of RPM and MP.   I leaned prior to the first data point by leaning the mixture until roughness developed, then richened it until it smoothed out.   I didn't touch the mixture the rest of the flight,  so the FF values at higher power are high.  The numbers on the plot replace symbols, and represent the number shown in the right-most column of the data sheet.

Test done at a DA of 6500'.   Interesting stuff.  The data are a little noisy.  Points 2 and 10 are out of place compared with the rest of the data.  Anybody else done this?  Someday I'd like to re-do at higher DA.

The second reason I wanted to fly today was to pass 1000 hours!   Success!!!

 

So #4 was about Carson speed for you? If so, it seems like it’s in a reasonable place?

[PilotX]

I have no idea what I am looking at but I am super curious now. Congrats on the hours! 

[0TreeLemur]

3 hours ago, Ragsf15e said:

So #4 was about Carson speed for you? If so, it seems like it’s in a reasonable place?

These results say that if I want to go fast and be most efficient while doing it, then 11, 9, and 1 are the most fuel efficient ways to do it.   Admittedly the data are pretty noisy.  They suggest that a really good, clean data set would consist of curves of constant RPM made up of points at different MP, with  gph/knot increasing with increasing MP.

Points 1 and 9 both use 20" MP.   Those results say that I should redo the test up higher, where 20" is WOT.   Engine power will be the same, but less drag because of reduced air density.   The reduction in speed for the reduction in MP is not as large as the reduction in fuel flow.

 

1 hour ago, PilotX said:

I have no idea what I am looking at but I am super curious now. Congrats on the hours! 

Thanks!  About 570 of those are in our M20C.

[philiplane]

Optimize your CG to get the most speed. You'll find best CG is about 75 percent of the way towards the aft limit. Push the seats back in cruise, and use ballast as needed to achieve the best CG. Optimum CG is generally worth 4 to 7 knots.

[jaylw314]

1 hour ago, 0TreeLemur said:

These results say that if I want to go fast and be most efficient while doing it, then 11, 9, and 1 are the most fuel efficient ways to do it.   Admittedly the data are pretty noisy.  They suggest that a really good, clean data set would consist of curves of constant RPM made up of points at different MP, with  gph/knot increasing with increasing MP.

Points 1 and 9 both use 20" MP.   Those results say that I should redo the test up higher, where 20" is WOT.   Engine power will be the same, but less drag because of reduced air density.   The reduction in speed for the reduction in MP is not as large as the reduction in fuel flow.

 

Thanks!  About 570 of those are in our M20C.

If you run at a lower altitude, say, 3000' MSL, you'd be able to get a larger range of MP values to test at any given RPM, since you're non-turbocharged.

In your chart, are the GS values calculated or GPS values?  They look like GPS values.  I suspect using GPS speed (corrected for wind and altitude to a "calculated" IAS) to measure airspeed will be more accurate than depending on the airspeed indicator for those small differences in airspeed.  Granted, I suppose changing winds could be a variable...

Also, for posterity's sake, I think the gph/v and airspeeds should be in IAS, not TAS.

[0TreeLemur]

17 minutes ago, jaylw314 said:

If you run at a lower altitude, say, 3000' MSL, you'd be able to get a larger range of MP values to test at any given RPM, since you're non-turbocharged.

In your chart, are the GS values calculated or GPS values?  They look like GPS values.  I suspect using GPS speed (corrected for wind and altitude to a "calculated" IAS) to measure airspeed will be more accurate than depending on the airspeed indicator for those small differences in airspeed.  Granted, I suppose changing winds could be a variable...

Also, for posterity's sake, I think the gph/v and airspeeds should be in IAS, not TAS.

TAS values calculated by GNS430W using JPI900 provided OAT and encoder Pressure Altitude after entering CAS. 

GS from foreflight.  Not used in analysis.

Flight lines selected to be approximately normal to the wind so that GS is a loose approximation of TAS (sanity check).

At bottom of spreadsheet see data used for 3-way speed run to verify CAS calcs.

I know from experience that our C uses a lot more fuel at any speed below 5000 ft.   I only cruise that low if winds favor it.   She is happiest between 7,000 and 10,000.

Anyone is welcome to take their Mooney out and collect a dataset at a different altitude though...    I'll provide the empty spreadsheet. 

[0TreeLemur]

1 hour ago, philiplane said:

Optimize your CG to get the most speed. You'll find best CG is about 75 percent of the way towards the aft limit. Push the seats back in cruise, and use ballast as needed to achieve the best CG. Optimum CG is generally worth 4 to 7 knots.

My test variables didn't include CG.  Maybe someday.   My typical mission involves a CG near the forward limit.

[PT20J]

3 hours ago, philiplane said:

You'll find best CG is about 75 percent of the way towards the aft limit

I've always thought the aft limit would be better. 

[A64Pilot]

1 hour ago, PT20J said:

I've always thought the aft limit would be better. 

In other aircraft it is, don’t see why a Mooney would be different.

On edit if you plan to fly at the aft limit, do a check at landing fuel load to make sure your still OK, from a glance Mooney seems blessed with very little CG change with fuel burn, that’s not true for all though.

But I don’t think highest speed was his purpose, I believe his purpose was to record how much more fuel each kt of speed cost, and if that’s the case CG so long as it remained relatively the same shouldn’t matter, it will change slightly with fuel burn, but so slight as to be very unlikely to make much difference.

If someone has the power required curve for a Mooney it ought to illustrate the cost of speed pretty well as I believe fuel burn and power follow each other pretty close, but I’ve never done any real testing with piston engines so I’m not certain.

But it ought to be good enough

 

[carusoam]

For long distance flying… I generally select an altitude over 10k’.

• decent glide range
• no O2 requirement
• WOT for best airflow through the intake
• LOP by a couple of degrees… to use 100% of fuel to produce bhp….
• %bhp is altitude controlled… <65%. Sooo 0% fuel used for CHT control…

My M20C was capable of getting similar mpg as my Firebird…

finding peak with the single analog EGT instrument was OK…

The carburetor got things close enough….

Unfortunately, my M20C didn’t have the instrumentation required to make the calculations timely….

oddly, my Firebird didn’t have the instrumentation either….  

Somewhere near 18mpg on a really good day…
 

 

Slowing things down to use Vz…. WOT, peak, and 2550rpm (max efficiency of the TopProp)

Would net some interesting mpg figures….

Adjusting the B in WnB is always interesting….

When I ran these calculations years ago… we always had the backseaters with us…. So B was pretty far back, but not engineered that way…   

 

Backseaters often make the mpg discussion more of a necessity….

Fortunately, Vz is too slow for backseaters…  soooo speed is still more important that efficiency….

 

 

PP thoughts only,

-a-

[Jerry 5TJ]

Looks as if the trend of your curve is still decreasing at 130 knots, suggesting you can slow more and further improve your mileage.

Try the power setting from the C model’s POH that’s around 1900 RPM and 18” and I think you will get about 100 KTAS at <5 GPH.  That’s around 0.05 on your scale.  

At that power setting you are still faster than a Cessna 150.  Which isn’t saying much.   

[A64Pilot]

These numbers are down low as I most often fly short trips and just OFO flights, but I think Carson’s speed is roughly 120 kts in my J and I can get there with 6.5 fuel burn

thats about as good as I can get without trying too many other things like low RPM etc. That’s roughly 18 ish MPG?

But here’s the thing, I can bump it up to 8 GPH and get 135 kts or so and still get 17 ish MPG. Beyond that at some point the fuel burn per kt gets steep, which if we could find a power required curve specific for a Mooney would illustrate that and give us numbers. It would look like this, just with numbers on both speed and power, knowing % power, or power in HP we could easily calculate fuel burn

https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node97.html

See the power required curve is what we used to refer to as bucket shaped, and yes there is one spot where it’s the lowest, but as it’s bucket shaped or actually more U shaped there are points where you can make significant increases or decreases in airspeed without much additional fuel required, but as you begin to climb the walls of the bucket each additional kt cost more and more fuel.

I don’t believe Carson’s speed is anything more than the forward point of the power required curve before it starts to climb steeply, not to belittle it, just it’s not rocket science.

To put that in perspective a neighbors Legend Cub cruises at I believe 80 kts burning 6 GPH. That’s 13 MPG,  which in my mind is horribly inefficient.

My C-140 as an example is a pretty efficient little airplane and cruises 90 ish kts burning 5 GPH for 18 MPG.

A Mooney though can carry four people at at least 50% greater speed than either of those little airplanes, and burn the same as the 140 but much less fuel than the Cub.

It was somewhat surprising for me to discover that I burned much less fuel over the same distance than a Cub.

 

 

[KLRDMD]

8 hours ago, carusoam said:

 soooo speed is still more important than efficiency….

Anthony! You're admitting that on a Mooney site! You sound like a Bonanza owner 

Edited November 20, 2022 by KLRDMD

[A64Pilot]

41 minutes ago, KLRDMD said:

Anthony! You're admitting that on a Mooney site!

Says the guy with the big motor Mooney.

Of course it is to him, or the big motor makes no sense at all, and he’s in good company, because almost since the beginning airplanes have been sold on speed, almost never have you seen an ad about how much less fuel it burns over the same distance than the other brands, just how much faster it is.

Why I like a J is to some extent you get both, increased speed AND increased efficiency.

But to continue the thread a decent idea of the cost of speed could be given based on average fuel burn and speed of one of the N/A 300 HP Mooney’s compared to speed and fuel burn of a J in normal cruise.

Internet says a J will burn 10.5 GPH and return 155 kts, which I’m pretty sure is ROP, but what’s the internet numbers for the N/A big motors?

On edit, my WAG is that you’ll get about half the percentage increase in speed compared to fuel burn. You should roughly burn 1/3 more fuel, but I’m guessing a 1/6 increase in speed?

[ilovecornfields]

35 minutes ago, A64Pilot said:

Says the guy with the big motor Mooney.

Of course it is to him, or the big motor makes no sense at all, and he’s in good company, because almost since the beginning airplanes have been sold on speed, almost never have you seen an ad about how much less fuel it burns over the same distance than the other brands, just how much faster it is.

Why I like a J is to some extent you get both, increased speed AND increased efficiency.

But to continue the thread a decent idea of the cost of speed could be given based on average fuel burn and speed of one of the N/A 300 HP Mooney’s compared to speed and fuel burn of a J in normal cruise.

Internet says a J will burn 10.5 GPH and return 155 kts, which I’m pretty sure is ROP, but what’s the internet numbers for the N/A big motors?

Internet numbers: My ovation does 185 kts at 4.5 GPH LOP.

Real numbers: 160 kts at 10 GPH. I have TKS so the normal ones are probably a little faster.

[aviatoreb]

Just now, ilovecornfields said:

Internet numbers: My ovation does 185 kts at 4.5 GPH LOP.

Real numbers: 160 kts at 10 GPH. I have TKS so the normal ones are probably a little faster.

My rocket runs on a 5gram pellet of enriched uranium.  It does 275knotx indicated and I only have to fill it up every 50 years.

[A64Pilot]

10 minutes ago, ilovecornfields said:

Internet numbers: My ovation does 185 kts at 4.5 GPH LOP.

Real numbers: 160 kts at 10 GPH. I have TKS so the normal ones are probably a little faster.

So you burn .5 GPH Less than a J, but are 5 kts faster?

I’ve run a few 520’s and 550’s, if run like most do for speed they burn 1/3 more and make 1/3 more HP than a 200 HP Lycoming that’s run ROP for speed, so they burn about 15 GPH when you run ROP as you will do if your after going fast.

I’d suspect that if run for speed your fuel burn would be closer to 15 GPH and your speed maybe 15 kts faster, rough approximations?

I believe your pulled back and LOP as that extra 100 HP has to be worth way more than 5 kts

[jaylw314]

2 hours ago, A64Pilot said:

I don’t believe Carson’s speed is anything more than the forward point of the power required curve before it starts to climb steeply, not to belittle it, just it’s not rocket science.

It's supposed to be the speed at which the force from drag (and, conversely, the force forwards from the propeller) is at its minimum.  You're right, though, that that doesn't mean its the most important variable for choosing a speed.

Aren't the NA big motors similar to the IO-360 in terms of BSFC?  I imagine any difference is mainly from weight and airframe drag 

Edited November 20, 2022 by jaylw314

[ilovecornfields]

2 hours ago, A64Pilot said:

So you burn .5 GPH Less than a J, but are 5 kts faster?

I’ve run a few 520’s and 550’s, if run like most do for speed they burn 1/3 more and make 1/3 more HP than a 200 HP Lycoming that’s run ROP for speed, so they burn about 15 GPH when you run ROP as you will do if your after going fast.

I’d suspect that if run for speed your fuel burn would be closer to 15 GPH and your speed maybe 15 kts faster, rough approximations?

I believe your pulled back and LOP as that extra 100 HP has to be worth way more than 5 kts

I’m not sure what a J does but I tend to cruise in the 10-12k’ range so that probably helps with the TAS. When I had a 200 HP airplane I rarely went above 8000’.

I also fly ROP although at those settings (<50% power) sometimes it’s pretty close to peak.

[A64Pilot]

1 minute ago, ilovecornfields said:

I’m not sure what a J does but I tend to cruise in the 10-12k’ range to that probably helps with the TAS. When I had a 200 HP airplane I rarely went above 8000’

It sort of doesn’t  matter, as both are NA airplanes and in truth both are about equally efficient, both can make similar % power at the same conditions, or said another way a 300 HP IO-550 can make 1/3 more power under the same conditions a 200 HP IO-360 can.

But and this is a big but, the conditions have to be the same, of course one can’t be LOP while the other ROP etc

[A64Pilot]

8 minutes ago, bluehighwayflyer said:

If you get 160 KTAS out of your Ovation at 10 GPH that is quite good.  My previous early J wouldn’t do that, although some J’s might.  It makes me want an Ovation, to be honest.  

He probably can because he can be LOP, a J can run LOP of course, but not at 160 kts

My old Maule had a 540, I could fly in formation with a 360 powered identical Maule burning less fuel, because to make the cruise power he was ROP, and my 540 I could be peak or slightly LOP

But I suspect most that fly behind 550’s in Mooney’s go much faster than 160 kts

[Shadrach]

11 hours ago, carusoam said:

For long distance flying… I generally select an altitude over 10k’.

• decent glide range
• no O2 requirement
• WOT for best airflow through the intake
• LOP by a couple of degrees… to use 100% of fuel to produce bhp….
• %bhp is altitude controlled… <65%. Sooo 0% fuel used for CHT control…

My M20C was capable of getting similar mpg as my Firebird…

finding peak with the single analog EGT instrument was OK…

The carburetor got things close enough….

Unfortunately, my M20C didn’t have the instrumentation required to make the calculations timely….

oddly, my Firebird didn’t have the instrumentation either….  

Somewhere near 18mpg on a really good day…
 

 

Slowing things down to use Vz…. WOT, peak, and 2550rpm (max efficiency of the TopProp)

Would net some interesting mpg figures….

Adjusting the B in WnB is always interesting….

When I ran these calculations years ago… we always had the backseaters with us…. So B was pretty far back, but not engineered that way…   

 

Backseaters often make the mpg discussion more of a necessity….

Fortunately, Vz is too slow for backseaters…  soooo speed is still more important that efficiency….

 

 

PP thoughts only,

-a-

That often makes for a dismal hit to speed this time of year. I too like to fly up high but it’s just not practical this time of year. Tale of two flights…same day, same airplane.  Playing winds and power settings correctly made the round trip faster than a no wind day.

 

[A64Pilot]

What I get from the “Internet” being I think Aviation Consumer is 

J model 155 kts and 10.5 GPH

Missile, which I think is a 550 modified J so identical airframes?

173 kts and 15 GPH

That seems to correlate with what I expected, if true. I expect they were at best power for both, because people want to read about speed.

For whatever it’s worth but 7 to 9 thousand or so is the generally accepted “best” cruise altitude for the average GA aircraft, my Maule was an outlier though, it was 10 to 12 K, I supposed because it had excess wing area for efficient cruise and or an inefficient airfoil, or both

[A64Pilot]

7 to 9 thousand isn’t considered by most to be O2 altitude? I flew the Maule up to 12K or so with no O2, but I was younger. I believe the older you get, it’s likely the altitude you should be on O2 gets lower

Efficiency wise I believe that’s the altitude where you can run LOP at WOT and indicated A/S is L/D max.

Too high for me, and likely too slow too, but I believe most efficient as in MPG, regardless of exhaust.