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75% power?

D

David-aviator

Well Known Member
I am attempting to determine the top speed of this RV-8 at 75% power.

Today I was at 8000' PA (29.92) with an OAT of 57F. The speeds I recorded via gps were not what I thought they should be - 174.46 knots or 200.76499 mph.
Maybe the engine was not developing 75% power.

When the 8000' PA number and 57F is plugged to density altitude calculator it comes up 9949'!

I assume the base line altitude (8000') for 75% power with WOT is density altitude, is that correct?
 
David-aviator said:
I assume the base line altitude (8000') for 75% power with WOT is density altitude, is that correct?
Click to expand...

Pretty close. But there is a temperature dependent correction for the intake flow speed. You need to look at a power chart for your engine.
But at a density altitude of 9700' (my calculation) you won't get 75%.
 
What was the fuel flow?

edit

And rated horsepower?

And RPM during the test?

edit again

And throttle setting during the test?
 
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David here is a handy calculator. http://www.digitaldutch.com/atmoscalc/ Yes, note that 8000 density altitude is .75 atmospheres. In this range the power is pretty close to proportional to in-man-p, if at rated RPM and full rich.

It seems you were about 69% power if running at 2700 and "power" rich.

So, you should get around 206 mph at 8000 density altitude. (corrected). Actual results are sure to vary!
 
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BillL said:
David here is a handy calculator. http://www.digitaldutch.com/atmoscalc/ Yes, note that 8000 density altitude is .75 atmospheres. In this range the power is pretty close to proportional to in-man-p, if at rated RPM and full rich.

It seems you were about 69% power if running at 2700 and "power" rich.

So, you should get around 206 mph at 8000 density altitude. (corrected). Actual results are sure to vary!
Click to expand...

I was looking for at least 202 with 180 HP. (Vans number)

The airplane is in the ball park but not as good as hoped. That's why I am looking at the atmospheric conditions. If the density altitude was almost 10,000' it is not a good test.

Today it was WOT, rpm 2760, fuel flow 12.4 leaned to 100 ROP. I should have noted the manifold pressure but did not. Altitude 8000 with 29.92 in the window. OAT was 57F.

I've got to come up with a way to get an honest 75% out of the engine.
 
David-aviator said:
I am attempting to determine the top speed of this RV-8 at 75% power.

Today I was at 8000' PA (29.92) with an OAT of 57F. The speeds I recorded via gps were not what I thought they should be - 174.46 knots or 200.76499 mph.
Maybe the engine was not developing 75% power.

When the 8000' PA number and 57F is plugged to density altitude calculator it comes up 9949'!

I assume the base line altitude (8000') for 75% power with WOT is density altitude, is that correct?
Click to expand...

Were you using GPS ground speed on a single heading, or were you performing 3 or 4 leg GPS runs and calculating your true speed with the NPS spreadsheet?

Skylor
RV-8
 
Working from memory....Using Lyc's performance charts, assuming 180 hp engine in more or less stock config, properly leaned, it will burn pretty close to 10 gph at 75%. That's 2700 rpm, full throttle at ~8000 ft 'standard day' conditions.

If rpm is truly above 2700, it sounds like the airplane is under-propped and running 'in a lower gear' (like a car in 4th instead of 5th gear). It *might* go faster that way, but with a typical prop, it might not.

If it's running extremely rich (20% richer than best power), it probably isn't making best power.
 
rv7charlie said:
Working from memory....Using Lyc's performance charts, assuming 180 hp engine in more or less stock config, properly leaned, it will burn pretty close to 10 gph at 75%. That's 2700 rpm, full throttle at ~8000 ft 'standard day' conditions.

If rpm is truly above 2700, it sounds like the airplane is under-propped and running 'in a lower gear' (like a car in 4th instead of 5th gear). It *might* go faster that way, but with a typical prop, it might not.

If it's running extremely rich (20% richer than best power), it probably isn't making best power.
Click to expand...

Charlie, this engine began its life on the Barrett dyno in Tulsa. Its BSFC is .51.
My calculations using that information (assuming fuel weighs 6#/gal) computes to a burn of 11.5 gph at 75% power. I use fuel flow numbers for cruise - 10 gph high cruise 170 knots and 8 gph low cruise 160 knots. It is not producing 75% power at 10 gallons per hour, more like 65%. This is leaned to peak most days.

I was running rich yesterday, I know it - the 12.4 was too high. I had leaned to peak or what I thought was peak and then went rich to decrease EGT about 100. I should have spent more time doing it.

No one has answered the question, how do you find 8000' density altitude while flying?

Next step is to look at GRT Sport EFIS features. It may compute density altitude while flying, that would answer the question. :) I am still learning that system.
 
David-aviator said:
Charlie, this engine began its life on the Barrett dyno in Tulsa. Its BSFC is .51.
Click to expand...

David,

I caution you with trying to a BSFC value obtained on the dyno to compare your mixture settings. BSFC is dynamic and largely dependent on mixture setting. At what phase of operation did Barret obtain the .51 value? Full rich, sea level power? Leaned to best power?

Skylor
 
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David-aviator said:
Next step is to look at GRT Sport EFIS features. It may compute density altitude while flying, that would answer the question. :) I am still learning that system.
Click to expand...

My GRT Sport displays density altitude at the bottom left corner of the Engine-Temperatures page. It must be the standard display, since I don't recall setting it up that way.
 
.45 lb/hp/hr is a more realistic number at 75%, & aligns with Lyc's numbers.

I just went back to the original post(s), & it looks like you're less than 1.25 mph away from the target. At over 200 mph, that's around 0.5% deficit. Isn't that down 'in the noise' of measurement? At that level, things like density alt, prop setting, mixture setting, even a far forward CG might account for the deficit.

(Forgot: fixed pitch or c/s?)

A mis-aligned or poorly sealed fairing could cost more than a 1.25 mph deficit. Flying the test 2k feet above the specified 'standard day' would be a pretty big hit on power, too. And I missed whether the test was flown as a calculated 4 way test.

Might be worth re-flying the test, at specified standard day conditions (or corrected to them), with the prop dialed to 2700, engine leaned to best power and an aft cg.

Charlie
 
rv7charlie said:
.45 lb/hp/hr is a more realistic number at 75%, & aligns with Lyc's numbers.

I just went back to the original post(s), & it looks like you're less than 1.25 mph away from the target. At over 200 mph, that's around 0.5% deficit. Isn't that down 'in the noise' of measurement? At that level, things like density alt, prop setting, mixture setting, even a far forward CG might account for the deficit.

(Forgot: fixed pitch or c/s?)

A mis-aligned or poorly sealed fairing could cost more than a 1.25 mph deficit. Flying the test 2k feet above the specified 'standard day' would be a pretty big hit on power, too. And I missed whether the test was flown as a calculated 4 way test.

Might be worth re-flying the test, at specified standard day conditions (or corrected to them), with the prop dialed to 2700, engine leaned to best power and an aft cg.

Charlie
Click to expand...

The prop is a 3 blade Catto.

I will do this again, thanks to Terry it appears the GRT Sport does compute density altitude so the test will be more realistic.

I agree, the performance is not off by much and it is a sure bet Vans had everything just right when they established their numbers. The sheet I have from their web site shows 204 solo and 203 at gross for this airplane, but with a CS prop not FP. I am off 3 or 4 MPH.

Thanks Skylor for your comments re BSFC. It is a subjective number.
Obviously the test is done near sea level, there is no other way to develop that much power turning the water pump. These are the basic numbers the 182 HP.

MP=28.8
F/F=16.2
RPM=2742
BSFC=.51

The .51 was the lowest indicated in the run. Most were .58 and higher.
Charlie, I don't see how one could run Lycoming at .43 and 75% power without damaging the engine, that is mighty lean.

I do not know how A/F ratio is controlled at Barrett's dyno, but it is. It would be interesting to see what it is at max power - probably 11-12:1.

Thanks to all for your thoughts.
 
Running at 100% power on the dyno they undoubtably used "full rich" mixture, richer than best power but need to prevent detonation.
At 75% detonation should not be a concern and "best power" mixture should be okay.
 
David-aviator said:
The .51 was the lowest indicated in the run. Most were .58 and higher. Charlie, I don't see how one could run Lycoming at .43 and 75% power without damaging the engine, that is mighty lean.
Click to expand...

Chart below is a 540K, big angle valve motor, more detonation-sensitive than your parallel valve, run under hot conditions (FAA Hughes Test Center). Mild detonation onset when leaned to about 0.43 BSFC. Best power mixture at this MAP-RPM combination is about 0.48 BSFC:



I do not know how A/F ratio is controlled at Barrett's dyno, but it is.
Click to expand...

With the mixture knob.
 
I offer this only for what it's worth as my engine and airplane are slightly different.

At 8,000 PA and standard temperature the ambient MP would be 22.2 (which you can exceed with ram effect). This is not theory; I have observed it repeatedly and I won't offer details here but simply say it has been proven beyond any reasonable debate that my 7A with 3-blade Catto can easily obtain more than 1" above ambient MP. Craig Catto and others confirm this observation.

So, for example, if I am at 8000 PA and 57 DegF my DA is 9692 but my ambient MP would be only 20.9. If I add back one inch that is about 5% so it is worth mentioning.

Your available pressure is only about 69% but with added ram effect it is back up to around 73%, very roughly estimated because of the temperature issue and other complexities. The fuel/air charge you actually get is a result of: A] available pressure for the manifold, B] the density of that air which is affected by both temperature and moisture as well as actual altitude in PA terms. Let discuss BSFC below..

175 kts in my airplane is about as well as it ever did/does. My engine was tested at specified HP by Superior (with the usual corrections) when it was at zero hours. Its actual performance at this time is, I think, as good or better. But remember that it takes a lot of HP to get measurable speed increases when you are already going 200 mph. With better ignition or, more importantly, better aerodynamics, you can go faster.

Superior says my absolute best SFC for maximum power is .48 but that is not at maximum engine performance. The performance cruise chart says best (max) cruise power is at 64 pounds per hour. I would be surprised if yours is much different. 64 pounds at 6#/gallon is 10.7 gph.

Let's quickly look at a 180 Lyco-clone. Let's assume 75% even though that is hard to get under the conditions we are discussing.
180 x 75% = 135. Now use .48 SFC and 6# per gallon and you get 10.8 gph. If you are at .50 then it is SFC=11.25 gph. This is assuming 2700 RPM actual. My tach is digital and accurate; is yours? If you are burning more fuel you are getting less power, in this case. Fuel is not exactly 6#/gallon. Instruments have some error, even mine. Yours? Likely. Humidity matters a little.

The engine makers also tell you that you get more HP for the same RPM and MP combo as you increase altitude and the reason is usually said to be due to decreased exhaust back pressure. I don't know if I believe that but they all say it.

OK, now for Van's numbers. His are 203-204 mph at whatever he thinks 75% is. Try yours at DA=8000 instead of 9500. You should have MP equal or greater than 22" on an accurate instrument, depending on you intake system.
 
200.7 mph is all it will do

I found 8000' DA today and carefully leaned to 100 ROP.
The best it will do is 200.7 mph about 2.3 mph less than Vans.
The issue is the prop, it turned 2770-2790, if I could pull it back to 2700, I believe it would make up that 2.3 mph.
Notwithstanding that little imperfection, I really like flying this airplane. :)
 
another poster asked how you are measuring your 200.7 mph. Are you using a 3 or 4 direction GPS ground speed calculation?
 
What about using the Lycoming document Kevin Horton has hosted on his site? It gives a thermodynamically sound method backed by plenty of empirical measurements to determine and set HP. It is old, but thermo hasn't changed much and the Lycoming detractors say our engines are ancient anyway so this should apply. :D

Link to Kevin's post is here
 
rzbill said:
What about using the Lycoming document Kevin Horton has hosted on his site? It gives a thermodynamically sound method backed by plenty of empirical measurements to determine and set HP. It is old, but thermo hasn't changed much and the Lycoming detractors say our engines are ancient anyway so this should apply. :D

Link to Kevin's post is here
Click to expand...

It is a method of setting HP, I use it for cruise but the fly in the ointment with it is what BSFC to use? My computer program is set for .51 because that is the number used to get 182 HP on the dyno.

Lycoming charts will show .48, but the number can not be that low running at 100 ROP for best power (a Lycoming term). I ran at 12.3 gph this morning and the program shows a BSFC of .54 with WOT at 8000' DA. That had to be as close to 75% "best" power as can be achieved with what indications we have assuming 100 ROP is where it should be. To lean it out to say 10.9 gph and a .48 BSFC, the engine is not producing as much power, it can not be because at .48 it won't be burning as much fuel and therefore not producing as much power.
 
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The document I took from Kevin Horton's web page shows bsfc computed by finding max egt, correcting to best power by taking about 85% of max egt and applying the BSFC applicable to engine compression. Mine is 9:1 which is just under 0.40 BSFC. This is all from my memory, so figures may not be exact, but are close. Take a look at the document for more info. Derived by Lycoming over many years from test cell data
 
David,
I find the lyc document to be more accurate than anything else I have used in the cockpit. Use the BSFC in the document
It is tailored for the method which includes subtracting friction HP at the end. Your dyno BSFC is a snapshot including the friction loss so it is not usable in the lyc calcs.
 
Bill and Brad,
The documents depict good performance but have either of you gone up and attempted to verify the information in your airplane?

This is what I am doing and I just don't believe the HP is there at .40 or.48 BSFC. If the dyno run had been leaned out to those numbers, there would have been detonation at a MP of 28.8 and in no way would ther have been 182 HP.

Lycoming has over the years published a lot of interestin data under controlled circumstances but performance has to verified in an airplane.
 
David-aviator said:
It is a method of setting HP, I use it for cruise but the fly in the ointment with it is what BSFC to use? My computer program is set for .51 because that is the number used to get 182 HP on the dyno.

Lycoming charts will show .48, but the number can not be that low running at 100 ROP for best power (a Lycoming term). I ran at 12.3 gph this morning and the program shows a BSFC of .54 with WOT at 8000' DA. That had to be as close to 75% "best" power as can be achieved with what indications we have assuming 100 ROP is where it should be. To lean it out to say 10.9 gph and a .48 BSFC, the engine is not producing as much power, it can not be because at .48 it won't be burning as much fuel and therefore not producing as much power.
Click to expand...

If this is a 180 hp engine, and the dyno test was at .51 for for 182 hp, then that's the full power bsfc. As far as I know (there are much more knowledgeable people here than me), air cooled IC engines have to be run well rich of peak (poor bfsc) at full power so the exhaust valves can live, but that isn't required up at 8000 feet and 75% power.

Any chance that would explain the discrepancies you're seeing in bsfc numbers?

To the point that .48 can't produce as much power as .51: I don't think that is necessarily the case. The reason for overly rich mixtures at full power in air cooled engines is that the excess fuel isn't burned in the combustion chamber, but is used to cool the exhaust valve. (That's why EGT's are lower than peak when running at full power.) It follows that the fuel being used for cooling isn't producing any power.

BTW, was the dyno run at sea level under standard day conditions? (Pretty hard to find one that actually is.) If run at a higher density altitude and/or other non-standard environment (lower true power) and 'corrected' to standard day conditions, then even .51 might be a bit leaner than would be safe for a truly full power run.

FWIW, Lyc used to say that you could run at least some of their engines *at peak* egt at reduced power settings (basically cruise power at altitude). I think that they have now rescinded that recommendation, probably due to their lawyers and having operators misinterpret the recommendation's conditions.

Charlie
 
Best power, assuming no detonation is present is usually made around 12.6 AFR but power is almost flat between 11.8 (RBT) and 13.2 (LBT) AFR, only about a 1% variation in power.

RBT (rich best torque) occurs at about 95% of peak EGT, we'll call that around 70-80 degrees ROP on an average atmo Lycoming, LBT would be at around 25-35 ROP for comparative purposes.

100 degrees ROP would be around 11.3 AFR, possibly making around 1.2 to to 1.3% less power than at around 50 degrees ROP which would be best torque theoretically. Engines do vary slightly in this regard but dyno data on a specific engine trumps the general theory (if gathered correctly- properly stabilized readings for EGT and torque).

The point I am making here is that you are talking about 180hp vs. 182 hp with these slight mixture variations. 2 hp is going to make very little difference in TAS at these speeds since speed varies as the cube of the hp.

BTW, friction hp is not something most of us have to worry about in our aircraft, it's there and and we can measure it but we are really interested in the hp at the crank minus all other parasitic losses which we can do little about once our engine is assembled and running. All the Lycoming power charts I've seen quote crankshaft hp, usually corrected for SL standard atmosphere conditions unless otherwise specified.

One other thing to consider is that EGT is likely lower at 75% power than it is a 100% power so it is not an especially accurate way to extrapolate true AFR and the resulting point of best torque mixture and different power settings. EGT is also influenced greatly by ignition timing if you have EI so also be aware of that variable.
 
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Good discussion re Lycompng docs and information so many subscribe to.

Weather here is good so tomorrow will do 2 more runs at 8000' DA. Same set up WOT, this time leaned to 50 ROP and at peak. Will compute BSFC based on f/f worked with the formula. TAS should reflect increase or decrease in HP.
 
Ross, I've about decided to go with one of Robert Paisley's EI systems later this summer. It will be interesting to see what the fuel burn and speed numbers are with it.
 
Cooling - it slipped my mind the engine is sometimes cooled with unburned fuel. Good point guys, thanks for pointing that out. That is also the reason CHT's are higher with EI, less fuel wasted for cooling.

The OAT is high here right now, will hit 95 today, will have to watch CHT at 50 ROP and peak. I noticed yesterday F/F at peak was 10 something before going to 100 ROP.

Don't really know why I am doing this.....just interested in how the airplane is performing. Usually fly around LOP to save fuel. It makes quite a difference burning 5-6 gph vrs 10-12.

The Paisley EI should interesting to watch with instrumentation showing dwell angle and A/F ratio. I will have the O2 sensor installed in #4, Robert says it is working well there with no problems of lead contaminatin.
 
David-aviator said:
Cooling - it slipped my mind the engine is sometimes cooled with unburned fuel. Good point guys, thanks for pointing that out. That is also the reason CHT's are higher with EI, less fuel wasted for cooling.

The OAT is high here right now, will hit 95 today, will have to watch CHT at 50 ROP and peak. I noticed yesterday F/F at peak was 10 something before going to 100 ROP.

Don't really know why I am doing this.....just interested in how the airplane is performing. Usually fly around LOP to save fuel. It makes quite a difference burning 5-6 gph vrs 10-12.

The Paisley EI should interesting to watch with instrumentation showing dwell angle and A/F ratio. I will have the O2 sensor installed in #4, Robert says it is working well there with no problems of lead contaminatin.
Click to expand...

Interesting indeed. While the dwell is not user programmable, you can play with ignition timing in flight to see the effect on speed, economy and EGT/CHT.
 
Had a good flight this morning and enjoyed it.

Not much to report except TAS is unchanged at 100 ROP, 50 ROP, and peak.
174 knots the whole way.
What did change was CHT. It increased from 389 to 413.

So - fuel flow 12.7 down to 11.4 is the difference. It costs fuel to cool the engine at high power.

I will calculate BSFC when i get home to the desk computer.
 
I may not know what I am doing here, the BSFC numbers do not make much sense. I can not get anywhere near the .48 BSFC on some Lycoming charts.

The density altitude was 8000' according to GRT EFIS, OAT 57F, WOT (actual altitude 5900'). The theoretical maximum power available is 75% due to atmospheric conditions. So here are the results.

At 100 ROP (fuel flow 12.7) My spread sheet computes a BSFC of .56 at 75%.
At 50 ROP (fuel flow 12.0) BSFC is .53
At peak EGT (fuel flow 11.4) BSFC is .505

The engine will run just fine LOP and fuel flow will fall off dramatically. I did not do that today due to high CHT's after running for a while for the gps speed legs.

Tomorrow I will see what speeds result from running 50 LOP and the fuel flow. That should bring BSFC down below .50.

This is not a Lycoming engine, it is a Superior clone. I wonder if it makes a difference?
 
Those flows sound quite high. If I remember, you said this bird was fairly new? Have you had opportunity to calibrate the fuel flow meter?
 
SFC for Superior

For DA=8000 when Temp=57 your pressure altitude had to be 6615 as opposed to actual altitude of 5900. The equivalent MAP is thus 22.2.

For the MAP you may be getting with ram effect, using 1.0", your equivalent DA is 6900 (this assumes you get that boost).

For that DA your pct pressure is 77.5% of 29.92 but Superior and others count 100% at a lower altitude. They use 29.5. So 77.5% corrected for that difference gives you about 78.6% available air density. My aircraft actually gets even more boost so these are not exaggerated numbers. The short version is that you have 5% more available power than you thought.

50.5 SFC / 1.05 = .48.

In my experience with the GRT, the SFC computation is just not truthful. It's useful, but not truthful. It is useful as a relative number. I can get mine to say .38 but it's not really doing that. That is simply because it is based on HP that is calculated without reference to some of the relevant factors.

But to make it even "better", Superior computes max HP at 2700 and you are running your engine somewhat faster. Kevin Horton's spreadsheets are likely the best for this, but just roughly, 2760/2700 = 102.2.

Going back to your earlier comment about going faster if you could slow the prop down a little - to 2700. That does not "work" for me and for the reason shown in my previous paragraph.

OK, now to contradict my earlier calculations:
Superior's "performance cruise" page (p.47 in the book mine came with) Shows SFC = about .525 at 72 pounds per hour at which the HP should be 138 or 76.6%.

If you don't have that page, PM me for a PDF of it.

IMHO, the Superior is actually a little better than the Lyco for stuff like this and some experts have told me it is also better on "volumetric efficiency". The guys at Superior, when I chat with them at OSH say that much of their charts are simply copied from Lycoming, though.

FYIO, I had my MAP readout checked when I had the airplane IFR certified so I am pretty confident in that. Fuel flow is another issue, though and it is worth checking. Also FYIO, when I try to check maximum performance I always use RPM as the final indicator, using my vernier mixture knob to get the RPM as high as it will go while on autopilot, just as you are doing. The EGT's usually come out where they should, but in my thought process, the maximum power is where the prop turns the fastest.


David-aviator said:
I may not know what I am doing here, the BSFC numbers do not make much sense. I can not get anywhere near the .48 BSFC on some Lycoming charts.

The density altitude was 8000' according to GRT EFIS, OAT 57F, WOT (actual altitude 5900'). The theoretical maximum power available is 75% due to atmospheric conditions. So here are the results.

At 100 ROP (fuel flow 12.7) My spread sheet computes a BSFC of .56 at 75%.
At 50 ROP (fuel flow 12.0) BSFC is .53
At peak EGT (fuel flow 11.4) BSFC is .505

The engine will run just fine LOP and fuel flow will fall off dramatically. I did not do that today due to high CHT's after running for a while for the gps speed legs.

Tomorrow I will see what speeds result from running 50 LOP and the fuel flow. That should bring BSFC down below .50.

This is not a Lycoming engine, it is a Superior clone. I wonder if it makes a difference?
Click to expand...
 
Thanks for the input H. Evans. Your comments are appreciated. You know more about this than do I.

I was flying too low today at 5900', manifold pressure was 24.5" and my brain did not pick up the discrepancy. A better way to find 8000' DA might be to climb until MP is 22.2 WOT. I will do it again tomorrow and also record data running LOP.

It would be comforting to gather some data that made sense. If the flight can be properly positioned in the atmosphere, maybe that will happen. WOT is a no brainer and so is pulling the mixture slowly and watching the EGT numbers.

Fuel flow has been calibrated. I believe it to be accurate as top off fuel is usually within a half gallon of fuel used.

Thanks again for your comments, they are helpful.
 
David-aviator said:
...The engine will run just fine LOP and fuel flow will fall off dramatically. I did not do that today due to high CHT's after running for a while for the gps speed legs...
Click to expand...

I'm confused by this statement. If "cooler" CHT was the desired condition, then going LOP was the quickest way to get there. I'm generally temp limited in climb (like almost everyone), and my most effective corrective action is to go LOP as a means to reign in my CHT. Just like turning down the flame in a BBQ, the temps will quickly follow.
 
Toobuilder said:
I'm confused by this statement. If "cooler" CHT was the desired condition, then going LOP was the quickest way to get there. I'm generally temp limited in climb (like almost everyone), and my most effective corrective action is to go LOP as a means to reign in my CHT. Just like turning down the flame in a BBQ, the temps will quickly follow.
Click to expand...

You are correct Michael, LOP will reduce CHT.

It was not part of the plan yesterday, I could have done it but decided to cool the engine descending and going home.

It is on the agenda today.
 
David, I have a Superior IO-360 and Catto 3 blade also. I find that my Rv-8 will run the same TAS at 2700 rpm's, regardless of other conditions. I told Craig Catto I wanted Van's numbers at 75% power. But I find I get the same tas at 2700 rpm's regardless of density altitude. It may be that your prop is pitched to get 201mph at 2700.
 
David-aviator said:
You are correct Michael, LOP will reduce CHT.

It was not part of the plan yesterday, I could have done it but decided to cool the engine descending and going home.

It is on the agenda today.
Click to expand...

OK,

I understand.

I sometimes overlook the fact that there are probably as many ways to manage an engine as there are pilots. We all adopt our preferred methods and frankly, there are not a lot of ways to do it "wrong".

As an example, I have evolved to running LOP by default - the ONLY time I'm ROP is in initial climb, or if I'm trying to go as fast as possible. Cruising around the neighborhood, cross country, descent, even cruise climb is typically done LOP. And my primary means of temp control is going lean to cool - unless I'm trying to out climb a mountain.

BTW, bringing my ranting into relevance for the thread, my 200 HP RV-8 will run to 178 KTAS at 7500 and 12.7GPH. That's about max speed I've found.
 
More of my OCD on this (smile)

Dave, I am only too glad to offer my OCD-ness and I'm grateful anyone even reads it!

If I recall correctly, you had a RV 7 or 7A that ran LOP almost a full gallon per hour better than mine at the same airspeed. That is a testament to good aerodynamics and I think you were using electronic ignition. Just saying I admire your work!

That said, finding 22.2 MAP won't necessarily give you 8000 DA if you are getting any ram effect. If you are getting any significant ram effect then 22.2 will be higher than 8000.

Here is how I "test" for ram effect. I use the GRT to tell me what it thinks the DA is and fly the airplane level, best power mixture at, for example, 150-160 tas kts. Then I keep the autopilot engaged and increase throttle to maximum without undue delay. The MP will jump up and be more or less steady with RPM. Then as the airplane gains speed I can watch the MP increase until it gets stable again. That period where the MP is gaining and the only thing that is changing is airspeed tells me there is a ram effect.

If my OAT is accurate (I've had to relocate it 3 times) and my pressure altitude is accurate and my baro correction is good (close to GPS tests for reasonableness), then I can compute my DA pretty well. It helps if the relative humidity is low, too. Higher humidity robs power. So if I "know" my DA and my MP is more than it "should be" then if the instruments are accurate it can only be explained by the increased pressure of the higher velocity air as opposed to static.

FWIW, my air intake system is by Sam and Will James and has a K&N filter so it is not losing much. When Cessna, Piper et al use 8000 feet as 75% they are assuming a less efficient intake system.

I note in passing that the SFC as shown by Superior is a curve, not a constant, even at "best power". That makes this harder.
David-aviator said:
Thanks for the input H. Evans. Your comments are appreciated. You know more about this than do I.

I was flying too low today at 5900', manifold pressure was 24.5" and my brain did not pick up the discrepancy. A better way to find 8000' DA might be to climb until MP is 22.2 WOT. I will do it again tomorrow and also record data running LOP.

It would be comforting to gather some data that made sense. If the flight can be properly positioned in the atmosphere, maybe that will happen. WOT is a no brainer and so is pulling the mixture slowly and watching the EGT numbers.

Fuel flow has been calibrated. I believe it to be accurate as top off fuel is usually within a half gallon of fuel used.

Thanks again for your comments, they are helpful.
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There are many small factors which can creep in to change power and/or calculated power and we also need to know what factors the Glass uses to compute.

If you have fixed timing, the engine probably will not make 75% power at the reduced MAP of 22.44 inches actually, it would have to be advanced somewhat assuming it was optimized on the dyno at SL in the first place.

Be aware that indicated OAT at 200mph is not true OAT, does the Glass figure in reduced back pressure at altitude for power calcs and how about humidity?

I've said it before, people tend to take the engine monitor power percentage as exact but it isn't- just a close approximation. In reality, I doubt if your WOT speed would change more than 1-2 knots between 7500 and 8500 DA. Power will drop off slightly but so does the drag due to reduced density.
 
rv6ejguy said:
There are many small factors which can creep in to change power and/or calculated power and we also need to know what factors the Glass uses to compute.

If you have fixed timing, the engine probably will not make 75% power at the reduced MAP of 22.44 inches actually, it would have to be advanced somewhat assuming it was optimized on the dyno at SL in the first place.

Be aware that indicated OAT at 200mph is not true OAT, does the Glass figure in reduced back pressure at altitude for power calcs and how about humidity?

I've said it before, people tend to take the engine monitor power percentage as exact but it isn't- just a close approximation. In reality, I doubt if your WOT speed would change more than 1-2 knots between 7500 and 8500 DA. Power will drop off slightly but so does the drag due to reduced density.
Click to expand...
Your comments are very interesting. I am not an engineer, nor mathematician, mechanic, aircraft designer, computer programmer or any other professional who may be involved in the calculations of HP/TAS/DA/MP/RPM or any other measure of performance. No, I am just an experimental airplane builder and pilot. Being such, I usually look at the relative aspects of these measurements rather than the minutia of the details of the measurements. I don't see how you engineer guys keep your sanity when attempting to calculate to the Nth degree measurements that have so many moving target variables. :p You definetly have a lot more patience than I do in tracking down and isolating all of those variables.
 
RVbySDI said:
Your comments are very interesting. I am not an engineer, nor mathematician, mechanic, aircraft designer, computer programmer or any other professional who may be involved in the calculations of HP/TAS/DA/MP/RPM or any other measure of performance. No, I am just an experimental airplane builder and pilot. Being such, I usually look at the relative aspects of these measurements rather than the minutia of the details of the measurements. I don't see how you engineer guys keep your sanity when attempting to calculate to the Nth degree measurements that have so many moving target variables. :p You definetly have a lot more patience than I do in tracking down and isolating all of those variables.
Click to expand...

I don't bother with the nth degree in most flight testing as unless you have glass smooth air and perfect technique and procedure (unlikely). The best repeatable data is only within about +/- 1.5 knots experimental error IMO.
 
Not sure the reasoning here is correct.

If it is demonstrated that it takes 135HP to achieve 174 knots, is it reasonable to assume it will take 124 HP to achieve 160 knots? That would be the result of a math equation but I do not know if it is true. Drag is not linear with speed so it probably is not completely true, but it may be in the ball park here.

If it is 124 HP, I achieved a BSFC of .42 this morning flying at 50 LOP. If it is 117 HP (for example) BSFC is .44.

Fuel flow was 8.6 gph, TAS 160, OAT 65F, altimeter 29.85, 6500 above MSL, DA 8500' (E6B and GRT agree) rpm=2660, MP 23.9, CHT 350.

I am wagging it figuring BSFC, I really do not know how it is done except I plug numbers into a formula created from dyno data on this engine.

For sure it is not perfect, but I am satisfied. :)
 
David-aviator said:
Not sure the reasoning here is correct.

If it is demonstrated that it takes 135HP to achieve 174 knots, is it reasonable to assume it will take 124 HP to achieve 160 knots? That would be the result of a math equation but I do not know if it is true. Drag is not linear with speed so it probably is not completely true, but it may be in the ball park here.

If it is 124 HP, I achieved a BSFC of .42 this morning flying at 50 LOP. If it is 117 HP (for example) BSFC is .44.

Fuel flow was 8.6 gph, TAS 160, OAT 65F, altimeter 29.85, 6500 above MSL, DA 8500' (E6B and GRT agree) rpm=2660, MP 23.9, CHT 350.

I am wagging it figuring BSFC, I really do not know how it is done except I plug numbers into a formula created from dyno data on this engine.

For sure it is not perfect, but I am satisfied. :)
Click to expand...

Actually it should only take about 105hp for 160 knots at this DA if we ignore induced drag effects over this small speed reduction.
 
rv6ejguy said:
Actually it should only take about 105hp for 160 knots at this DA if we ignore induced drag effects over this small speed reduction.
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If it is 105 HP, the BSFC would be .49.

How so your math Ross?

If 174/135=160/x, x=124 not 105.
 
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