percent power rule of 48 and leaning

I have finally gotten my static port location/shape/airspeed calibration testing done. My CAS is 3 knots slower than IAS and is linear from fast cruise all the way down to 75 knots indicated. I stall heavy and clean at 57kts IAS and dirty at 55kts IAS. Stall technique can be improved but those are the numbers I'll be using for now.

So I went out to check my real 8000'DA speeds and to see at what RPM my C/S Whirlwind 200RV prop is most efficient. My test plan was to fly WOT for everything. First vary rpm by 100 rpm from 2700 down to 2300, FULL RICH. Then pick a couple of "sweet spot" rpm's and test those with differing mixtures.

It was a hot day and 8000'DA was found at 5,500'. Here was my first surprise...I was at Wide Open Throttle and was getting 25"MP at 8K'DA. Shouldn't it have been around 22"? My problem was that I had always been using the "rule of 48" to guesstimate 75% power and wasn't comfortable leaning aggressively at this higher combined RPM/MP. But, I was at 8000'DA which is supposed to be 75% power with everything pushed forward, right? Full throttle, full rpm, full mixture. Have I screwed this up somewhere?

What I found was that at 25" and both 2200 and 2300rpm, I was uncomfortable as I approached finding peak egt and never allowed myself to get there. No way was I going to lean much at all at 25" and 2700rpm At 1420 egt on the hottest the CHTs were all above 400 and the engine didn't seem happy. At lower MP on previous tests peak EGT was typically found around 1450, 2300rpm, 23'MP and about 7.5gph.

Procedure Question: How do you establish 50 or 100 degrees rich of peak if you don't know where peak is and you're running too much power to lean to peak I know to look at take off egt and you're O.K. to lean to that in the climb. Beyond that I'm in the dark. Maybe I need to Re re re-read Deakins. Again.

Could I get some fuel flow vs. rpm/MP combinations from some of you who have been there before? I know this is in the engine books in theory, but I'm pretty sure that those numbers are out the window with higher compression ration pistons and electronic ignition. 9:1 pistons and dual PMags on the conservative curve.

BTW: it seems that the WW200RV has a sweet spot at around 2500rpm in terms of speed/fuel flow. Anyone else test this? I'm not sure if I'm going to cruise there or at the lower rpms (23 to 2400) I've been using, but I might test it on extended trips just to see how much a difference it makes vs. the higher pitched noise. I had the prop dynamically balanced using 2200 and 2400 test points.

Thanks for any help/observations/hints/tips!

Jeremy Constant
RV7A 120hrs

Manifold Pressure and Density Altitude

After sleeping on it I realized there were WAY too many questions in one post...sorry

I'm having a basic misunderstanding regarding the effect of density altitude on engine Manifold Pressure.

First question: is the MP dependant on density altitude or just altitude? I was at 5500' and it was 80 degrees F and I was seeing just under 25"MP. If the rule of thumb is approx 1"/1000' then at 5500', standard atmosphere I should have lost around 5.5"MP, or just under 25". That's what I had.

I thought MP would have been dependant on DA which under these conditions was 8000' which should have yielded a MP of just under 22". This would have gotten me close to the magic 75% power/rule of 48.

Thanks for the review of basics I thought I knew but don't

Jeremy

Jeremy, I don't know the answers to most of your questions.

My understanding is that if you set the colsman to 29.92 and then fly until the alt. show 8000 ft, you will be at the 8000 DA. I think that temp plays a small effect, but for the most part that adjustment can be ignored.

The MAP is effected by real pressure (IE DA).

Kent

kentb said:

My understanding is that if you set the colsman to 29.92 and then fly until the alt. show 8000 ft, you will be at the 8000 DA. I think that temp plays a small effect, but for the most part that adjustment can be ignored.

The MAP is effected by real pressure (IE DA).

Kent

Click to expand...

Are you perhaps thinking of pressure altitude? Density altitude is affected by temperature - at least at my home base where the AWOS broadcasts it when it gets above a certain temperature

I am not sure...

ChiefPilot said:

Are you perhaps thinking of pressure altitude? Density altitude is affected by temperature - at least at my home base where the AWOS broadcasts it when it gets above a certain temperature

Click to expand...

I am not sure that I understand the difference. From the Internet:
DA = PA + (120 * (OAT - ISA_temperature))
Where ISA_temperature = 15C - 1.98 * actual altitude in thousands of feet

So... (the below is a question)

If you want to test at 8000 ft you set the colsman to 29.92 and fly to 8000 ft (PA). Now you read you OAT and it reports 15 C.

Using the above formula your DA = 8000 + 120 * (15 - (15 - 15.84))
or 9900.8 ft

Now to do the testing do you need to descend until the DA reaches 8000?

Kent

Pressure altitude and density altitude

The manifold pressure will be closely related to the Pressure Altitude, which is what your altimeter shows.

If you set the Kollsman window to 29.92, your altimeter will read what your pressure altitude would be on a standard day in a standard atmosphere. If you set the Kollsman window to the appropriate setting given by ATC or FSS, then your altimeter will read the proper pressure altitude for that day and place. You are correcting for deviation in surface pressure from a standard atmosphere.

If the air you are flying in is a different temperature than the standard atmosphere, then your Density Altitude will be different than your Pressure Altitude. Lets say it is warmer. With a constant speed prop, at a given Pressure altitude, you will still see the same manifold pressure and can set the same RPM, but the engine will develop less power because there is less air going through the engine. It will run richer, and will require a somewhat lower fuel flow to peak EGT.
With a fixed-pitch prop, at a given Pressure altitude, you will see lower rpm at the same manifold pressure because the engine can't develop enough power to turn the prop as fast as it would on a standard day.

Depending on the design of your induction system, your manifold pressure is also affected by your flight speed, because you get some "ram" recovery of pressure. At 200 mph IAS, the highest this recovery can be is 1.4 in. hg. If you have an air filter, you will loose about 0.3 in. hg. of that. If you take air from the lower cowl plenum, you won't see any of it (plus, that air is even warmer, so you also will see additional loss of power at the same MAP)

You should use the tables in the Lycoming manual to determine your percent power at a particular combination of manifold pressure, density altitude, and rpm. **I think** (not totally sure about this) that if you take off at sea level, and the surface pressure is 29.92 but the density altitude is 3000 ft, you will make the same engine power that you would if you were flying at 3000 ft on a standard day, even though in that case, your manifold pressure would be 26.82. This is because the temperature difference offsets the pressure difference so in both cases, you have the same density, i.e. the same amount of air going through the engine.

I found 8000'DA via my GRT Efis, but verified it on the ground using an E6B and it is correct.

O.K. so I might be getting a bit of ram air recovery due to the efficient engine inlet thanks to Van. Let's be generous and call it 1" ram. Maybe also getting a bit of ram effect from the fat section of the 200RV prop blade lining up with the air intake. Call it 1.5" total ram effect although I'm guessing that's very generous.

So what I'm understanding is that my engine should be breathing air at the prevailing Density Altitude plus whatever ram I'm getting. I'm still not sure how I was getting 25" rather than 22 or 23.5 allowing for ram. That's still a mystery. If I recall, when I'm on the ground, engine off, indicated MP=altimeter setting. I'll check it again but that's the only way I know to verify MP reading.

Next question: Can I use the Lycoming manual percent power charts with an engine that has higher compression pistons and electronic ignition and so is NONE of the engines in the charts? I would have thought that fuel flow would certainly be different (higher horsepower) and other things would change as well.

Trying to wrap my brain around unexpected results...

Thanks for helping guys

Jeremy

Hi Jeremy.
It's common to see ram air pressure gains from the Van's induction system. Your 1 to 1.5 inches is consistent with what I've seen in my 9A/O320. I found that I needed to cruise at 9500 to 10000 feet da to be at 75% or below.

The Lycoming charts are not very useful in this scenario, so you need to calculate engine horsepower directly from MAP, RPM, PA and temperature, or specific fuel consumption (when lean of peak).

There's a thread on VAF (search power percent) that gives the formulae that I derived. However, the ultimate simplification that I use is that for a 160HP O-320, if I'm burning less than 8.2 gph, I'm less than 75% HP regardless of altitude. When rich of peak, your fuel consumption will be greater than this so you need to use MAP, RPM.

Rule of 47 or 48 works at lower altitudes just fine.

jjconstant said:

I found 8000'DA via my GRT Efis, but verified it on the ground using an E6B and it is correct.

O.K. so I might be getting a bit of ram air recovery due to the efficient engine inlet thanks to Van. Let's be generous and call it 1" ram. Maybe also getting a bit of ram effect from the fat section of the 200RV prop blade lining up with the air intake. Call it 1.5" total ram effect although I'm guessing that's very generous.

So what I'm understanding is that my engine should be breathing air at the prevailing Density Altitude plus whatever ram I'm getting. I'm still not sure how I was getting 25" rather than 22 or 23.5 allowing for ram. That's still a mystery. If I recall, when I'm on the ground, engine off, indicated MP=altimeter setting. I'll check it again but that's the only way I know to verify MP reading.

Next question: Can I use the Lycoming manual percent power charts with an engine that has higher compression pistons and electronic ignition and so is NONE of the engines in the charts? I would have thought that fuel flow would certainly be different (higher horsepower) and other things would change as well.

Trying to wrap my brain around unexpected results...

Thanks for helping guys

Jeremy

Click to expand...

Thanks for the info guys. It seems that one "rough and ready" approach may be to simply climb to whatever altitude yields 22" MP which is 8" less than standard at sea level and should be equivalent to 8000' and 75%. From what you say Vern, I'll be looking for it at 9500 or 10,000'.

I'll look at the thread for % horsepower on the lean side of peak...I remember reading it but I need to copy the formula down. My memory is really good but it's getting shorter with age

Jeremy

Hi Jeremy,

Your manifold pressure guage is just like your altimeter. If you are at 5500 feet on your altimeter at 2992 you will get the equivalent manifold pressure (I'm guessing 25 inches since that's what you saw). An altimeter reads pressure and so does a manifold pressure guage. DA is of course different because it's based on temp. I'm sorry I can't help as far as power settings at different RPM and manifold pressures.

-Andy