Density Altitude and HP

[hevansrv7a]

I know Van's and many others use 8000 feet for cruise speed comparison. I also thought I knew that this was supposed to be where - because of air density - you could put the throttle all the way in and not exceed 75% of rated power. I think that's what my Cessna manual said, too.

I was playing with various on-line calculators today and they all seem to say that the relative density of the air is 75% at a density altitude of around 9500' more or less, with humidity being a factor. It is significantly more than 75% at 8000.

So, question #1: we are talking about density altitude not MSL, right?
and question #2: ram air effects notwithstanding, how to account for the discrepancy?

I have a GRT and it computes density altitude ( I assume it is ignoring humidity but it knows OAT, pressure, etc. ). My observation is that I can easily exceed 75% at 8000' DA. I also have a fixed pitch prop, so the only way I can measure best speed at 75% is to answer these questions.

I am somewhat math challenged when I look at the actual equations. I know many of my fellow VAFers can answer these questions easily. Please and thanks.

 

[John Clark]

Density Altitude

Yes, it is density altitude, not MSL, for this calculation. I just ran a very quick calculation using a couple of charts and can't match your findings. On a "standard day" the sea level pressure is 29.92 and the pressure drops at about 1" per thousand feet. So the pressure at 8000 will be 21.92. If I run the chart in my Lycoming Operator's Manual I come up with about 145 HP at 2700 rpm and 22 inches. In the case of my 200 HP IO-360, that is 73% power. Again this does not consider moisture or gain/loss from the induction system.

John Clark
RV8 N18U "Sunshine"
KSBA

 

[Kevin Horton]

I know Van's and many others use 8000 feet for cruise speed comparison. I also thought I knew that this was supposed to be where - because of air density - you could put the throttle all the way in and not exceed 75% of rated power. I think that's what my Cessna manual said, too.

I was playing with various on-line calculators today and they all seem to say that the relative density of the air is 75% at a density altitude of around 9500' more or less, with humidity being a factor. It is significantly more than 75% at 8000.

So, question #1: we are talking about density altitude not MSL, right?
and question #2: ram air effects notwithstanding, how to account for the discrepancy?

I have a GRT and it computes density altitude ( I assume it is ignoring humidity but it knows OAT, pressure, etc. ). My observation is that I can easily exceed 75% at 8000' DA. I also have a fixed pitch prop, so the only way I can measure best speed at 75% is to answer these questions.

I am somewhat math challenged when I look at the actual equations. I know many of my fellow VAFers can answer these questions easily. Please and thanks.

You are quite right, that at a density altitude of 9500 ft the density is almost exactly 75% of the sea level standard day density. At 8000 ft density altitude, the density is about 78.6% of sea level density.

You also need to understand that engine power does not vary exactly with density altitude. Temperature also comes into play. That is to say, if you pick two conditions that give the same density altitude, but the temperature (and hence the pressure altitude) differ, then the power will differ. At least that is what the Lycoming power charts tell us, and I tend to believe Lycoming more than I believe the people who tell us that power varies with density altitude, as I don't know any of those people who have ever actually put their engine on a dyno in an altitude chamber.

If you take a slow aircraft with a poorly designed airbox (e.g. some Cessnas), and run at full throttle at 8000 ft maybe the manifold pressure will be such that you get around 75% power. But, if you take a fast aircraft with a well designed airbox (like an RV), you'll get quite a bit more MP than the Cessna had, and you will almost certainly get more than 75% power.

How to deal with this? Use your Lycoming power chart, rather than simply believe people who tell you that full throttle at 8000 ft = 75% power.

 

[Kevin Horton]

Yes, it is density altitude, not MSL, for this calculation. I just ran a very quick calculation using a couple of charts and can't match your findings. On a "standard day" the sea level pressure is 29.92 and the pressure drops at about 1" per thousand feet. So the pressure at 8000 will be 21.92. If I run the chart in my Lycoming Operator's Manual I come up with about 145 HP at 2700 rpm and 22 inches. In the case of my 200 HP IO-360, that is 73% power. Again this does not consider moisture or gain/loss from the induction system.

The pressure drop is not exactly 1" per 1000 ft. The standard pressure at 8000 ft = 22.2" Hg. At 180 kt, the ram pressure rise at the entrance to the air inlet on the cowl could be as much as 1.2", so the total pressure at the entrance to the air inlet could be as high as 23.4". There will be pressure losses in the air induction system, so the actual MP will be something less than this.

My IO-360A power chart shows that 2700 rpm and 22.2" gives about 145 hp at sea level (left hand side of the chart). The right hand side of the chart predicts that 22.2" MP would be reached at full throttle at about 6500 ft (assuming no ram pressure rise), and if so about 158 hp would be produced. But, at a given rpm and MP, the power produced increases with altitude. You draw a line between the two conditions, and extrapolate to 8000 ft, and get a predicted power of about 161 hp, or about 80% power. This assumes a MP of 22.2", and standard temperature.

What MP do people see at 8000 ft at full throttle in level flight?

 

[rv6ejguy]

Have to know what MAP you are getting as you may have restriction across the throttle even at WOT or intake manifold restrictions in general or as previously stated these could be offset by a bit of ram pressure. Standard atmosphere is 22.23 in. hg. Ab and 30.48F (490R), 0% humidity. We'd also have to take into account exhaust backpressure at 8000 feet as well vs. SL.

I agree with Kevin, using Lycoming's charts is probably most accurate but humidity is the wild card which can alter results by 10%. We really don't know without an altitude chamber dyno.

 

[Kevin Horton]

but humidity is the wild card which can alter results by 10%. We really don't know without an altitude chamber dyno.

However, the conditions must be very extreme before humidity has anywhere close to a 10% effect. As near as I can tell, the accepted theory is that the power loss due to humidity occurs because the water vapour has displaced some oxygen. The SAE dyno correction factor seems to agree quite closely to this theory. A 10% power loss would require that the vapour pressure of the water be 10% of the atmospheric pressure. This would require a temperature over 100 deg F with 100% humidity. How often does that occur?

At the temperatures and dew points more typically seen at altitude, the effect of humidity should be much, much smaller. At 8000 ft, with standard temperature, the maximum possible power possible power loss due to humidity would be about 1% (this is the difference in the SAE dyno correction between zero and 100% humidty). If the temperature at 8000 ft was ISA + 20 deg F, the max possible power loss, as near as I can tell, would be less than 2%.

Lycoming does not specify what humidity level is assumed for their power charts. If they use the criteria in FAR 23 (and they should, as these are the power charts they tell you to use if you want to show compliance to FAR 23 requirements), the power charts should assume 80% humidity at temperatures of standard, or colder. The assumed humidity level for temperatures above standard would be a straight line variation between 80% humidity at standard temp, and 34% at ISA + 50 deg F. If they have actually assumed these humidity levels, the maximum possible power variation from the power chart due to humidity is even less than I calculated above.

 

[vlittle]

Kevin, thanks for pointing out something that's been bugging me for a long time.

There are many, many posts, magazine articles and so-called expert opinions that say "I did this, took it up to 8000 feet density altitude and saw a 3 knot speed increase." (or something to that effect).

All of the number crunching I have done on engine horsepower is based on pressure altitude and temperature, NOT density altitude. Its quite possible for an engine to perform differently on different days even though the density altitude is the same- because the PA and T are quite different, even though the DA is the same.

Even the CAFE guys are guilty of doing this.

I calibrated my engine performance against the Lycoming charts and determined that I was getting about a 1 to 1.5" MAP boost due to ram air effects (similar to your number). The Lyc charts assume no ram air and are not usuable directly for RV's.

I need to fly at above 10,000 feet in order to get 75% HP, and I will redline my prop at lower altitudes and full throttle.

FYI, the empirical power curve for my O-320/160 mounted on my 9A with Sensenich prop is:

h=(H-((R-r)*(5.58-0.125*(M-m))/100 + 7.35*(M-m)) + 2.0*pa/1000)*sqrt((519-3.58*pa/1000)/(460+ta)) (in horsepower)

Where H is the sea level rated HP at R rpm. M is seal level max MP, r is actual rpm, m is actual MP, pa is pressure altitude and ta is ambient temperature in degrees F. Formula works only with excess fuel (rich mixture). Lean mixture uses Lycoming BSFC to determine power.

Vern

 

[gmcjetpilot]

Bench mark

8,000 feet (density alt) is not magic, just an unwritten benchmark to compare one plane or mod to another. In theory in a Cessna or Piper you are assured 75% power or less. As Kevin points out RV's have a better air-box, exhaust and we're going faster, which adds a little more dynamic (ram), so we make more than 75% at 8k or even 8.5k. However not every RV is the same. Some RV's might only make 75% at 8k. Of course the VFR hemispherical cruising rule means we fly at 8.5 or 9.5.

I do think many still assume 8,000 feet = 75% power in their RV, since that was the case in their C-172. Still many RV's with fixed props don't have a MAP gauge, like many GA planes, so there is some blissful ignorance. The difference is the GA planes where flight tested with MAP gauges and documented. Now most new RV's have multi channel engine monitors, MAP, FF and C/S props. If you have a MAP gauge you have a better grip on actual power than without. The 8,000ft=75% rule is a good assumption in a Cessna 172, not so good in a fire-breathing RV7 or RV8.

What about leaning? Lyc says lean only at 75% or less, SOME RV's should not lean at 8,000 feet with wide open throttle, since it's over 75% power according to Lyc. Can you or is there any way to safely lean at 76% or 77%?

To be realistic most RV's will not be much over 75% power @ 2,500 rpm & 8,500 feet. Also fixed prop guys may be running away with +2,700 rpm with WOT, and may likely be over 75% even at 8.5k'. I am assuming NET manifold pressure (same as ambient) for that std day altitude (e.g., 21.92" @ 8,000'; 21.16" @ 9,000'). BTW, net MAP is excellent, since internal loss in the engine is about -1 (in-hg). Most Cessna's & Pipers run at 1.5 less MAP than typical RV's for all the reasons already mentioned, even with the same engine. That's good for us. Much credit goes to Van and his FAB air-box and the great exhaust pipes avaiable to us. The engine just breathes more and makes more power. The extra dynamic pressure from higher speeds does not hurt either. A Cessna at 100 mph v.160 mph of a RV, is about 0.55 in-hg more dynamic pressure. The FAB air-box we use have better pressure recovery, which makes the most of the extra pressure. The exhaust is the second part of the formula.

So at 8k' we may be at 77% power, may be 78% at WOT. So what? Can you lean? That is up to you. Officially approved by Lyc? No. The caveat leaning over 75% is not Lyc approved, but Lyc assumes we don't have 8-channel EGT/CHT and FF. Also I doubt a Cessna or Piper pilot with a Tach only can estimate the difference of 74% or 77% power anyway. Just food for thought.

You can always close the throttle to get 75% at any altitude, but WOT ops is more efficient than closing the throttle slightly. A partially closed throttle adds internal loss and inefficiencies compared to WOT. That is why climbing to at least the min altitude to run WOT at 75% power, leaned of course, is most efficient, wind allowing.

Constant speed prop guys have an easier time setting power, since an exact RPM can be set. Fixed prop guys have to take the RPM they get with WOT operations. The higher the rpm the more power you make. At least with a constant speed prop, you can lower the RPM to get below 75% power even with WOT. For example at 8.5k' @ 2,600 rpm & assuming 21.5", power is 77%. Dial the RPM down to 2,450 RPM, its 75%. For the fixed pitch guys, IT WOULD BE BEST (efficiency wise) TO JUST KEEP CLIMBING TO 9,500' or 10,500' TO ASSURE YOU'RE AT 75% OR LESS WITH WOT. On super cold dry days you may have to get into 5 digits, since the engine will make power higher with the dense air. I doubt leaning at 77% power will damage the engine, but than again I have no engineering evidence one way or another, just Lycs limit, which I respect. It's your engine.

Also, in my opinion, leaning is a function how much you lean. There's a big differences between 150F ROP verses running at 30F ROP EGT. The reason for not leaning at high power is to avoid detonation. A super lean mixture at high power can spontaneously and abruptly burn (explode or detonate) verses a controlled combustion or burn. When Lyc sets its 75% number it has some margin in it, worse case scenario rolled into it. Still a limit is a limit and there is no reason we can't respect it. There are some techniques to lean with out finding peak first, using target EGT and/or FF; we'll leave that for other discussion.

Fixed guys more than the constant speed prop guys need to make your own Power v Altitude charts. To do that you have to flight test with a MAP gauge. I'm all for a MAP gauge in all RV's, including fixed pitch prop RV's. Other wise you have no idea how much power you are making. Put those MAP gauges in and flight test. Altitude v power charts for a fixed pitch Cessna or Piper are flight tested for that particular airframe. No other plane can use them even if the engines are the same. Engine + Installation/fixed prop affects alt v pwr charts. Same with RV's, each one is different. The basic Lyc engine charts are a good starting point (see below).

Bottom line Density altitude + MP, RPM and TEMP (and to a small degree humidity) and airframe (drag) determine max power for a given altitude, not just altitude and RPM.

 

[rv6ejguy]

My statement about humidity stands. Yes, from 0% humidity to 100% at higher OATs you CAN see 10% difference in hp. In North America it would be rare to have either condition. My point was that this is usually ignored by people. I have not been able to find out how/ if the the Lycoming charts take humidity into account. In real life, the effect of this in North America would likely reduce the mean variation down to around 2%. We can then throw AFR (air/ fuel ratio) into the mix. In the old days where we were leaning by feel, AFRs may not have been too consistent from day to day. HP varies quite considerably with AFR (around 8% from best economy to best power AFR). Using EGT will help narrow the AFR range but EGTs are affected by ignition timing and compression ratio as well so the more an engine strays from Lycoming's timing (EI) and CR that the charts were compiled from, the more in error the data may be for a particular engine.

Flight and engine testing demand controlled variables for accuracy and there may be many variables unaccounted for by glass displays in calculating % hp. The more assumptions you make, the higher the probability for error.

It is probable that using Lycoming charts and observed MAP (calibrated of course) and OAT that estimates within 2-4% of actual power percentage can be obtained which may be good enough for most of us. On a typical RV installation it is more than likely that the altitude where 75% power will be obtained WOT will be above the often used 8000 feet. DA is probably not what the engine is seeing downstream of the throttle plate so strictly speaking is a not a scientific way to compare power output. We should have induction temperature probes near the port as well as use observed MAP to correct charge density/ mass. The heated sump/ induction design of the Lycoming makes this even more important.

There are so many variables to take into account to get accurate, repeatable data with the instrumentation we typically have installed that it is really not possible to reduce errors below 2% on a consistent basis IMO even on a dead stock Lycoming with mags. All these factors were exactly the reasons why large radials like the R3350 used torque meters plus extensive instrumentation including spark plug oscilloscopes to monitor every aspect to get repeatable power settings and observe trends.

The topic has technical interest primarily but if you want to save fuel or go fast- well you know what to do. Whether you are at 68 or 71% power is pretty academic most of the time.