Is all 75% power equal?

[201er]

Particularly at lower altitudes there are many different configurations to achieve 75% power but does % power always equate to the same airspeed?

From POH at 2000ft: 23.3"/2700, 24.4"/2600, 26.8"/2400 all yield 159ktas. The POH gives the same speed value across all of these settings and it remains the same between best power and best economy mixtures. What I'd like to know is from your experience how true this is?

I was led to believe that in theory the higher RPM settings for equal power yield higher speeds while lower RPMs yield better gas savings. Between best power and best economy, I thought best power should yield a few more knots. Is 75% power based on actual power at best power, best economy, or peak? In other words do you get true 75% at best power and a little less at best economy? Or do you end up getting a boost of slightly more than 75% at best power? The POH does not compensate power variations based on mixture as the MP/RPM values are identical between best and economy powers. So how much power and speed difference is it in reality?

Also, I'm curious how LOP pertains to this. Everyone says LOP is slower. Is that just because for the same configuration it sacrifices power? But if you set power by GPH while LOP to provide 75% power, does it yield the same speed as ROP or is it still slower for some other reason?

[KSMooniac]

75% power is 75% power no matter how you set it, and yes, if you are low enough to develop 75% power while LOP then you will go the same speed as a 75% power ROP setting.  Flying LOP only has to result in a loss of airspeed when up high and you cannot develop as much power as you would like.

There is a minor effect on prop efficiency with RPM, so theoretically it *could* vary a little bit with different RPM settings, but it would be hard to measure this without flight test instrumentation, unless you compare something like 2700 RPM vs. 2200 RPM as a rough example (assuming equal power delivery).

[carusoam]

 

75 is 75 but...

The M20 R POH over simplifies the situation slightly.

It indicates that 65% power is achieved at the same combination of MP and RPM at either 50 LOP or 50 ROP.

The MP Required to reach this target % value also decreases with altitude.

 

Hp is MP (O2) limited when ROP

Hp is FF limited when LOP

At low altitude, back pressure is higher than at high altitudes.

Colder air is more dense than warmer air (at the same MP)

Fortunately air gets colder the higher up we go.

Overall the number of situations is large, but it can be simplified, and is simplified,  

When ROP, MP is the driver, but I follow the POH chart, and expect it to be truthful.

When LOP, Follow the FF, it is highly converted to power.

For proof, maintain altitude (MP) and rpm and vary mixture from deep ROP, through peak, deep into LOP.  Chart your airspeed.  Do this at altitudes where HP does not exceed 65%.

I think Byron gave the rule of thumb,  add about 2" of MP to ROP conditions to achieve the same HP LOP.

Best regards,

-a-

[Gone]

Quote: pmccand

LOP at 75% power?!!  How many folks are running LOP at higher power settings?  I thought the maximum LOP recommendations are at or below 60% power settings.

Phil McCand

[carusoam]

Phil,

...what Ned said +

I was referring to the IO-550G in the Ovation.  Where, in fact, the POH allows continuous operation at SL in LOP mode at about 83% power.

As you can see from my note above.  WOT Experimenting should be done above 8500' (< 65% my recomendation).

Engine science is complex, but engine operation should not be.

I like the JPI and the G1000 and other devices that calculate %hp for you using all of the proper inputs.

Best regards and go Halos!

-a-

[carusoam]

login incorrect...

[Shadrach]

LOP above 60% or 70% or what ever is not a big deal for most of us with reasonably balanced engines. I can cruise all day lone at 28" 40 LOP and never see a CHT over 340 even in warm OATs. It's the only way I fly on trips under 100nm. I do a regular 50nm trip that would take a lot longer in the car. It does not make sense to climb above 3K unless I have to. On many days I'm t 2500-3000ft WOTRAO 35LOP ~2500RPM. Under certain conditions this generates indicated speeds well into the yellow (152kts). I am guessing that I am over 80% power with the Ram-Air open.

[Parker_Woodruff]

Quote: pmccand

LOP at 75% power?!!  How many folks are running LOP at higher power settings?  I thought the maximum LOP recommendations are at or below 60% power settings.

Phil McCand

[KSMooniac]

Quote: pmccand

LOP at 75% power?!!  How many folks are running LOP at higher power settings?  I thought the maximum LOP recommendations are at or below 60% power settings.

Phil McCand

[jlunseth]

Horsepower, or brake horsepower (BHP) is a measurement of the power of an engine.  %HP is purely mathematical, so all "75% HP's" are in theory, created equal.  %75HP is .75 x max. BHP.  In the case of my 210 HP engine, that number is 157.5 and always will be regardless of circumstances.

The real question is whether, when you think your engine is producing 75%HP, it actuallly is.  The problem is that in our aircraft we do not have a measurement of BHP output.  We have measurements of associated settings such as MP, RPM, OAT, Alt., and we have tables in our POH's that say that if we set MP at a value, and RPM at a value, and OAT and Alt. are at known values, then we get 75%HP (or whatever setting you are looking for).  These values were derived from experience with the original test aircraft, aircraft which were new out of the factory and in presumably very good condition.  For LOP in particular, we also have formulas that say if you multiply Fuel Flow times the factor for your particular engine's compression (mine is 13.7), you get the %BHP at a LOP setting.

In reality, we probably rarely get what we think we are getting, especially those of us with older aircraft.  Too many variables, and some not accounted for in the POH.  An engine with poor compression, a camshat that is worn, spark plugs that are not at their best, timing just a hair off, valves leaking a little, propeller profile having changed from being "dressed" many times over many years, all change the "all other things being equal" assumption in the POH table.

Then there are externals that affect airspeed even if you are actually at the power setting you think you are at, antennas, landing gear doors not fully closing, etc.

So if one pilot in a particular model aircraft sets 75% per his POH, and another pilot does the same in his identical model per his POH, rarely will you have both aircraft (1) actually producing the same BHP, and (2) going the same speed.

Best we can do. 

And for those of us with 231's and intercoolers, where we have to take the POH settings and make further adjustments, and then adjust the adjustments for changes in temp. and altitude, it is more like witchcraft than art.   But fun to do nevertheless.  That makes a nice mental image for this time of the year, flying a Mooney "broomstick," I wonder if there is a way to make a game of quidditch work.

[jetdriven]

I tried climbing 75% power LOP in the 201 and it would do it, but the climb rate was so anemic that I would have to measure it to see if we actually save any fuel doing that technique. 

Another area with NA aircraft is the ability to make over 75% power LOP or ROP rapidly dimishes with altitude. We all know what the books say but in a 201 the ability to lean to 10.0 GPH LOP pretty much disappears around 4-5,000'.   We always seem stuck at exactly 8.7 GPH LOP, 2400 RPM around 4-7K feet. that ends up as right about 67% power.  Flying lower and running more GPH nets no real increase in airspeed, going leaner is slower.

As jlunseth points out, our airplanes are not ideal. I noticed our plane seemed a little slower than other 201s, truing out 145-147 KTAS at 5000 feet, 2400 RPM, over a 25 hour period when we went to KOSH. We were also running 40-50 LOP at 65% power. Our EGTs were also peaking at about 1585.  absolute flat out sea level WOT IAS was 183 MPH.

I suspected the mag timing was at 20 degrees, which is not approved for the -A3B6D.   We reset it to 25 and the difference is dramatic.  WOT IAS is now 187, and average Peak EGT is 1520, average TAS is 153 KTS.  The horsepower also does not fall off a cliff at 50 LOP.  More testing is needed, but it is worth verifying the magneto timing.