MP vs RPM for % power.

[Guitarmaster]

A long time ago I run across a chart that was a rule of thumb for figuring percent power if you don't have an engine monitor. I cannot find that anywhere. Does anybody have one they want to share on this thread? It's something like 47 is equal to 65% power or something like that.

 

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[Oldguy]

They are taught in the Mooney Safety Foundation PPP and are called "key" numbers. Goes like this:

(RPM/100)+MP = Key number

For my J, 44=55%, 47=65%, and 50=75%.

Not sure if your F would be the same values or not.

[Bob - S50]

And of course that is only true for ROP.  If you are LOP then power is proportional to fuel flow.  10 GPH = 75%, 8.7 GPH = 65%, 7.4 GPH = 55%.

[Guitarmaster]

They are taught in the Mooney Safety Foundation PPP and are called "key" numbers. Goes like this:
(RPM/100)+MP = Key number
For my J, 44=55%, 47=65%, and 50=75%.
Not sure if your F would be the same values or not.

This was what I was looking for. Thanks!




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[Robert C.]

I'm no engine expert but if I recall correctly the way to calculate hp produced goes as follows:

hp = (Fuel Flow (in gal/hr) * 5.8 lb/gal )/ BSFC

BSFC is "brake specific fuel consumption"

5.8 is weight of avgas/gal

BSFC can probably be found in your engine manual. I found mine (IO-550-G) by going to the Continental website. For my engine they give .39 for best economy and .41 for best power. Take the calculated hp and divide by the engine's listed max and you have the % pwr at which you operate. There is a shortcut of hp = FF * 14.9 which gets you to similar numbers.

Attached is the little table I printed and put on my kneeboard with all the other performance table junk

Robert

 

	Percent Pwr produced function of FF
	gl/hr	hp	pwr
	10.0	149	48%
	10.5	156	50%
	11.0	164	53%
	11.5	171	55%
	12.0	179	58%
	12.5	186	60%
	13.0	194	62%
	13.5	201	65%
	14.0	209	67%
	14.5	216	70%
	15.0	224	72%
	15.5	231	75%
	16.0	238	77%
	16.5	246	79%
	17.0	253	82%
	17.5	261	84%
	18.0	268	87%
	18.5	276	89%
	19.0	283	91%
	19.5	291	94%
	20.0	298	96%
	20.5	305	99%
	21.0	313	101%
	Note: IO-550-N and BSFC @ .4 or hp at 14.9 per gl

IO-550-N percent power table-v2.xlsx

[Oldguy]

38 minutes ago, Robert C. said:

I'm no engine expert but if I recall correctly the way to calculate hp produced goes as follows:

hp = (Fuel Flow (in gal/hr) * 5.8 lb/gal )/ BSFC

BSFC is "brake specific fuel consumption"

5.8 is weight of avgas/gal

BSFC can probably be found in your engine manual. I found mine (IO-550-G) by going to the Continental website. For my engine they give .39 for best economy and .41 for best power. Take the calculated hp and divide by the engine's listed max and you have the % pwr at which you operate. There is a shortcut of hp = FF * 14.9 which gets you to similar numbers.

Attached is the little table I printed and put on my kneeboard with all the other performance table junk

Robert

 

	Percent Pwr produced function of FF
	gl/hr	hp	pwr
	10.0	149	48%
	10.5	156	50%
	11.0	164	53%
	11.5	171	55%
	12.0	179	58%
	12.5	186	60%
	13.0	194	62%
	13.5	201	65%
	14.0	209	67%
	14.5	216	70%
	15.0	224	72%
	15.5	231	75%
	16.0	238	77%
	16.5	246	79%
	17.0	253	82%
	17.5	261	84%
	18.0	268	87%
	18.5	276	89%
	19.0	283	91%
	19.5	291	94%
	20.0	298	96%
	20.5	305	99%
	21.0	313	101%
	Note: IO-550-N and BSFC @ .4 or hp at 14.9 per gl

IO-550-N percent power table-v2.xlsx

I can understand this for sea level, but what is the effect of Altitude/Density Altitude on this table? While I see the calculations show the fuel flow for full hp range, on my NA IO-360, max hp is less than 100% shortly after TO.

[DanM20C]

11 minutes ago, Oldguy said:

There is a shortcut of hp = FF * 14.9 which gets you to similar numbers.

The 14.9 number works great but is only for LOP ops with  a 8.5:1 compression ratio.  When ROP HP is more a function of MP and RPM, hence why the "key" numbers work out well.

So when rich of peak use "key" numbers (what ever they are for your airplane) and when lean of peak use FF.

Cheers,

Dan

 

 

[carusoam]

Nicely covered, Dan!

• LOP %hp is a direct function of FF. Because all the fuel gets used to produce power...
• ROP %hp is more challenging because some amount of excess fuel is not used to produce power... and is hard to account for...
• In the LOP calculation the Compression ratio is an important part of the calculation...  So, each engine has its own specific factor for calculation...

Something I believe should be represented as well is engine ignition timing... but under cruise conditions, all the fuel that is going to burn, is being burned before leaving the exhaust valve...

%hp calculations using an engine monitor have improved over the years... when they first came out the numbers didn’t match the engine’s chart very well...

MAPA is a proponent of the Key Numbers for ROP.  Really easy to use in flight....

The engine performance charts in the LB POH are really good for the IO550 operations... 

• Best Economy is an LOP standard...
• Best power is an ROP standard...

Be sure to read the details on how many °F (ROP or LOP) are being used in those standards... can technically be 1°F, LOP... you won’t want to go to 50°F LOP if you don’t have to (low power)... ROP recommendations might leave you in the too hot CHT range as usual...

One of my favorite topics...

PP thoughts only, not a mechanic or CFI....

inviting @patrickf to have a look at Robert’s excell spreadsheet for the IO550N (my engine) the IO550G (standard O engine) will be the same in this case.  The N has different cooling fins on the cylinders...

Best regards,

-a-

[Bob - S50]

Just like LOP power is fuel limited, unless you go WAY ROP, ROP is oxygen limited.  The amount of oxygen that is pumped through an engine is determined by volume (RPM) and density (MP), thus ROP power can be figured using a key number.

Since SL pressure is about 30", a 1" change in MP equals about a 3.33% change in power.  Likewise, since most our engines have a maximum RPM limit of 2700 RPM, a 100 RPM change makes about a 3.7% change in power.  While they aren't exact, for initial power settings, those numbers are close enough.  Thus the RPM/100 + MP = a key number is pretty close.

[Awful_Charlie]

2 hours ago, Bob - S50 said:

...Since SL pressure is about 30", a 1" change in MP equals about a 3.33% change in power.  Likewise, since most our engines have a maximum RPM limit of 2700 RPM, a 100 RPM change makes about a 3.7% change in power.  While they aren't exact, for initial power settings, those numbers are close enough.  Thus the RPM/100 + MP = a key number is pretty close.

Except engines don't idle at 0" MP! I guess you've got the range of about 12-30 to play with at sea level

They also don't make a great deal of power sub 1000 RPM, but I agree for RPM in the narrow operating band we use, it's close enough

[Bob - S50]

On 11/14/2018 at 4:43 PM, Awful_Charlie said:

Except engines don't idle at 0" MP! I guess you've got the range of about 12-30 to play with at sea level

They also don't make a great deal of power sub 1000 RPM, but I agree for RPM in the narrow operating band we use, it's close enough

Actually, it still kind of works.  The number of molecules of oxygen is dependent on V x D.  So at 0" of MP you get (0/30) x (anything/27) = 0% power.  Same thing before you start.  (0/27) x (30/30) = 0% power.  At low altitude using the magic number of 47 for 65%: (24/27) x (23/30) = 68% or (26/27) x (21/30) = 67%.  Using 50 for 75%: (26/27) x (24/30) = 77%.

I doubt it is quite right, but at idle with the throttle essentially closed and guessing MP is about 10": (10/27) x (10/30) = 12% power.

[Peter Rejto]

On 11/14/2018 at 8:36 AM, Guitarmaster said:

 

A long time ago I run across a chart that was a rule of thumb for figuring percent power if you don't have an engine monitor. I cannot find that anywhere. Does anybody have one they want to share on this thread? It's something like 47 is equal to 65% power or something like that.

 

Sent from my Pixel 2 XL using Tapatalk

 

 

 

For the M20 J the "Magic Number" is 47. To get the number you take the RPM and drop two zeros and add the manifold pressure. Thus, 23 " MP + 24(00) rpm = 47 which is approx. 65% power. So is 24" MP + 23(00) rpm, etc, etc.

Peter

[OZMOONEYMAN]

Just wondering about the IO-550-G vs N models.  I note continental publish 280hp and 310hp respectively.  Could someone sort my curiosity around where the extra 30hp comes from given it seems the compression ratio, bore and stroke are the same and both hp ratings are at 2700 rpm.  I assume it's either or both advanced timing, and/or throttle body.  With throttle body on the G I'm guessing its restricted in its full potential, i.e. full open its flow is restrictive and FF is proportionally down (Just a guess here).

[A64Pilot]

I doubt HP difference is from restricted fuel flow, however I’d suspect possibly restricted airflow, I know nothing at all about those two engines but timing ought to be easy to look up. restricting fuel flow at high power can give bad results so I doubt that’s it.

I’d suspect differences in induction systems, and possibly how the engines are rated, many aircraft engines are rated conservatively, reduce some of that conservatism and get a higher rating. There can be a lot of power in “tuning” induction systems.

The Lyc IO-360 has the same rating at two different timing settings, pretty sure the advanced timing results in higher power, but also of course higher cyl head temps.

There is enough conservatism so that it still makes spec power with the timing turned down.

Engine monitors derive percent power from calculation of course and many I’ve seen weren’t very good at it.

 

Edited December 19, 2021 by A64Pilot

[A64Pilot]

On 11/14/2018 at 2:15 PM, Oldguy said:

I can understand this for sea level, but what is the effect of Altitude/Density Altitude on this table? While I see the calculations show the fuel flow for full hp range, on my NA IO-360, max hp is less than 100% shortly after TO.

In theory the formula still works as it is based on best power mixture so theoretically the fuel flow is reduced with manifold pressure drop

We take off and climb well rich of best power so it is what it is, there are other considerations like cyl head temp of course 

[N231BN]

Just wondering about the IO-550-G vs N models.  I note continental publish 280hp and 310hp respectively.  Could someone sort my curiosity around where the extra 30hp comes from given it seems the compression ratio, bore and stroke are the same and both hp ratings are at 2700 rpm.  I assume it's either or both advanced timing, and/or throttle body.  With throttle body on the G I'm guessing its restricted in its full potential, i.e. full open its flow is restrictive and FF is proportionally down (Just a guess here).

IO-550-G is rated at 2500 RPM.

[carusoam]

I have had both in my O1…

The G is what it was born with… 2500rpm max…. 280hp… max FF around 25gph

The N is what it graduated to…. 2700rpm max, with fancy cylinder differences…. Visible from the outside with their cooling fin arrangement… 310hp  … saving a few pounds per cylinder… max FF around 29gph

You can get the G turned up to 310… and 2700rpm… following the STC that requires the TopProp to go with it… the weight of the TopProp makes the N cylinders interesting…

Getting the FF to meet your preferences is interesting…

Really makes sense after a ground strike, or time for OH, or prop change….

Also makes sense if you have spare amus looking for a great performance project…. 
 

 

Best regards,

-a-

[OZMOONEYMAN]

Copy thanks.  The publication I had FROM continental had 2700 RPM for the G... its a mis-print.  I flew a C206 with N engine and a Roberson STOL... great machine