Peak and lean of peak horsepower?

[PT20J]

5 hours ago, airtim said:

HP/GPH is always directly related to compression ratio. A higher compression ratio extracts more energy out of the fuel. A 150HP to 160HP STC for a 172 with a O-320 simply increases the compression ratio to get the 10 hp. 

A downside is you can't run Mogas in the 160 HP

The not so popular 10:1 compression ratio stc for Mooneys makes more power because of the Compression ratio.

The lower compression ratio in turbo engines is mostly if not entirely for detonation margin.

A V35B and TC Bonanza came with a 520 making 285HP. The turbo model needed an extra 2.5" to make up for the lower compression ratio. 

I didn't mean to imply that there is no relationship between CR and HP. If you take the same engine and increase the CR, it will increase the HP.

What I was trying to point out is that there is no formula based on fundamental principles of which I am aware that allows calculation of horsepower based on compression ratio and fuel flow alone for an arbitrary engine. The formulas that originate with APS and others are empirical and based on observations of the characteristics of various classes of real engines. Considering BSFC to be a constant when LOP is an approximation (the curve is relatively flat, but not constant) and relating the minimum BSFC to specific compression ratios is a further approximation based on observation. Taken together they are approximations of approximations. So to answer Parker's question, it doesn't really matter whether you use 15 or 15.1 -- it's not accurate to that level of precision in the first place.

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[PT20J]

There seems to be a misconception about LOP operation among some that if a little is good, a lot must be better. This may be because 'high' CHTs are bad and they cool off when LOP (compared to say 50F ROP), so leaner must be better. But keep in mind that one reason CHTs decrease when LOP is that power also decreases. The horsepower vs mixture strength curve falls pretty rapidly when moving farther LOP. (Conversely, the same curve is relatively flat between peak EGT and 100F ROP which is why the power tables in the POH don't differentiate power output between best economy and best power mixtures).

Also, the BSFC curve is at a minimum between about peak and 50-75F LOP and then starts to rise again. This means that if you run leaner than about 50-75F LOP, you will put out less power and burn more fuel per hour to do it.

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[Tim Jodice]

Question regarding propeller rpm efficiency. How much more efficient is it at 2500 than 2700?

Based on this,

https://warpdriveprops.com/propspd2.html

A typical 74" Mooney propeller at 2700 RPM is nowhere near having a tip speed that would cause a loss in efficiency. Are there other reasons?

 

 

[Tim Jodice]

Choices regarding RPM and LOP/ROP

Referencing the POH at 7500 feet WOT, 2500RPM using 10.8 GPH is 75% power.

This give me 155 Knots

WOT 2700RPM 10.2 GPH also gives me 155 knots with slightly cooler CHTs maybe 10-15 degrees. I am about 20-30LOP.

Using 14.9HP per 1GPH LOP for NA engines I am making about 75% power.

All of this makes sense to me 75% ROP or 75% LOP= 155 knots

The part I can't understand is that many have said the propeller is most efficient at 2500.

If that is so why am going the same speed on the same power spinning the propeller faster? 

[jlunseth]

5 hours ago, gsxrpilot said:

I think this is a common issue specific to the 231's. There's just too much going on back and forth between throttle and mixture. So better to just lean to a known power setting.

The Lean Find function works well with the 252. I set the throttle and then pull the mixture back to a target LOP setting. But I can reduce the mixture very slowly and see each cylinder peak and move LOP. 

I expect it would work well with the NA engines as well.

Yes, exactly.  If you can hold MP steady as you vary fuel flow you can find a valid peak in LOP mode.  There is a way to do it in my machine also, which I have described before, but it is just easier for me to make a setting that works, ignore the %HP on the JPI, and manage TIT by fuel flow.

[PT20J]

2 hours ago, airtim said:

Question regarding propeller rpm efficiency. How much more efficient is it at 2500 than 2700?

Based on this,

https://warpdriveprops.com/propspd2.html

A typical 74" Mooney propeller at 2700 RPM is nowhere near having a tip speed that would cause a loss in efficiency. Are there other reasons?

 

 

While it's true that the tip drag goes way up when the tips approach sonic velocity (which reduces efficiency) there are other constraints on efficiency. Interestingly, propellers create maximum thrust when the airplane is sitting still, and the thrust decreases as airspeed increases. So, the ratio of airspeed to rpm is an important parameter. Engineers prefer dimensionless parameters because it makes it easy to compare different designs without having to correct for differences in physical dimensions. So plots of propeller efficiency are plotted as a function of the dimensionless advance-diameter ratio (usually just called advance ratio): J = V/nD where V is the velocity (true airspeed) ft/sec, n is the rotational speed in rev/sec, and D is the propeller diameter included as a reference dimension to make J dimensionless. A propeller efficiency curve always has a peak at some advance ratio. The effect of constant speed props is to vary the pitch with airspeed so that the effective peak is much broader. This means that the interplay between rpm and TAS is not as significant as it would be for a fixed pitch propeller. But it is still there. 

David Rogers did an extensive analysis on the Bonanza propeller efficiency: http://www.nar-associates.com/technical-flying/propeller/cruise_propeller_efficiency_screen.pdf

It's a different engine and prop than my M20J, so the results probably aren't directly transferable, but it's an interesting analysis if you are interested in propellers.

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[PT20J]

1 hour ago, airtim said:

Choices regarding RPM and LOP/ROP

Referencing the POH at 7500 feet WOT, 2500RPM using 10.8 GPH is 75% power.

This give me 155 Knots

WOT 2700RPM 10.2 GPH also gives me 155 knots with slightly cooler CHTs maybe 10-15 degrees. I am about 20-30LOP.

Using 14.9HP per 1GPH LOP for NA engines I am making about 75% power.

All of this makes sense to me 75% ROP or 75% LOP= 155 knots

The part I can't understand is that many have said the propeller is most efficient at 2500.

If that is so why am going the same speed on the same power spinning the propeller faster? 

Those are excellent observations. If you are getting the same TAS at the same density altitude, you are getting the same thrust horsepower.

Remember that thrust horsepower is brake horsepower * propeller efficiency. Also, with a constant speed prop, the efficiency is not going to be dramatically different between 2500 and 2700 rpm. So, what is happening is that the engine is putting out slightly more bhp in your LOP case to make up for the slightly reduced prop efficiency and give you the same thrust hp.

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[Ibra]

10 hours ago, PT20J said:

Remember that thrust horsepower is brake horsepower * propeller efficiency. Also, with a constant speed prop, the efficiency is not going to be dramatically different between 2500 and 2700 rpm. So, what is happening is that the engine is putting out slightly more bhp in your LOP case to make up for the slightly reduced prop efficiency and give you the same thrust hp.

Yes, I had the impression on a constant speed prop you get similar efficiency curves for all speeds as you offset prop slip
But surely there is an efficient rpm that is always hidden in the geometry of the base fixed pitch design (cruise propeller)
If this was the case, then it make sense to cruise at 2500rpm rather than 2700rpm max rated

If not then a consideration not to cruise at 2700rpm is the risk of over speeding?
Don't think "5min of 2900rpm" before cruise at 2700rpm work well on Lyco engines

But probably other constraints: prop clearance? speed at prop tips (I doubt these)

Despite their philosophy, did Mooney ever made any early makes/models on fixed pitch props?

Edited February 11, 2020 by Ibra

[Sandman993]

20 hours ago, gsxrpilot said:

I bought my Mooney to go fast as well. But there is no doubt the best speed mod is the ability to skip a fuel stop. Therefore if I can go A to B without a fuel stop ROP, then I'm going ROP and to hell with the fuel costs. 

But there are plenty of times that 9.5 gph LOP will allow me to go non-stop while 14 gph ROP would require a fuel stop. In that case, LOP is actually quite a bit faster, A to B.

I'm always calculating the best speed for my Mooney for any trip. Sometimes that's ROP and sometimes that's LOP. 

The 201 has fairly serious endurance...while I have flown long stretches in the past, I usually like a leg stretch every few hours, especially if not trying to outrun rotton conditions or loss of daylight (single engine night ops gives me some pause). And some destinations are simply out of reach even if I'm willing to sit thru a long leg. I'm pretty sure stronger headwinds and LOP don't do well as effective fuel management. Suffice it to say, rarely are two missions the same. When I get into LOP,  there's usually more like a 10kt loss...you gotta feed the ponies if you want em to gallop.

Kids, don't try this at home...these unusual attitudes, botched landings and peeing enroute stunts should be left to the experts.

[carusoam]

13 hours ago, Ibra said:

Yes, I had the impression on a constant speed prop you get similar efficiency curves for all speeds as you offset prop slip
But surely there is an efficient rpm that is always hidden in the geometry of the base fixed pitch design (cruise propeller)
If this was the case, then it make sense to cruise at 2500rpm rather than 2700rpm max rated

If not then a consideration not to cruise at 2700rpm is the risk of over speeding?
Don't think "5min of 2900rpm" before cruise at 2700rpm work well on Lyco engines

But probably other constraints: prop clearance? speed at prop tips (I doubt these)

Despite their philosophy, did Mooney ever made any early makes/models on fixed pitch props?

The M20D came originally with fixed gear, and a fixed prop...

Most have been updated with RG and constant speed prop...

Best regards,

-a-

[Tim Jodice]

20 hours ago, Ibra said:

A consideration not to cruise at 2700rpm is the risk of over speeding?
Don't think "5min of 2900rpm" before cruise at 2700rpm work well on Lyco engines

Why would you ever be at 2900 RPM? I suppose if you slam the throttle open you will get a brief overspeed. Where does 2900 RPM for 5 minutes before cruise at 2700 come from?

 

[Ibra]

Thanks, quickly checked that M20D POH and the fixed pitch prop (McCauley GL 52720) not much online but something fitted on PA22s

From the Tri-Pacers POH, it seems that fixed pitch prop on O-320 has max efficient cruise at 2450rpm (vs max 2700rpm) but does make huge ground rolls (1300ft)

 

[Ibra]

18 minutes ago, airtim said:

Why would you ever be at 2900 RPM? I suppose if you slam the throttle open you will get a brief overspeed. Where does 2900 RPM for 5 minutes before cruise at 2700 come from?

Some have that limitation takeoff as the engine (e.g. WW, Rotax, Gipsy) was designed to cruise at a max continuous rpm but allow for above that up to max rated RPM for short periods, this avoids pulling the propeller (or throttle back) while on takeoff ground roll or fast climbs, but the other scenario is RPM overspeed while flying fast

AFAIK, most Lyco engines have Max Continuous RPM = Max Rated RPM

Edited February 12, 2020 by Ibra

[jetdriven]

23 hours ago, Sandman993 said:

When I get into LOP,  there's usually more like a 10kt loss...you gotta feed the ponies if you want em to gallop..

If you’re losing ten knots you’re not doing it right. You should see a IAS loss of about 2-3kt.  I know from a lot of experimentation that the 3-5kt loss of TAS is worth around. 1-1.5 gph.  
It’s the difference between 15 and 17 or 18 nautical miles per gallon

[skydvrboy]

On 2/10/2020 at 7:39 PM, PT20J said:

There seems to be a misconception about LOP operation among some that if a little is good, a lot must be better. This may be because 'high' CHTs are bad and they cool off when LOP (compared to say 50F ROP), so leaner must be better. But keep in mind that one reason CHTs decrease when LOP is that power also decreases. The horsepower vs mixture strength curve falls pretty rapidly when moving farther LOP. (Conversely, the same curve is relatively flat between peak EGT and 100F ROP which is why the power tables in the POH don't differentiate power output between best economy and best power mixtures).

Also, the BSFC curve is at a minimum between about peak and 50-75F LOP and then starts to rise again. This means that if you run leaner than about 50-75F LOP, you will put out less power and burn more fuel per hour to do it.

Skip

You know way more about all things aviation than I do, but I don't think this is correct.  Wouldn't you continue to burn less fuel per hour the more you lean.  I think what you were trying to say was that the speed will decrease faster than the fuel burn and you will burn more fuel per mile while going slower.  Please correct me if I'm wrong.

[PT20J]

1 hour ago, skydvrboy said:

You know way more about all things aviation than I do, but I don't think this is correct.  Wouldn't you continue to burn less fuel per hour the more you lean.  I think what you were trying to say was that the speed will decrease faster than the fuel burn and you will burn more fuel per mile while going slower.  Please correct me if I'm wrong.

You’re right; I misstated it. The BSFC increases, but the bhp falls off rapidly and the fuel flow decreases. What I should have said is that you will take longer to get where you’re going and burn more fuel to do it. Thanks for the correction. 

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