I’ll take a rough stab at some of these, and leave the others for George.

 

1)       Most props are optimized for cruise at about 2500, as that is where most people run during cruise.  Several race props (probably what Klaus has, similar like Greenemeyer and Schrameck) are available and these are optimized higher.  The higher the RPM, obviously the more HP you can make.  On my io-540 300hp C182RG, the sweet spot is 2550 on a 3 bladed hartzell.  Anything over 2550 and I don’t gain any speed.

2)             a)   the pistons likely have less of a “dome” in them, which gives a smaller combustion chamber, thus increasing the CR

b)       not sure there

c)        

                                                               i.      when you say 90 degree reduction, do you mean across the board, or just at a certain setting?

                                                             ii.      Advancing the spark changes where the peak cylinder pressure occurs.  If I recall, optimally, you want this Theta_pp to be about 16 degrees.  Unfortunately, this varies at different power and rpm settings.  Retarding this value will have the effect of reducing the peak pressure (less hp, as hp is the area under the ICP curve) and making it occur over a longer duration.  This lower peak pressure resists detonation.  Advancing this value will result in a higher peak pressure (ICP) which will result in more HP (again, area under the curve).  However, with these higher peaks and higher hp comes more change of detonation.  Georges PRISM addresses all these issues, allowing you to run right up to the point of detonation (on whatever gas you have) and therefore maximizing the HP output.  It will be a glorious day when it is available.

 

 

If you are normally aspirated, if you are over 7000 feet, you are going to be under 65% power, so you can really run wherever you want with the mixture and be safe.  You just aren’t making enough power for it to matter.  Therefore, at anything over 7000 feet, I’m operating at 60-70 ROP, as that is where most power occurs.  Now, if you are below 7000 feet, you need to increase that to 100 at about 5000, linearly to about 250 at sea level.  If I’m looking for endurance, I’ll switch over and run 70 LOP above 7000.  I routinely run 70 LOP at least part of every flight so I can do a high power LOP mag check, as this really will show any problems in your ignition system…much more than any mag check you do on the ground.  This also serves to burn off any deposits that have accumulated during my ROP runs.

 

If you have a 1.5” ram air, you might want to increase the altitude to 8500.  If you are boosted, then you can either run about 150 ROP in high power (75-85%) cruise, or run 70-100 LOP and increase the MAP a couple inches to make up for the lost power.

 

This is all stuff that is taught at www.advancedpilot.com  Highly recommended.  All your questions and more will be answered.

 

-----Original Message-----
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Sky2high@aol.com
Sent: Tuesday, June 10, 2003 1:00 PM
To: Lancair Mailing List
Subject: [LML] LOP FLOP, ROP FOP Query for Guru(s)

 

OK, here goes ----

 While some of us have heard and read the words of George Braly and what’s his name (AvWeb guy - Oh, I remember John Deakin, with the twin), we are still confused.  Why?  Well, let me speak for myself.  I find no comfort hearing about high flying sixes, some turbo charged, with caveats like “fixed ignition timing”, dyno tested, etc.  Most flying Lancairs are equipped with little 4 cylinder engines, many pushed to their limits.  No matter, I have no cylinder envy, nor even inches (cubic) envy.  I just want to operate at best power, or max power (for racing) or even economy power for the strange reason of stretching endurance on a long cross country.  My confusion in understanding comes from the following: 

1.       “Prop efficiency goes down after 2500 rpm.”  Even though Klaus runs rings around larger engined planes when he is operating in excess of 3000 rpm?  Does the efficiency go down, but thrust HP is increased?  Am I losing power by operating at 2660 rpm (a felt sweet spot)?

2.       What is 75% power for my Lycoming I/O 320 engine equipped with a 70 inch Hartzell constant speed prop?  I have made the following adjustments:

 

a.       Different pistons were used to raise the compression ratio from 8.5 to 9:1.  How was that done?  Did it reduce my cubic inches or somehow increase the stroke?

b.       A non-filtered induction ram air system is used which raises the manifold pressure 1.5” Hg at 180 KIAS (195 KTAS).  What affect is this on power charts or do I just artificially add1500 feet to the chart scale?

c.       The added LASAR ignition system has 2 effects:

i.          A hotter spark, burning more fuel in the cylinder.  The consequences are a 15-20 degree rise in cylinder head temps and a 90-degree reduction in EGTs.  This seems to lead to increased torque, thus increased thrust HP because the prop pitch is increased to retain the RPM.  This is seen as a sprightlier take-off run, a higher climb rate and better general performance.  What is the affect on determining % power?

ii.       At some RPM/MAP point, the “spark” is advanced, resulting in higher power and more efficiency.  How does this affect % power at full power and best power?  What is the affect when running LOP?

d.       The addition of a harmonic dampener, which for fixed pitch props generally, increases the full power rpm by 100.  Does this improve my power also by increasing the flywheel affect (allowing an increase in prop pitch to retain rpm)?

 Another words, what are the gross parameters I can use to operate LOP?  What are the steps I can use when operating ROP and how many degrees (EGT) rich at certain power levels.

 With the above listed modifications that appear to affect power, should I increase the takeoff fuel flow to something greater than its current 15.1 gph?

 Someone help us little guys with answers to our questions!  How else can we achieve harmony with the universe? Huh? Huh? 

Grayhawk, AKA Scott Krueger
Sky2high@aol.com
LNC2 N92EX IO320 Aurora, IL (KARR)

PS Formulae accepted.