Input on performance #'s, 150HP

Hi all,

It was a beautiful day to fly so I decided I would do some detailed performance testing now that I have all the fairings on minus the lower intersection fairings. To be honest, I am a bit slower than I would like, and would love any advice or comparable data from others. I know that adding the lower intersection fairings will add a couple knots, but probably not as much as I'd like. My setup is:

O-320 150HP, FP metal Sensenich, dual mags

I flew at 8000' DA, OAT 63* F. Mixture was set ROP.

And here are the numbers (all speeds in KTS):



Vans lists 75% cruise speed to be 162 knots for the 160HP engine - My 148 knots seems way too low for just a 10HP difference.

Any ideas, or am I somewhat close to what others are seeing?

Chris

Speed is all about drag reduction and having the prop matched for your engine power/rpm and the speed of the airplane, so that the prop is operating in its efficient range.

So any fairings, gaps, mismatched panels, cowling, canopy, fairings - all these little details add up. I don't know enough about the engine numbers to pick out what is happening i.e. do those MAPs correlate to the power level you are expecting etc. Do you have the torque and power curve for the engine vs RPM for a cross check?

I have the power curves for the engine, which I also plugged into my EFIS, and that is where the % power numbers are coming from. Putting today's numbers into the graph yields a few less HP than I should be seeing at those % power settings, but they are close.

I am thinking that adding a few knots with the lower intersection fairings will also allow the RPM to come up a bit. I am limited to 2600 RPM with the Sensenich, so I know I can never get full rated HP. I am wondering though if I should be shooting for a bit more RPM at WOT. The prop is pitched to 77", the Sensenich guide says that 76 is standard, 75 is climb, and 78 is cruise, for the 9A with 150HP. So mine is actually somewhere between a standard and cruise pitch. I definitely wouldn't consider changing anything until bottom fairings are on though as I know they will affect RPM.

I don't plan on going speed-crazy, or I would've built a rocket, but I'd like to at least make sure I'm not giving up easy MPG's. Thanks!

Chris

Put the fairings on and switch to a Catto and you will get the speed.

When I had my 135 hp O-290 my 65% power cruise was 140 kts. (With the O-360 75% is 174 knots.)

What bill said above. Read my speed mods article in the articles section of this website. I took a similar plane from 136knots to 165 knots with the only expensive change being the prop. The article details all of my changes to 396DS.

We used to see 145-150ktas at <7gph in our -9A with a 150HP O-320 and both a Catto 2-blade and a Sensenich Ground Adjustable prop.

woodmanrog said:

What bill said above. Read my speed mods article in the articles section of this website. I took a similar plane from 136knots to 165 knots with the only expensive change being the prop. The article details all of my changes to 396DS.

Click to expand...

Can someone give the newbie a link?

Mark_H said:

Can someone give the newbie a link?

Click to expand...

http://www.vansairforce.net/articles/speedmods.htm

Mark,

Thanks,
Newbie

I don't think you can get 75% power with the Sensenich metal prop @8000', due to rpm restrictions on the prop.

Do you have Lyc's engine operating manual? Look at the power curves vs altitude.

In general terms, if the engine makes 100% power at 2700 rpm at sea level, then at 8000 feet (more or less; depends on how efficient the induction system is on your plane) 2700 rpm will be 75% power. That's just the physics of manifold pressure loss with altitude. 2550 rpm just won't do it for 75% pwer at that altitude. And unfortunately, IIRC the lighter Sen. metal prop is limited to 2600 rpm, so you're stuck with less than 75% power at altitude.

Check Lyc's rpm/mp/altitude charts; you might be very close to Van's book numbers for power.

Charlie

rv7charlie said:

I don't think you can get 75% power with the Sensenich metal prop @8000', due to rpm restrictions on the prop.

Do you have Lyc's engine operating manual? Look at the power curves vs altitude.

In general terms, if the engine makes 100% power at 2700 rpm at sea level, then at 8000 feet (more or less; depends on how efficient the induction system is on your plane) 2700 rpm will be 75% power. That's just the physics of manifold pressure loss with altitude. 2550 rpm just won't do it for 75% pwer at that altitude. And unfortunately, IIRC the lighter Sen. metal prop is limited to 2600 rpm, so you're stuck with less than 75% power at altitude.

Check Lyc's rpm/mp/altitude charts; you might be very close to Van's book numbers for power.

Charlie

Click to expand...

I think you may be right, although I have the curves programmed into the EFIS and it has generally seemed to be pretty accurate, as at 8000', WOT it was showing 75% power, and in general use it agrees pretty closely with the rule of thumb for calculated % power. I also checked the chart manually with yesterday's numbers and it seems pretty close. Also, the Lyc manual shows 75% power at 2450 RPM and 10GPM, which is fairly close to what I am seeing when I am set for 75%.

I think my plan will be to keep eliminating drag until I can get the prop to spin at 2600RPM at 8000', and then see where that leaves me speed wise. Will probably still be a little slower than some others, which I can live with, but at least I can get as efficient as possible until I decide to upgrade props.

Thanks!

Chris

Are you at full throttle when making your measurements @ 8k'? I should have mentioned that in my 1st reply. The Lyc numbers (any normally aspirated IC engine, actually) should show ~8k' full throttle, 2700 rpm (rpm for full power at sea level), leaned for best power is around 75% power.

10 gph sounds way too rich for 75% at 8k'. That's roughly the burn of a 180 hp engine at 75%/8k'. The BSFC of the 150 hp engine will be a little worse due to lower compression, but it shouldn't be anywhere near that bad. The 160hp O320 (.5-to-1 point higher compression) fuel burn is between 9 & 9.5 gph @ 8k' full throttle & leaned for best power.

If you don't have tuned fuel injection and/or electronic ignition, BSFC *when leaned for best power* is around 0.45 lbs of gas per hp per hour (6 lbs per gallon). That number doesn't really vary much between engines unless you raise compression. Timing highly retarded can drive efficiency down (and egt's up).

Charlie

rv7charlie said:

Are you at full throttle when making your measurements @ 8k'? I should have mentioned that in my 1st reply. The Lyc numbers (any normally aspirated IC engine, actually) should show ~8k' full throttle, 2700 rpm (rpm for full power at sea level), leaned for best power is around 75% power.

10 gph sounds way too rich for 75% at 8k'. That's roughly the burn of a 180 hp engine at 75%/8k'. The BSFC of the 150 hp engine will be a little worse due to lower compression, but it shouldn't be anywhere near that bad. The 160hp O320 (.5-to-1 point higher compression) fuel burn is between 9 & 9.5 gph @ 8k' full throttle & leaned for best power.

If you don't have tuned fuel injection and/or electronic ignition, BSFC *when leaned for best power* is around 0.45 lbs of gas per hp per hour (6 lbs per gallon). That number doesn't really vary much between engines unless you raise compression. Timing highly retarded can drive efficiency down (and egt's up).

Charlie

Click to expand...

Yes I was at WOT at 8000' DA as shown by my EFIS, was about 6500' MSL, 63 degrees F, altimeter 30.10"

I know the fuel flow numbers are a bit high - to be honest I didn't spend as much time as I should have leaning properly, I really just set it well rich of peak. I believe fine tuning would have those numbers about .5gph lower. Maybe the imperfect mixture is slightly affecting power, also. Once the last fairings are on I will try to do a better series of tests and pay a bit more attention to mixture.

Chris