[Ironflight]

After I decided not to get up at 0430 to try and make the Cedar Mills Breakfast, I was left with a decision of where to fly today when I DID get up....thought about BBQ, but then found out about a neighborhood evening BBQ here, so didn't need that....since I have a long trip coming up later this week, I really didn't need a cross-country anyways, so I decided to get a little current performance data to see how the Val is doing in cruise with almost 600 hours on the clock. At the same time, I could take a look at the area where Hurricane Humberto made landfall to see if there was much visible damage or flooding, so I launched up the coast with a step climb, taking data at each thousand feet of pressure altitude.

The premise was this - I wanted to see what kind of speeds and fuel flows i was getting the way I usually fly - not when I am going for maximum speed. That means I level off, bring the RPM's back to 2350, and lean the heck out of the thing - usually get slightly LOP, even though I am carburated. I try and maintain 75% HP until I reach an altitude where I can't, and then I am Wide Open Throttle (WOT) after that. SO don't' worry that the Val is slow - she's not - I can open things up down lower, increase the RPM to get more HP (and burn more gas) and scoot along at 175 KTAS if I want - but usually, I try to get a little more endurance than that.

Here's the data I took - you can see very clearly that the Density Altitude was about 2000' higher than pressure altitude, so the altitude at which I could maintain 75% was lower than 8,000 pressure altitude today. I usually flight plan 170 knots if I am cruising between 7,000 and 10,000, and that is about what I get. Lower than that, I can still get the speed with more HP. Higher than that, I am slower, but burn less fuel.

Pressure Density TAS Percent Fuel Flow MAP Notes
Altitude Altitude (knots) HP (gph) (inches)

3000 4400 165 75 8.5 24.2
4000 5350 166 75 8.2 24.0
5000 6600 166 75 8.0 23.8
6000 7750 166 75 7.9 23.8
7000 8900 168 73 7.7 23.2 WOT
8000 9945 170 70 7.4 22.3 WOT
9000 10925 168 67 7.2 21.4 WOT (170 KTAS @ 2550 rpm)
10000 12000 163 64 6.7 20.6 WOT (166 KTAS @ 2550 rpm)

All data taken at 2350 RPM (except as noted on last two altitudes - at those altitudes, I took the 2350 data, then increased RPM 2550 to see how that added to sped. Also increased fuel flows.)

Now here's a question to set off some discussion for the week. We all know that at lower power settings, there are multiple combinations of MAP and RPM that can be used to get a specific Percent HP. Percent HP is directly proportional to Fuel flow, so once you have decided on a HP setting, you have committed to a fuel flow. But should you use a higher or lower rpm (when you have a choice) to get that percent HP?

(Pin pulled on grenade, grenade tossed into forum with usual suspects, ducking for cover...)

Paul

Oh....Hurricane Humberto didn't leave much trace from 5,000', and didn't stop the big Gator Festival in Anahuac, which was in the target zone....

 

[doctornigel]

In theory, a lower RPM is more efficient, and usually quieter. Not as many revolutions that have to fight drag. Just a theory, YMMV.

 

[Christopher Murphy]

great numbers for comparison

your numbers are pretty close to the numbers I see with my 160hp RV-4.
with Catto prop. At wide open it will exceed 180kts TAS at any alt below 10,000.

 

[gmcjetpilot]

Get high save gas

Enjoy

Note: Max gas econ 28 statute MPG (24.4 nautical mpg) at almost 190 mph (163kts)! yea baby!

8,000ft to 11,000ft looks pretty sweet, no O2, fast, good econ, of course depending on winds.

Keep climbing, get better econ & range. How high? Depending on gross wt & engine HP, FL180?
Factpr climb fuel, leg length & winds as well. You save fuel but sacrifice block time, while
gaining "air-mile" range. 8k-12k feet looks like a good compromise. I know a guy who likes
to fly at 17,500 ft. He gets crazy range but he takes a lot longer to fly a leg and he is burning
O2 bottles.

 

[Kevin Horton]

Now here's a question to set off some discussion for the week. We all know that at lower power settings, there are multiple combinations of MAP and RPM that can be used to get a specific Percent HP. Percent HP is directly proportional to Fuel flow, so once you have decided on a HP setting, you have committed to a fuel flow. But should you use a higher or lower rpm (when you have a choice) to get that percent HP?

The engine will have less frictional hp loss at lower rpm, so for the same indicated power (i.e. the power that is generated by burning the fuel in the cylinders), you'll make a bit more brake horsepower at lower rpm. But, the prop efficiency variation with rpm is probably a bigger player. The data for my Hartzell seems to suggest that lower rpm is better in most cases. Lower rpm is also quieter.

 

[Tom Martin]

Mark Frederick clued me into this one years ago. Set your plane up for 2300rpm and 21", lean of peak. Note the IAS and ground speed. Now reverse the throttle and rpm settings to 2100rpm and 23". Speed goes up, fuel flow goes down; at least in the rockets I have flown it does.

 

[David-aviator]

Now here's a question to set off some discussion for the week. We all know that at lower power settings, there are multiple combinations of MAP and RPM that can be used to get a specific Percent HP. Percent HP is directly proportional to Fuel flow, so once you have decided on a HP setting, you have committed to a fuel flow. But should you use a higher or lower rpm (when you have a choice) to get that percent HP?

(Pin pulled on grenade, grenade tossed into forum with usual suspects, ducking for cover...)

Paul

If only we knew the BSFC for various engines, it would be a simple matter of setting fuel flow to arrive at a specific HP, rpm wouldn't matter and would simply be a matter of efficiency at arriving at the best setting.

I like flying on fuel flow and WOT and have found 2100 prop rpm produces better speed than 2300 rpm at a lower burn rate, but the speeds are not as good as yours for reasons I still do not understand. Could be the BSFC of the Subaru is not as good as Lycoming or we do not have adequate control of mixture. The pilot really is along for the ride when operating the engine with an ECU.

Thanks for gathering and posting the numbers. Your monthly fuel bill must exceed your mortgage payment. Fuel prices around here are bumping $5/gallon although out in the boonies it can be found for less than $4.

 

[Ironflight]

Your monthly fuel bill must exceed your mortgage payment.

Fortunately, I did away with the Mortgage payment awhile back through clever negotiation with the divorce lawyers....and yup, Quicken tells me that my greatest single expense category each month is Aviation Fuel...

Thanks for the plot George - I was going to play around in Excel, but you saved me the trouble!

I might just give 2100 rpm a try for awhile on the California trip later this week - gotta have something to experiment with!

Paul

 

[garnt.piper]

High Alt. Ops

I was commuting regularly for a while between home and Bundaberg, QLD. (445nm). I like to travel up between 8-10k, (Oz transition level is FL100-110, so we can't cruise there). At ~75%, 36-37L/hr, I could get there with ~20 litres remaining in 2:40 or so. I went up to FL115 a couple of times, but found the extra fuel burnt in climb negated any saving on that leg length, so I gave it away. I concluded that with the standard 120 litres usable in the RV-4, no benefit was to be had in climbing that high. I was/am too impatient to experiment with pulling the power back to find the best mileage, plus, without a fuel flowmeter I was just guessing anyway.

I have a FP wood prop, carburetted O-360-A1A, slick mags. At these powers I'm about red-lined! At FL115 I'd have to pull the power back a little to 18"MAP to stay at 2700rpm. One day I'll get a cruise prop to try.