Dollars/gallons per mile

[Greg Arehart]

I am building a 9A primarily for cross-country use and need to make some engine choices (well, at least choose ONE engine). Three questions:

1) presuming identical airplanes (including weight) with identical fixed-pitch propellers, am I correct in assuming that no matter the engine, if it is running the same RPM, I will be traveling at the same speed in both planes? If this is true, then wouldn?t the fuel burn be higher on a larger-displacement engine (more cylinder volume, therefore more fuel used), and therefore my dollars/gallons per mile would be higher for say, a 180 vs 160 Lycoming?

2) starting with identical airplanes in terms of drag issues, if I were to put a constant-speed prop plus the larger engine on one, would/could the fuel burn be the same or better than the fuel burn on the smaller engine (that is, would they have the same efficiency)? I realize that this would probably mean a weight increase because of the larger engine and CS prop.

3) can anyone comment on the gain in efficiency from electronic ignition systems? Lightspeed Engineering claims a 10-15% gain depending on whether one or two mags are replaced. What has been the experience of the group?

Thanks,
Greg Arehart

[buz]

Simple answer for question 1)

It takes the same horsepower to turn the prop a certain rpm, and this would mean a very similiar fuel flow.
The speeds would be the same also.

[Harvey L. Sorensen]

wouldn't we need to take in the different pitch on the prop for the different engines?

[Greg Arehart]

In order to keep this thought experiment constant, I was assuming the same prop. I realize that one could increase the pitch somewhat with the 180 hp engine, and therefore RPM would go down for the same speed. My ultimate question really is what is the most efficient combination of engine/prop for cross-country flying. Even though I fly out of Reno with sometimes high density altitude, almost any engine will get me off the ground sufficiently. In terms of mileage, probably a smaller engine would work better, but I also appreciate having the power when I need/want it - I just don't want to pay too high of a price....

greg

[RVbySDI]

Insert disclaimer here
First things first. I am not an engineer, mechanic or any other type of expert so the comments below should not be construed as expert advice. It is MHO and YMMV.

Ok, disclaimer noted.

Question 1.) My opinion is that the differences between a 160 HP (O-320) and a 180 HP (O-360) engine would not be great enough for any typical pilot to note a significant difference in fuel burn. It is possible that one or the other may produce a better fuel burn than the other but I do not believe the difference would be as a result of the different displacement size. Given that both planes are identical in every aspect except for the 20 HP difference and you ran both at the same RPM settings I would not expect to see any difference in the fuel burn.

However, were we to have one running a 160 HP (O-320) and one running a 450 HP (superduper screamer O-740 whatever) the displacement may be different enough that running both engines at the same RPM would affect fuel burn differently.

Question 2.) The difference between a constant speed (C/S) prop performance and a fixed pitch performance have a great many variances that I am sure some of the experts on this forum can address. Your query lacks some details to properly answer this question. A fixed pitch prop is "fixed" at a specific pitch to give the optimum performance for the aircraft at whatever corresponds to that pitch. Therefore when comparing a fixed pitch prop to a C/S prop the comparison will only be appropriate whenever the C/S prop is set at the same pitch as that of the fixed pitch prop. So discussions of efficiencies are only appropriate when both are set at the same pitch and the airplane is performing at that particular function.

Furthermore there would have to be some discussions of defining what "efficiency" means in this context. You have specifically asked about fuel burn so just keeping the discussion on that measure the fuel burn should be the same with the C/S prop whenever it is set at the same pitch as the fixed pitch prop, with one exception. As you stated, the C/S setup will weigh more. That difference in weight may be enough to skew the fuel burn in favor of a fixed pitch setup when the C/S is running at the same pitch as the fixed pitch.

Discussion of efficiency cannot just be measured in terms of fuel burn however. The decision to go with a C/S setup must be determined by many factors that deal with the overall mission of the airplane. Many other things such as climb performance, cruise performance, purpose requirements of the aircraft (is it to be used in formation flying for example) and several other factors need to be analyzed before making this decision.

Question 3.) I will definitely leave for someone else to answer.

You ask important questions that all of us have asked at one time or the other. I do believe the answers are not as easy to address as the question is to ask however. Much analysis has to go into each answer and often times the only one who can truly analyze the information is the person asking the question. Keep plugging away. I hope this helped. If not, well, c'est la vie and good luck with your build.

[woxofswa]

Im not an expert, but here's my two cents. Two motors of similar design but different displacement will burn comparable fuel to produce the same horsepower. The energy actually resides in the fuel, the engine simply converts it. Since weight is a factor in consumption, a heavier powerplant will have a slight penalty just to pull its own weight. The more powerful package will have times when it is advantageous however. It can give you quicker climbs or higher altitudes to favorable winds. Another factor in Reno, is higher altitude capability can mean more direct routing for lower trip burn. I own a TB9 with the O-320 which is the smallest of four engines for the airframe. As a trainer it was great because it forces students to understand performance limitations. Nevertheless, power is your friend, I would opt for displacement if given a choice.

[rtry9a]

You titled the thread dollars/gallons per mile. The best dollars/mile would assume cheapest fuel AND decent economy- probably a lower compression 160 hp engine that can run on mogas, possibly one of the alternative engines.

[kentb]

I think some before me have stated what I also think. The engine size doesn't have much to do with rate of fuel burn for a given amount of HP.

I thinks that other factors would favor the smaller engine as long as it will do the job that you want. Something to consider:
Don't know of anyone using a 320 that needed a better oil cooler. It seems that 360 user need to go with a better cooler. As long as you were only going to ask for the power that the 320 would deliver then you would not need to change this. With the larger engine you would need to plan to be able to get ride of the heat that it would make, even if you did use the HP all the time. I think that that means that you would need the cooling drag built into the plane to handle the power even if you didn't use it. If you have extra drag then you will need to use more power to get back to the same speed.

Don't know how much this would amount for in lowering the of the MPG of the plane, but I think that it could be measured.

Now for the prop, I think that the CS prop will do more for you on the 320 then going to a 360 without one. It will add weight and again should cause you to have a reduction in MPG.

By the way, I am a software engineer, so you know that I am a real expert - NOT.

I really posted to your questions to see if what I have learned from other over the last few years is a correct understanding.

Looking forward to my continuing education.

Kent

[alcladrv]

I thought I'd some real world numbers to this discussion. Maybe others could add numbers from their experience and we can all benefit from comparing the numbers.

RV-7A slider, ECI O-360 carbureted, mag on left side, Lightspeed Plasma II+ on right, fixed pitch Sensenich, all fairings in place. I typically fly 200 to 400 nm cross-countries at altitudes from 6,000 to 10,500 ft.,
IFR - 25% of the time and do not make local flights.

From 1/1/2007 through 10/15/2007:
Hours flown: 83.4 hrs. tach time
Landings made: 77
Avgas used: 716 gal
Nautical miles flown (minimum): 11614
Every time I topoff, I look back and list all the places I've made landings, then I total up the point-to-point distance flown from topoff to topoff. I disregard miles flown while in the traffic pattern, because I very, very rarely fly the pattern more than once at any airport and because it's too difficult to estimate accurately.

Crunching numbers yields:
Average leg length: 1:08 hrs.
Average fuel usage: 8.6 gal/hr. (I try to lean to peak EGT)
Average ground speed: 139.26 knots
Gallons per mile: .062 gal/nm

Just my .02 worth.

Mike

[Radomir]

use Search feature.. there's a wealth of content on this forum regarding this. IMHO, most importantly, there's plenty of REAL WORLD data posted.. so... opinions and engineering theories aside..

In particular, I'd recommend reviewing posts made by Dan Checkoway, Bryan Wood and James E. Clark.. those guys posted real world "here's what I am seeing... " type of info... especially when flying side-by-side long cross-country types of trips.