Greg Arehart

Greg Arehart

Well Known Member
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
 
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.
 
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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
 
hmmmm

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. :D
 
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.
 
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.:eek:
 
Great questions Greg.

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
 
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
 
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.
 
Radomir said:
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.
Click to expand...

FWIW, with my 200hp IO-360 I average 1+ gph LESS fuel burn than the previous poster with a 180hp O-360. My average fuel burn is more like 7.5 gph on cross countries.

Fuel consumption is NOT proportionate to horsepower and displacement. Economy and performance has as much to do with operating technique as it does with engine size/output.

If I were building an RV-9[A], there is no question. I would have an IO-320 (yes, definitely injected! with Airflow Performance injection) with at least one Lightspeed electronic ignition and definitely a CS prop. I personally think the IO-320+EI is the best bang for the buck on a -9[A]. The economy "available" from that setup can be ridiculous...that is IF the operator knows what he's doing. :D

The -9[A] is so "fuel limited" to begin with -- my 2 cents is that if you gotta build a -9[A], and you want to travel long distances with it, going with a system that maximizes miles per gallon is the way to go!

Just my big fat opinion.
 
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The throttle is your friend

The greatest improvement in economy is a consequence of slowing down. Maximum MPG somewhere near max glide speed but no one goes that slow.

That being said, optimizing your engine for low load operations can make a big difference. Balanced LOP, higher compression, and ignition advance are all things that contibute measurably to economy at very low loads, (50% load and less) that you use when running 120-140 kts.
 
I'm making

About 160kts at 7.5GPH running LOP e/Pmag combo and Sam James cowl....This is the same for 100ll or 92Oct mogas.

Winds make a huge difference on the actual MPG number you end up with which is more about luck and selecting the altitude thats most favourable/least harmful.

With a small headwind for half the trip and zero HW for the other half I made 25MPG or a "driving equivilent" of 28Mpg on a 2.5 hour X country.

Frank 7a TMX IO360 hartz Ba C/s, 2*EI
 
Here's where this forum is so great. You can make your choice based on the data and your wallet. As I stated in an earlier post, I burn 8.6 gal/hr. using a 180 hp carbureted O-360 fixed pitch. Dan Checkoway burns 7.5 gal/hr. LOP using a 200hp IO-360 with constant speed prop.

So, over a 2000 TBO, I'll burn 1.1 gal/hr. more than Dan and use 2200 more gallons of fuel. At about $4/gal, that's $8800. At $5/gal, it's an $11,000 difference . So, the question is: For that $8800 -$11,000 difference, can you buy an IO-360 instead of an O-360 and a constant speed prop instead of a fixed pitch? Are you willing and able to operate your engine LOP to save 1.1 gal/hr.?

At my flying rate of 125 hrs/yr, I've got another 14 years before I'll reach that 2000 hr. total. Fuel costs will likely continue to rise over that time span. Engine and prop prices will rise. Maintenance costs will rise.

My point is that over the lifespan of the plane and your involvement in it, the variables are like statistics where you can justify any result you want based on your desires. So, get what you want. In many ways the costs are like the old Fram air filter commercials. "You can pay me now or pay me later." I chose to pay as I go, hence the carburetor and fixed pitch prop.

A note on performance: With Sensenich fixed pitch prop, I was willing to accept the lengthened takeoff roll, because the engine can only turn 2300 rpm at the beginning of the takeoff roll.

But the extended landing roll because of the lack of braking effect from the engine idling at 600 rpm has been a greater adjustment. My personal limit for hard surface operations is 2000 ft. because I prefer to let the plane decelerate more aerodynamically and, in the process, go easier on the brakes and tires. My final approach speed is 65 kts. with full flaps and land at around 50 kts. with my -7A.

Good luck and happy flying,

Mike
 
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First, I'm no expert, either. I can't comment on 320 vs. 360 as I have only flown the 320 in my plane. I do know that I can get incredible mileage if I settle for 55-65% power. Even carbureted, I can lean LOP by pulling the throttle a little and pulling carb heat. I get very even temps and 5.5 gph at 150+ mph. To me that's great. I do have one E-Mag which I put on at about 30 hours. It DID cut fuel burn for the above situation by at least 1 gph. YMMV.

Now, on the FP being something that won't slow you down on landing. At 700 rpm idle, that's true. Now with the E-Mag, I have a smooth idle at 380-400 stopped on the ground and get pretty good deceleration. It wouldn't idle that low before the E-Mag. I plan on adding a P-Mag and expect a little better results.

Bob Kelly
 
My experience

My airplane (RV-6, O-320 CS) has been flying 10-years (September 17) and has 2,059.2 hobbs hours. About 1/2 the hours are cross country and usually with another RV. Average fuel burn is 7.3 GPH. When I turn 2,300 RPM, and the 180 HP side by side is flying with me at my speed, we burn the same fuel. As I push up the RPM (throttle is already full forward) I burn more fuel than the 180 doing the same speed. The BSFC of the 160 HP Lycoming is HIGHER than the BSFC of the 180 thus the parallel valve 180 uses less fuel for the same horsepower at higher power settings. An RV-4 with 180 CS and FI will burn about 1 GPH less than I do on a trip when I turn 2300 RPM. Dan burns about 1/2 to 3/4 GPH less (IIRC) than I do when he slows down to the speeds that I fly. With my carb, I typically cruise as LEAN as I can so am at Peak EGT on at least one cylinder. IF I run ROP, my fuel burn can go up 1.5 GPH and speed will go up 10 Knots at same RPM and manifold pressure.

The 150 HP 320 will burn more fuel for the same horsepower. IF you look at the Lycoming charts, it has a HIGHER (needs more fuel) BSFC than the 160 HP. Higher compression pistons are more efficient and use less fuel for the same horsepower.

EI and high compression pistons: Typically anything that makes more power will allow better fuel efficiency IF you run the same speed you did before you increased HP. They will also use MORE fuel if you run at max power. Remember the old saying that there is no such thing as a free ride. Everything else being equal, you need more fuel to go faster.
 
How Fast?

As someone else mentioned, how fast do you want to go? I can run my RV-9A literally all day (10+ hours) if I want to poke around at 98kts/3.3 GPH. But I'd never do that on a XC.

I'll agree with Dan C that the ultimate economy RV-9(A) would have an IO-320 with Electronic Ignition and a CS prop, running LOP. I think the EI is the the key to very lean ops. Several of us have found way to run LOP with a carb, but it requires some futzing. (with carb heat)

I understand you asked about fixed pitch. I don't have any experience with a FP prop. My setup is an O-320/160/ Hartzell/EMag/PMag. I get 148-150kts TAS on 5.9-6.1 GPH at 7500 den alt. I can do a bit better if I go higher. This gives me 5 hour endrance and 700+nm range. I've done that once, from Bozeman to Minne - it was enough for me.

Mike had it right above - choose your options, your price, and enjoy. Do keep in mind that gas prices are not coming down, so if you fly a lot, an efficient setup does make sense.
 
The theory is..., but the practice scuppers it...

Theory says that the smaller engine will be running at a higher manifold pressure and will be closer to optimum ignition (MBT), so it should be more efficient and yield better FC.

However, an O-320 has a ~10% worse combustion chamber surface/volume ratio than the O-360, so despite having the theoretical fuel consumption advantage of being a smaller engine, it throws it all away in heat transfer losses via the cylinder head.

An 'O-327' hybrid with O-290 cylinders and an O-360 crankshaft would be much more comparable (<1%) to the O-360 in surface/volume ratio so you might begin to see the FC benefit that should be there.

A
 
Andy_RR said:
snip

However, an O-320 has a ~10% worse combustion chamber surface/volume ratio than the O-360, so despite having the theoretical fuel consumption advantage of being a smaller engine, it throws it all away in heat transfer losses via the cylinder head.

snip
Click to expand...

I am not sure I understand. The head on a 320 and 360 is the same. The 320 and 360 have the same bore only the stroke has been changed in the 360 to increase the displacement.
 
Go for the gusto!

Flying never has been cheap and I built an RV for the speed it gives. I like running near 200 MPH and I go for it. My fuel burn is around 9.6 GPH but it's sooo much fun. Life's too short to go slow..;)

Pierre
 
the clearance volume has to reduce on the O-320 to maintain the same compression ratio but the surface area is more-or-less constant (fractionally less, actually).

For the O-320 to make the same power, the charge air mass must be the same, which in a smaller cylinder means the peak cylinder pressures - and therefore temperatures - are higher. Higher peak temperatures means higher the heat transfer rate for the same(ish) surface area.

A
 
I agree with Pierre

I have owned 2 cherokees and a 182. I was always the "slowpoke", got there last, took a bird strike from the rear, etc. NOW, the only time I fly slow is when I am in formation. The rest of the time if I'm going somewhere, the throttle is against the stop and I set the prop at 2350 RPM. This usually gives me 170+ knots at 8-9K DA and between 8 and 8.5 gals/hr. I like to go fast! This is still 24 - 25 statute mpg, good enough for me and better than my car!
 
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