What's your MPG?

Insomnia struck tonight and my mind woke up thinking.......

Why do cars and planes have such a big difference in fuel economy? It seems likely it has to be induced drag due to speed. I remember reading once that to pedal a bicycle at 28 mpg, it takes 80% of the energy required just for wind resistance.

Based on experience of a few cars at 80 mph, I've had 2 that were around 25 mpg and one had 190hp and the other 287hp. At 25 mpg at 80mph, you get a fuel burn of 3.2 gallons per hour. A 400 mile trip will take 16 gallons and 5 hours (no stops such as tickets or need for fuel).

Some of the planes I've flown have fuel burn rates far less. A PA32-260 that was about 13mpg, Bonanza S35 ~15, and C172~17. Efficiency of the plane vs the car doesn't exactly look so good. Even my wife's big SUV gets 20mpg.

I don't know about my RV7A yet, but I'm guess somewhere around 19-20 based off of gal/hr that I've picked up off of some of the posts. Maybe a smidge higher (I'm sure we'll have some posters here that can verify what the numbers are they are showing).

To travel the same trip in the RV, the distance is supposed to be about 15% shorter for the plane. The same 400 mile trip would really be ~ 350 miles. In a perfect world, at 200mph, it would take 1.75 hrs and 17.5 gallons. Just under 10% for the trip in fuel use difference. However, if the plane was able to get 25 mpg, then it should only take 14 gallons.

Similiar horsepower between cars and single engine planes. This leads me to believe that RV's are one of the most fuel efficient planes out there. No way could I get that kind of fuel economy from some of the other planes.

So....what are folks getting for actual miles per gallon?

If you don't mind, it be nice to know info such as speed, HP, fixed or CS, etc.. you might want to share.

Here are two trips with my RV-6 160 HP with Constant Speed prop. Both legs were run at 2,300 RPM WOT. There is now 2,130 hobbs hours on my RV and engine.

Saint Johns AZ (KSJN) to Borger, TX (KBGD) with a headwind all the way.

398 NM
3 hours Hobbs
18.3 gallons
120.6 NmPH
21.7 NmPG
138.687 MPH
25.01 MPG


Next leg had a tailwind. I was seeing 180 Knots GPS GS most of the time.
Borger TX (KBGD) to Pittsfield, IL (KPPQ)

556 NM
3.3 Hobbs
168.48 NmPH
21.62583 NmPG
24.8697 MPG
193.7576 MPH

My GMC has about the same HP and does not get this good of fuel economy. My VW TDI has 75% the HP and gets double this fuel economy but takes 3.5 times longer to go the same distance.

See everyone at Oshkosh 2008.

About 19-21..

........in our 180 HP 6A/Catto three blade.

Remember though, that an RV can run 100 MPH on less than 3.5 GPH, a feat few other homebuilts can achieve, yielding better than 28 MPG.

Regards,

RV4 MPG(US).

That is a coincidence. I have been flying my triangles to calibrate TAS etc., and calculated just that, this am.

Two runs I trust so far.I am getting:

RPM 2230 2250
MP 22.7 22.4
Alt 3000 3050
Temp 63F 66F
TAS 168mph 170mph
literes/hour 26 25
MPG(US) 24 26

Its an RV4, solo, with a carb'd O-320 and 2 blade c/s MT prop and dual p-mags.

I expect I can get the litres/hour down but its a new engine so only gentle leaning. The attempt at 22.5 square is because I sense that is the point where the airspeed has dropped only a little, but the fuel consumption quite a bit. It feels like the point where I will end up in the cruise as I get to know the aircraft. Perhaps I should be looking more at 23.5 and 2150 rpm. Havnt tried that yet.

(PS It is almost exactly half what my diesel Skoda car gets. Similar to VW Passat.)

My RV7A with a Superior IO-360 and dual P-Mags gets 24 to 29 mpg at 10'000 feet according to my GRT engine monitor. Of course that depends on wind aloft. My TAS at that altitude at 2300rpm and 20" mp is 173kts @7.4 gph.
That puts my fuel economy in the same league as a car like a Camry. Considering I travel as the crow flies the car would use more to get to the same destination.

Martin Sutter
building and flying RV's since 1988

6A 180/CS/FI/EI trues at around 158 to 162 knots at 22" and 2300 in the range of 7500 to 9500'. Depending on temperature, fuel burn is 7.0 to 7.4 gph. At 160 knots true and 7.2 gph, statute mpg is just over 25.5.

I've been flying it at these general power settings for over 1000 hours.

Here's some competitive info for you. The guys with the RG Lancair Legacys with IO-550 - the big 6 cylinder. They are able to run 180TAS, on 6.6gph If you do the math on that, you'll get 31.6 mpg. That's at 1900rpm and 16" of map at 8500' and 50d LOP.

I was in CA a few weeks ago and did some performance testing with a couple of Legacy Pilots, both were able to do the above with no problem. I know of one other that I've flown that was able to do it on 6.1gph for 34.5mpg.

Oh, and yes, the first part of the performance testing was calibrating the IAS/TAS it was good to +-1.5 kt.

This past weekend I made a trip from Suwanee, GA to Latrobe, Pennsylvania. I made it in 2.6 there, 2.5 back. This was definately not in an economy cruise setting. I was at 21 inches, 2400rpm there and back.

I burned 23 gallons there, 22 coming back.

LZU->LBE
Distance: 436nm, 501 miles
Average Speed: 167kts, 192mph
Average Fuel Burn: 8.8gph
Fuel Economy: 18.95nm/gal, 21nm/US gallon

Compared against driving (based on Google maps routing):
Distance: 651 miles
Time: 10:37
If we drove my wife's Mazda (30mpg): 21.7 gallons

I would say that the RV competes favorably with a 30mpg car. If I would have slowed the airplane down to 150kts or so we could have easily burned less fuel than the hypothetical trip in the Mazda.

As far as time spent traveling....there's no comparison...the RV wins hands-down.

My wife was asking me about how efficient the RV is vs. a Cessna 172. Well, I told her, the new Cessnas use basically the same engine we have but we're 35% faster than the Cessna...so for any given power setting we're 35% more efficient than the Cessna.

More data points

I can run LOP at 7.5 gph or less and get 155 ktas which is 23.8 smpg at 178 mph. Obviously I can do better at lower speeds and worse faster. I could also do better up high. Superior IO-360 XP+, Catto 3-blade cruise pitch. With the wind is better and against the wind is poorer, but the numbers above are based on airspeed. I'm sure that with EI and a CS prop you could do better.
Not too many cars can beat an RV for mileage at speed.

aadamson said:

Here's some competitive info for you. The guys with the RG Lancair Legacys with IO-550 - the big 6 cylinder. They are able to run 180TAS, on 6.6gph If you do the math on that, you'll get 31.6 mpg. That's at 1900rpm and 16" of map at 8500' and 50d LOP.

I was in CA a few weeks ago and did some performance testing with a couple of Legacy Pilots, both were able to do the above with no problem. I know of one other that I've flown that was able to do it on 6.1gph for 34.5mpg.

Oh, and yes, the first part of the performance testing was calibrating the IAS/TAS it was good to +-1.5 kt.

Click to expand...

Typically in my Lancair 235 with a 9.7:1 O-235 I get 198 mph TAS at 5.7 gph at 14,500' dalt; that's 34.7 mpg! Three-blade ELIPPSE-Catto, dual Plasma I EIs.

Don't forget SM vs. NM per gallon, some people are reporting in NM.

Webb said:

Why do cars and planes have such a big difference in fuel economy?

Click to expand...

Easy...they don't.
My Alon Aircoupe, a 45 year old design, flies at around 105-110 smph at 5.5 gph which is about 20 mpg...not great, but similar to an American car of like vintage. RVs are well known to routinely achieve 25-30 mpg when run at 55-65% power...again, similar to a modern small car.

Real life long trip...

Even a slower certified plane is better than driving....

I just returned from a long trip in my Tiger. I used 234 gallons of 100LL and covered 3100 sm - straight line distance, not distance actually flown - giving around 13.2 mpg (sm) Over the 24 days, headwinds and tailwinds essentially cancelled each other out.

Just for fun, I entered the stopping points in Google maps, and got 3800 sm road distance and 60 hours drive time. Engine on time (not flying time) was about 25 hours for our trip.

And this is at Tiger speeds (just under 150 mph), not RV speeds...

gil A

Lets see "287hp 25mpg and no tickets" would that car be a Porsche 993? Really didnt think this would be my first post!

Quick Data Point

OK, so this thread gave me something to do on this morning's flight! Here is a quick data run done at 8,500' over the Gulf Coast, data recorded by hand. The MPG and Fuel Flo are taken right off my GRT EFIS after waiting a minute at each power setting to let them settle down. You can really get the mpg up if you are willing to go slow! Of course, slow is relative - 30 (statute) mpg at 145 mph is a WHOLE lot better than doing the highway thing for the same mileage in a subcompact at 70 mph.....



Observations/Notes:

1) I played wth RPM and saw very little variation - got a bit better mileage when I pulled it down to 2000 rpm, but 2350 is normal for me - and it runs so smooth there!
2) Wind was not a factor - it was light at altitude, and what there was I put directly on th eleft wong, so I had zero tailwind/headwind component. TAS was equal to GS.

Coming back from Osh last year at 13K we were burning 6.5 at 180mph. That is about 28 statute mpg. I was leaned to -100 peak. Leaning more aggressivelyand pulling the power back a bit more should have touched 30 mpg.
I find the real savings is time. I usually can skip a motel room expense on longer trips. Also the distance by going direct and cutting the corners that ground bound travelers are required to drive.

I would think that some of the differences in efficiency stem from the generally higher speed of airplanes and the concurrent increase in air resistance. The faster you want to go, the harder you have to work to get there. Still, it's amazing to look at the efficiency of many composites like the Lancair and Rutan series, and given the essential compromise that is the RV it's still remarkably efficient.

Webb said:

Why do cars and planes have such a big difference in fuel economy? It seems likely it has to be induced drag due to speed. I remember reading once that to pedal a bicycle at 28 mpg, it takes 80% of the energy required just for wind resistance.

Click to expand...

A car engine provides thrust; an airplane engine must provide both thrust and lift. There's gotta be a cost associated with staying up there.

AND THE WINNER IS..............

elippse said:

Typically in my Lancair 235 with a 9.7:1 O-235 I get 198 mph TAS at 5.7 gph at 14,500' dalt; that's 34.7 mpg! Three-blade ELIPPSE-Catto, dual Plasma I EIs.

Click to expand...

IF YOUR NOT JUST TALKING RV'S............

by Klaus Savier

As a result of the ever-increasing fuel cost, I decided to demonstrate the efficiency of the Delaminator, a highly optimized Vari EZE, a grocery getter and 25-year-old ?Technology Demonstrator? for Light Speed Engineering.

This airplane is well known for winning all kinds of races since 1984. From the CAF? efficiency races in the late 80?s to the AirVenture Cup race in 2007, where it won the 400 mile race from Dayton to Oshkosh at an average speed of 254 mph. In 1990, two closed course world records were set in the C1-A weight class for the 1,000km and 2,000km distances at over 200 mph. They are still standing.

The Delaminator?s configuration for this endurance flight was identical to that used in the AirVenture Cup:

- Same LSE Composite 64x86 Propeller,
- Dual Plasma III CDI,
- Our own Timed Sequential High Pressure Electronic FI,
- No extra tank- standard 29-gal Vari Eze fuel system,
- About 25 pounds of luggage: minimal tools, no spare parts.

On Monday April 7th, just prior to Sun ?N Fun, the weather looked great for a non-stop flight across the country, coast to coast, 2000 miles. I hopped in the Delaminator as early as I could bear and set out for Florida. The tailwinds were good but not quite as strong as predicted so I had to slow down a little to increase my range. It took 8 hrs and 58 min for the total distance of 1985 statute miles, SZP, CA to PFN, FL via ELP TX to avoid the restricted area.

Total fuel used was 25.8 gallons of the 29.2 carried in the standard tanks, leaving more than an hour worth of fuel remaining. Average fuel flow for the entire flight was 2.87 gph. Average speed was 220.6 mph. Tailwind average was around 30 mph at 17500 ft. The density altitude was above 19000 ft. Of course, I was on oxygen for the entire flight.

Now for the technical run-down on engine configuration and the methods used to achieve this level of efficiency. In the Delaminator, the maximum manifold pressure available at 17500 is almost 16?. See picture 1 and note the fuel flow of 4.2 gph at peak power giving a true airspeed of 204kts, resulting in 55.9 statute mpg. As shown in picture 2 taken during this cross-country flight, the throttle was reduced by almost 1?. While our custom FI automatically adjusts the mixture for all engine conditions, it can be biased just like the Plasma CDI timing. The automatic mixture adjustment was thus manually leaned to almost 300 degrees past peak EGT, where slight roughness occurs. Manual increase of timing advance returns some power lost under these conditions and eliminates any lean misfiring. As a result of these settings and lean burn, all temperatures are near the low limits, oil pump outlet temp is below 150 F and the cylinder head temperatures are all below 300 F. See picture. While leaning this far on the lean side of peak reduces power significantly, it is slightly more efficient to reduce power by leaning rather than by closing the throttle because the pumping losses of the engine are lower.

The best ?no wind range? is normally achieved at best glide speed. For the Delaminator, this is about 100mph when fully loaded. Most of the trip was flown at 130 mph indicated at 17500 ft. Prior testing has shown that the airplane achieves well over 80mpg at best glide speed. Flying at 130 mph, the Delaminator still achieves over 60 mpg. The tailwind brought the mpg to nearly 80 mpg.

While I could think of a faster airplane with a higher wing loading and a bigger engine, such a configuration could not come close to the efficiency demonstrated here at high altitude. It seems that this combination of a highly optimized 0-200 engine and high aspect ratio wings offers a combination of efficiency and speed that simply cannot be beat.

Klaus Savier 4-08

Efficiency

If we are drifting off RVs...

Can I count my glider Diamond Distance flight over a pre-set course of 312 sm as infinite mpg (or perhaps 100 mpg if I allow 3 gallons for the 2000 ft tow)....

Paul.... note that your figures can become a little like the DoT highway/city mileage numbers - few folks get the rated numbers.
You need to calculate mpg over a real world trip, with take-offs, landings, climbs and stuff...

gil A

REAL WORLD............

I can (and do) fly seven days a week for 45 minutes a day (enough to get my fix) and I will use 26 gallons or less.

6A 0320 fp LSI lite......

Its just a passing breeze...

Webb/Szicree: Parasitic drag is the product of frontal area, speed and drag coefficient (ignoring density). Things that are big, flat and fast have more drag (waste more energy).

A bicycle (with a rider) has a small fronta area and is horribly inefficient at moving air out of the way since the rider is essentially a flat plate and there are all kinds of protrusions sticking into the airstream. But bikes usually go slow enough where the weight/convenience/cost of mounting fairings to improve aerodynamic efficiency are not worth it.

A car has a large frontal area but is more efficient at moving air out of the way. Attention to detail has been shown to improve mileage at highway speeds.

An aircraft, RV-7/9 for example, have a smaller frontal areas and are even more efficient at moving air than cars. Small cross-sections and streamlining enable an aircraft to go considerably faster than cars for a given amount of energy.

If parasitic drag is the cost of obtaining speed, then think of induced drag over your wing is the cost of generating lift. Hence the focus on light weight materials for aircraft. But don't assume that a car has zero cost. Your car keeps its distance from the Earth though its tires and suspension which flex and warm up. That heat came from its fuel.

With a car engine in an RV6A, we typically average about 20 mpg (statute) at 8-9000 feet. Not too bad at 155mph.

Subie mpg?

rv6ejguy said:

With a car engine in an RV6A, we typically average about 20 mpg (statute) at 8-9000 feet. Not too bad at 155mph.

Click to expand...

Hi Ross,

Why is that so much lower than others have reported on this thread? For myself,
I can easily get 24 mpg statute @ 170 mph running LOP with my IO-360. Also a -6a. This almost sounds like the Lycos are more efficient than the Subie. Why would that be? Gearbox losses?

Always good to see your informative postings...

Jerry Cochran

There is no doubt that the Lyco is a little bit better for SFC that my Subie. My installation is pretty draggy too and we really don't know how the IVO prop stacks up. I can't get the rpm down where I'd like- not enough pitch.

I do a bit better up high with the turbo but I usually don't cruise there so I try to make the comparison as close to where a Lycoming guy would fly.

I report what I see- not trying to fudge it. My engine only cost about $5K though so I've been earning interest on the other $13K for the last 4 years.

I could apply the typical evil auto conversion vendor factor of 50% and beat all you guys in fantasy land with 30 mpg at 200 knots.

I was reading a paper lately on fuel consumption of various modes of transportation. It mentioned that the criteria for aircraft is quite different than other transportation means. Other means of transportation cars, trucks, ships and trains try to minimize fuel consumption per mile/Km aircraft minimize fuel consumption per time. Speed is the most important aspect of air travel. I will try and find the graph and post it. It is quite interesting that aircraft of all types could get better fuel economy per mile but the time to travel would increase and air travel optimizes time.

The mileage that the various people have quoted is very interesting and quite competitive with long distance road travel. The time however for the road travel would have been quite a bit longer that the time in an RV.

Bob Parry

Here are some figures for my 9A, 0-320 (carb), Hartzell C/S, Plasma III and mag that I consider are accurate. The TAS figures are calculated from 4 leg GPS runs with the results put into the National test Pilots School Spreadsheet http://www.ntps.edu/HTML/Downloads/ The FF figures are calculated by draining and then filling one tank with a known quantity. Flying for exactly 20 min on that tank. Draining to calculate fuel used, then multiplying by 3.

DA 8,500ft, TAS 146kts (168sm), FF 5.9US g/h, MPG 28.5 (statute) - WOT, carb heat, LOP on all 4 cylinders

DA 8,500ft, TAS 155.5 kts (179sm), FF 6.7US g/h, MPG 26.6 (statute) - 2245 rpm, WOT, carb heat, around peak EGT

The MPG would be better up higher, but around 8,500ft DA is my typical cruise altitude.

Fin
9A

I've checked and rechecked and I am seeing almost exactly 25mpg...however if this were a car travelling down the highway from point a to point b to the same destination, that number is probably about 25% low because we fly direct (normally). Not a bad view either!!

Car Type

phopkins said:

Lets see "287hp 25mpg and no tickets" would that car be a Porsche 993? Really didnt think this would be my first post!

Click to expand...

It's a Nissan 350Z. I've had 2. One standard, and one automatic. Standard was 25, auto was 23. Funny part, 90-95 was still 25 mpg in the standard.

gasman said:

I can (and do) fly seven days a week for 45 minutes a day (enough to get my fix) and I will use 26 gallons or less.

6A 0320 fp LSI lite......

Click to expand...

Is that why you're called gasman?

I don't have the exact numbers

But O-360 with 9.5:1 pistons I can get around 7 gph at around 175 mph (assume no winds). That is about 25 mpg. FP Catto prop on a -6A. One Lightspeed EI. Plus above 12,500'

I need to find my last data run to verify the 7 gph figure.

Real numbers...

Ron Lee said:

.....
I need to find my last data run to verify the 7 gph figure.

Click to expand...

Nah... do a several hundred mile cross-country round trip flight and work out the total gallons used for the straight line distance.

That would be much more realistic than the aeronautical equivalent of a car cruising at a single freeway speed....

gil A

Gil, that does work but actual MPG efficiency independent of wind makes more sense to me and allows people to compare relative efficiencies. Regardless, my RV gets better mileage than my SUVs.

Question

My CYYJ to KOSH trip ( once the WX clears up here on the coast) gives me the opportunity to calculate MPG among other things (Cruise performance).

What important figures do I have to gather? For sure....distance, fuel used, power setting, speed, altitude. What else? Do we want EGT and CHT?

Actual trip mpg...

Ron Lee said:

Gil, that does work but actual MPG efficiency independent of wind makes more sense to me and allows people to compare relative efficiencies. Regardless, my RV gets better mileage than my SUVs.

Click to expand...

My thought was to get real world figures.... if the trip is a round trip, or enough actual trips averaged out, the wind effects should cancel out.

My previous Tiger mpg number was for 9 seperate legs over 3 weeks... some flights had head winds and some had tail winds ...some flights where lowish, and some had 10,000 ft mountains to cross.... which is what happens on real trips.

You wouldn't plan fuel quanities for a cross country trip using the efficiency numbers you quote...

And a small SUV might get the same mpg if run at a steady speed on a flat freeway with no wind.... but it's that starting and stopping (the taxying, climbing stuff in a plane) that makes the mpg "real world".

The Toyota RAV-4 claims 30 mpg highway, and would do much better than our RVs if run at a single highway speed.....

...but the plane gets there faster and with much more fun....

gil A - don't like 10 hour days driving any more....

Simple is OK

RV9798 said:

My CYYJ to KOSH trip ( once the WX clears up here on the coast) gives me the opportunity to calculate MPG among other things (Cruise performance).

What important figures do I have to gather? For sure....distance, fuel used, power setting, speed, altitude. What else? Do we want EGT and CHT?

Click to expand...

Keep it simple... as a real world trip.

Point-to-point miles, and # of gallons 100LL used.....

Hope the wx clears for you soon

Comparative MPG/Speeds car/airplane

One thing I've seen missing in these posts is the fact that airplanes run at or near full throttlewhile cars are normally run partial throttle. A Vegas run with a car and airplane both at full throttle would be a more realistic comparison between the two-and I'm sure the plane would win that one in both speed and less fuel burned for the same engine power output. I guarantee my Dodge 360 will get less than 25 MPG at full throttle!

az_gila said:

You wouldn't plan fuel quanities for a cross country trip using the efficiency numbers you quote...[/I]

Click to expand...

True. I assume 150 kt groundspeed for three hours as a start (450 nm). I don't use mpg at all. Predicted winds aloft factor in then once airborne I use "fuel time remaining" versus "time to destination" to modify my planned fuel stop as required. I always land with plenty of fuel reserves.

Flight Planning

The 155 kts at 7.5 gph which I cited earlier is the figure I use in my Golden Eagle Flight Planner and it works our very well. Of course, the planner also has a climb and descend entry. Also, that's still a no-wind figure. My point is that Az-Gila is right; you need realistic figures for flight planning.

az_gila said:

If we are drifting off RVs...

Can I count my glider Diamond Distance flight over a pre-set course of 312 sm as infinite mpg (or perhaps 100 mpg if I allow 3 gallons for the 2000 ft tow)....

Paul.... note that your figures can become a little like the DoT highway/city mileage numbers - few folks get the rated numbers.
You need to calculate mpg over a real world trip, with take-offs, landings, climbs and stuff...

gil A

Click to expand...

You're right, Gil! Those are just cruise numbers. I'm sure the overall would fall to something like 28-30 mpg on a 4 hour trip. But those numbers of Klaus' are the REAL thing!