I think that at the far-subsonic speeds we fly at, the cockpit volume is the biggest driver of “MPG”.
If you want more MPG, get a smaller plane 
Peter wrote:
If you want more MPG, get a smaller plane
Vari Eze and similar planes prove that point. 130 kts at 100 hp and so on.
Pal of mine who used to fly one said the best 4 seater would be one where everyone sits behind each other. Would look weird to the max but probably yes.
The main problem is the frontal area exposed to drag. The better that is done, the more MPG you get out of it.
But as many examples show, it’s not only making the cabin smaller. Roy LoPresti was the guy who managed to get loads of speed and MPG out of airframes such as the Mooneys, the Grummans and others with his speed mods, which dramatically reduced drag to the extent that you get up to 20 kts speed increase. If you look at the examples of the Mooney pre/post LoPresti, that is darn obvious. The direct predecessor of the Mooney 201 is the F model, which has a gas mileage of 15 MPG. The 201, which is the identical airframe with LoPresti Mods, has 18 MPG and a 200 NM increased range with the same fuel capacity and engine. Not to mention the speed jump from 140 to 160 kts. Or the second example: The Grumman AA5 Traveller vs the Cheetah. 15.5 vs 17.5 MPG and 15 kts increased speed. No wonder he is also the guy behind the Mooney 301 which turned into the TBM eventually.
Or other speed designs like the Comanche or the Twin Comanche. Both planes which achieve extreme performance out of very few HP without actually loosing too much space.
I guess that is the reason why quite a few of those UL contraptions get very decent performance out of 100 hp engines, apart from their weight problems. If some of those planes would be developed into a full airplane without the silly weight restrictions and to the max of their performance, I’d think a 160 kt airplane on a rotax would be possible to achieve.
I don’t buy the +20kt – at the same fuel flow.
Rotax engines don’t do more SFC than Lyco or Conti. They just happen to go into little planes, so everybody goes around saying how little fuel they burn
I don’t buy the +20kt – at the same fuel flow.
I have flown M20F and M20J with same IO360, +15kts is about right on 75% cruise with same fuel flow and leaning…there is an STC for M20F to bend windshield and install various speeds modifications and reduce the speed delta
I also flown Arrow with similar 200hp engine, I would say it’s +25kts on same fuel flow at 75% power but that is likely related to cockpit size
Peter wrote:
I think that at the far-subsonic speeds we fly at, the cockpit volume is the biggest driver of “MPG”.If you want more MPG, get a smaller plane
Agree
To add: and fly it at the best speed for fuel economy.
Re the VariEze and similar planes, assuming we’re talking canard layout concepts, I’m not so sure. Owned and flew a VEZE, aka the Plastic Toy, myself for 210hrs. Yes, it was typically cruising 130KTAS burning 23lt/hr. WOT barely returned 150 at 25+ lt/hr. It was impressive performance when the design came out in the 70’s… but the world has moved on… and there are quite a few airplanes around nowadays providing similar or better figures whilst offering more than a toothbrush for baggage allowance.
The biggest handicap of canards has not changed: the canard HAS to stall first to prevent the craft entering a deep stall situation. Preventing this with different airfoils or incidence values, VGs, or any combination of the previous, negates the brilliant concept of 2 lift producing surfaces
The problem with VariEze, Rutans and Velocity is convincing a pax who is not a pilot to fly in one, their designs is so slick that is perceived as “death trap”
The runway requirement is very limiting (people only fly them to places where Mustang and CJ4 are parked)
I am not sure why you would assume the fuselage is the main drag component for fast-cruise or practical-cruise efficiency (reading the above nobody seems very interested in Carson speeds. and the likes).
In “fastish” cruise mode, while it is true that at low altitude the relative important of lift-induced drag is lower (ie IAS is relatively high), the best cruise efficiencies are achieved at high altitudes where there is a proper balance of lift-induced drag vs parasitic drag (lower IAS, higher AOA for similar or higher TAS). In other words: do not underestimate the importance of lift-induced drag for cruise efficiency, just look at this recent thread, for example.
But even if you do, the fuselage component of drag on a light airplane is lower than the above posts would lead you to believe.
Take for example, the relatively draggy, half-boxy cross-section, fixed-gear fuselage of a PA28-181 aircraft:
Even if it is possible that the main gears are included in the parasitic drag of the wing in that particular study (unclear) , it gives an idea of the relative orders of magnitude and the fuselage will be nowhere near the dominant drag source. This is going to be so unless your light airplane looks like this:
If you look at more modern, more efficient and retract-gear aircraft the fuselage may be an even lower proportion. Can someone post actual data for a Cirrus, a Mooney or a C210? Perhaps @Pilot_DAR ?
Bottom line is that in most aircraft, but clearly for light aircraft, wing-surface and wing design is as important as fuselage cross section and design: both contribute significantly.
the fuselage only accounts for 23% of the total parasitic drag:
I find that really amazing, but can’t dig around much due to lots of work. OTOH it is possible that while it may be true (it is conceptually easy to accept that the wings, HS and VS make up much or most of the drag) there isn’t actually anything one can do about it. Putting it another way, a given size of plane requires a given size of these protruding objects to make it fly and to achieve acceptable behaviour (e.g. a useful speed range, adequate elevator authority at Vs, etc).
I say this because I have never come across a verified case of a big difference in (say) 4 -seaters doing very different MPG. Not when you compare properly i.e. same altitude, TAS or IAS, etc. Something is in play here which prevents designers achieving anything too far away from the median.
An example I have often posted is that the TB20, the Cessna 400 (a.k.a. TTX), the SR22, and even the DA42 all do ~140kt IAS at 5000ft at 11.5 USG/hr (I tested all these myself). The C400 test was with @Cobalt and the SR22 and DA42 were with Fuji who died a year ago. It was funny that the 11.5 USG/hr was the combined fuel flow so the DA42 was doing damn well, dragging the two lumps along (a bad example).
Here’s a puzzle in light of @Antonio’s post about parasitic drag versus induced drag. The Bölkow Monsun below belongs to a friend of mine in Florida, the book cruise speed is about 135 kts on 160 HP with a very elegant tapered wing design and retractable nose gear. 
The RV-7A below has a constant chord wing and fixed nose gear, but cruises at over 160 kts TAS on the same engine. It also stalls at significantly lower speed, climbs much faster and weighs only perhaps 50-100 lbs less. The question is why, if it’s not fuselage drag? One factor might be a relatively small horizontal stabilizer on the RV.
My guess is that the difference is mostly detail design in areas such as landing gear fairings, wing root fairings, windshield design and so on. The cabin is about the same size. You’ll note that my friends plane has non-standard RV wheel main fairings… What amazes me is that the simple wing on the RV appears to work better, not worse.
It is puzzling indeed since I checked and both have similar wing surfaces, and definitely the boxy Monsun fuselage must account for a decent part of the delta!
FP vs CS prop?
Wing profile?
Monsun is Airfoil: NACA 64215 at root, NACA 64212 at tip
RV7 airfoil is NACA 23012 so in theory 13% thinner and less draggy, but this in itself is not enough to justify
SOme sources actually quote RV7 as NACA 23013.5 which would put it in the same place as Monsun in terms of thickness…
ooooh, wait a minute! I got it:
Monsun was certified, but anarchistic RV-7’s are not bound by the laws of aerodyamics!
