Practical aircraft for everyday use -concept

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Himat

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Basically without running the numbers, my expectation for required cruise L/D would be >25:1. And also the engine power would be such that for takeoff there is good power to weight ratio while in cruise the power is like 25% of total available. We can assume a fuel injected Rotax 912 is sufficiently powerful.
Best L/D > 25?
Then you are either flying slow or high. Autoreply and others know sailplane polars much better than me, but what I have seen of sailplane data they have their best L/D at a speed lower than your requirements. An Airbus may have a cruise L/D, but that is at high altitude.

And the Diamond DA-40?
Best sea level L/D of 14(?) at 80 knot? Or am I misinformed by just Googling for information?

The large problem I do see in your requirements are utility and ultimate fuel efficiency. An useable airplane that is time efficient and reasonable fuel efficient can be made, but I would be very happy with a cruise L/D of 10 at 100 knots or higher than 100 knots for such a plane.
 

karoliina.t.salminen

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Pipistrel Taurus G4 electro has cruise L/D of over 30 at 160 km/h. There is nothing in sailplane wings which would make it slow. In RC world some people are cutting wings shorter and shorter and making planes to look like jet fighters yet the fastest planes for power happen to be the hotliners which coincidentally are almost identical to sailplanes, but made for fast speed. Due to military planes many have misconception that high AR would mean slow speed. Aspect ratio nor wing span has nothing negative to do with speed if aeroelasticity (strucutural) is not considered.

My simulations with XFLR5 do indicate that the high L/D can be moved to lower Cl by designing airfoil which peaks at lower Cl than a typical sailplane airfoil and then by increasing wing loading which moves the cruise Cl to higher than typical sailplane in cruise. The special airfoil obviously becomes a compromise - as the high L/D spot is moved to lower alpha/Cl, the L/D max gets lower. Also reducing the camber that helps the trim drag in the cruiser, reduces the absolute L/D max. But in modifying high L/D airfoils for the cruiser, there is spare to lose as the good sailplane airfoil peaks L/D max over 150 for infinite wing. To get as near as possible to the infinite wing, the long wing is drastically more like the infinite wing than the short wing.

The only negative effect for speed which comes with high AR are structural.

If you have 100 kg/m2 wing loading for sailplane -like wing, it is going to cruise at higher Cl than more lightly loaded wing at the same speed. The lightly loaded wing will lose faster the cruise L/D in function of speed as the Cl gets under 0.2 and no airfoil has spectacular L/D under Cl 0.2.

I think the pilot operating handbook of DA40 promises L/D max of only 10 which is disapponting figure.

Now to get the 100/kg/m2 wing up from water will require some special stuff like blown wing with slotted fowler flaps. This explains why I mentioned the flaps in earlier post. The no flaps case of the sailplane wing is not going to work because the slow flying and slow stall speed is not a feature of the sailplane wing. It is affected by the wing loading and Clmax and the Clmax can be augmented with flaps and active boundary layer control. In actual sailplane the wing is lightly loaded and it has L/D polar which peaks at slow speed. Lightly loaded is not ideal for going fast at low altitude. Sailplanes too use water ballast when going fast.

For efficiency, the steepness of lift curve slope has an effect. Sailplane airfoil has steep lift curve slope and the high AR also increases the steepness of lift curve slope resulting in high Cl at low alpha.
 
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rv6ejguy

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The very high AR ratio wing to give you the good L/D ratio presents some problems in real life like tip clearance to the water and taxiing on less than smooth wave days plus storage which will drive landing gear length to support foils/ floats which equals more weight for these and a simple folding mechanism (probably has to fold both wings simultaneously to avoid tipping over on the water).

It's a tall design task to meet all your objectives- certainly a very challenging design and building exercise. Lots of overlapping compromises.

Best L/D on the D40 looks to be about 13 to 1 http://www.slideshare.net/MichaelDitzler/diamond-da40-analysis-ditzler-aero-2200-final but the glide ratio does not seem to be impressive- 8.8 to 10 to 1 depending if prop is stopped on not.
 

Vigilant1

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The only negative effect for speed which comes with high AR are structural.
But that's a big issue. More structure=more weight=more hydrodynamic drag in the water. Getting off the water is your biggest challenge.
Now to get the 100/kg/m2 wing up from water will require some special stuff like blown wing with slotted fowler flaps.
Which will require more power compared to a"smooth" airfoil that yields the same Cl (the blown or active BLC wing obviously requires engine power, and even the best Fowler flaps have higher Cd than a smooth airfoil producing the same Cl. Yes, it is great that we can pull in the flaps and get the wing better optimized for the cruise performance you want, but during that critical takeoff run and initial climb, the wing will have more drag than a "smooth" high-lift airfoil. So, a bigger engine will be needed just for that portion).

This is an old but interesting NACA technical paper (NACA TN 2404). The paper examines the issues surrounding the design of small planes able of using short, rough airstrips. They examined various flap types, aspect ratios, etc. In the end, they settled on a moderate aspect ratio and single-slotted full-span flaperons. It's no surprise that the shortest takeoffs were accomplished by (proposed) designs with the lowest power loadings and span loadings. Here's an interesting part that was counterintuitive to me: high aspect ratios were >not< associated with the shortest takeoff distances(Bold added):

Aspect Ratio and Lift Coefficient
Since, for any set of values of weight, span, and power there
is an optimum take-off speed, and since W = L = qCLS, in order to obtain
the optimum take-off speed the product CLS remains a constant.
Thus it appears that the optimum take-off speed, and to the first approximation
the minimum take-off distance, can be obtained by the use of a
narrow-chord, high-aspect -ratio wing with a high lift coefficient or
with a low-aspect-ratio wing with a correspondingly low lift coefficient
.
In figure 17 the take -off distance is plotted against aspect ratio for
one combination of span and power loadings , and the maximum lift coefficients that will give the optimum take -off speed in each case are noted
on the curves (the take-off being made at 0 .9CLmax). It is apparent
from this curve that the shortest take -off distance was obtained with
the very lowest aspect ratio for which computations were made, 2.5
, and
that the optimum lift coefficient for this case was only 1.2. For an
aspect ratio of 7.5 the take-off distance to a height of 50 feet would
be only 5 percent longer, however, assuming that a lift coefficient of
3.2 could be obtained at the drag value shown in figure 1.

The explanation of the shortest take-off run's occurring with a
very low value of aspect ratio can be obtained from an examination of
the curve of CL against CD (fig. 1).
To see figure 1 and 17, please go to the PDF at the link since I apparently can't paste them in here. But Fig 17 depicts the takeoff distance and max velocity of four planes, all with the same power loading (15 lbs/HP) and span loading (W/b^2 = 2.0). The planes differ in aspect ratio (and differ in wing area,weight, and power, since span loading and power loading are held constant), the plane with an aspect ratio of 2.5 has a shorter TO and climb to 50 ft distance than the plane with an aspect ratio of 10. This surprised me. Basically, because the plane with the lower AR has the biggest wing, it has a lower required Cl and this reduces the drag enough to make up for both the higher TO weight and the reduced efficiency of the wing.

Obviously, there are other good reasons to have a high aspect ratio.

Side Note: The lead author of this paper was Fred Weick, who was a bit of a legend at NACA and the main designer of the Ercoupe.
 
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Sockmonkey

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That may be a bit counter-intuitive but it does match what we seen in nature. The heavy-bodied ground-nesting birds that rely on a quick takeoff for survival use low-aspect wings.
 

Autodidact

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The lift curve slope modification due to AR is relatively slight compared to the increase in wing area due to changing the AR while holding span loading constant. You're talking about an increase in wing area from 160 ft^2 to 640 ft^2 for AR 2.5 to 10 with a 40 ft span and MAC of 4 to 16.
 

Vigilant1

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Right, a big change in wing area. Weight and power also changed. I'm sure if they had kept wing area (rather than power loading and span loading) constant then AR would have been high.
 
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ultralajt

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Hi!

It will be a hard task to fulfill all demands and features from your list on the first post, but it would be also an interesting design journey at same time, whatever the outcome.

My opinion is that if somehow design cant fulfill all design task (for instance gasoline expences), hey, the way of doing transport (by air in custom build hidroplane) is way cool-er than in a car by road, so this fact (coolnes and wow factor!) can turn the scale in your side! :D

I would not went to sailplane alike wings as wingspan could be your enemy on the ground or water, in terms of space, handling, designing suitable tail (tail area and arm influences fuselage length and that influences on complete craft weight and cost .... a snowball effect)

If you build wings with effective retractable flaps and slats, you achieve STOL, yet in "clean" configuration you still have a sleek high AR wing for speed and L/D.

And materials? Best choice is composite structure from molds. This way you can change internals structure and skin design by your will, depending of the prototype test results, and of course if some new materials arrive to the market... this way you are safe for many years of building that airplane from the same molds, while internal structure can follow the mainstream of modern composite materials...

As I express in many different threads, I am always happy to see some sketches, drawings or renderings of the design in development, so I am anxiously waiting for yours.

I wish a nice day to all of you!

Mitja
 

ultralajt

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In RC world some people are cutting wings shorter and shorter and making planes to look like jet fighters yet the fastest planes for power happen to be the hotliners which coincidentally are almost identical to sailplanes, but made for fast speed.
Hotliners must be capable of making sharp turns (as low radius as possible) at very high Gs (high CL) at the competition task, and high aspect ratio wings can produce high lift at lowest possible drag, so at "corners" on the task route they loose less speed and energy than wings with lower aspect ratio. Of course lower aspect wing ratio can withstand higher angles of attack, but that has nothing to do with efficient "cornering" the competition field. So all "speedsters" in that class evolved high AR wings. In straight line AR doesnt matter so much (well, depends of the wing loading..less wing loading is better)
 

karoliina.t.salminen

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Best L/D on the D40 looks to be about 13 to 1
Without prop.

http://www.slideshare.net/MichaelDitzler/diamond-da40-analysis-ditzler-aero-2200-final but the glide ratio does not seem to be impressive- 8.8 to 10 to 1 depending if prop is stopped on not.
With prop. For the cleanliness of the wing and fuselage this is not exceptionally good figure and it is not much better than a Cessna which has rivets hanging all over the place.
A similar looking design than the DA40, the Pipistrel Panthera, is however, superior to the DA40. That is well designed. The DA40 is well designed in a sense that it is a docile, very stable and safe plane to fly IFR around, but
it is not stellar in performance, performs less than it "looks like". There are multiple not well designed details in the DA40: one of them is the wing fuselage junction which (as surprise to myself as well) is not
a CNC part that would be result of accurate calculations for optimal shape for the joint between the fuselage and the wing. Instead it is made of microballoon-epoxy mix and my suspicion is that someone at Diamond just carves the shape
and it has nothing to do with science. When flying in rain with the Diamond, it can be seen that near the fuselage junction, the air on the wing surface is still. The droplets do not move like they move in the outer portion of the wing.
This indicates that the boundary layer is thick on that area which means high drag. The wheel pants also are of poor design, someone got testing results that the plane was a little faster without the front wheel pant.
I removed the front wheel pant and did not share this observation, our plane is a little slower as expected, without the nose wheel pant.

As slight OT to the topic, about the Diamond:
The DA40 is a good compromise as a product for a IFR GA plane, but it is not a daily commuter type plane by any means. It is good for longer trips, only. At higher Cl (and alpha) the plane is fairly efficient compared to the competitor planes made of metal.
However, not efficient enough to be remarkable. We are typically cruising at 50% power leaned at peak resulting in fuel flow of circa 25 liters per hour and the speed depends on the altitude, being 110-115 kts at low altitude and up to around 130 kts TAS
at 10000 ft depending on the air temperature of the day. For sightseeing cruise at low altitude we can lower the consumption to around 20 liters per hour (5.0 gallons per hour) but the speed reduces to 100 kts (100 kts IAS/TAS at 1000 ft) and the alpha is a little further higher. Any slower than that start to require more power because it becomes a slow flight attitude and the speed reduces drastically if power is not increased, so this is about as good as the DA40 gets (in case of our plane) with reasonable cruise speed. The fuel consumption is still quite high considering that there are typically two on board and no much baggage and the Lycoming is drinking the 20 liters minimum all of the time and the fuel cost becomes a significant portion of the operation cost when flying the plane (because fuel here is extremely expensive).

Fun calculation for the DA40 as daily commuting plane:
Consider this (imaginary salary taken as example): One engineer would receive 100000 per year as salary.
That is circa 8300 per month. After taxes that is circa 4800 per month.
Now consider the AVGAS price of 3.7 eur per liter. Diamond tank is something like 150 liters. It costs 555 eur to fill the tank.
If you do not eat or do anything else in the month, you can fill the tank 4800/555 eur = 8 times. 4800 dividided to working days in a month is 240 eur per day salary.
If you are in hurry and are going with faster than economy cruise, it drinks 10 gallons per hour. That is 38 liters per hour.
With the tank you can thus cruise 150/38 = 3.9 hours (as opposed to 5-6 hours with our economy cruise setting).
Lets assume there would be airfield in both ends (there isn't, so this is imaginary). The length of the flight is around 40 minutes (0.6 hour => 23 liters fuel consumed)
to one direction. Plus the taxi, takeoff at full power, climb, and traffic pattern consumes additional ~10 liters at least in addition to the cruise fuel consumption.
We can assume that one way trip takes 33 liters of fuel. The trip needs to be done two times per day. Therefore resulting in 66 liters of fuel.
The fuel cost per day is 66 liters * 3.7 eur/liter. Resulting in 244 eur per day for fuel alone. If we consider the salary gained from doing this daily commute, we end
up at 240 eur per day. This means that instead of gaining money from work, the person would be losing money every day 4 eur per day, in the gasoline price alone.
This is why it is important how much fuel is consumed per kilometer. Compare that to figure of Toyota Prius. The Prius uses gasoline that costs 1.5 eur per liter.
The fuel consumption is 4.2 liters per 100 km, resulting in 3.78 liters consumed for the trip. The fuel cost for the whole month is just 226.8 eur as opposed the Diamond
taking the same cost per one single day. To compete with the Prius, the plane needs to perform the trip with the same fuel consumption or maximum not be much worse.
It can be a little worse, but not that much worse. Definately not in the realm of the Diamond. The daily consumption can be lowered to around 120 eur per day with the Diamond
by cruising the the economy cruise setting we always fly. However, that is still enormous cost compared to the cost of fuel for the Prius.
If the car would not be Prius, but it would be Tesla. The electricity is a lot less expensive than the fuel. It would be almost free to commute with the Tesla, considering the energy cost
(and not operating cost and price deduction of the car). Hereby a plane with similar economy and fuel consumption than the Diamond DA40, is not viable product as daily commuter
for much anyone else than people who have income levels that of Tim Cook or Elon Musk.
 
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BBerson

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There is no seaplane as fuel efficient as Prius.
If your lake cottage was say 20 miles direct across a fiord then a seaplane makes sense. But if you are flying the same highway route, then no.
Need folding wings to use most marinas. And they usually need to be designated as seaplane bases.
So your examples prove my point.
Consider the world record of miles per gallon for a land vehicle is about 8000 miles per gallon, I think.
No aircraft can match that. Weight isn't so much a factor for a vehicle.
But the L/D of a large span airplane can be rather high, but then the useless airframe weight is excessive. The drag per pound of payload is really what matters, L/D of gross weight is irrelevant for a powered airplane. Barnaby Wainfan has shown how a low aspect ratio airplane of low weight can match a high aspect ratio design on drag per payload basis.

So if you could build a low drag ultralight (high L/D) then it would be fuel efficient.

It would be interesting if you could test the L/D of your Prius on a long downhill road glide.
 

bmcj

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the world record of miles per gallon for a land vehicle is about 8000 miles per gallon, I think.
No aircraft can match that.
What vehicle is that? Is it electric or hybrid, or proof of concept with no real usability? If so, then there is the Solar Impulse that used zero fuel for its round the world flight.
 

Riggerrob

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There is nothing rigidly tying you to burning Avgas (3.7 euros per litre) when many small airplane engines run happily on automobile gasoline. Supplementary Type Certificates allow many of the smaller (lower compression ratio) Lycomings and Continentals to burn 100 low lead (aviation gasoline) or auto-gas.
Rotax 912 burns 87 octane gasoline (90 octane in Europe) and Rotax 914 burns 91 octane (95 octane in Europe). Rotax engines run cleaner on no-lead gasoline.

The other advantage of automobile gasoline (1.5 euros per litre) is that most marinas sell 87 to 95 octane auto-gas/petrol, saving you a visit to the airport.
 
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BBerson

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What vehicle is that? Is it electric or hybrid, or proof of concept with no real usability? If so, then there is the Solar Impulse that used zero fuel for its round the world flight.
Miles per gallon of fuel. Looks like the record is now 12,000 miles per gallon, see here: https://en.m.wikipedia.org/wiki/Shell_Eco-marathon

The Solar Impulse is a joke to [me], with a build cost of $100 million+ per occupant, so hardly practical.
 

Riggerrob

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Vigilant1,
Good point about high aspect ratio not affecting .....

Aspect ratio determines rate of climb ..... along with power-loading.
Another way to say that is low span-loading equals great rate of climb. Most successful floatplanes (Cessna and deHavilland of Canada) have AR in the 7 to 10 range.

Touch-down and take-off speeds are primarily affected by wing-loading. That is why many Cessna and DHC float-planes are retrofitted with wing-tip extensions (Wing X) or leading edge extensions (Sportsman STOL) to increase wing-area. The primary function of dropped leading edges (Robertson STOL) is to increase the leading edge radius, correcting a discontinuity in the original airfoil. When you cruise a lightly-loaded wing at shallow angles of attack, they produce little drag.

I would avoid fancy double-slotted or blown flaps - on a commuter - because the added cost, weight and complexity wil not pay for itself. Also consider dispatch reliability because an overly-complex airplane will suffer too much down-time for repairs.
 

autoreply

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So your examples prove my point.
Consider the world record of miles per gallon for a land vehicle is about 8000 miles per gallon, I think.
No aircraft can match that. Weight isn't so much a factor for a vehicle.
Hold your horses there. Going slow will always be more efficient in terms of MPG. I know a few world champions of the Eco marathon. At 20 mph, that's not terribly practical.

Many single-seat sailplane can do 70 mph @ 3-5 hp. That's about a quart per hour or on the order of 250 MPG. Plenty of two-seaters that can still do 200+ mpg @ 70 mph.

Efficient airplanes by definition will use less fuel as a comparable car. It's not terribly complicated. Aircraft can get laminar flow. Cars can't in any practical circumstances. Major hindrance for the guys of the Nuon solar team for example.
But the L/D of a large span airplane can be rather high, but then the useless airframe weight is excessive.
Well, no. Maybe in the old days of relatively weak, flexible and dense materials, but not today.
 

Vigilant1

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Efficient airplanes by definition will use less fuel as a comparable car. It's not terribly complicated. Aircraft can get laminar flow. Cars can't in any practical circumstances. Major hindrance for the guys of the Nuon solar team for example.
But the car can have >zero< induced drag--no airplane can do that. The "road friction" of bearings and highly inflated tires will be tiny.
Also, while laminar flow is great, it's not magic. "A surface with turbulent flow will have a friction coefficient as much as 3 times as high as one with laminar flow." (ref: Raymer AD:ACA, Ed 1 pg 280). But a car doesn't need the huge surface area of the wing. When it comes to wing profile drag, "zero" is less than "very little."
I suppose the proof would be in the doing. If we look at highly efficient cars and planes with similar useful cruise speeds and useful loads, which is more fuel efficient? I admit I do not have data at hand to answer that. But there's no readily available airplane (is there?) that can carry 4 people at 70 MPH and a burn rate of less than 1.5 GPH, and many cars can do this. And our airplane does have to climb: it will take 10 HP to lift a 1600 lb airplane at 200 FPM, so to climb to 5K feet AGL will take 25 minutes. That's 1.9 gallons right there, added on to our fuel used for cruising (though we'll get some of that back on descent, we won't get it all.)
I suppose none of this answers the OP, since the car can't take off from the water (but I don't think a plane that meets the stated requirements can do that either, without some tricks).
 
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Jay Kempf

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But the car can have >zero< induced drag--no airplane can do that. The "road friction" of bearings and highly inflated tires will be tiny.
Also, while laminar flow is great, it's not magic. "A surface with turbulent flow will have a friction coefficient as much as 3 times as high as one with laminar flow." (ref: Raymer AD:ACA, Ed 1 pg 280). But a car doesn't need the huge surface area of the wing. When it comes to wing profile drag, "zero" is less than "very little."
I suppose the proof would be in the doing. If we look at highly efficient cars and planes with similar useful cruise speeds and useful loads, which is more fuel efficient? I admit I do not have data at hand to answer that. But there's no readily available airplane (is there?) that can carry 4 people at 70 MPH and a burn rate of less than 1.5 GPH, and many cars can do this. And our airplane does have to climb: it will take 10 HP to lift a 1600 lb airplane at 200 FPM, so to climb to 5K feet AGL will take 25 minutes. That's 1.9 gallons right there, added on to our fuel used for cruising (though we'll get some of that back on descent, we won't get it all.)
I suppose none of this answers the OP, since the car can't take off from the water (but I don't think a plane that meets the stated requirements can do that either, without some tricks).
I would guess that very few cars have zero induced drag. Almost all cars will leave a wake just like a plane taking off with circulating lateral vorticies. I would guess that is evidence that the lump that is round on top and flat on the bottom creates a net lift for all but a scant few race cars that have advanced aero that sacrifices drag for negative lift or downforce. I suppose also that interpolating between those two extremes that that it is possible to shape a car such that is has no net lift but few bother. When on the track I always have to convince myself that with a car with no downforce on an off camber corner that there is still gravity and therefore weight which is a form of downforce and is not zero but worth several thousand pounds of force normally.
 

autoreply

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But the car can have >zero< induced drag--no airplane can do that.
Road cars often have considerably more induced drag than your regular Cessna in a slow cruise. A very low-AR profile will do that in a bit of a cross wind.
The "road friction" of bearings and highly inflated tires will be tiny.
Also, while laminar flow is great, it's not magic. "A surface with turbulent flow will have a friction coefficient as much as 3 times as high as one with laminar flow." (ref: Raymer AD:ACA, Ed 1 pg 280). But a car doesn't need the huge surface area of the wing. When it comes to wing profile drag, "zero" is less than "very little."
That's why I mentioned the Nuna:

The drag of the PV area (basically all the horizontal area) is negligible, it's the wheels and pilots position that cause the majority of the drag. Why?

In real-life conditions you have cross-winds and a turbulent boundary layer because you're near the ground. Those two combined make aircraft using laminar flow potentially much more fuel-efficient than a car at higher speeds, say anything above 55 mph or so.

Also note that that Raymer quote is talking about drag over flat plates. If we're talking about real-life 3D bodies, the difference between laminar and turbulent flow can be over a factor of 10 ;)
I suppose the proof would be in the doing. If we look at highly efficient cars and planes with similar useful cruise speeds and useful loads, which is more fuel efficient? I admit I do not have data at hand to answer that. But there's no readily available airplane (is there?) that can carry 4 people at 70 MPH and a burn rate of less than 1.5 GPH, and many cars can do this.
An Arcus @ 700 kg needs about 5.8 THP to propel itself horizontally @ 70 mph. So call it 10 HP if we assume a horribly inefficient prop. That's 2 liters an hour, or about 0.55 GPH. Admittedly only 2 seats, but about a third of the fuel burn. Or about 130 MPG for a 2-seat existing airplane.
And our airplane does have to climb: it will take 10 HP to lift a 1600 lb airplane at 200 FPM, so to climb to 5K feet AGL will take 25 minutes. That's 1.9 gallons right there, added on to our fuel used for cruising (though we'll get some of that back on descent, we won't get it all.)
You'll get all but the last 1000 ft (pattern altitude) back without significant losses. Potential altitude is pure energy.
 

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Victor Bravo

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The highest "L/D" or overall efficiency of any transportation device is still the big railroad trains, (even without the future super-conducting mag-lev system) and with all their drag and hundreds of wheels and unfathomably high "tare" weight.
 
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