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.