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Upgrading DA42 with CD-155 engines

Peter wrote:

because a best-perf climb is not good for engine management reasons

My climb profile in autorouter is the one I follow. Not book performance. It is up to the user to configure the aircraft profile to reflect their real-life profile.

Hourly fuel burn is the same at FL180 as it is at FL060, but TAS is 16% better, hence the better MPG. On the other hand you do burn fuel to get up to your chosen cruise level. I burn 26 gph during climb vs 16 gph duirng cruise, and I climb at a lower TAS than cruise which contributes towards a higher total MPG. If getting up to the chosen cruise level takes 30 minutes, I will have burned 10 USG more than in cruise, at 120-130 IAS.

Autorouter explicitly says that the “Optimum FL” screen takes the climb performance into account in the computations, but it does not mention the descent, so I assume that it does not account for the reduced fuel burn during descent. The descent from FL250 takes 35 minutes during which I burn 12-14 gph instead of 16 gph, and the TAS is somewhat higher than during cruise.

Here is a better example with same headwind component at several altitudes which does show lower fuel brun at the higher altitudes.

Last Edited by Aviathor at 01 Jun 10:09
LFPT, LFPN

I don’t think the Autorouter climb profile is ever actually flown, because a best-perf climb is not good for engine management reasons. Also the wind forecast is from GFS…

However one should definitely get better MPG at a higher altitude. I know this has often been questioned and we have had countless threads on it, but I certainly see that. Some 10% better at FL100 versus say 2000ft. A turbo, such as Emir has, should not change this principle, but it ought to extend it as one climbs higher unless other factors come in which degrade the efficiency.

Administrator
Shoreham EGKA, United Kingdom

AdamFrisch wrote:

This is a perfect example to illustrate Carson fuel economy. And that is that you have negligible advantage on fuel economy and range by going higher in a piston.

There is a new feature in the autorouter which computes the time and fuel burn at various altitudes so you can find the optimal altitude for fuel burn or time. It takes not only the fuel burn to reach that altitude into account, but also the wind forecast. Quite interesting to see those tables. The one below is for a turbocharged engine with critical altitude of FL180

Last Edited by Aviathor at 01 Jun 06:25
LFPT, LFPN

Interesting figures, Emir.

I had a flight in a DA42 about 10 years ago (so presumably that tells you what the engines were?) and at 140kt IAS, 3000ft, it was burning 11.5 USG/hr – exactly the same as my TB20.

A Cessna 400 I flew in soon afterwards was doing the same, although obviously it could go a lot faster.

What is your fuel burn for 140kt IAS at 3000ft?

Administrator
Shoreham EGKA, United Kingdom

Miles per gallon (per engine):

FL160:
- 85% = 28.83 mi/g
- 80% = 29.33 mi/g

FL140:
- 85% = 27.66 mi/g
- 75% = 30.76 mi/g
- 65% = 34.94 mi/g

FL110:
- 85% = 26.52 mi/g
- 75% = 29.46 mi/g
- 65% = 34.05 mi/g

This is a perfect example to illustrate Carson fuel economy. And that is that you have negligible advantage on fuel economy and range by going higher in a piston. However, you have huge advantage by slowing down to closest to best glide or Vy.

Emir or Dave Phillips – I would love if you could slow down to best glide speed next time you’re at altitude and see what FF is?

Last Edited by AdamFrisch at 31 May 17:49

Here are some new cruise data for CD-155 – IAS, TAS.

FL 160:
- load 85% 137 kts, 178 kts, 2250 RPM, 7.1 g/h per engine
- load 80% 135 kts, 176 kts, 2200 RPM, 6.9 g/h per engine

FL 140:
- load 89% 142 kts, 180 kts, 2300 RPM, 7.6 g/h per engine
- load 85% 141 kts, 178 kts, 2230 RPM, 7.4 g/h per engine
- load 80% 138 kts, 175 kts, 2120 RPM, 7.0 g/h per engine
- load 75% 135 kts, 171 kts, 2040 RPM, 6.4 g/h per engine
- load 70% 132 kts, 167 kts, 1990 RPM, 6.0 g/h per engine
- load 65% 126 kts, 158 kts, 1950 RPM, 5.2 g/h per engine

FL110:
- load 90% 146 kts, 175 kts, 2180 RPM, 7.8 g/h per engine
- load 85% 144 kts, 173 kts, 2150 RPM, 7.5 g/h per engine
- load 80% 141 kts, 170 kts, 2100 RPM, 7.0 g/h per engine
- load 75% 136 kts, 164 kts, 2030 RPM, 6.4 g/h per engine
- load 70% 133 kts, 160 kts, 1990 RPM, 5.9 g/h per engine
- load 65% 128 kts, 154 kts, 1950 RPM, 5.2 g/h per engine

LDZA LDVA, Croatia

Well, I’m considering TAS 140 to 150 kts. Obviously, lowering it can increase range but usually it’s not practical (7-8 hours is small aircraft to cover 1200 NM or so).

LDZA LDVA, Croatia

With the DA62 you can bring the load back to about 40% giving a tad over 10hrs endurance – TAS at that load is about 115kts. I’ve loitered at 30% and then endurance shoots up to 13+hrs but you’re down at about 3000ft or lower and TAS is 95-100kts. Next time I’m flying I’ll set a ridiculously low load and take a picture of the “Range Ring of Death” :)

I don’t recollect precise details of the 42, but in general terms I’ve always thought that the 42 and 62 seem to match each other from a load/endurance perspective.

Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

Cool.

I seem to recall one of the DA40 demonstrators did St John to Porto direct after Oshkosh many years ago. He pulled the throttles back to 35% or something – way below the published max economy in the POH. Over 1900nm. As I understood it he had just the original LR tanks, not a ferry tank. I wonder if you could get similar range with the DA42 pulled back?

Yes, I have LR tanks, total usable fuel on board is 76 gal. I don’t know the numbers for max economy for the new engines yet but I expect range 1100 NM – same as for the old ones.

LDZA LDVA, Croatia
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