DA42-VI v Super 700 Aerostar

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ultraturtle
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DA42-VI v Super 700 Aerostar

Postby ultraturtle » Mon Feb 26, 2018 9:00 pm

New thread started to curb topic drift on another. Over there I was making the point that efficiency greatly impacts the payload any given aircraft can carry over a significant distance. Beyond a certain distance, a less efficient aircraft with a large useful load ends up with the ability to carry a smaller payload than a more efficient aircraft with a smaller useful load.

Since the Super 700 Aerostar conversion was thrown into the conversation, I thought it appropriate to run the numbers on it compared to the Diamond Twins:
Screen Shot 2018-02-26 at 3.31.36 PM.png

At maximum takeoff weight and maximum published cruise speed, the Super 700 Aerostar hauls the same payload as a DA42-VI for a distance of 675 miles with 45 minutes reserve. The Aerostar can haul more for any shorter distance, and the DA42-VI can haul more for any greater distance. The Aerostar does so at an impressive speed advantage of 37%, albeit at a fuel cost nearly 2.3 times that of the Twinstar.

Assuming a 45 minute reserve, the DA62 hauls more than the Aerostar over any distance.

I left climb consideration out of the analysis for simplicity, lack of an Aerostar POH, and the likelihood that the Aerostar would come off worse in the efficiency comparison. It may be able to reach 25,000’ a bit more quickly than the Diamonds can reach 16,000’, but the Aerostar does so at a whopping 66 gph, vs 19.4 gph for the DA62 and 16.6 gph for the DA42-VI.
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Re: DA42-VI v Super 700 Aerostar

Postby TimS » Mon Feb 26, 2018 9:41 pm

I had the gross weight increase on the 700. With that, MTOW is 6850. With my plane, I had an empty weight with all five seats in the plane of 4399.9 (call it 4400). That gives you a UL of 2400lbs. With only 700lbs of fuel, you have a UL potential of 1700lbs. You then have to be concerned about MZFW which I do not recall.

You also should compare with the same cabin altitude. With the cabin pressure increase of 5.5 PSI; at 28,000 ft my cabin was around 10K. No O2 required.

You can also pull the power back on the 700. In the low 20s, you can run LOP get a solid 220 KTAS burning 24 GPH total. That last forty knots of speed you really need to pour on the fuel. But many is it fun to fly. Oh, you also missed on the take off fuel flow. Closer to 85 GPH :D I normally ran this only for about five minutes. Full power, full rich. In five minutes I was usually up around 10K; this was running about 140 KIAS and gave me a solid 1800 FPM climb. If I really wanted to climb fast, at 120 KIAS I could sustain almost 2600 FPM to the high teens at MTOW. I then switched to a cruise climb and speeds.

The planes are totally different with totally different missions. The Aerostar is like a miniature airliner, the Diamond's are closer to beach cruisers.

Tim
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Re: DA42-VI v Super 700 Aerostar

Postby Tommy » Mon Feb 26, 2018 9:59 pm

Thank you Tim. Finally someone that gets it. An Aerostar owner of course.
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Re: DA42-VI v Super 700 Aerostar

Postby TimS » Mon Feb 26, 2018 10:10 pm

Tommy wrote:Thank you Tim. Finally someone that gets it. An Aerostar owner of course.


lol, former owner. I was living next too BSFAirpark.com (where I still own a lot and hangar if anyone interested) and had a meeting located at Knoxville airport. Roughly 50 miles away over a mountain range. Two hour drive on windy roads or twenty minutes in the Aerostar. Then I calculated that it was four gallons of gas in the car at 2.50 a gallon vs forty gallons of 100LL at 6.50 a gallon. As soon as I did the mental math, I knew I had to sell the plane.

I loved the plane, but even after 300 hours in it in just over 18 months, I never did get used to the gas price sticker shock.

Tim
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Re: DA42-VI v Super 700 Aerostar

Postby Karl » Tue Feb 27, 2018 2:33 am

TimS wrote: I calculated that it was four gallons of gas in the car at 2.50 a gallon vs forty gallons of 100LL at 6.50 a gallon. As soon as I did the mental math, I knew I had to sell the plane.

I loved the plane, but even after 300 hours in it in just over 18 months, I never did get used to the gas price sticker shock.

Tim


Using the figures above how much would it have been in a DA42 out of interest?
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Re: DA42-VI v Super 700 Aerostar

Postby TimS » Tue Feb 27, 2018 2:56 am

Use 134 KIAS as base line for the DA-42.
50 / 134 = 37%
16.8 * .37 = 6.3 gallons

Tim
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Re: DA42-VI v Super 700 Aerostar

Postby Antoine » Mon Mar 05, 2018 7:15 pm

Oh man how I envy your fuel costs... In Europe it's double the money for AVGAS. makes the Aerostar a no go... And that's even before you take into consideration the Enroute fees from Eurocontrol.
My Extra 400 is as crazy as it can get before going bust... just escape the Enroute fees by 2 (two) lbs
Here the numbers at FL 250
Speed (at 75% pwr): 220 KTAS
Fuel flow: 24 GPH
Fuel density. 6 lb/USG
Fuel flow (mass) 144 lb/hour
Fuel cost: (ouch): 240 USD/hour. For a 675 NM trip: 740 USD! thats' over one third more than a faster Aerostar in the US:.......
Just realized how much money I'm throwing at this! CRAZY!
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Re: DA42-VI v Super 700 Aerostar

Postby Tommy » Mon Mar 05, 2018 8:54 pm

Antoine, your'e making me feel better. That's almost what I burn (26 gph) with TWO engines at 15,000', 50% power and 198 KTAS. No matter how you cut it, avgas, diesel, or jet A, it doesn't matter. If you want to go fast, you're going to burn a lot of fuel. Incremental increases in speed require exponential increases in fuel flow. There is no getting around it.

I can do 192 KTAS at 15,000' 22 GPH if I want to slow down. If I want to slow down even more I can get
180 KTAS @ less than 20 GPH.
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Re: DA42-VI v Super 700 Aerostar

Postby Antoine » Mon Mar 05, 2018 9:58 pm

Yep... drag is proportional to the square of speed...
I think your Aerostar 600 and my Extra are within 5% of each other at 15'000 ft. That's funny!
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Re: DA42-VI v Super 700 Aerostar

Postby TimS » Mon Mar 05, 2018 10:09 pm

Antoine wrote:Yep... drag is proportional to the square of speed...
I think your Aerostar 600 and my Extra are within 5% of each other at 15'000 ft. That's funny!


And kind of amazing to think the Aerostar design was almost thirty years old when the Extra 400 was designed.


Tim

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