Engine Selection Criteria

Ideas and suggestions on OS Aircraft Design optimized for CNC.

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John Nicol
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Re: Engine Selection Criteria

Post by John Nicol »

These are great ideas and the links are really interesting. I hope that the sugar battery comes on to the mainstream very soon!

We have been thinking of electric for a while, but wanted to get the reference design for the airframe down before changing it. The great thing is that once the 3D files are up for any of the aircraft, it shouldn't take a lot of effort by a knowledgeable person to change the design to accommodate the engine and batteries. When I was at Oshkosh in 2012, I had some very long discussions with a couple of companies that were building electric powered aircraft and it seemed that it wasn't quite there. Flight times varied from 20 minutes to an hour max. I even had a discussion about on-board generators charging the batteries while in-flight and one company had done a lot of research in this area and said that the generator would be too heavy.

Anyhow, there is also a new announcement I have been tracking from NASA and this might be how we could get this going. It is the LEAPTech project and apparently it has been going since 2014. It uses multiple electric motors per wing and is apparently very efficient.
"LEAPTech’s big innovation is using 18 small engines to blow air directly across the wing, the part of the aircraft that generates the most lift. Traditionally, aircraft have a big engine or two that are used exclusively for forward propulsion, and lift is generated by the wing as a side-effect of that forward movement. As a result, wings have to be relatively large in order to provide sufficient lift at lower speeds (takeoffs and landings), and at higher cruising speeds, but all of that wing area just slows the aircraft down.

The LEAPTech X-plane tightly integrates engines with wings, so the engines all work together to maximize the amount of air moving directly over the lift surfaces. By doing this, the wing can be optimized for cruising speeds instead of takeoff speeds, drastically improving cruise efficiency. The only reason this concept works is that with electric motors, you can just slap 18 of ‘em on there, because of how well they scale in size, weight, and power."
http://www.nasa.gov/centers/armstrong/F ... ptech.html
http://www.jobyaviation.com/LEAPTech/

It is part of an initiative called the Convergent Electric Propulsion Technology (CEPT).

Image

So this might be one way to do it? If only we could get a grant from Quebec to get this going here!

John
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John Nicol
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Re: Engine Selection Criteria

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My 2 cents worth:
Let's assume we want to cruise @ 100 knots (115 mph, 185 kph, 51 m/sec) for 5 hours (range = 575 miles).
Also assume we cruise @ 70% throttle with a 100 HP engine (=70 HP = 52 Kw) and the overall efficiency of the whole system including the prop, motor, electronic speed controller (ESC) and battery is 80% (optimistic!).
Now let's do some math to find out how much energy we need: We need 52 kw for 5 hours @ 80% efficiency = 325 kw.h.
Now let's look at the battery technology: The best and safest affordable battery technology as of today is Lithium-Ion, with a maximum energy density of about 0.24 kw.h per kilogram. In order to store 325 kw.h we need 325/0.24 = 1354 Kg (=2985 lb) of battery.
Fuel cells, "sugar power" and other technologies will probably have a much higher energy density but as of today they are either still a long way from mass production or horrendously expensive.
So, I apologize for being the party-pooper, but let's be realistic about electric power - we're not there yet.
Jeffrey
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They said "Cheer up, it could be worse", so I cheered up and sure enough it got worse.
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Re: Engine Selection Criteria

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Hey Jeffrey,

Good stuff. Hopefully we will see that revolution in energy density to weight ratio come down soon! Bring on cold fusion....

John
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Re: Engine Selection Criteria

Post by John Nicol »

Hey Guys,

Here is a site to keep a track of the electric power technology: http://sustainableskies.org/

I also met up with Tomas Broderskift at AirVenture 2013 in the Innovation Hanger. He had a booth opposite MakerPlane there. Really interesting project and very sexy design. We had a long conversation about electric power and using a hybrid concept. What he was looking at doing was potentially using a generator on-board to charge the batteries in-flight. At the time he said that the generators were too heavy to produce any good results, but looks like he may have found a solution potentially.

http://sustainableskies.org/tomas-brodr ... ar-at-sas/

John
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Re: Engine Selection Criteria

Post by aeromomentum »

Hey Jeff,

While your method of calculating the weight of the batteries is great I think you made a few assumptions that are not valid or are a little pessimistic. For example you lumped the prop efficiency with the engine, controler, etc efficiency. This is not really valid since you also need a prop with a IC engine. So just including the "engine system" you should have about 95% efficiency. Of course the prop may be 85% or lower efficiency but this is the case any power system and is not part of the power system. Next your 70% power rating is a bit high since all of the large IC engine makers recommend 65% as maximum for meeting TBO. A more realistic range would be 2.5 hours. I fully understand about wanting 5 hours. But is it really needed for most missions. So assuming 100 hp, 65% power, 95% power system (but not prop) efficiency, 2.5 hour range you get 128 kw-hr. Using 0.30 kw-hr/kg you get about 940 lbs. Still not practical but we are getting closer. Now taking into account the difference in weights of the engines we have the gasoline engine at 175lbs plus 27 lbs of fuel plus the weight of the fuel tank and you get about 210lbs. The electric motor, with controler and wires is about 60 lbs. So for a difference of 150 lbs. Now apples to apples the electric plane is 790 lbs over weight. You did give up some range that is rarely used but you massively reduced maintenance and fuel cost. Is it practical? Maybe not unless we change the mission more. With a 1.5 hour range that gives 410 lbs weight penalty. How about reducing power to 75 peek? That gives just a 260 lbs weight penalty. Some would consider this a very practical training aircraft!

Now consider that with no major change in technology but with the current rate of improvement the batteries should be about half the weight for the same power in about 7 years. Now it starts getting practical for 2.5 hours. In 14 years it should be 1/4 the weight so it should be practical for 5 hour range on low power aircraft.

Mark
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jeff54il
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Re: Engine Selection Criteria

Post by jeff54il »

Hi Mark,

Thanks for the input - I see you have gone into the subject quite deeply.
I stand by my numbers and even emphasize that they are optimistic. I will answer you in due course and I want to do your comments the justice they deserve, but I have some pressing work to complete, so I'll get back to you in a few days.
Jeffrey
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They said "Cheer up, it could be worse", so I cheered up and sure enough it got worse.
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jeff54il
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Re: Engine Selection Criteria

Post by jeff54il »

John, this topic seems quite closely related to the new All-Electric MakerPlane topic. Suggest you move the posts over also.
Jeffrey
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They said "Cheer up, it could be worse", so I cheered up and sure enough it got worse.
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