Electric Airplanes, contra-rotating propellers

[Northstar1989]

14 minutes ago, mikegarrison said:

Making thinner wings would not be terribly useful for an airliner. Modern airfoil design prefers a relatively thick wing for transonic flight anyway. Supersonic is different -- the thinner the better.

Also, if you take the weight of the fuel out of the wing and put it into batteries in the fuselage it would very structurally inefficient due to the effect it would have on the wing root bending moment. It's much better to have the weight distributed out on the wing. The first place they would want to stuff batteries would be the wing.

Yes and No.  The wings would be *relatively* thinner.  This does not necessarily mean thin in absolute terms.  Just a bit thinner than they are now.  Not thin enough to create issues with transonic flight.

And, although removing the fuel mass from the wings *would* exasperate the root bending-moment issue, there *are* workarounds for this problem- like switching to a dual-fuselage design (this would work best with smaller aircraft, where passengers tend to walk out on the runway to load anyways...) with a section of wing between the two fuselages, or adding extra engines further outboard on the main wing (which would allow each engine to operate at an exhaust-velocity closer to cruise-speed: each engine would generate less thrust, but consume even less energy- and the total airflow accelerated would be greater...)

4 minutes ago, mikegarrison said:

First: you were simply wrong when you said that Ve=V0 maximizes thrust. That actually produces zero thrust. Thrust is maximized by "moar boosters" -- just get yourself bigger and more powerful engines if you need more thrust.

I didn't mean matches as in "equals".  I meant it as in "is based on".  Nor did I use the term "Ve=V0" in my original phrasing- you introduced that.  YNM accurately expressed my idea when he said:

"Thrust generation comes best when the exhaust comes close to the airspeed."

Note the phrase *close to*.  In the real world, you want Ve to be *close to* V0, but some definite amount larger than it...

[mikegarrison]

7 minutes ago, Northstar1989 said:

I didn't mean matches as in "equals".  I meant it as in "is based on".  Nor did I use the term "Ve=V0" in my original phrasing- you introduced that.  YNM accurately expressed my idea when he said:

"Thrust generation comes best when the exhaust comes close to the airspeed."

Note the phrase *close to*.  In the real world, you want Ve to be *close to* V0, but some definite amount larger than it...

You guys are both not getting it. You are not talking about "thrust generation". There really is no maximum for thrust generation. Just slap more engines on if you want more thrust.

You are trying to talk about thrust efficiency.

[Northstar1989]

17 minutes ago, mikegarrison said:

Second: Propulsive efficiency isn't everything. Thermal efficiency is the other half of engine efficiency, and then you have to consider weight, cost, engine size, noise, desired cruise speed, etc., etc. It is true that when you put everything together, a good propeller engine has an advantage at low speed over a good jet engine, and a good jet engine has an advantage over a good propeller engine at high speeds. Turbofans exist because they are kind of a hybrid of the two and work pretty well to bridge the gap over all the operating range of a high-subsonic cruise speed airplane.

I never said it was, and these are several additional factors that are also important in engine design.  But the basic point stands- high exhaust velocity engines work better at high speeds than do low exhaust velocity engines, and vise-versa.  Which is why electric turbofans are likely to see more widespread use than electric propellers in passenger aircraft...

3 minutes ago, mikegarrison said:

You guys are both not getting it. You are not talking about "thrust generation". There really is no maximum for thrust generation. Just slap more engines on if you want more thrust.

You are trying to talk about thrust efficiency.

We're talking about how much Thrust you can generate with a given amount of energy/fuel- which is pretty much the definition of thrust efficiency.  So yes, that is *exactly* what I'm talking about, and I *do* get it...

Edited January 25, 2018 by Northstar1989

[mikegarrison]

10 minutes ago, Northstar1989 said:

And, although removing the fuel mass from the wings *would* exasperate the root bending-moment issue, there *are* workarounds for this problem- like switching to a dual-fuselage design (this would work best with smaller aircraft, where passengers tend to walk out on the runway to load anyways...) with a section of wing between the two fuselages, or adding extra engines further outboard on the main wing (which would allow each engine to operate at an exhaust-velocity closer to cruise-speed: each engine would generate less thrust, but consume even less energy- and the total airflow accelerated would be greater...)

Just off the top of my head: engines farther out from the centerline mean more yaw problems if you lose one.

Dual fuselages seems entirely unworkable for a passenger airplane. How do you load it and unload it quicky? How do you certify it for emergency escape? (Would you allow passengers to escape towards the center of the airplane?) Right now airplanes get loaded from one side and serviced from the other, but if you loaded from both sides where would you service it?

Really the best place for batteries would be right where the fuel is now, but that certainly would cause problems with the idea of just popping battery packs in and out during airplane turnaround.

[Northstar1989]

4 minutes ago, mikegarrison said:

Just off the top of my head: engines farther out from the centerline mean more yaw problems if you lose one.

If you have multiple pairs of engines (remember I said add *additional* outboard engines) you can just shut down the opposing engine, or throttle down multiple engines on the opposite side to remove any yaw tendency...  It's already something pilots of multi-engine aircraft are trained how to do...

Edited January 25, 2018 by Northstar1989

[Northstar1989]

5 minutes ago, mikegarrison said:

Really the best place for batteries would be right where the fuel is now, but that certainly would cause problems with the idea of just popping battery packs in and out during airplane turnaround.

You assume that mature electric plane designs would look anything like modern airliners.  I've already made the case there would be several major design differences- biplane designs instead of monoplanes (to improve range at the expense of energy-efficiency), thinner wings, and possibly dual-fuselage designs (one possibility might be to have the fuselages merge somewhere along their length, creating a "V" shaped fuselage bridged into an "A" shape by the wings.  Passengers would simply have to walk further to reach seats on the opposite side of the plane...)

And such ideas only skim the surface.  Flying-wing designs (like the B-2 Bomber) might be easier to manage with a Center of Mass that doesn't shift due to fuel-consumption, for instance...

[mikegarrison]

13 minutes ago, Northstar1989 said:

You assume that mature electric plane designs would look anything like modern airliners.  I've already made the case there would be several major design differences- biplane designs instead of monoplanes (to improve range at the expense of energy-efficiency), thinner wings, and possibly dual-fuselage designs (one possibility might be to have the fuselages merge somewhere along their length, creating a "V" shaped fuselage bridged into an "A" shape by the wings.  Passengers would simply have to walk further to reach seats on the opposite side of the plane...)

And such ideas only skim the surface.  Flying-wing designs (like the B-2 Bomber) might be easier to manage with a Center of Mass that doesn't shift due to fuel-consumption, for instance...

Well, the thing is, 25+ years of actually being in the airplane design business has left me pretty conservative about what design elements should be messed with. It probably takes younger people who haven't learned all the reasons why things won't work to actually try them out and see if they work.

[YNM]

2 hours ago, mikegarrison said:

No I'm not. Just think about it using Newton. If Ve=V0, where do you get any thrust from? Statically that would mean there is no air coming from the engine at all.

That's the point - if you can somewhow not produce thrust while maintaining a speed, it'd be good.

I bet there's more to the story. The universe is as chaotic as you can imagine, so yeah. Like this thread resurrected a few months after by a tangent...

Edited January 25, 2018 by YNM