NASA Hybrid Electric Hyper Efficiency Aircraft Concept

[andymckee53]

Can you explain why the propulsor system is not efficient at low speeds please? Is take it there no known 'work around' to make it more efficient?

[NASAPAV]

Actually, the problem is not efficiency but simply how much thrust you can get out of the small nozzle. The BLI fuselage propulsor is optimally sized for cruise to achieve maximum efficiency, and therefore does not have nearly the thrust required for takeoff and climb. If you just try to throw more power, than the disk area is so small, that you would have to throw a LOT of power at it - and you would end up with a terrible solution.

[andymckee53]

@NASAPAV OK - understood. Can the nozzle be made to have a variable opening - such as the nozzles on fighter jets are? Also wondering if you have thought about the asymmetric problems that propellors on the extreme edge of the airframe cause if one were to fail? Have you got enough yaw control to overcome this? I don't suppose there is a possibility of locating a propellor after then end of the propulsor nozzle? I would be good to see cut away drawings that show how it all works.

[NASAPAV]

A variable diameter nozzle would still require enough power to get enough thrust and defeats the entire strategy of the concept to be efficient and low cost. I would not entertain doing wingtip propellers except for the fact that they are driven by electric motors (2 on each side with an overrun clutch). So they are as reliable as anything could be - so yes, I have thought a lot about the potential for asymmetric thrust and would never attempt this with conventional IC engines.

[EVAUnit4A]

I am not a pilot, but I can only imagine that you have to have some mad skills to take-off/land with the wings soooo low to the ground; it looks like the slightest crosswind or pilot error could roll it over easily-!

Still, cute-looking plane.

[yrmyahdaleth]

by what method does this concept aircraft provide control about the pitch axis?

[NASAPAV]

Great question! The aircraft planform is based on the Swift glider, designed and developed by Steve Morris and Ilan Kroo at Stanford. Steve documented this in a AIAA paper (92- 4604), and turned this PhD optimization topic of ideal lift distributions for tailless aircraft into the highest performance foot launched sailplane ever built (L/D ~ 25). Ilan also has a great paper on this (AIAA 00-4336), that explains that optimizing the sweep, twist, taper without the need for reflexed airfoils...

[NASAPAV]

could achieve a balanced design with high L/D, while achieving a reasonable CLmax, and CG excursion (key problems of tailless designs). SWIFT stands for Swept Wing with Inboard Flap for Trim; because the inboard flap is the pitch trim surface. Read these papers to understand further, or talk to glider pilots who have had the pleasure to fly the Swift - it is an outstanding design. So I'm leveraging this prior work to achieve high L/D in powered flight, with an ultralight, low cost structure.

[Allexxx96Reloaded]

@NASAPAV This plane could have control of the pitch axis by shifting the center of gravity forward or backward

[yrmyahdaleth]

@NASAPAV Thanks for the reply. Unfortunately I am not a member of the AIAA and cannot retrieve that paper. However I am also wondering, about the boundary layer control thruster, I have read Goldschmied's papers (thanks to CAFE), is it mostly ineffective at low speed? Or are the wingtip recovery turbines/propellers meant to act as propellers on take-off merely to further reduce take-off roll distance? Also, I have seen this concept on CAFE's and NASA's website, is it likely to be built? Thanks.

[NASAPAV]

Actually, I supplied all those papers to the CAFE website :)

The rear fuselage BLI propulsor is optimized for cruise flight, and has poor static and low speed thrust (insufficient for a 2000 ft field length and reasonable climb rate. The wingtip props/turbines provide supplemental thrust at takeoff and climb. I am doing detailed conceptual design on this study now, with wind tunnel testing of the BLI propulsor. I'd love to see this built - perhaps as a coop with a small company

[bobert4him]

I hate to say it but maybe solar cells in the wings would greatly increase the climb rate and range.

[NASAPAV]

Sorry, but no. Covering the entire wing would add about 2 hp... Solar flux is simply too weak to be powering real aircraft (not aircraft like the Qinetic Zephyr that have ridiculously low wing loading). We can get about 10 hp just from the wingtip props - so that is far more effective.

[bobert4him]

@NASAPAV Thanks. I was thinking about Larry Mauro's Solar Riser, which was quite aerodynamically dirty, McCready's Solar Challenger and the Heilos. Yes, it's more of a drag (added weight) during non-optimal exposure. I just figured that a free in-flight charge would be a boon. Or at least a boost.

By the way. Best of luck on the boundary layer intake and wingtip motor/generators. That is inspired. I see that you are maximizing surface effect too.

[NASAPAV]

We are doing an initial sub-scale Boundary Layer Ingestion inlet wind tunnel test with Cal Poly SLO right now. We've already flight tested the wingtip propulsors, and have an excellent understanding of those effects. But it is a highly coupled aero-propulsive concept that needs a lot of detailed analysis to put together.

[buzzyno]

I don't know too much about aerodynamics. But "Bumblebee" comes to my thoughts.

Me like plane!

[niflap]

What will be the sourse of electricity? A fuel cell would be the closest you can come to making electricity an energy soarse. It can be fuel efficient, but vey inefficient weight & bulk wise. A lithium battery of the right size may take care of that problem, but it wouldn't have the equivalent of a fuel cell's efficiency

[NASAPAV]

Fuel cell is not as efficient as battery in terms of energy conversion, but potentially a fuel cell could have better energy density and be lighter. Doesn't matter for this concept because there is not much stored electrical energy so a battery is easier and better.  All the cruise thrust comes from a small reciprocating engine in the back, the wingtip propulsors are just to provide extra thrust for takeoff and climb so that the cruise propulsor can be optimal for the cruise flight condition

[tomview1]

Looks like it will suffer the same ineficiancies that all flying wings have.

[NASAPAV]

What are you talking about??? It's about 42 ft, far less than LSA motorgliders which are typically 50' wing spans. Wing folds at the dihedral break would not be hard, but would add weight. The fact is, if you are interested in high efficiency, you need span - there's no other way to get it. The wing end plates help to reduce the span compared to the motorglider spans.

[pytoche]

where did you find those HDR for the background?

[squirrelflight]

Are there any publications describing details?

[NASAPAV]

No, I have not had time to publish any results on this, but I am hoping to do so towards the end of this year. While I think this is high priority work, I can't get others at NASA to agree with me on that :)