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Flying wing for high efficiency

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karoliina.t.salminen

Well-Known Member
Joined
Oct 7, 2008
Messages
407
Location
Finland
Hello,

I picked up one comment by autoreply in Synergy thread about flying wing for attempting very high efficiency. So I went and wrote to him a private message about it because I have felt some interest towards this configuration for quite some time, largely for two reasons 1) looks cool and 2) simplicity, potential for fewer part count. I have searched some old threads about this topic also from this forum and elsewhere and have read them. I had sort of dismissed the configuration earlier but have came to second thoughs about the applicability of the flying wing for high efficiency concept that targets for cutting the cost/mile to similar level than airline ticket on the same distance, in other worlds, not flying just for fun, but doing practical travel (and having fun along) with private aircraft where today's GA is largely unsuitable for (I fly Diamond DA40, but even for domestic trip I have to go with airline if I want to pick the less expensive alternative, although the queues at airports are super annoying and same applies for the ridiculously small leg room, I have to wonder how 2 meter tall guys can travel in economy class, the Diamond is luxury in comparison). However, my speculation is about efficiency for practical aircraft that can be used to fly from point A to point B (medium to long distance, with minimal fuel cost and also being able to cover that distance also partly because the trip gets the more expensive the more there are stops in between as there are landing fees and navigation fees and even noise charges for every unnecessary landing between point A and B).

However I have seen few challenges on the flying wing concept when the aim is high efficiency: instead of dragging a fuselage around, one has to drag around extra wing area simply because the shape of the plane makes the wing area rather large. The lower achievable Clmax for the tailless design does not become a problem for achieving less than 61 kts stall speed that easily because the rapid expansion of the wing area when sizing a flying wing up sufficiently so that e.g. four people can be accommodated inside the wing center section, or almost inside it without canopy or blisters for doing that but instead blending the cockpit in seamlessly (as the aim was high efficiency).

I noted his comment about blowing and suction, but then raised a concern (in the private reply) that it would be possibly challenging system in real world use where bugs, dirt and ice can block the suction.

Autoreply gave me permission to include his private message reply on this topic. I was requesting him to create the topic, but ended up creating it myself after all.

Here is his answer (copy paste with permission):
Boundary layer suction works essentially the same on a body (fuselage) or airfoil. Suck off (a part of) the boundary layer and thus reduce your drag by having more laminar flow and a thinner boundary layer, both caused by the different pressure distribution when following the chord.

Essentially, you can have zero or even negative drag by sucking off the boundary layer, even if you don't exhaust that airflow anywhere. Naturally, a perpetuum mobile doesn't exist and you still need power to suck off that boundary layer and of course, exhausting the sucked-off air will give you some extra thrust.

When you turn to reality, sucking off properly is a nightmare. A layer with holes gets quickly clogged up by dust and insects; a slot very quickly leads to problems in the distribution of the suction. At this moment, I think the only way to make it work in a practical application is have reasonable large slots (2-3 millimeter or so). While some gliders can extend the laminar flow over the flaps, in this case, full span elevons and suction at the hinge is probably the best way.

By now, you might have understood that, what I was talking about was a full-span suction slot on a BWB, the Atlantica closely resembling it, though an unswept design would work too. Having the right suction at the right (spanwise) position is still very complicated, having an appropriate pump isn't that hard (turbocharger).

The difficulties don't stop there though. No turbocharger blowing will produce thrust, sufficient to even get the aircraft rolling. Thus you very likely need a different propulson during slow flight (take-off and landing). But even there it doesn't stop. To produce full-body laminar flow, you can basically not have cut-outs or openings. That not only makes the landing gear a difficult trade-off (fixed with the extra drag, or a tiny opening and accepting the turbulent flow), but also things like an opening to the cockpit and air inlets for the cooling a nightmare.
I have been reading of course all kinds of papers about active boundary layer control. In Goldschmied papers it has been discussed for fuselages and in some others it has been discussed for glider wings. However, I have been thinking some time that the thick airfoil of a flying wing (especially center section) could be a candidate for boundary layer suction.

Obviously the plane can't fly with blowing the sucked air alone, but needs some main propulsion. Whether that would be a propeller (pusher behind the trailing edge) or several smaller pusher props on top of the trailing edge or series of ducted fans is an interesting question also. Several fans or several props could be more easy to arrange in case of electric propulsion. However, the power plant for electric power generation and battery pack would be quite heavy with current battery technology and would lead to rather poor power to weight ratio. But large drive shafts, couplings and gear reduction drives are not without weight penalty either. I have been speculating that in case of a flying wing, since the wing area is large and wing loading is low, the capability for additional weight penalty is somewhat greater (example of this is actually some of the old Northrop wings, they had rather poor power to weight ratio) than on traditional configuration with minimum wing area.

So I have the following questions initially:
1. The wing area for reasonably sized aircraft becomes very large on flying wing. So the wing loading will be light (much lower than necessary for the stall speed and climb performance). Does the lack of fuselage compensate for this unnecessarily large wing area?
2. Flaps might not be doable in this configuration. On the other hand, since wing loading is low, they are not that much needed. Does dragging around the extra wing area compensate for the lack of fuselage drag?
3. Aspect ratio tends to become lower than on fuselage + high aspect ratio wing configuration. It is of course possible to design a high aspect ratio wing, but still, the center section is rather low in aspect ratio. Question then is that which one is worse: lower aspect ratio or dragging around fuselage for no benefit. On the other hand, the flying wing can have equal span loading with lower aspect ratio than high aspect ratio conventional layout. I been thinking this a long time and I do not have a clear answer. What is your opinion on this?
4. If the main propulsion is with propeller, which pusher propeller configuration is lesser penalty: propeller behind the thick wing or propeller behind a small high aspect ratio wing, taking in account the boundary layer control opportunities from the wake propeller vs. potential for undisturbed flow on small wing with large diameter prop. Any takes on this?
5. If the suction is in the full span elevon hinge, where would be air then blown out? Consider that there would be one, two or several propellers at the trailing edge, where would be the exhaust or exhausts where the air would be blown out?

These are just some thoughts and questions on the flying wing and high efficiency. I would be interested to hear your opinions on the flying wing configuration.

Best Regards,
Karoliina Salminen
 
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