Facet Opel

You've either been reading Dan Raymer's "Aircraft Design, A Conceptual Approach" or something very similar! I believe he uses the Boeing B-47 in his comparison, but also uses the Vulcan for the low-AR case. It really does go against the conventional 'what pilots are told' wisdom, doesn't it?

One of the 'nice' things about how a flying wing (with a pod) compares to a 'conventional' plane in terms of how the wetted area is distributed is that the former concentrates the wetted area on a part of the airframe ( the wing) where it's relatively easy to obtain substantial laminar flow, whereas that's rather difficult on most practical fuselages. I believe that for most practical flying wing/flying plank variations, the total wetted area is roughly the same as would be a conventional airplane. I haven't done a broad numerical comparison. However, as mentioned above, most of that wetted area is in a relatively low-drag part of the airframe, so I would say that's a point to the tailless aircraft, in terms of actual parasite drag. Getting a reasonable induced drag on a swept flying wing while still preserving good stability and post-stall characteristics is rather more difficult, as I'm sure you've both seen in your own work. While I'm not working on a sailplane, my budget dictates a rather smallish engine, so I do have to pay attention to that to obtain a reasonable climb rate. The extra wing area does help that, however.

Hans, that's one nice-looking airplane! If you've used Nickel & Wohlfart's methods for a 'trimless' camber flap, then great. Just looking at it, I'd be afraid that there would be a trim change with flap deployment, but it's very hard to eyeball such things. With regard to your control surfaces, I've scanned over the section on multi-element pitch/roll controls, so if you can gear that mechanism simply, I imagine you'd get all the advantages N&W claim for that. I know Strojnik (Laminar Aircraft Technologies, etc.) claims that such a mechanism is simple to do, but I've never seen a drawing nor tried to design one myself. This is where the R/C model guys have it made with their fly-by-wire systems: they just 'dial in' whatever mix of control movement they want and the processor in their radio gear does all the work. Lucky so-and-so's...

I think the only concern I'd have about your design - and believe me, my qualifications to say this are minimal - would be the delta-esque center section when it comes to near-stall conditions (high AOA). You'll probably get some vortex lift on that section (because of the highly swept leading edge), and I'd be concerned that it might be enough to keep that part of the wing 'flying' beyond the point where the tips stall. It might also make the trim a little unpredictable in the middle-AOL ranges, as the vortex lift begins to form, perhaps with a little instabilty at first. Hard to say. Orion and/or Wimpso would probably be the guys to ask. I'm still learning this stuff, myself.

My 'paranoia' with tip stall stems from the fact that while one can certainly design an aircraft in which it's unlikely to happen, even that small remaining chance has consequences that are very drastic. Discovering new and creative curse words as the ballistic parachute lowers my broken airplane to the ground is the best possible outcome.

One last thing to note: X-plane has been discussed at length on this forum regarding testing of new designs. The consensus amongst the more experienced designers/engineers (Orion, etc.) is that while it's a decent tool for very rough checks on your design, it's not accurate enough to use for a definitive analysis of real-world flight characteristics. Even an R/C model would give a more-accurate picture, apparently. Just a word of caution; you may already be familar with that information.

Originally posted by Norman
Flying wings built with little or no incidence are notorious for their inability to rotate for takeoff, especially if the thrust line is a little high. The Delt-air 250 tested fine in the wind tunnel but couldn't pull the nose off the ground under full power, even well beyond the minimum flying speed. When the pilot cut power the nose down moment disappeared and it jumped into the air.

Yeah, that's a nasty thing, isn't it? Orion had mentioned about having the aircraft 'on the gear' at the takeoff attitude, so that you just 'fly off' when the airspeed is right.

It seems to me that if the main gear is carefully placed and you limit yourself to paved runways (smooth), then one might have enough elevator/elevon power to rotate for takeoff. If the pilot is forward, you can't keep the airplane from rocking back on the tailskid once the pilot is out of the plane, why not optimize the gear to allow a 'normal' rotation takeoff. I haven't run any numbers on that yet, though. Don't know if it's possible, or if it would put the gear so far forward that even a small bump on the runway would pitch the plane back prematurely.

Worth a trade study or two, I would think.

Topaz,

we seem to have a very similar libary...

Thanks for the thumbs up on my (very preliminary) design.

As to the delta center section. yes, this will only work if enough vorticity is generated. At that operating point, the two outer surfaces of each wing will be deployed completely up, creating a lot of wing twist in the right direction. I think tip stall can be controlled, all the more so because of the low pressure side of the winglets should help in this respect. There are other measures one could take: small vg's on the control surfaces; those thingies that all EZE-derivates seem to sport on the leading edge of their main wing, etc...

As to X-Plane: checking out this product with plane models that I have flown for real, I'd say that it's pretty darn good as long as you stay within the linear aerodynamics limits. Stall modelling stinks. Other than that, I feel it can be used for conceptual/preliminary design.

I have other software at my disposal, but I don't like most of them.

bye
Hans

Originally posted by h_zwakenberg

There are other measures one could take: small vg's on the control surfaces; those thingies that all EZE-derivates seem to sport on the leading edge of their main wing, etc...

The LE thingies are vortilons. Here are some links I have bookmarked.

This is the notice telling Long-EZ builders that vortilons are now mandatory:
http://www.ez.org/cp47-p9.htm

This is some pictures from a Velocity builder's log:
http://www.rguerra.com/velocity/wings2/wings2.html

This one has a drawing of the vortilons as used on VariEzes:
http://www.ez.org/cp42-p5%28.htm

This one shows all the excresences on a 737-200:
http://www.b737.org.uk/flightcontrols.htm
The vortilons are just past half way down.

These are fairly old so a google search would probably find a lot of new stuff

Oops, forgot the picture. You can see a vortilon on the LE and two fins on the TE. Alan Shaw is calling these TE fins "feathers". He says that these trailing edge devices improve the effectiveness of the elevons. Both surfaces block the spanwise flow of the stagnation line and deflect it aft. In the case of the vortilon this deflected material rolls up into a vortex and has a similar effect to a fence. To get the lowest drag you need to know where the stagnation point is at CLmax and place the vortilon so that it doesn't block it until you're at an AoA where separation is starting to become a problem.

Originally posted by h_zwakenberg

As to X-Plane: checking out this product with plane models that I have flown for real, I'd say that it's pretty darn good as long as you stay within the linear aerodynamics limits. Stall modelling stinks. Other than that, I feel it can be used for conceptual/preliminary design.

Dose X-plane model unsteady flow at all? My impression has always been that it's an adequate flight simulator for pilots but not an engineering tool. The developer used to let people think that it was really modeling the complex physics of flight and could be used to predict the flight characteristics of a new design but he's gotten better about not exaggerating its capabilities so much. I haven't used plane-maker so I'm not sure but don't you have to supply the lift characteristics? Be very suspicious of lift predictions for deltas above 7 or 8 degrees AoA.

X-plane has been discussed herein numerous times. Basically its capabilities should not be considered any more accurate or usable for design than a good video game. It might be OK for trying out ideas or just playing around with concepts, but as you transition from playing around to actual design, then you need to go elsewhere. One of the biggest problems of the package is that there is no numerical or otherwise quantatative output. Furthermore, since it is designed for virtually anyone to use, its inherent "simplicity" forces it to be less than useful or accurate beyond very limited application.

Originally posted by orion
X-plane has been discussed herein numerous times. Basically its capabilities should not be considered any more accurate or usable for design than a good video game. It might be OK for trying out ideas or just playing around with concepts, but as you transition from playing around to actual design, then you need to go elsewhere. One of the biggest problems of the package is that there is no numerical or otherwise quantatative output. Furthermore, since it is designed for virtually anyone to use, its inherent "simplicity" forces it to be less than useful or accurate beyond very limited application.

if you remain within the linear aerodynamics part of the envelope, X-Plane is quite accurate. Outside this range it's not that good and other tools should be used instead.

As to tools, I'm looking to put effort into CFD. What kind of (preferably free) tools are you guys using for CFD analysis? I saw a number of CFD tools on SourceForge. What do you use?

Personally I've played with CFD a little but generally don't depend on the tool(s) for the design process. Even some of the better codes have sufficient limitations to make the results less than useful. Only when particular procedures are in place that assure that the modeling environment and initial assumptions are accurate, can the results be representative of the eventual product. This either takes a lot of experience with a particualr product or, a combination of that and a very pricey system.

Several years back I was in a meeting with a design engineer who specalizes in CFD for United Technologies (Pratt and Whitney). When the subject of CFD for up front design came up, he smirked and indicated that as far as usefuleness is concerned, in his industry the rather pretty pictures are commonly refered to as "clown puke". In other words, they are pretty, make for good presentations, impress the non-technical crowd but in reality, unless there is a verifiable baseline in place, the results shouldn't be depended upon for a high level of accuracy.

So, to answer your question, I do use CFD from time to time but we actually subcontract the work out to an organization (AMI - Analytical Methods, Inc.) that specializes in this analysis and has the procedures in place to do it right. The two primary tools we use are VLAERO and VSAERO. The first is for derivation of the neutral points and control authority while the second is for performance prediction and to assure ourselves that we're not running into any flow problems (interfearance, flow separation, etc.).

All up front work however is purely based on experience and our own in-house software. The CFD is used only for verification and applications where the upper end of the performance envelope is significantly over 300 mph. Below that, the extra tiny bit of information you gain is usually not worth the expense.

Drifting ever more away from the topic...

Hans, you might find these interesting, if you haven't already seen it.

http://www.twitt.org/BLUMENTHAL.htm#JERRYDREAMS

Originally posted by Topaz
Drifting ever more away from the topic...

Hans, you might find these interesting, if you haven't already seen it.

http://www.twitt.org/BLUMENTHAL.htm#JERRYDREAMS

Thanks Topaz, although I knew this page already. I find Jerry's Raspberry especially appealing...

The Rasberry is an interesting concept and from the bit of curiosity analysis I did several years back, it actually stood a good chance of success. Contrary to what the short blurb next to it indicates, the landing speed might have been crisp, but not excessively so. The highly straked body actually creates a significant amount of lift and as such, the actual planform loading is quite low. Furthermore, the wing stubs act sort of like winglets in that they provide the airplane with behavior that seems to be more efficient than you'd have with such a low aspect ratio planform. Coupled with the potential for making use of vortex lift, this could've been quite fun and not a bad handling airframe.

The comment regarding the engine placement however does have merit. Given the short coupled nature of this small design, the high placement of the prop would most likely have had some interesting pitch trim characteristics as a function of throttle. Ideally the prop should be much lower, which would then of course affect things like prop selection and of course landing gear design.

The issue I was most concerned with however was the pitch chracteristics as a function of aoa. The behavior of the body strake would have to be very carefully tailored for predictable vortex formation. Most likely it would need a fairly generous LE radius in order for the flow to stay attached to fairly high angles of attack since an early separation would form the vortex and thus shift the Cp a bit too dramatically for linear control. This aspect would require significant testing and optimization.

Looks like fun though.

http://adg.stanford.edu/aa241/highlift/sstclmax.html

Vortex Lift

Think the Rasberry could still be made?

The document you reference deals mostly with wings with sharp leading edges. It would not be as applicable to how the Rasberry would need to be designed, except in the most general sense.

With a bit of work, yes, I think the Rasberry could be a viable airplane or kit but as with all flying wings, it would have some limitations and would probably require a somewhat more experienced pilot. Getting the configuration to be not only stable but predictable and linear in its behavior would be quite a bit of work I would guess.

But given time and money, there have been many wierder things in the air, so yes, eventually the Rasberry could most probably be configured into a pretty slick little airplane.

Oh... well further down the page it appeard to me they were talking about vortex lift and things... with all those triangle drawings and stuff....

I thought these were shaped similarly to the nose of the Rasberry.