Nodd

longsoar

Hey!
Nice project! Here is something to consider (I would thnk):
Design will be quite edgy on CG - you'd have to get it just right.
Elevator control will be quite edgy too.
As far as mixing-in brakes it will be quite a challenge.
Thrust angle (aligning the motor) has to be just right too.
I'd move fins a bit more backwards to give it a bit more directional stability since your fus has a lot of side footage.
Looking forward for some flight trials to see if I am right or wrong.
cheers,
Mark

Nodd

Thanks for the feedback Mark. Yeah I've played around with flying wings before & yeap they can be interesting to setup. I think that's why most these days are heavily swept back to improve pitch stability. I don't really want to fly a Zaggi though. I've seen plenty of planks like this, on the forums & YouTube, happily zipping around.

I must admit I'd planned to set the vertical thrust angle to 0° as this is a symmetric design. I'll do some poking around online & see what other folks are using.

As suggested I may play with the vertical fin design some more although I understand one of the odd quirks with the KFm-4 airfoil is that it can apparently be flown with no vertical stabs at all. Somehow its natively yaw stable *shrug*. Not sure I believe that but I've heard it mentioned a few times. This bird will have fins though.

Over on one of my mirror threads its been suggested that my carbon-fiber spar could be improved & have to agree. As soon as I ordered it I realized I should probably have gone with a vertically oriented spar & not horizontal. So I ordered some different spars...



One spar instead of two maybe all I need, I'll make that call once the wing is together & I can gauge its rigidity.

A few folks, not surprisingly, are wondering why I'm not just using full span elevons. The reason for that is I maybe flying this in confined areas with small landing areas. I'll need air-brakes to slow her down. If you attempt to use elevons as flaps you end up pile-driving into the ground. With a four servo wing however you can use the outboard ailerons to counter the downward pitching from the inboard "flaps". Basically mix in crow braking...



I expect it'll take some fine tuning to get the deflection angles right but from what I've seen, this is the way to stop a plank.

Okay well this is sounding good. I hope to begin construction soon. Stay tuned.

PatternPilot

My Feedback: (58)

Another good choice for a fiberglass fuse....

Scott Anderson

longsoar

Well, from what I have seen those wings that do not use fins usually have wingtip angled leading edge to counter sliding. Others (B-2) use aileron brakes to have directional control.
Something has to be done. Some ideas work better, the others' are not so good. Aileron brakes introduce an extra drug. I'd go for fins on RC model since we're not after stealth technology and fins work just as good on slow and high speeds.
Birds use their tail by twisting it as well as wingtip feathers as a fins but they have software far more advanced than we have to control the airframe.
I was about to propose the swept wing, but I like your design in general.
As far as spar goes - what's the expected weight of the plane? My guess is about 800 grams ~(27 once) all up? You can use one 90 degree molded carbon fiber spar for that. That way it would be very light and strong. Foam will hold it against the twist load. Or you can buy a square box tubing, cut two identical halfs and then do the exact fitting on each of them, or just use one if it's long enough.
Infinite solutions as far as design goes.
just a few outloud thoughts.
cheers,
Mark

Nodd

Yeah don't get me wrong, I'm keeping the fins. I was just making the point that there's something about the KF airfoil that supposedly makes them yaw stable all on their own. So I figure my current fins might be good to go if that's the case. I may still play around with them, possibly enlarge the fins on the ducted-fan too. I could model this in X-Plane & see for sure I guess. My feeling is the design is good to go but a couple of people have commented on the fins now so yeah maybe I'll look at that some more.

Weight is an interesting subject. I'm actually leaning towards ballasting the heck out of this. I'm not interested in a light-air floater here. I want something that'll rip when the winds up, something that'll have a good amount of stored inertia to carry her through some big maneuvers. If you've flown the Plank fully ballasted in PicaSim, that's what I'm looking for here. So yeah I'm liking the new double CF spars, foam-board/cloroplast laminate design. I think at only 40" long that'll produce a bullet-proof little wing for sure. I like some of your ideas too, as you said, infinite solutions.

Yeap Scott, the fuselage will be glassed. I'm planning to carve a foam plug of the fuselage then fiber-glass/carbon-fiber/kevlar that, cut out a canopy/battery hatch & then remove the foam. That'll leave me with a nice strong fuselage for those not so perfect landing areas at the slope. I've been meaning to give that technique a try so this is a good opportunity to play with that.

Hope to get the build started soon, fun fun.

longsoar

Yaw stable airfoil, as I understand it will not recover from yaw but rather will be somewhat immune to it.
It's just airfoil will still produce enough lift for the aircraft to be still controllable. Controllability also depend on many factors besides airfoil alone: size and placement of control surfaces, CG, side geometry of the aircraft and much much more..
When you fly Blanik - it'll let you yaw the hell out of it but when you fly high performance glass ship - yaw can get you in deep trouble in a hurry!
"Dropping" controls will recover Blanik if there is enough altitude and not all glass ships will recover with ease.

As always - speed is the king and if it's not there yaw will get you in trouble earlier.
Once I was told that 2-33 glider will not stall at all - when I showed flight instructor at LISA how easily it is stalled on any speed - she was beyond the surprise after flying for 50 years. The answer is: rudder/elevator bad combination. When there is no rudder - elevator along can do the trick.

Flying wing has one inherited feature that is hard to get rid of - unrecoverable stall. How bad it is - depends on a design. You can control/minimize it in classic design by using leading edge slots, bigger stab whatever but on flying wing there are limitations introduced by design.
Looking forward to see how this project unfolds!
cheers,
Mark

iron eagel

Nodd,
Your change of the flat horizontal single spar to two is a major improvement.
Your idea for zero thrust angle and symmetrical wing works quite well for a wing.
You may want to shift the intake opening for the DF a bit more forward on the wing itself if your looking for really fast.
You may want to take a look at this little wing planeform, it has a very gentle stall, and might give you some ideas.

Nodd

That thing is gorgeous. I could take the swept wing approach, they're definitely more stable & easier to setup but I like a challenge. I do like the blended fuselage. I was thinking maybe I'd blend my EDF's housing with the wing some. We'll have to see about that.

Question... why do you feel moving the duct intake forward will improve performance?


Design in progress
Based upon feedback here on the forums I've made a few tweaks to the design. As suggested, I moved the duct intake forward a tad...



I also increased the vertical fin area some...



So here's the latest & greatest...



Thanks again for the input.

iron eagel

Thanks!

If your looking to soar power off, the fan enclose will generate a lot of drag on the order of badminton birdie (I would think). First thing that springs to mind is Rutan's dethremalizing re-entry system for his Space Ship One. I would think you need to have your intake for the DF enclosed and puling the air in through a NACA style intake that is in the top of the wing, so with power off it isn't creating a lot of drag like it would having the df unit hanging out in the back. With power on a area of low pressure occurs on top of the wing. Is that a bad thing? I also wonder about any blanketing or interactions the control surfaces my cause relative to the df unit intake.

As an aside... Have you noticed that in many of today's experimental designs you see a lot of the Star Trek type of pods for the fans on them?

One interesting concept you may want to look at...
With a DF you have the option of incorporating vectored thrust. I added a rudder to the exit duct of a Miglet DF flying wing, you could get that thing to fly knife edge. Add elevon function as well and it gets "entertaining".

Yet another question. Those 3 fins are going to generate a lot of interference drag with the duct tube won't they? All of those junctions with the fuselage/duct, do you need that side area?

wyowindworks

...

BMatthews

I suspect that the badminton birdie like drag will come from the non running fan regardless of the duct work being short or longer. I do agree that fewer fins would create fewer corners and that means less drag. But then I suspect that the steps on the KF airfoil are going to generate a pretty good portion of drag anyway. After all, for all the years that the KF airfoil has been in existance the only folks that use it are model builders. That should suggest if the hoopla is all snake oil or not.

It would be interesting to build two of these models. One with the KF4 airfoil as you are planning on already and another with an easy build symetrical airfoil that uses the rear 1/3 of the airfoil having flat lines to allow building on a flat bench. Then fly the two versions and see which is faster and holds its energy longer.

iron eagel

Bruce,
I have to agree with you about the fan generating drag when it is not running, despite the intake position. It was my hope while not eliminating the drag entirely by using an intake at 90 degrees to the airflow that it would reduce it significantly as opposed to having it directly in the airflow, and hopefully even power off it may contribute to lower air pressure on top of the wing generating some lift as a tradeoff.
As far as the airfoil I have to agree with you as far as any real benefits from it, other than being very easy and quick to build, are minimal. I have to agree that a symmetrical traditional type of airfoil would be the better choice. Perhaps he could model both to test in X-plane and see if there are any differences in the performance of one as opposed to the other.

Nodd

More great feedback, some really good points made. Thanks once again folks.

Less fin
It's possible I got a little fin-happy there. Here's a simplified fin setup that I think should offer plenty of yaw dampening, while minimizing interference drag...



Now you see it, now you don't
I've been pondering how to make the EDF go away when she's gliding. If this were a regular pusher-prop I could just use a folder to streamline the airflow. As far as I know, no one has invented a folding ducted-fan, so here's what I came up with instead...



A cylindrical sleeve covers the intake ducts until power is needed. That then slides back exposing the intake when it's go time...



I have a few ideas how to do this, will probably use a pull/return-spring system hooked up to a long throw servo. Slots in the fins & wing will act as guide rails.

So that takes care of the intake duct's drag. Now you're going to point out that I still have a blunt end where to duct exits. I'm okay with that I think. Especially as many cars are deliberately designed with blunt rear-ends supposedly to improve fuel economy somehow...



Yeah I'm not sure I'm buying that either but it is in keeping with the KF airfoil's tech... eddies creating aerodynamic shapes for the air to pass over. To be honest I'm not too worried about the exit duct, it is what it is. Anyway so that's the latest design...



Still looks cool & that's what's important right? *wink*

iron eagel

It almost has a bit of Buck Rogers look to it. lol
The intake cover is a very interesting idea it will give you a clean airflow over the skin but I am not sure about the blunt end thing, but it has got to be better than a fan sitting dead in the airstream.
You might want to extend your slots into the cover and decease the slot in the fin itself for a bit more rigidity.
It does look pretty cool!
I really like the shape of the wing it may be a lot more pitch stable than your typical plank.