MCSEVZ

Lately I have seen quite a few locals with flying wings and I have been quite impressed at the variance in flight quality, some I have seen are super fast, others not so much, some fly quite stable, others seem like a miscalculated blink could sent the plane plummeting to the ground.

I own (have owned) two planes and feel the need for speed, but also want to keep in tact with slower flight too! So here are my ideas!

I want to build a flying wing this summer, but I have some basic questions about aerodynamic behavior, construction techniques, possibly new ideas for flying wings, and the less known woes of flying wings.

Ultimately I want to make a wing that meets the following criteria:
1. Capable of flying at 10mph-100+mph (Focusing on slower speeds)
2. Capable of gliding for a while
3. All electric powered
4. Easily seen at longer distances
5. Moderately stable flight, but capable of all scale flight maneuvers (maybe even some not-scale maneuvers such as flatspins)
5. Budget stays under $500 USD (TX/RX not included in price)[X(]

Here are some of my current ideas for the design:
1. 48"-72" wingspan with a shorter chord length will increase gliding properties and allow for slower flights
2. Have a thicker center chord for more stable faster flight and more structural strength plus room for electronics
3. Have streamers connected to wingtips for better visual aid

Here are some of my questions:
1. Is it best to have a standard or pusher prop setup?
2. If pusher is better, would at rear of plane or in middle (with center of wing cutout) be optimal for flight characteristics?
3. What is the best overall setup for control surfaces (split elevons, standard elevons, etc...)[sm=confused_smile.gif]
4. Any well known airfoils to stick with or avoid?
5. Any thing that should definitely be avoided?


Any comments, suggestions, concerns, knowledge, hints, tips, crazy ideas, and cruel analysing of my thoughts are ALL WELCOME!

Ram-bro

My Feedback: (101)

I happened to do a flying wing and it flew beautifully and it all happened by chance. I crashed my Great Planes F14 but the wing cameout unscathed. We got bold and daring and strapped a os 60 on the front and it flew great right outta my hand....so if you wanna take a chance, there you go.

CoosBayLumber

My Feedback: (1)

Back in about early 1970's editions of RCM was an article by Andy Lennon about how to design flying wings. He had several illustrations, but main thing was CG location. In formulae, he told how to make faster and more stable. Main thing was that it was going to take more than one prototype to accomplish with gear you wanted to install and use.

(This is back in the era of when RCM had real thick editions)

Wm.

TFF

The Simitar designs have been a good series. Gliders to pattern planes and in between with pretty much the same airfoil.

BMatthews

You're obviously new to this design stuff as there's a few requirements in your list that are totally at odds with each other.

10 to 100 mph plus soar on occasion? Pretty much not going to happen. At best two designs would be needed to cover that much speed range and flying style. More likely 3.

Your judgement of flying speed of the models you've already flown is likely way out. This is pretty typical of all modellers so it's not a slight against you at all. A more typical slow flight and landing speed for even lighter sailplanes is in the 15 to 20 mph range. Maybe a little less with some super light models. To see what I mean have someone cruise by in a car at 10 mph on the speedo and you'll see that it's almost painfully slow. Have them do the same thing going up in 5 mph increments and focus on one wheel from a typical slow flyby distance to get a proper size impression and you'll soon see what I mean. Similarly a fairly small to medium size model flown fairly close in may LOOK like it's doing 100 but in reality it's far more likely to be in the 50 to 60 mph range. I've seen some true 100 to 120 mph models and they go past like a bolt of lightning but there's just no way for them to slow down to a sub 20 mph landing or soaring speed. World class F3B and similar soaring gliders being the one exception to this general rule. When ballasted up for the speed runs they can typically hit speeds around 100 mph but when in unballasted soaring trim can fly in the mid teens to low 20's. But now you're talking SERIOUS design and building efforts.

So a more realistic single design speed range for something capable of soaring would be more like a 15 mph stall speed up to maybe 50 to 60 mph top speed. Especially if you want the wing to be big and light enough to soar.

The longer span you want is a good size to see out at a decently long range for soaring or even general flying. If soaring is high on the list I would not even consider the 48 inch size. Stick with the 72 inch to 2 meter (84 inch) range. Don't go too narrow on the wing chord. The reduction in reynolds numbers will make it LESS efficient overall when it comes to the speed range. Also in keeping with using a thicker airfoil in the root area to house the gear a higher reynolds number avoids possible issues with the airflow separating early. The reynolds number is related to the chord width and airspeed. You either need to keep the chord wide or the airspeed up to avoid the really small numbers where the airflow predictions break down badly. Also with narrow chords comes thin wings. Something you want to avoid to ensure both wing strength and room for the radio gear. On a 2 meter size span a center area chord of around 10 inches would not be a bad size. This much chord would allow you to use a 12'ish% thick airfoil for a max thickness of 1.2 inches. Enough to hold any of today's modern radios. Even at the tip sections I would not go under a 5 inch chord to keep it at least sort of in range with the center section reynolds numbers.

Tip streamers? Nope, WAY WAY too much drag. You may as well hang a drag chute off the model. Flying wings are definetly harder to see at distance but you'll just need to train yourself. At first do not fly out as far as you would with a regular planform. Make sure you keep in close enough to maintain an accurate perspective of the wing. As you get more comfortable with the visual clues it presents and more familiar with the appearance in each direction you'll find yourself automatically flying it out farther. But at first keep it in close to train yourself. Some serious colors on the wing tips and bottom vs top of the wing will help a huge amount as well. Light on top with strongly contrasting and different tip colors and a dark color on the bottom with one tip very light for a sizable patch to help see the contrast even at long distance.

On a swept wing design where slow to medium flight and soaring is intended I'd go for a rear motor and pusher prop. It'll help a lot with the correct CG location. For a plank style wing it'll need a forward prop and a stub nose section. If you change your mind about the soaring part and want to go for a fast model then a forward motor is best. The small props and high RPM required for fast flying favour having the prop in clean air. And there's no way to get clean air for a pusher prop.

Here's a teaser doodle sketch I did up for a past thread on making an ultra fast model. It's a flying wing for a few reasons. First is that you loose the drag from the fuselage. Second the required washout in the tips helps reduce the strength of the tip vortices. It uses the center fin instead of tip fins as someone rightly pointed out that with any swept wing there is a strong spanwise airflow. So tip fins need to be strongly toed inwards to allow the air to see their minimum angle of attack to the airflow. But since that aspect is strongly linked to speed I went with some shark tips and a center fin to avoid the drag that would be unavoidable. This particular one is powered by an engine so that's why the odd looking side cowl. An electric motor would obviously get away from that. The prop is set forward a little to avoid the propwash beating on the wing's leading edge and generating a siren like howl and the drag that comes with that.

Hopefully I've inspired you rather than discouraged.

MCSEVZ

Thanks for the long post...

So overall, you might suggest a 84" span with 10" center chord going down to 6 or 7 at wingtips...a pusher prop, and only a small angle for the swept wing (10*-25*)?
Colors should be contrasted between Left & Right on top, and also on bottom (eg... Neon green/black on top and orange/blue on bottom)

Would it be necessary to have vertical wingtips?

Make sure I have one thing straight....the higher the Renolds #, the better correct? (I'll have to do some reading on that too)

Also, I'm thinking 1.2" might not be thick enough for my 3S 1300mah 11.1v batteries... would it be better to get thinner batteries or thicken the wing to 1.5"-1.75"???

batchelc

My Feedback: (7)

This is a great hobby. You will have much more sucess and less failures (you'll still have them) if you do some reading before you get started.

Here is a full scale book on aerodynamics:

http://books.google.com/books?id=C8Z...esult&resnum=3

My copy is tattered and has lots of margin notes. Really understand this stuff.

Once you've got your head around that stuff, tackel the differences in the scaling down to R/C Airplane sizes. There are several books on that depending on what your intersted in.

On yea, and everone is a professional engineer on these forms right? Don't believe everything that you read. Fear the posts that start with "I think" or "This guy at my field" when it comes to airplane design.

Of course then you get from the aero into the materials and construction and that too is a whole nother ball of wax.

BMatthews

Given that the prop will be a pusher in the center then I think you're stuck with tip fins. What I'd do to try to at least guess about the spanwise flow to the tips is set them up with a toe in angle of around 2 to 3 degrees and use a good airfoil with a known lower drag over a fairly wide angle of attack like the Selig 8020. It is an airfoil that was intended for all flying stabilizers originally and has low drag over quite an angle of attack range.

What if you sit the pack in spanwise? The other thought is to better fit the gear, batteries and motor is to set the wing up with a strong flair into the center "stinger" that has the prop on the end. Something like the sketch below. Yeah, it's an old one too. I've been a fan of flying wing concepts for years.... In this case the strong widening in the middle increases the chord quite a bit so even a 12% airfoil will have a lot of thickness. The original sketch is based on a 2 meter wing. The tip chord is only 5.5 inches in this sketch but the root about 3 inches from the centerline motor stinger is just over 17 inches with the nose to spinner tip distance being 21 inches. With a 12% airfoil that 17 inches gives you a hair over 2 inches of thickness at the deepest point. That enough for ya?

Getting the smooth curves of this shape would be murder in balsa. I planned on a straight taper wing from root to tip and then add on some outer surface secondary ribs as well as the trailing edge extension for the swoopy bits to act as support for a thin fiberglass molded center section cover. That cover would extend out about 8 inches to each side and feather into the straight portion of the wing to get the smooth manta ray like center section swollen shape. This would have been done right on the model itself using styrofoam blocks set between the swelling ribs and sanded to shape using the ribs as guides. The molding would then be done in place using sandwich stretch wrap as a mold release membrane. The lower cap would have been built in place and the upper would have been removable for access to all the internal organs. The foam blocks obviously would be removed at least on the upper surface to gain acces back to the insides. On the lower surface I'd likely leave the foam in place and glass directly onto the foam using epoxy laminating resin (not epoxy glue) instead of polyester resin so it didn't melt the foam. The extra support would have been welcome during landings. All this stuff sounds pretty heavy but there's a HUGE amount of wing area on this design.

EDIT- If you're doing a swept wing like this you MUST get a copy of the Excell spreadsheet for the Panknin wing twist process. It's an excellent tool that allows you to determine amount of twist to be built into the wing without doing too little or too much and ending up with the elevons deflected all the time. Doing that just adds drag and it's obviously best if it can be avoided.