Schira-DOW 3rd counter rotating propellers
 

My project is on the fantasy aircraft Shira-DOW 3rd, I'm determined to model this around what I'm expecting to be a 9x5ish pair or propellers. Obviously no such propeller exists in the shape this fantasy craft needs for a completely scale look.

As a new member of this forum I can't post pictures so please excuse the lack of images. Googling "Shira-DOW 3rd" should bring you to some images of the aircraft I am speaking of. Again I apologize for any inconvenience.

Setting aside the impracticality of it, what guidance might you provide and what issues do you see that I will encounter?

I see the propeller being difficult to construct in a way that will withstand the incredible centrifugal force at 9500 rpm, especially because of its huge rearward sweep.

Second to that, I see the second propeller possibly requiring a different pitch and/or different rpm than the leading propeller.

My thoughts are; carbon fiber roving and epoxy, undercambered airfoil for the blades for structural strength and weight reduction.

Hopefully there is some input on this project.

I've purchased the plastic model (waiting on delivery) to guide me in creating the aircraft itself.

Pictures maybe...
 

https://cimg6.ibsrv.net/gimg/www.rcu...3321c834fc.jpg

Shira-DOW 3rd propeller
A sketch to show the crazy propeller design.

I'm fairly sure there are a million things wrong with this propeller as a whole. Because it's a scale model of a fantasy aircraft I am limiting myself to keeping the profiles faithful to the original.

I think I have a bit of freedom to mess with the airfoil of the propeller blades though. In the sketch I picked an under-camber airfoil with maximum thickness at around 25% chord (no other reason other than I feel that might help with keeping propeller weight down and strength up).

Maurice

Having spent much time on fantasy scale canard designs I can tell you it may become very frustrating if you actually hope to build and FLY a model of this craft. You have not indicated a scale or size that you hope to work to, that would probably be an appropriate starting point. After that, determining the general construction approach would be important, if you haven't already done this. Selection of a power source (either electric or glow) would also be fundamental.
You have correctly identified the significant issue of making such an unusual propeller and having it withstand the dynamic forces involved. Frankly it may not be possible without many difficult stages of complicated engineering, testing and evaluation. Would you consider having a scale (static) propeller that has the look you want but a flying propeller (like an APC) for the initial flights at least? Believe me, many scale modelers go this route.

Dimensions are very rough at the moment; still waiting on the plastic model to get here for me to take measurements from.

What I've worked up to is 800in^2 total wing area and a flying weight not to exceed 175oz. (I'll draft out a more accurate wing area when the model gets here)

As for construction, I'm leaning toward fiberglass 0.75oz.yd^2 to form the exterior with balsa formers and some stringers. I'll be doing the tried and true carve a plug, make a fiberglass mold process. I've made a few things like this (car parts mostly) but never an RC airplane.

How many layers of glass am I looking at needing? Yeah i know, that's a loaded question, and i know that. Let's just say I'm guessing this is going to end up being a fairly heavy model 10+lbs, hows the glassing go on those (layer wise)?

e-calc propCalc has given me a pretty good idea on what the power system is going to weigh, so I've got to go about looking at control surface areas to pick out servos.

If I have any weight room left I'll know how beefy or feather light I need to make the airframe.

At least that's how I'm looking at this right now.

I will absolutely swing an off the shelf 4 blade prop of whatever pitch and diameter will easily keep the plane well above stall speed and leave me room to power out of a situation if need be.

Maurice Pudlo

Edit: not so sure exactly how I came up with 10lbs for a total flying weight, as the design process moves along I'm thinking I can bring that weight down considerably. My main issue is the weight of the electronics I'd like to stuff into this airframe.

There are so many control surfaces that I want on their own servos.

Canard flaps x 2
Ailerons x 2
Rudders x 2
Split-flaps/Dive brakes/Spoilers x 2 (4 servos)
landing gear x 4

I'd like to install a 3 axis gyro, however I know next to nothing about this bit of electronics. (I'll ask about this in more detail in the appropriate section of the forum)

I'm considering using an E230 airfoil for the main wing, Im not so sure about the canard airfoil yet though; it's a fairly large flying surface and will no doubt interfere with flow over the main wing.

Maurice Pudlo

For flying I STRONGLY suggest skipping the swept blade props. By all means make some "scale props" for static display. But stick to commercial props for actually flying. As you say you don't have enough data to fully engineer a properly safe prop. And you can't just toss carbon fiber at this and trust that it'll hold. Carbon fails too when the forces get powerful enough.

800 square inches of area is a lot of wing. And a model that size at 10lbs is not out of the question at all. But you WILL need a lot of power for a model of that size. Power that will use bigger props than 9 inch. For performance that matches the design role of this sort of fake scale design you'll want around 100W/lb of model. So 1000w of motor power.

Will you be running actual contra-rotating props or fake it by using two props on one shaft both turning the same way? If the first then both can have the same pitch or the second can be one pitch amount less because of how it meets the air off the first prop. If you fake it and go with two props on the one shaft then the rear prop should be the same or slightly greater pitch. And you might well find that this combo works best when it's not quite at 90° so the front prop acts as a prop and the rear acts sort of like a booster flap for the second one. But if in doubt just put them at 90 and run the same pitch for both.

.75oz cloth is the super light stuff that is used over balsa as a prep finish for painting. It is not very strong. If you're going to use epoxy and fiberglass over foam you will want to use cloth layups similar to that used on vacuum bagged large glider wings. And I believe that this is more often 2oz cloth with second patches at higher stress points.

Making a plug and then a proper female mold for such a plane would be a major amount of work if you are not planning on making and selling at least a few copies. If you're after a one off a far easier way is to make up a skeleton of vertical and horizontal keels and formers and then insert blocks of Styrofoam in the openings, carve and sand it all to the right shape and then glass over the foam and framing with epoxy and glass. Also this way you can add in hard points for landing gear and wing mounts. The foam in this case is roughly hollowed out so you leave a generous amount of foam but at the same time leave yourself lots of room for the radio and battery and stuff. For a one off this is a lot faster and easier. And in the end if you do decide on making a female mold to make more then you can use the prototype as your plug for making the mold.

It's certainly a funky looking design. Good luck with it if you go ahead and I hope my suggestions help out.

Thanks for your input, I was hoping someone had built some exceptionally funky props and might have some fairy dust to sprinkle over my hopes... unless I find a perfect fit product to actually get contra-rotating props to fit within this design I'll just do the static display thing and run a single prop.

I'm not looking to go into production, It will most likely end up being on static display more often than anything else. Having molds to do it all over again though will be nice if the thing refuses to fly well.

The whole scale thing is a real trick, different drawings show different details for the same parts...fowler flaps, split flaps, perforated lower flap with sectioned upper flap, leading edge slats, flaperons, and on and on.

I'm considering making a smaller 3ish foot span version to get a bunch of things ironed out. I'm looking at no less than 10 control surfaces with a stupid quantity of mixing involved, plus I'd like to toss a gyro in the mix simply because it's a funky design.

As for the construction using 0.75oz s-glass I've tossed around a few ideas in my head but this one seems to stand out the most.

2 x layers of the 0.75oz glass on each side of 1/32" balsa plank/sheet, all set up in male/female molds. That's around 5.7 oz./yd.^2. At this point for the smaller version this sounds reasonable if not a bit heavy.

I think the fuselage will need an additional stage to assemble the balsa core (boat planking) as there will not be any time once epoxy is mixed and the outer layers of glass are wetted out.

The wings shouldn't be all that hard, the glass/balsa/glass skins over a carbon fiber laminated spar.

Maurice Pudlo

https://cimg3.ibsrv.net/gimg/www.rcu...88466f2919.jpg
Working on getting a better idea of wing loading...this is based on doing a 35ish inch wingspan version.

Maurice Pudlo

For smaller size like this to maintain good lower speed manners you need to keep the model quite light. Add to this that the lifting surfaces of a canard cannot be pushed to as high a coefficient of lift so you cannot slow a canard down as much as you can a conventional setup. All this means that for a 35 inch span you need to keep the model fairly light. It'll fly with the best performance if you can keep the weight to 20 oz or less. But up to around 24 would be OK but the slow speed manners will not be as nice. Start crowding up to 26 or more though and you'll find that the stall speed is high and the model needs to be kept moving fast ALL the time. And at that point it may start trying to snap out of things like steep fast turns or fast loops if you pull too tight

So my suggestion is to not try to get too fancy with the scale detailing on a model of this size. And pick construction methods which you know will result in a lighter structure. Keep things light in the structure and that could allow you a bigger allowance for weight in a nice paint job where you can add shadow lines and panels and such with an airbrush to give the model a really nice "full size" look. Paint like this is fairly heavy though so keep an eye on the structure weights as you go. You'll want to start with a target of 20 oz ( 560 gms). Subtract the weights for the radio package, motor and flight battery. Then give yourself a 2 oz (60gm) allowance for covering and paint just to be sure. Then split up the weights from there into your components in proportions you hope to achieve. Then hopefully picking a construction method that is suitable start building and watch the target weight vs real weight as you build. If your first component or two is way over the target then clearly you are not going to make the target weight overall and need to shift to a lighter manner of building.

If you are wondering the reason why canards cannot be pushed to wing angles as high as a conventional model is due to the fact that there really is only one sort of wing layout. At least as far as the air matters. On a canard the front wing is still the "main wing" from an aerodynamic standpoint. So it can only rise up to an angle where it stalls and then it produces a lot of drag and the aircraft slows and falls away. Meanwhile the large rear stabilizer is restricted by the design's trim angles to fly at a slightly lower angle. And that means that while it is the major lift producer it still never gets to produce as much lift as it might simply due to being restricted on the angle of attack it can achieve due to the limit set by the canard surface. And that would be why we don't see canards used in styles of aircraft where such things matter.

Hope that helps out with the design exercise.