Waterman Arrowbile

[André Wind]

I've started this project a few years ago and it is now time to finish it.
It's a tricky model to build and I've build a few but never succeeded really.
This one is, until know, a light model and I'm trying to keep it around 80 grams.
The wheel pants and wheels are printed without filler and just one layer.
Wingspan 75cm ( 30 inches).

[André Wind]

To build a perfect flying model it's necessary to use a perfect airfoil.
I don't any thing about it 🥴
Can someone me an advice for this tailless Aero car?
Ore maby a suggestion for one of those 3 airfoils?
PW 106
S 5020
MH 45
Ore a HQ profile

[SP250]

Well tailless aircraft are complicated.  You have about 22* to 24* of sweepback and a significant amount of up elevon to stabilise the pitch. A plank flying wing uses reflex to act as the tailplane, but swept wings use twist and little or no reflex in the aerofoil.  I suspect if there is no twist (washout) on the plan towards the tips then that is why there is up elevon to compensate.  My thoughts are to go with a little twist (about 4*) with a none reflex section and that will reduce the up elevon needed for pitch stability but also will lower the max lift available.  You could of course go with a non-reflex root and a reflex tip to help a bit more, but it's your call.  I'd maybe do a couple of quickbuild chuck glider test models of alternative aerofoils and see what works best first before committing.  I've been studying tailless for 30+ years and corresponded with Hans Jurgen Unverferth, Peter Wick and Mark Drela for insights and have a bookshelf full of full size and model tech stuff and I'm still not sure!  Good luck.

John M

[dputt7]

  I had a thought that Doug Mc Hard built one so I looked on OZ, but it wasn't there but there are 3 plans 2 of which are rubber models, so maybe worth a look.
  https://outerzone.co.uk/search/results.asp?keyword=Waterman+Arrowbile
                                                 Dave

[André Wind]

I had a thought that Doug Mc Hard built one so I looked on OZ, but it wasn't there but there are 3 plans 2 of which are rubber models, so maybe worth a look.
  https://outerzone.co.uk/search/results.asp?keyword=Waterman+Arrowbile
                                                Dave

Hello Dave.

I'm building this model from the outerzone plan( 30 inches) but it's a really old plan and maybe someone can tell me what typ of modern airfoil is the best for this flying car.

[André Wind]

Well tailless aircraft are complicated.  You have about 22* to 24* of sweepback and a significant amount of up elevon to stabilise the pitch. A plank flying wing uses reflex to act as the tailplane, but swept wings use twist and little or no reflex in the aerofoil.  I suspect if there is no twist (washout) on the plan towards the tips then that is why there is up elevon to compensate.  My thoughts are to go with a little twist (about 4*) with a none reflex section and that will reduce the up elevon needed for pitch stability but also will lower the max lift available.  You could of course go with a non-reflex root and a reflex tip to help a bit more, but it's your call.  I'd maybe do a couple of quickbuild chuck glider test models of alternative aerofoils and see what works best first before committing.  I've been studying tailless for 30+ years and corresponded with Hans Jurgen Unverferth, Peter Wick and Mark Drela for insights and have a bookshelf full of full size and model tech stuff and I'm still not sure!  Good luck.

John M

Thanks for your answer John, but I know nothing about airfoils.
Regularly I'm following the airfoils from the plans, but this is a very old plan and there must be better modern airfoils for this model.
Kind regards,
André

[OZPAF]

Andre - John's(SP 250) advice re wash out is far more important than the exact airfoil for models of this size.

However - the PW106 appears slightly better than the S5020 but how accurately can you build a tissue covered wing with such a small amount of reflex?

I would suggest that you use the tip washout suggested by John and the PW106 and adjust the elevon's to suit.

Good luck.

John

[André Wind]

Andre - John's(SP 250) advice re wash out is far more important than the exact airfoil for models of this size.

However - the PW106 appears slightly better than the S5020 but how accurately can you build a tissue covered wing with such a small amount of reflex?

I would suggest that you use the tip washout suggested by John and the PW106 and adjust the elevon's to suit.

Good luck.

John

Thanks John, and also John.M. for the input.
I will build the wing with the PW106 with a washout of 4°.

Kind regards,
André

[lincoln]

I think those three airfoils are meant for much higher Reynolds numbers than your model will see. For instance, Mr. Hepperle himself has written that the MH45 is for Reynolds numbers of 100,000 and above. They also are meant for wider speed ranges than you need for a free flight model, where there's no point in going much faster than minimum sink*. I'm guessing that the wing will be at around 35,000 or 40,000, plus or minus, although that depends on what lift coefficient you can get out of it. If you could get a Cl of 1, it might be around 30,000 though I think that's unlikely. Anyway, I suspect that a thinner foil might be better. Maybe something like the foil Barnaby Wainfan used on his Cyrano II P-30, only with less reflex. OTOH, it probably had a slightly higher Reynolds number. https://4.bp.blogspot.com/-VQtj4rwd5qo/UjYjpHCYe3I/AAAAAAAADH0/FgordfDRSCQ/s1600/Cyrano-II-P-30.jpg According to The Incomplete Guide to Airfoil Usage the Arrowbile, or at least one of them, had a NACA 23015. That's too thick for a model, though. Maybe a 24017 would work better. NACA 5 digit airfoil generator I'm just guessing, though. Maybe you'd want a bit more reflex?

This is all, to some extent, polite fiction. Stick and tissue models without sheeting generally have a bunch of different airfoils, unless you make them with a series of straight lines except for the leading edge. I think, though, that many CFD programs will barf when fed such things.

*Except for magnetically steered models, most free flight models circle. Getting nowhere fast isn't better than getting nowhere slow. (Pardon my grammar.)

-------
The Arrowbile appears to have a constant chord wing. This means you don't need as much washout as a tapered wing. You probably do need more than an unswept wing, though. If you want to minimize elevon deflection, the amount of twist depends to some extent on the pitching moment of the foil. Of course, usually we don't know that. I think we can assume that more reflex means a more positive pitching moment, requiring less twist. At least until the flow in the underside near the trailing edge separates. 

Speaking of separation, airfoils at these low Reynolds numbers often benefit from turbulators. Turbulent flow stays attached better than laminar flow. A turbulator could be something like a doped on thread, or a piece of striping tape, or rough paint. Many foils at high or moderate lift coefficients benefit very significantly from turbulent flow, at least according to Xfoil. Many modellers have used them on duration models, but I'm sure scale models can benefit from them too, if they don't ruin the appearance. Typically, you might see a turbulator on top, at 15 or 20 percent of chord, though I don't know if that's always optimal. For instance, with the Eppler 205, at Reynolds numbers typical of RC gliders, forced transition to turbulent flow seems to work best at 40 or 50 percent of chord. According to Xfoil. I don't know if Xfoil's forced transitions have more or less drag than a real turbulator. An overly thick turbulator sticks up too far out of the boundary layer and creates extra drag, while an overly thin one doesn't work. The boundary layer gets thicker as you go along the chord. It's possible to calculate an approximate thickness at a given point, at a given speed, but I don't know what percentage of that thickness a turbulator should be.

Maybe this is more than most people want to get into, though.

Keep in mind that I'm mostly an armchair aerodynamicist, though I have messed around with them on models a little. They really helped a small RC glider I built with an approximation of the 7043 airfoil.

[André Wind]

This is the airfoil on the plan.
It's a really old plan, but maybe this is also useful?

[lincoln]

When full scale aviators started building planes, they relied on data obtained with small airfoils in wind tunnels. Therefore, they used airfoils that worked well in those tests, and would work well in our models. Later, the full scale guys figured out that much thicker airfoils can work as well or better. Wings with those foils allow substantial internal soars, so much or all of the draggy bracing wires and struts could be dispensed with. Then the modelers started using those thick foils, which may have looked scale, but had inferior performance. The RAF 32, which I think was also used on full scale bombers,was popular. Some of the designers at Comet didn't seem to be taken in by this,nbut a whole lot of other people did. If you look at Wakefield designs, you can see that those flyers started wising up around 1950.

On the other hand, there's no lack of power in electric models these days, so maybe performance can be sacrificed for a more scale airfoil that's also easier to work with. Especially if you only need 20 seconds of flight. Therefore, much of what I've written above is pointless unless you're using rubber power, or want a good glide with the motor off. At least if the stall behavior of the thick airfoil is mild. I mostly fly gliders and rubber power, so it took me a while for this to sink in! It's fun to go chasing after high oerformance. Was there a particular advantage you were looking for from "modern" airfoils?

[André Wind]

Was er een specifiek voordeel waar je naar op zoek was bij "moderne" vleugelprofielen?


Absolutely not.
I haven't  knowledge at all about airfoils, and my thoughts about a drawing from 60 years ago and the knowledge at this time could be different.
That's the reason of my question about it.

[lincoln]

Airfoil design has advanced tremendously, but in this case I doubt it can give much of an advantage. On the other hand, if you look at what modern sailplanes can do, provided that you wipe the bugs off the wings...

[lincoln]

BTW, Megow went out of business in 1949, so the plan has got to be at least 76 years old. I'm guessing more like 85. The Arrowbile first flew in 1937.

[André Wind]

I've became an advice from an authority person in free flight scale models.
His advice is to use the original airfoil on the plan with a slightly thinner profile.
Below his drawing of the airfoil.