Reynolds Numbers??

[170flyer]

I have been reading these posts and learning alot from all of you. I am about to start work on a foam (ala Keith Sparks), scale Twin Otter. I have been looking over the three views and planing my attack. I would like to use the original airfoil (comments please). I am seeing guys talk alot about the
Reynolds Number. What is it and is the original airfoil on the Otter going to be a problem if I scale it down to, say, 1/11 scale.
Thanks

[jkettu]

Any of the links on first page of http://www.google.fi/search?q=Reynolds+number will answer your first question. As to the second part.. that would take a bit more research. At that scale it is safe to assume that the original NACA 63A516 will not be the optimal choise. But based on the fact that the aircraft itself is somewhat draggy and you are probably not interested to get that last bit of performance that would win a world championship, I would guesstimate that it should be good enough.

[MarkusN]

Yep. On a plane with two air acceleration devices up front almost any 16 % thick section with some camber should work. In that relatively small scale it may be a good idea to thin it down to about 12 %.

Rough formula for calculating Re: wing chord in mm times speed in m/s times 70 = Re

Which would make in imperial...

wing chord in inches times speed in mph times 800

[CloudyIFR]

I'll take a stab at Reynolds number. It's a number that can be used to compare how something will work in the same air i.e. fluid. You can scale the airplane down but you can't scale down the air molecules that the airplane will be flyin in. So even though it's exactly like the original, in effect it won't be unless you'd modify the airfoil. However, like was said before, to what detriment would this be, I'll leave that one for others.

Curtis

[170flyer]

So using the same airfoil to keep it scale would be ok but i'll thin it down to 12%
also, you have given me a formula.... is there a Re range that I am looking for.
Thanks for all the help!

[MarkusN]

also, you have given me a formula.... is there a Re range that I am looking for.
Since you have decided on a scale, not really. You should strive for the lowest weight possible and that will more or less give you a minimum airspeed and thus a Re that your airfoil must manage. if it doesn't drop well below 100'000 you should be OK.

If the "scale" aspect is important to you, just stick with the original airfoil. With two props to accelerate air over a large part of the wing, and turbulent air at that, there sould be no issues with low Re performance.

I just noticed a thing about my formulae for Re: Re is a dimensionless number, and therefore should be independent of the unit system (metric or imperial) used. However, this is only true if the practice in imperial is to compensate for different units used in speed, length, and dynamic viscosity. I assume this is so. If not, Re in imperial is different than Re in metric. Can eanybody confirm that this is not the case?

[170flyer]

Wow, that third paragraph is way over my head! but..I'll stick with the original scale
airfoil (might drop it to 12% if the Re is well below 100'000)
Thanks again for all the info from all of you.

[kcaldwel]

I just noticed a thing about my formulae for Re: Re is a dimensionless number, and therefore should be independent of the unit system (metric or imperial) used. However, this is only true if the practice in imperial is to compensate for different units used in speed, length, and dynamic viscosity. I assume this is so. If not, Re in imperial is different than Re in metric. Can eanybody confirm that this is not the case?

Re is independent of the dimensional system used. All units have to fall out to give you a dimensionless result, otherwise something is wrong.

Kevin

[MarkusN]

Re is independent of the dimensional system used. All units have to fall out to give you a dimensionless result, otherwise something is wrong.
Thought so, but thanks for confirming.

[kcaldwel]

I can't find any low RE test data on the NACA 63A516 airfoil. Looks like the Twin Otter used a modified (in some unspecified way) version anyway:

http://www.aerospaceweb.org/aircraft/commuter/dhc6/

A somewhat similar airfoil behaves quite well at Re = 150,000:

http://turbulence.kmip.net/doc/Science/Fluid-Mech/Flow_Control_and_Design/Airfoil_Design/Airfoil_Database/show-polar-e.phtml@id=205

I used to work on the Twin Otter a bit when I was with DeHavilland at Downsview, Ont., in '79, '80, mostly control system stuff. It was still in production then.

One note about Reynold's Numbers: the Re for an aircraft is typically calculated based on the wing chord, but any reference length can be used. For other objects, it is good to list which feature you are using as the reference length.

Kevin Caldwell

[170flyer]

Hey Kevin, you mentioned you mostly worked on control system stuff. I have another
question I'm not sure of. Q: looks like the ailerons droop as the flaps come down.
can you confirm this and if so tell me when they droop and how much.
Thanks agian to all of you for the info.
Frank

[170flyer]

Would you recomend the 64A010 over the 63A516 for a scale model such as this one?

[MarkusN]

Would you recomend the 64A010 over the 63A516 for a scale model such as this one?
No. Two reasons:

The 64A010 is a symmetrical section. Good for the tail, but not for producing lift.

The 64... line have a more aggressive laminar flow design. This affects other properties unfavorably, namely stall characteristics. (Altough the 64 series are not REALLY aggressive yet. Try 66 for that...)

Frankly, I don't see how the wind tunnel data for the 64A010 can be used to predict usability of the 63A516. It's much thinner, thus less prone to present problems at low Re. It's symmetrical, these sections have a lot less nasty influences to deal with. Plus the stall is not rally measured or at least not plotted in the data. This would be where low Re influences would show most (plus drag increase in the lift range around the center of the laminar flow low drag bucket.)

[170flyer]

Thanks, I'll stick with the 63A516. Any ideas about the "A". is that the designator for "modified"?

[kcaldwel]

Frank,

Yep, the ailerons come down with the flaps, but at a different rate. I would be very hard pressed to tell you how it worked after 27 years! It is quite complicated, because the flaps and ailerons deflect down, then move back to form the Fowler flap, and then deflect more.

To be honest, I can't figure out the numbers given in this manual for flap and aileron deflections:

http://www.fas.org/man/dod-101/sys/ac/docs/a9ea.pdf

The 64A010 is a symmetrical airfoil, so it isn't really suited to an Otter model. It just has a roughly similar thickness distribution and design criteria to 63A516 and seems to work at low Re OK.

Personally, I'd use maybe a Selig 7075 (3% camber, 10% thick). If scale thickness is important to you, it still might be possible to find something with some reasonable low Re test data:

http://turbulence.kmip.net/doc/Science/Fluid-Mech/Flow_Control_and_Design/Airfoil_Design/Airfoil_Database/list-polar-e.phtml

Kevin

[kcaldwel]

The A indicates the that the section is "substantially straight on both surfaces from about 0.8c to the trailing edge." I think the "mod" means DeHavilland modified the airfoil themselves.

Kevin

[170flyer]

Well, Can't thank you guys enough for what I have learned. Kevin, just one more question about the "A". I don't want to assume, so, I am not sure what the "0.8c"
is in reference to?

[kcaldwel]

0.8c means 80% back along the wing chord line from the leading edge.

There were 10 whole sets of DHC-6 (Twin Otter) manuals on eBay, but they just ended it early for some reason. Too bad, they'd have all the flap movement data:

http://cgi.ebay.ca/De-Havilland-Canada-DHC-6-Twin-Otter-Aircraft-Manuals_W0QQcmdZViewItemQQitemZ280147229241

Edit: And the Twin Otter is back in production:

http://www.vikingair.com/

[Sail 'n Soar]

Would you recomend the 64A010 over the 63A516 for a scale model such as this one?

I would advise against any of the full scale "laminar flow" airfoils. Since it isn't the scale foil, why are you considering it anyway? The 64A010 is a symmetrical foil. while the 63A516 is highly cambered. Again, why that particular foil? Choose any of the common airfoils that are used for models, to include the very common Clark Y. There are also a wide range of Eppler and Selig, etc., airfoils to chose from that were designed for RN ~200,000 or less, that should work well.

[170flyer]

Sail 'n Soar, I was opting for the 63A516 to keep with the scale apperance of the aircraft model. I have not been happy with the plans out there as they are not scale so I have decided to draw up my own. the airfoil is giving me the most problems though.

Kcaldwel, did not think about ebay, I have been looking everywhere to find as much info about the otter as I can. I contacted viking air and have not had any luck with them as they do not want to give out to much info about the otter. I think ebay is where i'll look next.

[CloudyIFR]

http://www.astroflight.com/articles/scalespeed.html?sid=0001ZGnAr7xZW3Qz4E2k7L0

[kcaldwel]

S'n'S, the 63A516 (mod) is the real Twin Otter airfoil. The A already refers to removing the cusp in the TE, so the mod may be the wedge they used to reduce the control surface moment.

The 63A516 might be OK, but there just isn't any low Re data that I can find. It isn't exactly a radical laminar flow airfoil, with the minimum pressure point at 30%. The 65 and 66 series are not good at low Re typically.

The 63A516 is a high lift airfoil, with the 5 meaning a design lift coefficient is 0.5. Here are the NACA reports on the 6A series:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980223594_1998378678.pdf

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091970_1993091970.pdf

A NACA 4415, or Clark YM15 would be safer bets, at near the scale thickness. They could be thickened to 16% if you wanted.

Kevin

[170flyer]

thanks Kevin, I'll consider those two. I am interested in building a scale otter. Most of the plans I have seen do not even round off the fusalage, so I'll design my own. I am interested in a scale airfoil but am not going after any awards, just a nice flying aircraft. Having said all that I will look into the 4415 and the YM15, they may just be my best bet!
Frank

[170flyer]

CloudyIFR, excelent read. thanks for the info

[macboffin]

Some years ago saw a scale Twin Otter, chord about ten inches, with scale section. Bad flyer! Two turbulaters, strips of1/32 balsa 1/4 wide were later mounted at the high point on the section, half inch apart ; this improved things a little, but not much. Finally a Clark "Y" section at 12% was used for a new wing and Bingo, flew well!.

[Sail 'n Soar]

S'n'S, the 63A516 (mod) is the real Twin Otter airfoil. The A already refers to removing the cusp in the TE, so the mod may be the wedge they used to reduce the control surface moment.

The 63A516 might be OK, but there just isn't any low Re data that I can find. It isn't exactly a radical laminar flow airfoil, with the minimum pressure point at 30%. The 65 and 66 series are not good at low Re typically.

The 63A516 is a high lift airfoil, with the 5 meaning a design lift coefficient is 0.5. Here are the NACA reports on the 6A series:


Kevin

None of the NACA 6-series foils all have a poor pressure distribution for even reasonable, let alone, good performance at model Re's. They are almost guaranteed to have a bad flow separation at the 30% cord for the 63xxx's, 40% cord for the 64xxx's and 50% cord for the 65xxx's when built at model scales for other than large/fast models. These foils were among the very first "laminar flow" foils and intended to operate with Re on the order of millions.

Andy Lennon, who has had multiple models published in Model Airplane News, is enamored with the Eppler 197. This 13% foil would have the desired look of the scale foil, but with better performance at scale. This airfoil, too, might require a turbulator for good performance at your scale and speed - a thin tape that could be part of the trim should suffice. Other than that go thinner and go more common model foil - such as the 12% Clark Y. The diversion from scale will be noticed on the ground by only a few and the improved and scale-like performance in the air will more than compensate for the ground scale points.

[kcaldwel]

I've run a quick Profili analysis of four potential airfoils at an an Re = 200,000. Profili has the NACA 63A612 built in, so I just used it as a facsimile of the 63A516. I'm new to Profili, so it will take me a while to get the 63A516 in it for analysis.

S'n'S was correct. The 63A612 has a sharp, strange stall, that is likely due to a laminar separation bubble. I don't think I'd use it, and the 63A516 is likely to be similar or worse, due to it's greater thickness.

The E197 doesn't look too good at stall either - fairly sharp, with a low Clmax, but not as strange as the 63A612. It does have a funny kink in the lift slope at mid Cl, which is some sort of separation.

The Clark YM15 has a nice soft looking stall, and at a high Clmax, but it too has a kink in the lift slope at mid Cl.

The NACA 4415 looks the best of them all to my eye - nice looking rounded stall at high Clmax, and a nice straight lift curve.

I need practice with this program, so if you want to look at other airfoils, or give me some idea of the size and weight of the model so I can better estimate the minimum Re, I can run more polars.

Kevin

[kcaldwel]

Juts figured out how to modify the 63A612 to 16% thickness, and 3.5% camber. This should be very close to the 63A516. I also recomputed all the polars rather than use the pre-computed points in the data base, so all the curves may be slightly different.

The facsimile 63A516 actually looks better than the E197. A fairly sharp stall, but at a higher Clmax, and a bit less weirdness in the mid-Cl range. The 4415 still looks best of the four to me, but a 63A612 modified to 3.5% camber, and 16% thick looks usable if the E197 is.

Kevin

[Sail 'n Soar]

The facsimile 63A516 actually looks better than the E197. A fairly sharp stall, but at a higher Clmax, and a bit less weirdness in the mid-Cl range. The 4415 still looks best of the four to me, but a 63A612 modified to 3.5% camber, and 16% thick looks usable if the E197 is.

Kevin

I've used the various NACA 4-series foils, to include the highly cambered NACA 6412, with great success at Re ~ 100,000 - 200,000. They are almost always a safe bet. The Clark Y's have the construction advantage of being flat over most of the lower surface, eliminating the need for jig's, tabs or other construction to build a straight wing.

[Sail 'n Soar]

Juts figured out how to modify the 63A612 to 16% thickness, and 3.5% camber. This should be very close to the 63A516. I also recomputed all the polars rather than use the pre-computed points in the data base, so all the curves may be slightly different.

Kevin

Kevin,

Interesting graphs. Please run your calculations again with a Reynolds Number closer to the proposed model stall speed to see how the CLmax changes. The modified 63A612 is doing better than I would have expected.

Gerry

[kcaldwel]

Yeah, good idea, I lost sight of how small this model is going to be.

1/11 scale gives a scale chord of 7.05". The minimum stall speed at minimum weight, flaps full, of the full scale Twin Otter is 42 knots. At 1/11 scale, this would be 12.6 knots. or 21.266 ft/sec. This gives a minimum Re = 80,000, which is really low.

The "almost 63A516" looks terrible down this low, and the E197 is not that great either.

I ran it with a 15% Clark Y and the NACA 4415. Either one looks usable. The 4415 has lower drag at high Cl, which is where you would be operating at this low speed.

I've used the 4415 before too, and it has worked well.

Kevin

[Sail 'n Soar]

Yeah, good idea, I lost sight of how small this model is going to be.

1/11 scale gives a scale chord of 7.05". The minimum stall speed at minimum weight, flaps full, of the full scale Twin Otter is 42 knots. At 1/11 scale, this would be 12.6 knots. or 21.266 ft/sec. This gives a minimum Re = 80,000, which is really low.

The "almost 63A516" looks terrible down this low, and the E197 is not that great either.

I ran it with a 15% Clark Y and the NACA 4415. Either one looks usable. The 4415 has lower drag at high Cl, which is where you would be operating at this low speed.
Kevin

Isn't science wonderful?

In Eppler's book, "Airfoil Design and Data," he indicates that the Eppler 193 was designed for an Re > 200,000. And the 193 is a 10.22% airfoil! The NACA 63A516 should be terrible at slow model speeds and the E197, which is designed for Re > for the 193, can probably be made to work with a strategically placed turgulator. This suggests going to the old freeflight standards for Re ~ 100,000 or using such more recent airfoils as those designeb by Selig, Donavon/Selig or Heperle, all of which are relatively thin.

Give up looks and go for performance.

[kcaldwel]

Yeah, the newer airfoils would be best, but the old NACA 4415 looks pretty good. Here are two more plots, one at progressively higher RE that would cover this model's range, and one at Re = 80,000 with a 25% flap deployed 37.5 degrees.

It is just a plain flap, not a fowler like the real Twin Otter, but it still makes a pretty impressive Clmax for an old airfoil. I think the 4415 would work pretty good for this model, and it is near the right thickness and the camber is only slightly forward from scale.

Kevin

Edit: Ran the E193, and it doesn't look good at Re=80,000 either. I think the 4415, or one of the Drela or Selig airfoils would be best.

[170flyer]

I'm very impressed with the help I'm getting on this. One thing I really did not want to do was to go with clark Y. Really wanted to go a bit more scale. Looks like the 4415 might be the way to go. I'm not sure about the weight estimate just yet ( I assume the lighter the better for any a/c and airfoil). would you add any washout with this airfoil?

[kcaldwel]

170flyer,

Weight is a bit tricky. Higher weights actually help airfoil performance, because the speeds get higher and therefore the Re gets larger as well. Of course this is at the expense of higher stall speed, maneuvering speed, etc.

I believe the Bob Boucher article referenced earlier means that the wing loading should be between about 3 oz/ft^2 and 4 oz/ft^2. This is really low, and not realistically achievable. So I guess you build it as light as possible. A higher weight will mean the minimum Re will be higher than the 80,000 I ran the airfoils at, which means they will behave better.

The rectangular planform of the Twin Otter should give a forgiving stall without using washout. Some washout in a rectangular wing will give a better lift distribution though (closer to elliptical), so I put in 1 to 2 degrees. The 4415 has a nice forgiving stall, even at low Re. It should make an enjoyable airplane, and the centre section flaps should make that even better.

Kevin

[170flyer]

Thanks again for the "class" on airfoils and Re. I think the 4415 with 1 or 2 degrees of washout should give a nice flying aircraft and have the scale look I'm looking for!
Frank

[kcaldwel]

I always loved watching the Twin Otters when I worked at DeHavilland. The STOL approaches are amazing to watch.

The company pilots used to do deliveries of Twin Otters to South and Central American countries that were on the US bad guy lists. They couldn't fly through US airspace with Cuban, Nicaraguan, or Bolivian markings, so they used to go outside the 200 mile limit and fly non-stop for days sometimes. They loaded the fuselage with 45 gallon drums of fuel, with wobble pumps to pump it up to the wing tanks. The plastic bags for "waste disposal" were the least attractive part of it, although 48 hours non-stop in a Twin Otter would have been a bit trying to start with!

Good luck with the model!

Kevin

[Sail 'n Soar]

170flyer,

The rectangular planform of the Twin Otter should give a forgiving stall without using washout. Some washout in a rectangular wing will give a better lift distribution though (closer to elliptical), so I put in 1 to 2 degrees.

Kevin

The 1 to 2 degrees will give a slightly lower induced drag because of the improved lift distribution, but it will be at the expense of having the outer panels operating at an even lower Cl compared to the root sections. This will lower the Cl average at stalll, raising the stall speed slightly.

The practical impact of the washout is the slightly better stall characteristics - less likely to stall away from the root sections even in turns. Constant cord wings have pretty benign stall characteristics as is because the outer portions of the wing are just loafing compared to the root sections. This will also make the stall behavior impacted less by slight differences between the as-fabricated airfoil shapes between the two wings.

All that said, unless I intend to fly the aircraft inverted or am trying to coax the last fraction of L/D out of a given wing planform, I typically add a degree or two of washout even for constant cord wings greater than AR ~ 6 - 7. I've forgotten, do you plan to have ailerons? The washout will also help to assure the section with the down aileron keeps from stalling.

[170flyer]

keven, thanks, I used to load and fuel otters for Pilgrim Airlines in Groton, Ct in the late 70's and early 80's. Always wanted to build an r/c version as I have been building and flying since then but have never come across a good set of plans. thanks again for all the foil help as this will enable me to design my own r/c aircraft from the bottom up!
S n S, thanks also for the info and yes I do plan on using ailerons. I'm also thinking that maybe from what I'm reading that maybe I should plan on an aircraft that is a little bigger. maybe as much as a 100" wing span.

[Ralph Walton]

Be carefull about scaling. Not everything scales down well. I'm not familiar with the plane but generally if you use exactly scaled down ailerons and elevators it will be a dog to fly . They are simply not big enought for a model to handle well

If you must use the original wing section it will fly. If you want it to fly well, consider using a section such as the SD 6060 I have helped several friends use this section on warbirds with great success. Its a good all round foil, not too much drag, good lift, and not too speedy.

If you can reduce the size of the fuselage relative to the wing and increase the size of the horizontal and vertical stabilisers you will have a better flying model.

I guess you hae to decide on how well you want it to fly, and how far from exactly scale you can accept.

My personal formula is to, exactly reduce the whole plane to whatever scale you are aiming for say 10:1 I usually pick a wingspan, and believe me bigger is better. 60" and up is nice. If you spend the money on getting bigger wings cut , you can use standard servos, which cost a fraction if micro ones.
Total cost of bigger wing with std servos is cheaper.

Thin the fuselage to down to approximately 80% and keep the original length- use your own judgement on what looks right.

Increase the area of the horizontal and vertical stabs by approx 20%

Enlarge the control surfaces to sensible sizes

Move the wing back slightly, just so you can't notice its been moved. - maybee 10 or 15 mm. This helps alot with trying to get the model balanced. warbird models always come out tail heavy, so the less lead you need up front the better.

Consider increasing the wingspan and chord slightly. It helps to increase the area of the wing. Scale warbirds usually suffer from high wing loadings, this creates various problems- high landing speeds , short flight times etc etc.

I have used these techniques on a 67" corsar, a 68" mustang a 68" me109 and a 60" vampire. and they work to produce scale looking, good flying aircraft.

Good luck with your project!

[kcaldwel]

Ralph,

Have you seen a Twin Otter? Not exactly a war bird! It's a STOL aircraft with a big wing and tail surfaces, and a long nose.

http://photo.net/philip-greenspun/photos/digiphotos/200207-nahanni/vf-dock-twin-otter-2.half.jpg

[Sail 'n Soar]

This thread began discussing Reynolds numbers, but has drifted into general model design criteria. In addition to the extensive discussion relative to the penalities of using the scale airfoil, Ralph Walton's comments on the downside of using scale control surrfaces is also right on. A number of general rules of thumb have been derived that help assure a good flying model. You can use these rules of thumb to judge the relative flying qualities of your "exact" scale project. One of those is that the model aileron area should be ~ 10% of the wing area. The 10% figure applies whether you are considering strip or barn door ailerons. If the scale subject has less than 10%, chances are its roll characteristics will be less than stirling. Similarly, the "tail volume" should be ~.4 - .5 (= tail moment arm X horizontal stab area/ wing area X mean cord) for a modeler with at least a fair degree of RC flight experience. The larger the number, the greater the pitch stability. Good trainers will have even higher tail volumes. If the scale aircraft has a TV < .4 it is going to be much more sensitive to elevator inputs or will require a forward CG to regain a reasonable static pitch stability. If the TV for the full scale aircraft is >.4, go with it. if it is less, you might want to consider enlarging the horizontal stab. Full scale aircraft also often have more vertical stab area than is the common model design practice. This one I'll let you do your own research and calculation because the "experts" provide differing, and often contradictory, rules of thumb.

[zitron]

And to answer the original question of what is the Reynolds number, it is essentially a ratio between the innertial and the viscous effects of a fluid. The higher the Re, the more air behaves like an ideal fluid with no drag, the lower the Re, the more it will act like honey, with forces dominated by viscosity. Or you can think of it this way, if Re was infinite, and you push on a bit of air, the bit of air will carry on pushing other bits of air and keeps moving on forever, but in reality, the effect is damped out by viscosity and eventually everything stops.

Because of this, most airfoils have lower Cd at higher Reynolds numbers, and the Re at RC sizes is not really optimal for best performance for most airfoils.

-Z-