Frank RKS

I had a (very) brief introduction to RC flying in the early eighties - like about one flight. I have recently returned and am now flying a trainer - solo - as of last weekend.

I have a copy of a Nov '78 issue of RCM which has an article on a model called a Magnum 40. The plans are still available at:-
http://www.rcmplans.com/index.php?ma...oducts_id=1559

This particular model has a wing with a Double Reflex Airfoil. Obviously, the article says the model flies brilliantly but I have had some rather negative comments by experienced modellers re the viability of this sort of airfoil. I've found other comments on the web which say the double reflex airfoil is common with modellers but I've not found another plane that has it.

I've always been interested in building this model as it is that little bit different and am seriously considering it as my next aircraft.

The designer was Dan Santich. His son Steve is still selling the plans (30 years on) and his emails indicate the model is still being built.

Two questions.
1. Does anyone have experience of this model and can comment on its performance.
2. Can anyone provide serious comment on the pros and cons of a double reflex airfoil for model aircraft.

Cheers
Frank

wellss

Very similar to a Joukowski profile, which is a natural streamline shape and the earliest mathematical or theoretical airfoil application, dating to 1910 or so. Good for lift, but draggy. Therefore, may work well at low Rn. I have no experience with such a profile and it would be very difficult to quantify any difference from any other profile, I'd imagine.

However, in real world application, the Republic P-47 used a modified Joukowski section ( with finite trailing edge thickness, ie Karman-Trefftz ) wrapped around the NACA 230 mean line to give minimal pitching moment while avoiding the rather abrupt stall with the NACA thickness distribution. Hmm, Dan Santich designed the original Top Flite P-47 with a similar airfoil as he mentions it having a little of the double reflex built in.

I flew a Hots years ago, but I can't remember what airfoil it had...but it flew well. I'd say that it will depend on the chordwise location of the maximum thickness. The further back it is, the more abrupt the stall will likely be. Joukowsky airfoils are at 25%. Modifications can move that point rearwards. NACA sections ( 4 and 5 digit ) are at 30% and are considered general purpose sections. Beyond that, your goal is to reduce drag at the expense of maximum lift and stalling characteristics.

BMatthews

I remember that one and remember the very odd looking "polywog" airfoil. Frankly I think it would be a pain to build and even more to cover and I doubt it does anything extra. As for the aerodynamics it's likely a wash. A few years ago the fun fly models all used some big fat but proper looking airfoil that they then stuck a wide flat aileron onto the end of. The result from an aerodynamic standpoint was a really strongly "double reflexed" like airfoil. It didn't hurt them and it won't hurt the Magnum. Just keep it as light as you can and it'll fly superb.

But frankly if it was me I'd ditch the refex curves it uses and just make those parts flat and use a flat aileron. Easier to build and cover and it'll fly the same as the original polywog looking thing.

rmh

That shape was used on the Veco Warrier, and Cheif control line models of 1949-
way too much is made of airfoils on these models
in this case the thich section make for a rather numb response (smooth in pitch) and is structurally very good and is light.
the flat aft section either flaps or ailerons is a mixed blessing - at slow speeds not a problem but structurally not very resistantat to flutter or warping under airloads.

TFF

I dont think Dan Santich kept using that airfoil; I know the Hot Hots does not, but it does not mean it is not a good model. I think it is good looking and have always wanted to build one. There was a .40 size pattern plane in Model Aviation a year or two later that lifted the airfoil from the Magnum.

rmh

Frank RKS

OK guys, thanks for the feedback - must say that overall, it doesn't sound really promising.

BUT, as a challange, I have decided to go ahead and build this just to see what it's like - ordered the plans from RCM.

BTW, my son who is an aerospace / mechanical engineer is also unconvinced of the viability of this airfoil. He thinks its more appropriate to the fuselage of high speed jets to decrease drag.

When it gets in the air, I'll let you know what happened. Time will tell.

Cheers,
Frank

wellss

Could you scan the profile from the magazine? If I knew the thickness ratio and chord location of maximum thickness, I could construct a similar profile and run it through some analysis software and compare it to a typical NACA section, for example? Or, you know what....I'll construct such a profile with maximum thickness at 30% chord, say 10% and 15% in thickness and compare to NACA 0010 and 0015. We'll see what happens...stand by

combatpigg

My Feedback: (3)

In a model plane speed application where the wings are very thin to begin with.....executing this idea will make an inherently weak design even weaker and more prone to twisting.

Pretty hard to execute this cleanly without full sheeting which adds too much extra weight for some models.

If I could see an intended goal or purpose............
You would need to build 2 wings and test them side by side on the same plane going through an identical gaunlet of inflight tests and clockings.

Dick, inverted flight for C/L hadn't even been "invented" back then, huh?

rmh

We flew inverted -and next years when the Barnstormer was hooked up - flew 8s -all the stuff Even the Stuntwagons going very fast would do most of th stuff - a Stuntwagon and a Torpedo engine was a fast, agile combo
The Cheif and others had the polywog airfoil .
The Zilch stuf did not but with large wings and really lowloadings were extremely agile I had a Super Duper Zilch

HighPlains

My Feedback: (1)

I think the reason they used the polywog airfoil on those early Veco kits was to fit the ribs into the die cut sheet. $$

rmh

actually the shape was to give max strength thu thickness and blend into the flat flaps used. this was a common practice on the big stunters
the thick foils went as high as 20% which really was of no concern -as far as excess drag is concerned
the lines added more drag than the wing .unless one made the plane faster -bigger etc..
the use of braided .015 lines reduced drag a lot. When I flew 1/2A controline-Ifirst tried the various dacron lines - all tooo draggy
th fine braided .008 provided less drag
My 8 ounce stunter flew the entire pattern on 50 ft of these lines!
On dacron - 35 ft was all It would handle,
for those who may actually have flown stunt - the plane was a scaled down Coyote with a Te Dee .049 on lotsa nitro
BTW I am looking for a complete Squaw or Warrior or Cheif -or even a Brave if anyone has one kicking around. These were Veco kits

HighPlains

My Feedback: (1)

I built this Warrior over 40 years ago. From the newer of the two boxes.

Dad built his from the older kit 55 plus years ago.

My Warrior survived due to a crappy Fox Rocket with minimal damage. Time for a rebuild.

I'm pretty sure that the original designs did not have the polywog ribs. I read somewhere Joe Wagner stated that it was the kit engineering that changed them for the reason I stated.

wellss

I don't know if this profile is the same, but maybe similar? I "reflected" the curve aft of the highest point, to the trailing edge. It looks like this,


The analysis at Reynolds number of 200000 ( The Magnum flying at 25 mph ) doesn't look promising, when compared to the NACA 0012. It is inferior in every way. The higher drag leads to earlier stall and quite possibly very abrupt as the code was unable to perform calculations beyond the end of the line, whereas the NACA profile clearly shows a gradual approach to maximum lift.

Frank RKS

The science of this is beyond my level of knowledge but please don't let that stop the discussion.

Here is the actual airfoil from the plan if that helps.

Frank RKS

BTW,

can someone please tell me how to get this image in fullsize

Frank

rmh

..

BMatthews

What you see is what you get. RCU resizes pictures such as this to avoid having to process huge image files through its servers.

Oh, what you see in the post is just a thumbnail. If you click on the picture it'll give a triple size bigger image which gets the idea across just fine.

Frank RKS

I only asked because I wondered how wellss posted his proposed airfoil and graphs apparently full size in the forum topic.

No drama - I am still interested in further discussion about this wing. As I have said, I'm going to build the Magnum 40 with this airfoil just to see what it does. Just waiting on the plans from RCM - maybe another 2 weeks or so.

A few of quotes from the original article by Dan Santich from the original article in Nov '78. Just to get tongues wagging or fingers typing.....

"The Magnum 40 has the greatest speed differential of anything I have ever flown.....with a little breeze, it will actually fly backwards"
"The performance capability of this airplane and its unusually good flying charastics are for the most part, due to the airfoil."
"...when an acceptable method becomes the 'norm' we tend to resist any changes, especially when it is somewhat radical or counter to our understanding."
"The theorists can sit around all day and show you in figures why something won't work and then someone like me comes along and blows their minds."

The last quote is what I really like - challenge the accepted norm and see what happens. Isn't that how science and technology progresses.

Give me a couple of months and I'll tell you either success or failure - to build a new wing (if it doesn't work) is not going to cause another global financial crisis.

Cheers,
Frank

mike 71

I built this plane from the plans when it first came out in rcm and it flew great with a super tigre 46 in the nose. It was easy to control and would land like a trainer. I sent the plans to lazer works in texas and they cut me a short kit for $58.38. Check them out if you're interested. The quality is first-rate. I plan on making this one electric. Mike.

Frank RKS

Thanks Mike71. Nice to have someone provide some POSITIVE feedback instead of all the negatives.

My plane is about 1/2 built. The wings are done but not covered. I have installed a servo each side for each aileron so that I can have flaperons IF I wish. Should make it even more interesting. Fuselage is about 1/3 done so still quite a way to go. Haven't had a lot of time lately with a new job but that's OK. I'm dropping an OS46AX in the front so should have more than ample power. Anothe month or two depending on time availability and we'll see how it goes.

BTW, I'm not going to Texas for a lazer cut kit - not from Australia that is............................... I have a really good workshop so accurate cutting is not a problem.

mike 71

Hi Frank, Good luck with your magnum. It really is a nice flying plane. I've been sitting on my kit for a little while now and after hearing about yours I think I'll get going on mine. Post a picture whenever you get it done. Mike.

turbo.gst

My Feedback: (8)

We had a guy briefly in our local club that flew a Magnum 40. It flew pretty well. I remember him using a 45 engine. Good power for the way he flew it.

turbo

Tall Paul

Any lightly loaded overpowered plane will fly well despite the shape of the airfoil.
Fat airfoils behave nicely at slower speeds and lots of power.
There wouldn't be any obvious benefits to the polywogs, other than being different.

dlach

hey frank,

well the airfoil its called a double reflex because it has 2 camber lines, which are the first curve on the leading edge almost to the half of the chord, and another not so curvy camber line from the middle to the trailing edge, because of the airfoil is symmetrical, its the same on the extrados, and the intrados (upper and lower part) the airplane is suspected to behave pretty much same, and I make myself clear, pretty much, not exactly, because the wing will generate lift on either way if its inverted or not, the only thing that affects the lift will be the angle of attack, using that airfoil is mostly because they wanted to make a rare thing, because a NACA symmetrical airfoil can behave the same, maybe on higher Reynold's number, that meaning higher velocities, higher wing chord, and different density's, that specific airfoil can prove very different, and r/c models being almost always overpowered the airplane will fly great as long as you have that extra punch the engine gives you.

Enjoy and post your progress,

Daniel