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Old Sep 13, 2002, 11:00 PM
Motors beat engines!
Milwaukee Wisconsin, United States
Joined Feb 2001
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Airfoil for pattern plane?

I'm trying to redesign my Gravity Abuse sp500 pattern plane for greater duration and speed.

I attended a electric fly in in June and saw a similar sized very impressive aerobatic plane that flew over 7 minutes at 65 or 70 mph on the same batteries I was using. ( my plane flys 4 to 5 minutes at 50 mph.)

I quizzed him about it and he attributed alot of its performance to the fairly thin almost flat bottomed airfoil he was using. Despite the flat bottom, his outside performance was very good, which he attributed to getting the cg just right.


So, here I am, ready to start form scratch and ditch my very thick full sym 1016 airfoil ( and therefore the entire plane) for something better, and I would love to hear about airfoils anybodys had good success with for aerobatics. I know a flat or semi sym will improve duration, but which one(s) will still perform?


All suggestions welcome please.

Dean in Milwaukee
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Old Sep 14, 2002, 01:34 PM
Motors beat engines!
Milwaukee Wisconsin, United States
Joined Feb 2001
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Anybody?
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Old Sep 14, 2002, 05:22 PM
Registered User
Oak Harbor, Washington, United States
Joined Jan 2001
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Dean, I don't really know anything about airfoils myself, by have had some recommended to me by those that do. For Pattern type planes they recommended Eppler 193, 195, or 374. These are 11 to 12 % airfoils.

Norm
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Old Sep 14, 2002, 08:46 PM
Motors beat engines!
Milwaukee Wisconsin, United States
Joined Feb 2001
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Hey thanks Norm! Thats exactly the sort of info I was looking for.

Now to start plugging it into motocalc and figure my area and span.


Thanks again,

Dean in Milwaukee
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Old Sep 14, 2002, 10:11 PM
Scott Black, Montreal
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montreal quebec Canada
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I don't think there is any great benefit to be had abandoning the symmetrical airfoil for an aerobatic airplane. Duration is drag. If you want to improve your duration, or compare it to that of another airplane, you have to look at the whole picture which includes motor, prop, airframe etc. The airfoil profile drag is one part of the equation.

You didn't mention what motor the other fellow with the similar airplane was using. A brushless motor makes an enormous difference in duration and/or performance.

An SD6060 will perform fine inverted, as will most any of the 10% sailplane sections with low camber. The amount of down elevator required for inverted flight will be a function of cg. Anybody who has seen Keith Shaw fly his Spitfire and Geebee might be surprised that he uses the Clark-Y. But for an all out aerobat, or pattern plane, symmetrical is the way to go - say an NACA 63012 or 64012. I have used both with success. My caprise flies 10 minutes on 8 CP1700s with an Astro 05BL. 8 minutes if I really horse it around.

If you want major duration, make sure your power system is optimized, build a clean airplane (keep the horns internal where you can, and make it light. Unless you were using a 16% thick funfly section, don't expect changing airfoils to make a huge difference. A better motor and/or a more optimum choice of prop will have a much greater impact.
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Old Sep 15, 2002, 09:07 AM
Visitor from Reality
United States, VA, Arlington
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Hi Dean
As my new pattern baby - okay, 16 cells - is using one of Scott's recommendations, I really have to go along with them!

Right now, have two big aerobatics - the CAP with a thick symmetrical D box structure and the 4*40 with it's who-knows-what mostly flat bottomed easy build section with spars all over the shop. The latter, when looked at hard, only has curves at the ribs, the rest of the wing is small, flat areas where the 'kote stretches over the spars.

Despite all that, I have to report that the 4*40 will do anything the CAP can. Even more sickening, it appears largely indifferent to which way the wheels are pointing.

My beloved Embat also has an 'easy build' section, it's some kind of Eppler, but now flat from spar to TE - which I believe negates Mr E's efforts. It too is happy any way up and will do control line eights effortlessly and with equal sized insides and outsides.

Agree with Scott - light weight, clean model, do everything possible to optimise everything. Big, fat fuselages, thick wings and mounting the radio gear mostly on the outside is okay with a big gasser to drag it all along. We don't have that luxury.

These anomalies go a long way - the full sized Bucker Jungmeister has a flat bottomed wing section and plenty of inverted capability.

Regards

Dereck
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Old Sep 15, 2002, 03:49 PM
Motors beat engines!
Milwaukee Wisconsin, United States
Joined Feb 2001
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Okay, you guys raise a bunch of interesting points.

First the other plane: All built up original design, looked sorta like a sleek 4 star 40, had about same span and area as my plane, 48" and 480 in/sq.

His batteries were 8 2400's, ( not sure if rc or cp) on an astro brushed motor. I belive his prop was a 11x7.

Mine runs a kyosho atomic force, 3.8:1 with a 12x8 apc-e on 8 cp2400's. I belive the astro brushed is not significantly more effiecient than the kyosho, certainly not enough to account for the duration and performance difference.

Clearly, with the extra speed and duration he had, his average current draw must have been much lower.

Our max climb angle and rate was quite similar, but his speed was much higher

My wing is quite thick at 18%. It was chosen last year when I knew less about this stuff but did know that a thick symetrical design would give very gentle stalls and fly the same inverted as upright. ( both true BTW ).

The potential reduction of frontal area alone should make this exercise worth the trouble.

Fuse frontal area = 12 in/sq
current wing frontal area =69 in/sq. (1.5" thick)

10 % wing frontal area = 39 in/sq

so a total of 51 in/sq frontal area down from 81 in/sq. for a reduction of 38%!

I also can see that now at lower speeds when throttled back, the entire planes angle of attack goes noticably nose high, which must create tremendous drag. This then requires that I keep the throttle at a fairly high position except for downlines, else it really slows down fast. Part throttle is therefore mostly for landings.

Part of the idea of using a semi-sym profile came from playing in motocalc. If I would subsitute almost any semi sym profile for almost any full sym, part throttle cruise duration would jump up almost 50%.
My thinking is that this is due to the plane no longer needing to go so dramatically nose high at lower speeds to stay airborn, reducing drag and therefore needing less power.

So now you guys have me in a quandry. Should I just go to a thin full sym, or would semi be a better choice? Aargh!


Dean in Milwaukee
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Old Sep 15, 2002, 05:15 PM
Scott Black, Montreal
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montreal quebec Canada
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You haven't mentioned the most critical aspect of the 2 airplanes - the weight. My first impression is that this is a huge airplane for 8 cells. If your weight is around 3 lbs, you have something like 15 oz wing loading. To have decent aerobatic performance, and I don't mean being able to hover and flit about like a 3D airplane, but good sport aerobatics, you want to be up around 18-20 oz wing loading i.e. a smaller airplane which amounts to less weight and drag. My 10 cell (2400s) pattern ship has 350 sq in and uses a 10% section.

A huge airplane, with an 18% wing section, is not going to have good endurance at high speed. Make it smaller and lighter, or else put more cells in, then use a 10% thick wing, symmetrical or slightly cambered doesn't make a whole lot of difference.

The most fundemental parameter in airplane design, the one that has the biggest impact on how the airplane flies, is the sizing vs the power system and weight. The choice of wing section is not nearly so important.

Now I want to be clear here - there are lots of funfly type 3D airplanes with 12 - 15 oz/sq ft wing loading, and if that is what you want they fly great. But if you want to do large, smooth, scale type pattern or IMAAC style flying you need something with some power that can fly fast. You need 18-20 oz wing loading and 75 watts/lb+.

The wing loading will determine the speed at which the airplane flies and hence its flying "style".

An 18% thick wing is appropriate for a 3D airplane, but almost 100% too thick for a fast, smooth, pattern-type airplane.

So I don't think your decision is as difficult as you think. Chose the type of airplane you want, then go for the wing thickness that the chosen type requires. Whether it is symmetric or just mildly cambered doesn't matter all that much.
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Old Sep 15, 2002, 06:03 PM
Visitor from Reality
United States, VA, Arlington
Joined Dec 1996
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Dean
All of the above, plus the best thing to add to an eight cell model is two more cells! Even if you have to keep the amps down because of motor considerations, adding 2 x 2oz cells to a 48oz model is a percentage weight increase of just over 4%. The power is hiked up around 25% if you reprop to maintain the amperage to ensure your motor survival. Even the lowly Mag Mayhem does fine on 10 cells, as long as you stay at 25A max.

The main difference between an AF and a ferrite - I've got an AF035G that's been running on 10 cells and 30 - 32A for five years or so, pretty much continuously. I looked at the brushes last week - still there.

Scott's right about funfly models - first off, do you want that envelope? They are fun to flit around and mess with, but very few of us will ever aspire to fly them like Gary Wright and a very few others. The Embat is classic pattern on the small size - it will pull huge round maneuvres and really long rolling maneuvres, but really is as little wood fuselage as can be decently wrapped around a direct drive 300W+ brushless / 10 cell package.

If you can track down an Embat plan, it is about spot on for this level, in terms of size, 'numbers' and weight/loading.

One of my models for next year could look a lot like an Embat wing on the bottom of a slimmed off 'pylon racer' type fuselage - a glow powered racer, not the electric sort.

When you do it - do the first one real light and well made, but with a basic finish and low in fiddly bits. If it works, tart it up plenty and go fly while you do an even better one. If it doesn't - less time and effort lost

Regards

Dereck
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Old Sep 15, 2002, 06:36 PM
Motors beat engines!
Milwaukee Wisconsin, United States
Joined Feb 2001
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I dunno what his plane weighed, but its likely heavier than mine due to its construction.

Mine is 48 oz. ( good guess!)

I am pulling an estimated 32 amps per motocalc so watts/lb are right around 90. I am considering going to 10 cells as well due to my two 8 cell paks apparently starting to downgrade based on performance. ( they do have ALOT of flights on them.) If I have to replace, I might as well upgrade.

I think I will take a cue from you guys and try it around 350 in/sq with one of your suggested airfoils.
I did make an experimental ship last winter using a 44" span 350 inch wing with a e205 airfoil that came in at about 42 oz. The goal was speed, ( moderate success) but what really suprised me is that it was a real floater. Landings would draw WAY out.

I am expecting my new one to be lighter with the smaller wing, perhaps 4 oz less.

I do know what you mean BTW about making a prototype first and making a nicer one once its proven out. My current plane is actually the second prototype, repairing many of the weaknesses of the first. These are all pink foam planes BTW.

My goal is to have a finely finished plane for the next electric fly next june at this same field. As it was last year, I got a lot of suprised comments when they found out it was'nt brushless.
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Old Sep 15, 2002, 07:24 PM
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A requirement for flying smooth, precision aerobatics is to have a model with the same pitch trim upright or inverted. If you have an airfoil with anything other than a symmetrical section this is very dificult to achieve.

The drag in inverted flight is high because the model has to fly nose up to generate the required lift and if you push into an outside loop the drag rises very rapidly. If you push too hard you will stall the wing easily which could in turn lead to a flick roll. Not nice!

This is not just theory but based on my own experience and that of a couple of my flying buddies with a supposedly aerobatic model with a typical "electric" cambered section. Eventually it caught us all out. I built a new version of the model with a symmetrical section. Hey presto a smooth neutrally trimmed pussy cat to fly.

Bob P
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Old Sep 15, 2002, 07:43 PM
Scott Black, Montreal
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montreal quebec Canada
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I was flying my Hangar 9 Aresti today, doing outside loops (well, trying anyway). I still have the cg a little far forward so I need about 1/3rd to 1/2 down elev for inverted. The required elev. for inverted is a function of cg more than anything. I need to move it back a bit.

If a model stalls during an outside maneuver, it is just as likely that there was not enough excess speed to start the maneuver rather than because of inverted camber in the wing. According to Keith Shaw, you should start a loop at 2 - 3 times the stall speed of the airplane. In that configuration you should be pretty far from stalling.

My preference is a symmetrical section but I have seen guys fly great aerobatics with a Clark-Y, assuming it is well set-up. Getting a neutral set-up, or close to it, with a cambered section is not as difficult as you might think if the cg is far enough back.
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Old Sep 16, 2002, 06:36 AM
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Stalling is not a function of speed but of angle, and the stall angle for a particular airfoil is constant. If you push /pull too hard at any speed you will stall the wing.

A problem with electrics has always been lack of umph (which is why the advent of outrunner motors like the AXI has produced such profound effects) This leads to problems with the high drag of cambered section when inverted. Like sblack says he needs 1/3 to 1/2 elevator just to fly level inverted, just think of the drag. Flying an axial roll is very hard when you have to push so hard just to maintain a straight line when inverted!

Bob
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Old Sep 16, 2002, 10:12 AM
Motors beat engines!
Milwaukee Wisconsin, United States
Joined Feb 2001
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Ok guys, I have the new planes specs about finalized.

span 44"
area 350 in/sq
weight about 48 oz
airfoil semi sym, probally e205 or similar

powertrain:
kyosho atomic force ( 17t)
4.6:1
11x10 apc-e
10 cells, either cp2400's or the new gp 3300 nimh
( amps are only 28 with 42 oz thrust and 70 mph ps speed!)

Compared to the old plane I should expect: ( all motocalc predicted)

climb about same at 1700 ft/min
top speed goes from 50mph to about 70
cruise duration up about 50% with the cp2400's and 100% with the 3300's!

If I even realize 80% of these improvements in real life, I will be extremely pleased!

PS, I'm debating if I should add a spoiler or flap function, as the minimum sink rate is going to go from 350 ft/min to 260, and at 350 it already floats in enough to make short field landings a bit challenging.


Thanks everyone for all the valuable input. This forum really is a treasure trove of info and knowledge.


Dean in Milwaukee
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Old Sep 16, 2002, 09:58 PM
Scott Black, Montreal
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Quote:
Originally posted by Bob P
[B]Stalling is not a function of speed but of angle, and the stall angle for a particular airfoil is constant. If you push /pull too hard at any speed you will stall the wing.
There is a direct relationship between speed and AOA. The faster you are flying, the further from the stall AOA you are when you do your pullup. If you have lots of thrust, you won't bleed off as much speed on the pull-up and hence won't approach the stall AOA. Speed is the best protection from a stall.

Quote:
Like sblack says he needs 1/3 to 1/2 elevator just to fly level inverted, just think of the drag.
I am not having to push to maintain a large negative AOA, which of course would be draggy. I am having to push just to combat the cg being too far forward. That airplane has a symmetrical section.
Quote:
Flying an axial roll is very hard when you have to push so hard just to maintain a straight line when inverted!
That is mostly due to cg, unless you are talking about some god-awful massively undercambered thing. You can move the cg back on a Clark-y and have very nice axial rolls, and a minimal down requirement for inverted flight. Just use the dive test and set it up neutral. I have seen Keith do all sorts of outside maneuvers with his numerous Clark-Y winged airplanes.
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