ice cream cone airfoils?

[dave siegler]

I have been told that the airfoils on full scale airplanes like the CAP 21 and Extra 300 do not get draggy when they are loaded up.

The are draggy in level flight but the drag increases only a little at higher G loadings.  Might this make a good control line airfoil? 

They are called ice cream clone airfoils due to the blunt nose , forward high point and straight sections aft of the high point.


Also why does it work? They look kind of funny.   It ought to be easy to build. 



CAP 21 (TraCFoil) airfoil
Max thickness 15.6% at 15.7% chord
Max camber 0.6% at 23.9% chord
Source UIUC Airfoil Coordinates Database

[Howard Rush]

I have been told that the airfoils on full scale airplanes like the CAP 21 and Extra 300 do not get draggy when they are loaded up.

The are draggy in level flight but the drag increases only a little at higher G loadings.  Might this make a good control line airfoil? 

They are called ice cream clone airfoils due to the blunt nose , forward high point and straight sections aft of the high point.


Also why does it work? They look kind of funny.   It ought to be easy to build. 



CAP 21 (TraCFoil) airfoil
Max thickness 15.6% at 15.7% chord
Max camber 0.6% at 23.9% chord
Source UIUC Airfoil Coordinates Database

From what I've seen, that might be about right for a combat airfoil.  Gary James came up with something similar, if I remember right.  You can use XFoil or JavaFoil (free) to check it out.  The camber number above says that that particular airfoil is not quite symmetrical.

[dave siegler]

ok not an areo guy.  Can you tell from this? 

[Brett Buck]

From what I've seen, that might be about right for a combat airfoil.  Gary James came up with something similar, if I remember right.  You can use XFoil or JavaFoil (free) to check it out.  The camber number above says that that particular airfoil is not quite symmetrical.

   Any time I have flown an airplane with this sort of airfoil (and flaps), in a bunch of different configurations, the control loads were inordinately high. Perhaps the demons and/or wizard that inhabit your magic box has an reason, but it doesn't leap out at me why that would be the case.

     Brett

[Howard Rush]

ok not an areo guy.  Can you tell from this?  

Looks suspicious.  The Reynolds number range is too low for much but half A: stunt is about 400K, combat 700K or so.  The Cl is pretty low.  I found the stuff Gary sent me.  He was getting a Cl of about 2 at 1,100,000 Reynolds number, but that is a pretty high Reynolds number for us.

  Any time I have flown an airplane with this sort of airfoil (and flaps), in a bunch of different configurations, the control loads were inordinately high. Perhaps the demons and/or wizard that inhabit your magic box has an reason, but it doesn't leap out at me why that would be the case.

Me neither.

[Igor Burger]

   Any time I have flown an airplane with this sort of airfoil (and flaps), in a bunch of different configurations, the control loads were inordinately high. Perhaps the demons and/or wizard that inhabit your magic box has an reason, but it doesn't leap out at me why that would be the case.

     Brett

That could be reason why Beringers used so small flaps 

[Dan Bregar]

I have heard that the most recent airfoil technology that improves on the Ice cream cone airfoil is NASA's new Bannana Split Airfoil.  Still very hush hush.   

[Dennis Adamisin]

I have heard that the most recent airfoil technology that improves on the Ice cream cone airfoil is NASA's new Bannana Split Airfoil.  Still very hush hush.   

I tried the Blizzard-foil but I got - DQ'd

[Dan Bregar]

Dennis

I'm smiling broadly 

[Steve Helmick]

Gordan Delaney, Pat Johnston and (as Igor stated) the Beringers use airfoils very much like this. Too bad they can't work any better than that!  Steve

[dave siegler]

I am having issues with the xfoil part of Profili 2, I can't get it to change the Reynolds number, or aoa to what I want.  Looking to cut some cores for some 80mph combat planes and a structures guy ( combat flier brother Tom ) suggested this type of airfoil. 

I am mostly interested if the claim is true that it is kind of draggy in level flight but the drag doesn't go up quickly when turning. Perfect for 80 mph combat if true.  I will have to mess with the software some more to see if I can clac it.   


What would be a good airfoil to compart it too?  a 0018?   

[Bill Little]

Gordan Delaney, Pat Johnston and (as Igor stated) the Beringers use airfoils very much like this. Too bad they can't work any better than that!  Steve

That's right, Steve.  I mean Remi Berringer only won the World Championship..............

BIG Bear
RNMM/AMM

[Brett Buck]

Gordan Delaney, Pat Johnston and (as Igor stated) the Beringers use airfoils very much like this. Too bad they can't work any better than that!  Steve

  One of the clearest examples of the "high control forces" was on Gordan's maroon Pathfinder. The CG was way aft, the controls were set up about like you would normally do it, it was very similar to a lot of other models, but it had astronomical control loads. The only airplane I ever flew that had more was the B-17. Flying it back to back with the second Infinity, we both had the same observation. As I said, I dont know why but that's what it did in numerous cases.

    Even a tiny bit of curvature (like on the inestimable PTG's Diva and his other airplanes, which are similar except for the dead straight lines) and it goes away.

   As an aside, Phil has found something. His skinny airfoil with reasonably-sized flaps appears to be more capable than something like the Patternmaster-derivative Strega airfoil that is at least twice as thick with giant flaps.

      Brett

[Dan Bregar]

This is very interesting. 

[Steve Helmick]

  One of the clearest examples of the "high control forces" was on Gordan's maroon Pathfinder. The CG was way aft, the controls were set up about like you would normally do it, it was very similar to a lot of other models, but it had astronomical control loads. The only airplane I ever flew that had more was the B-17. Flying it back to back with the second Infinity, we both had the same observation. As I said, I dont know why but that's what it did in numerous cases.

    Even a tiny bit of curvature (like on the inestimable PTG's Diva and his other airplanes, which are similar except for the dead straight lines) and it goes away.

   As an aside, Phil has found something. His skinny airfoil with reasonably-sized flaps appears to be more capable than something like the Patternmaster-derivative Strega airfoil that is at least twice as thick with giant flaps.

      Brett

It would be interesting to try different airfoils on the same design, since we know that flap and elevator chords as well as control system geometry can also increase control loads. Why would the airfoil make that much difference? Also, if the flatback airfoil would make that much difference in models, wouldn't it also do the same in the aerobatic plane?  I'd think it would be even more disagreeable to the pilot of a 1:1 scale aerobatic plane.  Steve

[Brett Buck]

It would be interesting to try different airfoils on the same design, since we know that flap and elevator chords as well as control system geometry can also increase control loads. Why would the airfoil make that much difference? Also, if the flatback airfoil would make that much difference in models, wouldn't it also do the same in the aerobatic plane?  I'd think it would be even more disagreeable to the pilot of a 1:1 scale aerobatic plane.  Steve

    They invented aileron "spades" about the time this airfoil showed up.

    All I can say, is that every time fly an airplane with an airfoil like this it works pretty much the same way. Jim Aron's "Frankenstunt" was another. This was the airplane about which Bobby Hunt, at the 2001 NATs, said (to Jim) something like "you have a 200 mph airplane and 100 mph skills". After which I suggested he give it a try himself, because I had flown it and it wasn't going to fly a lot better than Jim flew it even with Jim Walker himself at the controls.

    Brett

[Tim Wescott]

    They invented aileron "spades" about the time this airfoil showed up.

I thought those things were just counterbalances.

[Brett Buck]

I thought those things were just counterbalances.

  Aerodynamic counterbalances, to *reduce the control loads*, which seems on point.

   Brett

[Tim Wescott]

  Aerodynamic counterbalances, to *reduce the control loads*, which seems on point.

Sorry, I didn't express myself well.  I'm looking at a very nice 3-view of an Extra 230, and the counterbalances are on pronounced levers but they are just cylinders with rounded ends -- so they're going to produce very little lift.  The 3-view is accompanied by photos of some three different planes, all with counterweights that match the drawing.  The book is copyright 1998.

However, I could see how replacing that weight with a flat plate would do just what you're talking about -- I assume that's used on this sort of airfoil in full-size practice?

I did try looking in the article to see why that airfoil is used and whether the plane is known for excess control forces, but the book is in German, which I know just well enough to feel that I ought to know what I'm reading, but not well enough to pull sense out of it without slowing down to about four paragraphs per hour.

[Tim Wescott]

I don't know if it applies in the case of those ice-cream-cone airfoils, but full-scale aerobatics has a number of maneuvers that are done in a stall; that airfoil may be an attempt to get a specific stall characteristic as well as a specific drag characteristic.

CLPA doesn't do stalls, so if that's where the advantage lies it's no advantage to us.

[Brett Buck]

Sorry, I didn't express myself well.  I'm looking at a very nice 3-view of an Extra 230, and the counterbalances are on pronounced levers but they are just cylinders with rounded ends -- so they're going to produce very little lift.  The 3-view is accompanied by photos of some three different planes, all with counterweights that match the drawing.  The book is copyright 1998.

   I think you are talking about mere mass balances. Spades are aerodynamic devices:

[Howard Rush]

I eschewed those mofos in favor of balance tabs, which I thought would be more linear.

[Mark Scarborough]

I eschewed those mofos in favor of balance tabs, which I thought would be more linear.

and no doubt easier to paint and polish around,, not to mention they probably do not snag cats, dogs or clothing nearly as much,,,

[Howard Rush]

  Any time I have flown an airplane with this sort of airfoil (and flaps), in a bunch of different configurations, the control loads were inordinately high.

Airplane Performance Stability and Control by Perkins and Hage, a book that sits on Keith Trostle's shelf, says, "The variation in the hinge moment parameters with airfoil section arises mostly as a function of the included angle at the section trailing edge.  The hinge moment parameters will be more negative for small included angles and more positive for large included angles."  That looks like what Brett felt.  Notice Igor's airfoil.  

This is an example of oversimplification in Wild Bill's articles.  He shows a graph of hinge moments without a reference and leaves out the ugly parts.  

[dave siegler]

just one more question is there anything to be learned by wasting anymore time on xfoil?


What charastic am I looking for to prove or disprove the original quandary (no large increase in drag when loaded) ? 


Also I see Berringers fly every summer at Oshkosh.  They borrow an old oilsoaked flight streak with one a fox and fly a very superior pattern. 

So I can't say the airfoil they picked made a differnce.   

[Igor Burger]

May be following pictures answer something

The first airfoil is my which I used on MAX, the second is NACA with thickest point moved forward and LE radius enlarged to match. That was closesd I was able to make similar to that with flat sides with that airfoil generator.

It shows two things.

1/ It shows that distribution of pressure coefficient is flatter on my airfoil (higher pressure coefficient in 50% of the chord), while that second has higher peaks on LE and at corner of hingeline. The pressure coefficient is fuction of curvature of the airfoil surface so it is reason that my round airfoil makes it higher in middle and that the corner at hingeline of the second airfoil will make such peak. My airfoil is designed not to have such corner at hingeline on purpose - it is 18% NACA airfoil WITHOUT such place.

2/ it also shows that those flaps are counterproductive. Thay make stear on lift curve and it is last I want to have on stunter wing (when I make "up" on handle, it will make less lift - it makes fine contrilling inpossible). That is reason why it is better minimize flap area. And that is also reason why we do not use flaps on indoors with flat wings, they do it on flat wing even more and even stronger.

[Tim Wescott]

Dave: Igor's results don't show a hugely different characteristic in drag (it's hard for me to tell -- he didn't show Cd vs. AoA which is what I trained myself to, at the same time that the world went with Cl vs Cd curves).  It does show a well-defined stall characteristic, which may or may not be the point.

Igor: "Thay make stear on lift curve..."  Sorry.  Usually your English has a slight "written accent" but is otherwise excellent, and you make things perfectly clear.  In this case 'stear' isn't helping me.  Do you mean they make a kink in the Cl vs. AoA curve?

[Igor Burger]

hhh ... sorry I mean stair not stear ... simply that curve goes somewhere and then abruptly continues at different level, it is not only kink, it some kind of discontinuity, it will make probably also hysteresis to some extent

[Igor Burger]

Dave: Igor's results don't show a hugely different characteristic in drag (it's hard for me to tell -- he didn't show Cd vs. AoA which is what I trained myself to, at the same time that the world went with Cl vs Cd curves).  It does show a well-defined stall characteristic, which may or may not be the point.

remember, those my polars are for FLAPPED airfoil, unflapped airfoil can make some extra drag (compared to nicely curved NACA 0016) at 0 AoA because of suboptimal transition from that elliptic to straight part of surface

[Howard Rush]

Thay make stear on lift curve

stear? 

[Brett Buck]

stear? 

   As explained above, I think he means a "stair" - you and I would have said "step" - in the response. One wonders how well that it modeled in the software.

     Brett

[Howard Rush]

just one more question is there anything to be learned by wasting anymore time on xfoil?

Good question.  I don't think I understand it enough to get good results.  I've been told that Ncrit should be reduced to 6. 

Also, sometimes it refuses to plot stuff I tell it to.  I had some success fiddling with the XFoil settings submenu of the Polars menu: stuff like alpha angle step and max alpha range.     

[Tim Wescott]

   As explained above, I think he means a "stair" - you and I would have said "step" - in the response. One wonders how well that it modeled in the software.

We English-speakers like these little linguistic traps.  Of course "stair" means essentially the same thing as "stairs", and any one part of a "stairway" is a "step" or a "tread".  How could that not make sense?

Dave, I don't know if you could see what Igor was saying about flap moment in the xfoil output, but in the graph of pressure distribution it clearly shows that flaps on straight-backed airfoils suck.  Wait -- I mean that in the graph of pressure distribution it shows a much more pronounced pressure loss above the flap than the all-curved one; this in turn means there's a large restoring moment on the flap.  In English that "large restoring moment" means "high control force".

[proparc]

I have heard that the most recent airfoil technology that improves on the Ice cream cone airfoil is NASA's new Bannana Split Airfoil.  Still very hush hush.   

I've heard of them too. They use ice cream sprinkles for turbulators.

[Brett Buck]

Good question.  I don't think I understand it enough to get good results.  I've been told that Ncrit should be reduced to 6.  

Also, sometimes it refuses to plot stuff I tell it to.  I had some success fiddling with the XFoil settings submenu of the Polars menu: stuff like alpha angle step and max alpha range.    

   That's why I had pursued getting the CFD code from NASA so I could dig into the guts of it, and see why it did things unexpectedly. I don't care much about the windowing part of it, I can figure out how to plot it myself. I have it somewhere but I never got it working properly and then my spider-monkey-like attention span expired.

   I also have to question how well the hinge line is modeled. I can see what it shows in the figure but we have been of the opinion that even tiny details of the shape around the hinges seem to make a huge difference in the results. For instance it appears that merely knocking the corner off to about 1/8" radius keep the air going over the flap a lot better. That would make even more difference with the sharp angles involved here (as opposed to Igor's {and Al Rabe's)) airfoils that are intentionally shaped to fair in the flap better while deflected.

   As an aside   -  Al's car hood VS. the thousand of man years behind CFD programs. Discuss!

    Brett

[dave siegler]

I can't get re above 500000, but this may be it.  What do you think?   

[Gordan Delaney]

  One of the clearest examples of the "high control forces" was on Gordan's maroon Pathfinder. The CG was way aft, the controls were set up about like you would normally do it, it was very similar to a lot of other models, but it had astronomical control loads. The only airplane I ever flew that had more was the B-17. Flying it back to back with the second Infinity, we both had the same observation. As I said, I dont know why but that's what it did in numerous cases.

    Even a tiny bit of curvature (like on the inestimable PTG's Diva and his other airplanes, which are similar except for the dead straight lines) and it goes away.

   As an aside, Phil has found something. His skinny airfoil with reasonably-sized flaps appears to be more capable than something like the Patternmaster-derivative Strega airfoil that is at least twice as thick with giant flaps.

      Brett

   


Brett, I half to disagree . It was not the airfoil that was the problem it was the guy who trimed the plane. I move the C/G back a little more and took out some flap. That took the heavy load you felt when you flew it. When I flew it after you I knew I had to fix that which I did. I have the same airfoil on my Pathfinders. Every one that has flown my Little twin thought it was a very good plane. I don`t remember if you flew it or not. Howard Rush likes it a lot.Ted Fancher liked it. Bobby Hunt Thought is was one of the best planes he had ever flown. So I think the airfoil is fine. You can move the c/g back further that a conventional  airfoil. Not saying there is anything wrong with the curved airfoils. Most stunt ships use the Conventional airfoil for ever.

Just my thoughts,
Gordy

[Brett Buck]

   


Brett, I half to disagree . It was not the airfoil that was the problem it was the guy who trimed the plane. I move the C/G back a little more and took out some flap. That took the heavy load you felt when you flew it. When I flew it after you I knew I had to fix that which I did. I have the same airfoil on my Pathfinders. Every one that has flown my Little twin thought it was a very good plane. I don`t remember if you flew it or not. Howard Rush likes it a lot.Ted Fancher liked it. Bobby Hunt Thought is was one of the best planes he had ever flown. So I think the airfoil is fine. You can move the c/g back further that a conventional  airfoil. Not saying there is anything wrong with the curved airfoils. Most stunt ships use the Conventional airfoil for ever.

     I didn't mean to imply it was bad but that it was different. Jim's airplane was the same way.

    Brett

[Gordan Delaney]

Brett.
Which Jim? Was it Jim Hoffman`s ?

Gordy

[Brett Buck]

Brett.
Which Jim? Was it Jim Hoffman`s ?

Gordy

    No - the inimitable Uncle Jimby. His airplane had a very extreme version of the same thing (very close to the drawing shown above) and it was bad in many ways, not the least of which was that it took two hands to get it out of level flight or an absurdly aft CG. It had other problems, too.

      Brett

[phil c]

ok not an areo guy.  Can you tell from this? 

Dave, the reason the "ice cream cone" works for combat is that from a lot of experimentation it gives much better controllability than almost anything else.  The graphs show pretty much what I have seen.  Any reasonable airfoil has a fairly low drag up to ~.5 Cl.  Some of them, the ICC included have a fairly wide flat drag bucket so to speak.  That means the control feel doesn't change much in the useful range.  Nobody flys a combat model anywhere near stall because the plane will slow down and get sloppy really fast so the max lift doesn't even enter into it.

Stunt is another story.  Al Rabe's and Igor's modifed airloils are designed to work with flaps and produce significantly more lift.  They do that well.  So you can build a heavier plane or use less control(more precision) for the same turn.  Using a flap on a flatback airfoil can work too.  But it does have the possibility of that "step" in the lift curve.  Hit the controls hard enough and the lift suddenly drops a significant amount.  I've seen it happen a number of times on somewhat overweight models that might hit a gust at a bad time, or get slowed down overhead and simply fall out of the air.  A reasonable explanation is the "step" in the lift curve.

[frank williams]

One interesting thing about the "we" (wedged ellipse) airfoils is the potiential for hystersis.  This data is from UIUC wind tunnel data.  The E472 has a definitely higher max Cl, but after stall it doesn't return by the same path.  These airfoils are more "combat" looking than "stuntish".  The tested Re is 500k.  I repeated the chart with the up and down lines highlighted.

[EddyR]

My 1960 profile combat models all used what you are calling ice cream foils. My not so strong motors would put them OK. I built some with Flite Steak airfoils but they were to slow. Speed was more important at my level.     I used K&B 35's and later non ball bearing ST/35 motors.
Ed

[phil c]

I am having issues with the xfoil part of Profili 2, I can't get it to change the Reynolds number, or aoa to what I want.  Looking to cut some cores for some 80mph combat planes and a structures guy ( combat flier brother Tom ) suggested this type of airfoil. 

I am mostly interested if the claim is true that it is kind of draggy in level flight but the drag doesn't go up quickly when turning. Perfect for 80 mph combat if true.  I will have to mess with the software some more to see if I can clac it.   


What would be a good airfoil to compart it too?  a 0018?   

Dave, you can't go wrong with that kind of airfoil for combat.  It works remarkably well on a unflapped stunter too.  Typical version has the high point at 18%+/- and a thickness that is convenient,  ~15% at the root and 18% at the tip.  Use a fair amount of taper.  The tip should be no more than 50% of the root chord.  Another newer trick is to leave the trailing edge squared off 2-3% of chord.  The another is to match the weight and speed so the plane has lots of line tension at all times.  Keeps it from getting blown in by a gust.  Various designers are doing things like engine offset and building offset in aerodynamically to keep the plane pulling out hard.

[Brent Williams]

Here is a picture to illustrate the "ice-cream cone" style Pathfinder airfoil compared against the airfoil from a Sig Twister.

[Dennis Toth]

If you look at some of the work that Al Rabe did when developing the Mustang's (Development of a Thoroughbred article) he started with the Nobler airfoil which is close to the Ice cream cone foil and found that it was pretty good but with flaps you could get more lift with added thickness and a high curved aft portion of the foil (similar to the Twister section Brent posted). He also found that building in the flap as part of the aft airfoil did not work as well as the simple Nobler design.

Seems the ice cream cone airfoil would work well on a non-flapped ship but with flaps more thickness and rear curvature works better for stunt.

Best,     DennisT

[Avaiojet]

Den,

You flew my Pathfinder. It has the ice cream cone airfoil.

[Dennis Toth]

I think the point being not that the ice cream cone airfoil won't work, but rather how is it compares to other airfoil options you could use. The Twister/Rabe type airfoil can care more weight for a given area then the ICC airfoil. However, the T/R airfoil is more difficult to build on a simple building board without a special jig. Always trade-offs.

Best,     DennisT

[Ted Fancher]

I don't know if it applies in the case of those ice-cream-cone airfoils, but full-scale aerobatics has a number of maneuvers that are done in a stall; that airfoil may be an attempt to get a specific stall characteristic as well as a specific drag characteristic.

CLPA doesn't do stalls, so if that's where the advantage lies it's no advantage to us.

Blunter leading edges in general will stall at a higher angle of attack than sharp leading edges but when they do stall they stall much more abruptly and completely which is great for untethered type stalled maneuvers like snap rolls, spins, lomcevoks (sp?), etc.

The Imitation has an airfoil similar to those (very blunt and with a high point at about 21%...yes, including the flaps) and, under power, pretty much couldn't be stalled. It flew fine patterns effortlessly.  It wasn't, however, wise to try to stretch a glide as when it stalled it was "done" as in "belly flop" versus a "swan dive!"  Unlike the ice cream cone airfoil there were/are no flat planes anywhere on the airfoil profile.  The airplane also utilized very narrow chord flaps (and the now ubiquitous smaller chord elevators) and the control loads were never anything but light  and comfortable with excellent feedback to the flyer.  Easiest stunt ship to fly well I've ever had.

Ted

Edited for spelling.

[Brent Williams]

Here is a plan sheet for the Imitation.  Gives a good view of Ted's airfoil.

Such a great looking design.