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Author Topic: Tip Shape  (Read 2132 times)

Online Ken Culbertson

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Tip Shape
« on: August 22, 2018, 11:47:20 AM »
When I was designing in the 70's I did everything to look cool and if it was light it flew OK.  Now we have a reason for everything but unfortunately I was absent the day we wrote the instructions.  So when I have a question I look at the pictures of the newer designs and search the archives.  If I don't find the answer I ask another of those "we have been over this before" questions.

So, Does Tip Shape matter?

Not the top view.  We have plenty of discussions on that one I can find.  I am talking about the "airfoil" or what you see from the front.  All of the pictures I find seem to carve the block by extending the airfoil and rounding the end (A).  I have always tapered the tip (B) and I use 3" blocks.

Does it matter?  Am I creating a tip stall when it could be avoided?

Ken
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Offline L0U CRANE

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Re: Tip Shape
« Reply #1 on: August 22, 2018, 01:08:07 PM »
Hi, Ken,

Of course it matters. Everything matters, the question then becomes: "How much does it matter, and why?"

Not a whole lot, and various successful models have had drastically different tips in both front view and plan view.

SIG Twister has just a thicker last rib, square to the top and bottom surfaces. Its near twin, Banshee, has tips rounded in plan view and tapered to that outline from spar height in front view.

TF Flite Streak has the tip increase span from LE to TE in plan view, with straight riblets from tip rib height to the tip outline sheet.

Many I-Beamers had no 'riblets' from last rib to the plan view tip outline, which in turn appeared in various thicknesses.

That 'sagging' tip covering sort of looked like a Horner-type tip, which, most often used on planes not intended for inverted or outside g maneuvering. The idea is a quarter-round curve out from the bottom to the top skin, could re-aim the tip vortex and recover some usefulness of upper surface's wing area.

Aldrich's outward swept plan view sort of showed the same thinking. Tip vortices form when a wing is lifting. the lifting side develops lower pressure - could be thought of as 'sucking' that surface upward. The lower surface has higher pressure over its area. Well, duh...

Think of that difference when you get to the tip zone. The lower pressure area also draws in local air as well as the higher pressure air tending to flow outward. Outward? Sure. Fuselage acts as a divider. So the direction of local flow tends outward below, and inward above (thinking of lifting direction. The outward flow increases as you look at locations increasingly far from the divider at the fuselage, and increasingly slanted away from the fuselage.

When you look at the flow at the tip, you can visualize how it will roll-over from higher pressure to lower. That spin forms a vortex. It too tends to flow at an angle, and widens as it moves back along the chord. That interferes with the lifting action on the upper tip area.

The Horner form changes the flow by leading it up a section opposed to the roll-over tendency of the airflow. The Flite Streak outward swept tip delays when and where flow can roll over both upright and  inverted... Its thin tip sheet may affect the depth available for roll-over, which may make the vortex smaller.

Are these all significant enough to deal with seriously? Opinions, I'm sure. will differ on that. We have Stunt models which pull serious lift for upright and inverted figures. A single direction Horner section might work one way, but be worse the other way. The "sagging" covering on an I-Beamer's tips may have some effect both ways.

But the general conditions of CL flight may make all this worrying into a much less crucial thing. We have greatly overpowered models in almost all cases. (A heavy scale model may be an exception, but few of those pull hard maneuvers.)

We fly a large, circular path, so the airflow is affected somewhat. We have a turbulence causing condition at the inboard tip - the lines and clips.

Square end rib tips, according to studies by some, are at least as "effective" as any other form.

My opinion? Don't get into  a major sweat about the front view. It may be better to avoid large rounded cross section, which conforms to the roll-over tendency, practically encourages it. Your second sample may be slightly more effective....

Do what looks good to you, it all works.

That's some theory on it and it seems to work. How much effect on control-line models? I don't know...
\BEST\LOU

Online Ken Culbertson

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Re: Tip Shape
« Reply #2 on: August 22, 2018, 01:21:06 PM »
From the many types of tips, I think it matters only to the viewer. D>K  And of course I know I am wrong. Other will have esoteric views and ideas and theories. H^^
Thanks a whole bunch!  Like I said, I was absent during the great technological revolution where we became obsessed with aerodynamics.  I much prefer the parabolic tip, I was afraid I was creating a problem - not good.

Ken
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Online Ken Culbertson

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Re: Tip Shape
« Reply #3 on: August 22, 2018, 01:43:06 PM »
You should have asked, "in 25 words or less, does it matter how they are shaped?"   LL~ LL~ LL~ LL~ LL~ LL~ LL~ LL~ LL~ LL~ LL~ LL~ LL~ LL~ But then Lou is an injuneer. H^^
That's OK, I am just as interested in "why it doesn't matter" as I am in "if it matters".

ken 
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Re: Tip Shape
« Reply #4 on: August 22, 2018, 03:06:51 PM »
I think that for utility, it matters. To me, having some place to mount the adjustable lead outs and tip box could help decide the tip design. After that, or before that, depending on what matters to you, is aero design considerations. After that comes aesthetics.

So for me, tip design would be:
1. Function/build ease/accuracy
2. Aero concerns
3. Looks

I am wanting to build a foam cored, vacuum bagged, carbon fiber wing, and the tip treatment has caused me a lot of pondering. I might and likely will, go with the Twister style tip for first designs, due to simplicity. I thought that I could make some neat molded tips after that would give it a better look, but attaching them would be problematic.


Interesting topic!!
Regards,
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Re: Tip Shape
« Reply #5 on: August 22, 2018, 06:13:38 PM »
..
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Re: Tip Shape
« Reply #6 on: August 22, 2018, 06:26:05 PM »
I am very partial to the ĎVí tip shape.  They arenít as attractive perhaps but they do wonders flying through turbulence to smooth or stabilize the rocking motion.  Bob Gieseke did as well and would suggest in his last few years to anyone building his airplane to use the V.  I think they have a sort of polyhedral effect like a free flight airplane which is just rock steady in flight.  Many of the early I beamers had tips like this- not sure if they understood the aerodynamic bonus or they were just simple but most of those airplanes cut the wind like hot butter and needed to given the low power engines of the day.  If I have anything negative about some of my own earlier designs itís that they DIDNíT have these tips and were more temperamental in less than good flying conditions.

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Online Ted Fancher

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Re: Tip Shape
« Reply #7 on: August 22, 2018, 06:27:59 PM »
Back in the late 60s/early 70s I built two reasonably successful airplanes airplanes that should put your concerns to rest.  Although they didn't have actual names they came to be known as "Moby Dick" and/or "the Whale" because neither was what you would call a light weight.  The latter, however, which is the only one I have any pictures of was quite competitive in pretty stiff local competitions and placed in the top 20 at the 1976 Nats so it wasn't a dog.  Take a look at the pictures.

The first is a picture taken reasonably recently hanging on the wall in my den after hanging in the local hobby shop for, literally, decades under flourescent lights that managed to suck all the red out of the trim on the top of it.  Pretty standard "jet era" look with the exception of the wheel pants masquerading as drop tanks and the curled down wing tips which aren't readily visible.  The other picture is of the left wing tip which fairly clearly shows it was not a "standard" type of tip.   


It's the best picture I've got of the wingtip showing the "Horner effect' tips. I've been asked thousands of times if it would turn outside and thousands of times I said "yup, just fine." It was, however, a sensible question, one that I asked with my fingers crossed while gluing them on.

The Horner tips, by the way, were and likely remain more or less SOP for STOL (Short Take Off and Landing) aircraft, especially on early STOL  Cessnas, etc. modified by after market modifiers. The theory was they would minimize spanwise flow and the resultant high levels of drag and turbulent vortices off the wingtips at high angles of attack and allow flight at lower airspeeds. The Moby Dick didn't appear to testify to the worrisome reverse assumptions made by most who think about it!  Don't ask me why, however.  All I know is that I wasn't aware of any difference in response to outside vice inside maneuvers.

Ted

Not sure why but you may have to "scroll" the full top view picture to see the whole thing.  I'm not very "nerdish" with this stuff.

P.S. Modified to replace the former "...at risk" to the orange highlighted "...to rest" in the first sentence of this message.  Not sure how the original text came about other than the sentence must have been edited at some point and the unintended two words not modified.

P.P.S. FWIW the first Moby Dick also had the Horner type tips on the stab/elevator, again no adverse performance resulted.
« Last Edit: August 28, 2018, 04:58:47 PM by Ted Fancher »

Offline Matt Spencer

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Re: Tip Shape
« Reply #8 on: August 22, 2018, 08:52:25 PM »

Offline Matt Spencer

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Re: Tip Shape
« Reply #9 on: August 22, 2018, 08:55:03 PM »
And the Jack Sheeks ' that looks round to me ' ( not being derogatory - just descriptive , as it is a master of visual deception , as to ' Round ' . )





Dunno if anyone enirely believed the explanation , as to the photogaph .

----------------------------------------------------------------------------------

And a fairly good one of the ' End Plate ' ( on ailerons here ) type set up .



Where if your tips run full chord , can be on the flap or tip ??

----------------------------------------------------------------------------------

intresting to see the Vortex eliminator type tip Ted .
On the Phanton ( Aeromodelor Mustunt Wing  & moments / areas )
tips were parrallel to ribs , but 45 Deg angled , looking for & aft  .
Theory was evened lift , upright & inverted - qwith the flat 4 in wide center fuselage underside . .

However , this one at 47 Oz. , got a bit of mushing / unequal ( inv./uprt). under duress.on INSIDES , flying slow in hot or disturbed air .

Extensions of 1/8 sheet added to the 45 Dg. angled tips , from the high point aft - about 1 1/2 wider going back ( at the T E )
Going down to Bill Bells , was blowing firmly but steady .
First test , ' Liftier ' inside resonse , rounds & squares .
Trimmed sweep angle , 1/2 in . + a bit less unequal .
Trimmed to 1/2 O.A. wider at  T E , = Ripped round repeated Sq. 8 , horiz , equal reponce ? but right side ( insides )
6 - 12 in lower than left / outsides .

this in a firm steady 15 Kt plus ' good country air ' . near sea level (Flying  requireing your undivided attenion )
 / 500 Ft maybe . temp around 15 C.

The most noticeable thing was , the Tips werent ' Walking Around ' , as they had previously , in V similar conditions .
where the forces were apparenty sufficent to presumeably pull the VORTICES about . The Vortices pulling the Wing Tips, wobbly like .

The      Extensions appeared to eliminate the Vortices . Or cut them from 20 - 30 foot to 2 or 3 foot trail .
Definately eliminated the Tail ( tips ) wagging the Dog .
 ( This was the white then Camo. F4E , NOT the Blue One - Which had a development of these )

« Last Edit: August 22, 2018, 09:19:50 PM by Matt Spencer »

Offline Matt Spencer

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Re: Tip Shape
« Reply #10 on: August 23, 2018, 11:34:37 PM »
Not Mine .  :(


Online Ken Culbertson

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Re: Tip Shape
« Reply #11 on: August 24, 2018, 06:46:12 AM »
Back in the late 60s/early 70s I built two reasonably successful airplanes airplanes that should put your concerns at risk.  Although they didn't have actual names they came to be known as "Moby Dick" and/or "the Whale" because neither was what you would call a light weight.  The latter, however, which is the only one I have any pictures of was quite competitive in pretty stiff local competitions and placed in the top 20 at the 1976 Nats so it wasn't a dog.  Take a look at the pictures.

The first is a picture taken reasonably recently hanging on the wall in my den after hanging in the local hobby shop for, literally, decades under flourescent lights that managed to suck all the red out of the trim on the top of it.  Pretty standard "jet era" look with the exception of the wheel pants masquerading as drop tanks and the curled down wing tips which aren't readily visible.  The other picture is of the left wing tip which fairly clearly shows it was not a "standard" type of tip.   


It's the best picture I've got of the wingtip showing the "Horner effect' tips. I've been asked thousands of times if it would turn outside and thousands of times I said "yup, just fine." It was, however, a sensible question one that I asked with my fingers crossed while gluing them on.

The Horner tips, by the way, were and likely remain more or less SOP for STOL (Short Take Off and Landing) aircraft, especially on early STOL  Cessnas, etc. modified by after market modifiers. The theory was they would minimize spanwise flow and the resultant high levels of drag and turbulent vortices off the wingtips at high angles of attack and allow flight at lower airspeeds. The Moby Dick didn't appear to testify to the worrisome reverse assumptions made by most who think about it!  Don't ask me why, however.  All I know is that I wasn't aware of any difference in response to outside vice inside maneuvers.

Ted

Not sure why but you may have to "scroll" the full top view picture to see the whole thing.  I'm not very "nerdish" with this stuff.
Ted - Do you think that this type of tip might affect wake turbulence on calm days?  It certainly redirects it but does it help or hurt.  I have used them on sailplanes both pointed up and down.  Both helped with tip stalls on slow tight turns and I don't recall if one way was better than the other. We don't do slow tight turns but we do get tip stalls!

Ken
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Offline TDM

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Re: Tip Shape
« Reply #12 on: August 28, 2018, 09:25:39 AM »
Ken your goal should be elliptical lift distribution on the wing. That is what makes the model fly well. So for sure B is better than A. If you do that along the wing span is even better yet.
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Online Ken Culbertson

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Re: Tip Shape
« Reply #13 on: August 28, 2018, 09:50:32 AM »
Ken your goal should be elliptical lift distribution on the wing. That is what makes the model fly well. So for sure B is better than A. If you do that along the wing span is even better yet.
Thanks - I am just now getting used to doing things because they work better instead of the way we did it in the 70's which was to make it look good first then hopefully make it fly well.  I am glad that at least one thing I do is right!  I don't like the boxy style of the current wave of piped planes and I am not good enough at building to do a Walker style plane so I am stuck in the middle trying to make something that looks good to me fly like the big boys.  That is what makes it fun.  To quote Brett, Stunt it is a journey not a destination.

Ken
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Online Howard Rush

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Re: Tip Shape
« Reply #14 on: August 28, 2018, 04:33:24 PM »
Ken your goal should be elliptical lift distribution on the wing. That is what makes the model fly well.

How do you figure that?
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Online Ted Fancher

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Re: Tip Shape
« Reply #15 on: August 28, 2018, 05:26:30 PM »
Ted - Do you think that this type of tip might affect wake turbulence on calm days?  It certainly redirects it but does it help or hurt.  I have used them on sailplanes both pointed up and down.  Both helped with tip stalls on slow tight turns and I don't recall if one way was better than the other. We don't do slow tight turns but we do get tip stalls!

Ken
Hi Ken,

You could probably make the argument that they "should" reduce tip vortex generated wake turbulence in "upright" level flight but, if you accept that premise as gospel, you'd be obliged to expect increased turbulence  due to accelerated tip vortices in inverted flight in calm air conditions.  There were lots and lots of flights on both ships and their performance was nothing out of the ordinary for yet another couple of modestly overweight modified Noblers.

FWIW (not much nowadays), the only wing planform/paraphernalia parameters I felt had verifiable "significant" impact on stunt ship performance were the entire wing's aspect ratio and the flaps % of chord and span. (+, to a more modest degree whether the builder/flier sealed the flap hinge lines...which probably doesn't fall neatly into our discussion).

Ted

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Re: Tip Shape
« Reply #16 on: August 28, 2018, 07:28:57 PM »
Hi Ken,

You could probably make the argument that they "should" reduce tip vortex generated wake turbulence in "upright" level flight but, if you accept that premise as gospel, you'd be obliged to expect increased turbulence  due to accelerated tip vortices in inverted flight in calm air conditions.  There were lots and lots of flights on both ships and their performance was nothing out of the ordinary for yet another couple of modestly overweight modified Noblers.

FWIW (not much nowadays), the only wing planform/paraphernalia parameters I felt had verifiable "significant" impact on stunt ship performance were the entire wing's aspect ratio and the flaps % of chord and span. (+, to a more modest degree whether the builder/flier sealed the flap hinge lines...which probably doesn't fall neatly into our discussion).

Ted
Since we fly a whole lot more level upright than inverted it should help more than it hurts.  The one that gets me the most is the down leg of the hourglass.  I seem to hit that wake at least once a session.

Thanks for the response - Ken
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Offline Matt Spencer

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Re: Tip Shape
« Reply #17 on: August 29, 2018, 09:50:49 PM »
ANTI VORTEX .

On this outstanding quality infinate resoution  :-\ picture ,

You can see , effectively - The Outermost Rib / is at 45 degree to vertical . AND Horizontal .



Flying at the Pink Pig , in a severe blow , irregular air . The Tips were walking all over the place . about 5 or 6 inches
yaw / roll , etc & so on . !

However ,
On Return to the Secret Laboratry , out Hero Glued 1/8 Sheet from the extremity there at the ends , on the Top even ! -
to the rear bit , right out wide . So the span went up a few inches or more . Then Doped and filed in smooth .

Flights in blustery Autumnal Air revealed 1/2 outer sweep to be something like right , if not perfect . So + 1 in O/A , to length at spar
at the T. E. now .
Returning to the same magnificent field , similar but steadier windflow . From the South West . S?P

Seemed dicideably more ' locked in ' to straight flight .

Double - ish initial force at handle ( From not much to now  asper a 40/46 ship ) to ' kick ' into Square corners & deflect into rounds .

As In it was LOCKED IN to Level / Stright mode decidely & observeably .

From above ' ground effect ' ( wind ) windflow hight , where previously it was acting fishilly ( Literraly , as a fish . tension Wise )

It Had Steadied up decidedlly . Primarilly in YAW , and decidedly in ROLL , So Now Flying more smoothly, with S F A divergance ,
but decidedly greter contol loads & tension .

Wind wouldve had a KITE firm & steady going up decidely . No Calm Spots about . Almost a ' no hats ' day .

Another Outstanding Picture  ??? of the original , circa 1976 . at the Forrest Hill Resivior . Overlooking Auckland ,on the North Shore .

Had the Same everything bar full span flaps . The ' 2000' one had the end few inches of flap fixed , so the extensions were persuaded
to flow through flat on the rear portion , after having followed the Top from forward of the high point aft ( as if from a ruler laid to the ends ).



Wont Throw a picture of the third , which had refined versions of the tips described , when built .
Appears a larger higher A/R , but is only + 1 in odd spa. A few in at the T E .
The ' evaluation' is based on many hundreds of flights on that same airframe .

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Re: Tip Shape
« Reply #18 on: August 29, 2018, 09:54:25 PM »
o.k. THEN .  :(

You can see them fairly clearly ? here .



No Brainer to fly if wind under 18 Kt/ mph ish . gotta doa nuther or two , maybe a .46 one also . Bigger .

it was BUILT from THIS Plan .  S?P


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Re: Tip Shape
« Reply #19 on: August 29, 2018, 10:45:00 PM »
FWIW (not much nowadays), the only wing planform/paraphernalia parameters I felt had verifiable "significant" impact on stunt ship performance were the entire wing's aspect ratio and the flaps % of chord and span. (+, to a more modest degree whether the builder/flier sealed the flap hinge lines...which probably doesn't fall neatly into our discussion).

Yep, anything you do with wingtips or lift distribution to affect induced drag (tip vortices) can be done with a wee change in aspect ratio.  And who cares about getting the aerodynamically "best" wingtips when the aspect ratio (with the wee adjustment for tip shape) is picked to be what it needs to be to balance drag with windup in turns.  Just pick a tip shape that looks good to you and doesn't do anything perverted in yaw. 
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Re: Tip Shape
« Reply #20 on: August 30, 2018, 07:14:07 AM »
Howard I would think any aerodynamicist  would confirm that lift distribution is important to get the best handling on a plane. Actually in real life you see that in all high end performance machines and can be accomplished with straight taper with morphing airfoils, as well as the elliptical platforms. Windi's Spitfires flew great in the wind as well as the Sharks and they all have on thing in common at very least, I will let you figure it out.
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Online Ken Culbertson

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Re: Tip Shape
« Reply #21 on: August 30, 2018, 07:50:00 AM »
....with straight taper with morphing airfoils, as well as the elliptical platforms.
How do you do that?  Are you referring to the taper in thickness?

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Re: Tip Shape
« Reply #22 on: August 30, 2018, 09:07:54 AM »
Same airfoil with elliptical wing shape, or taper wing with thinner tips will accomplish about the same elliptical lift distribution more or less. Either way the wing will gradually get thinner towards the tip.
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Re: Tip Shape
« Reply #23 on: August 30, 2018, 10:13:56 AM »
Same airfoil with elliptical wing shape, or taper wing with thinner tips will accomplish about the same elliptical lift distribution more or less. Either way the wing will gradually get thinner towards the tip.

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Online Ken Culbertson

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Re: Tip Shape
« Reply #24 on: August 30, 2018, 11:08:29 AM »
Same airfoil with elliptical wing shape, or taper wing with thinner tips will accomplish about the same elliptical lift distribution more or less. Either way the wing will gradually get thinner towards the tip.
I thought so.  This is what I am doing.  It was the morphing of the airfoil I thought was a problem.  Appears it is a +.  That thick nice curve at the root gets pretty flat at the tip....made me nervous.

Ken
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Re: Tip Shape
« Reply #25 on: August 30, 2018, 01:44:54 PM »
Since we fly a whole lot more level upright than inverted it should help more than it hurts.  The one that gets me the most is the down leg of the hourglass.  I seem to hit that wake at least once a session.

Thanks for the response - Ken
Ken
We also spend a  lot of time in outside maneuvers, so you may want to give that a bit more thought, Also I have built tips  both ways, and it seems  to NOT matter  on my planes, as long as they are accurate, and top vs bottom  exact.

Regards
Randy

Online Ken Culbertson

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Re: Tip Shape
« Reply #26 on: August 30, 2018, 03:22:37 PM »
it seems  to NOT matter  on my planes, as long as they are accurate, and top vs bottom  exact.

Regards
Randy
That seems to be the consensus.  We used to just glue, sand and paint till we looked at it and smiled, then hoped it flew.  Now, everything seems to have a chapter in a physics book.  Brave new world!

Thanks - Ken
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Offline Trostle

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Re: Tip Shape
« Reply #27 on: August 30, 2018, 03:26:54 PM »
Howard I would think any aerodynamicist  would confirm that lift distribution is important to get the best handling on a plane. Actually in real life you see that in all high end performance machines and can be accomplished with straight taper with morphing airfoils, as well as the elliptical platforms. Windi's Spitfires flew great in the wind as well as the Sharks and they all have on thing in common at very least, I will let you figure it out.

You make several comments here that may be more applicable to full scale aerodynamics than directly to our CLPA models.  Elliptical lift distribution may be desirable for many full scale aircraft designs where lower drag, range, speed and payload become important design criteria.  None of those requirement really come to play in our quest for the ultimate CLPA design where reducing drag and improving range and speed and payload  is of little consequence.  If reducing drag was all important, all of our top designs would probably have retracting gear.  We would be looking for a more "streamlined" airfoil rather than our 18%+ sections with rather blunt leading edges.  Sure, we want a "clean" airplane, but our current designs pull all of that drag (thick airfoils, extended LG, induced drag, lines) with horsepower.

What is "best handling on a plane" that you are looking for. For our CLPA designs, that is a rather elusive criteria.

Yes, a straight tapered wing on our stunt ships has some advantages aerodynamically and structurally.  However, in my humble opinion, actual tip shape seems to have little affect on the performance of the CLPA model.

Yes, the elliptical wings look nice as do those with at least elliptical looking tips.  It would be interesting to do some research on the tip shapes of the designs in the last 30 or 40 years that have been in the post qualifying rounds at the Nats.  Yes, the Palmer Thunderbird has performed well over the years, but that is not a true elliptical wing either.  Dave Gierke had his elliptical wing Novi IV but we saw him normally fly one of his straight tapered wing designs at the Nats.  In fact, I think there have been very few models (at least a definite minority) in the top 20 over the last 20 or 30 years with elliptical wings or elliptical shaped tips.  (Yes, Windy's Spitfires are an exception.)  Maybe there is a reason for this.  Indeed, if improved performance (or "better handling") was to be had, these top flyers would build and fly the things even though they may be more difficult to build.

I left the elliptical wing camp many years ago after I built a straight tapered wing model (using a P-51 type wing and tail planform) with the same "numbers" (areas and moments and weight) and airfoils as my previous elliptical wing and elliptical tail design.  There may have been other factors involved which I did not or could not understand, but the straight tapered wing flew better in my opinion and had more success with it in competition.  (Just for the record, that design with the straight tapered wing was good enough for two 5th places and the Walker Cup at the Nats.)

Good luck with your ultimate CLPA design quest.

Keith
« Last Edit: August 30, 2018, 06:54:02 PM by Trostle »

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Re: Tip Shape
« Reply #28 on: August 30, 2018, 06:15:19 PM »
I hear a LOT of Sailplane design in what TDM says frequently.
After reading a lot here and seeing U/C fly myself, I have to wonder how much matters with regards to fine details like perfectly elliptical lift patterns and such... I mean the plane isn't even free to fly in a straight line ever (hopefully!).

As Mr. Trostle pointed out, a lot seems like it is masked by having hopefully bountiful thrust happening up front. And of course the trim of the plane probably matters even more.


Making one's own design, and having complete faith in it might even make the plane fly better than one that you didn't design. You might fine tune it better, then proclaim it to be a better design?

In the end, all that matters is the end result. It seems pretty hard for a hobbyist to prove that a certain tip shape works better than another. Too many other variables most times. Having faith helps though!

Being a former/future sailplane nerd myself though, I will say that TDM has some nice cutting edge composites techniques up his sleeve, and I'm surprised that more of us are not using them. I will, if I ever get to have normal human hours.
 Might have to wait til retirement, sadly....
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Re: Tip Shape
« Reply #29 on: August 30, 2018, 07:55:50 PM »
Howard I would think any aerodynamicist  would confirm that lift distribution is important to get the best handling on a plane.

He would confirm that an airplane would have a lift distribution.  An elliptical lift distribution generally has the least induced drag for a given aspect ratio, but for stunt we aren't trying to minimize induced drag.  We're trying to get the right amount of induced drag: not too little, not too much. 

Actually in real life you see that in all high end performance machines and can be accomplished with straight taper with morphing airfoils, as well as the elliptical platforms.

The subset of real life of which you write is airplanes like sailplanes and transport airplanes, for which minimizing drag is important. Such airplanes are also designed to be optimized for flying right side up.  Hence, their wings can have twist to get the right lift distribution from a straight-taper wing. 

I hear a LOT of Sailplane design in what TDM says frequently.

Me, too. 

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Online Howard Rush

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Re: Tip Shape
« Reply #30 on: August 30, 2018, 07:58:39 PM »
Now, everything seems to have a chapter in a physics book.

More like a magazine article written by a guy who didn't understand the physics.
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Re: Tip Shape
« Reply #31 on: August 30, 2018, 08:47:38 PM »
It was the morphing of the airfoil I thought was a problem. 

You thought correctly about the thinning of the airfoil along the span, although I wouldn't worry about the last couple of inches at the tip.  One misconception that folks have is thinking that airfoils have different lift curve slopes. They don't until they start to stall.    Changing the airfoil along a symmetrical straight-taper stunt wing to get an elliptical lift distribution won't work and wouldn't help if it did work. 
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Offline Trostle

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Re: Tip Shape
« Reply #32 on: August 31, 2018, 12:00:36 AM »

An elliptical lift distribution generally has the least induced drag for a given aspect ratio, but for stunt we aren't trying to minimize induced drag.  We're trying to get the right amount of induced drag: not too little, not too much. 

 

Not trying to contradict some of the points being made about the practicality of an elliptical wing for our CLPA designs, but I recall there was a combat ship with an elliptical a few years ago (like 45) that had the then current trend twin booms and stabilator and had roots (airfoils) to a thing called the Nemesis.  Neat looking combat ship.  We do not see many combat ships with elliptical wings.

Keith

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Re: Tip Shape
« Reply #33 on: August 31, 2018, 12:27:20 AM »
Not trying to contradict some of the points being made about the practicality of an elliptical wing for our CLPA designs, but I recall there was a combat ship with an elliptical a few years ago (like 45) that had the then current trend twin booms and stabilator and had roots (airfoils) to a thing called the Nemesis.  Neat looking combat ship.  We do not see many combat ships with elliptical wings.

Keith

Neal White's Bosta.  It was difficult to build, but Neal was a fast builder.  Combat benefits from reducing induced drag.  The Bosta was a humdinger of a combat plane. 
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Re: Tip Shape
« Reply #34 on: September 01, 2018, 03:14:39 PM »
I agree that tip shape isn't so relevant to optimizing pure stunt performance. I think though that wing shape affects performance in other ways than simple efficiency. Due to tip effects, spanwise lift distributions aren't directly proportioned by wing chords. So, elliptical wings do not necessarily have elliptical lift distributions anyway, while straight tapered wings come closer than seems obvious. Wings with spanwise elliptical lift distributions do, however, concentrate the lift closest to the root, as far as I can see. This should be an advantage, regarding gusts, since wind direction relative to the wing varies along the flight circle. Rolling  moments from most gust upsets ought to be lower. So to the extent that elliptical distributions and their approximations relate to tip shape ('smoothie,' 'Freebird," etc?), maybe tip shape has some importance.

FWIW, sometimes we forget - it seems - that our calculations relating to MAC (MGC) position are based on the assumption that lift is uniform throughout the wing's area; i.e. it's the same at any point within the wing's plan view. We are actually finding the center of mass of wing shape cut from a material of uniform thickness and density. It gives a good initial approximation and good basis for comparison, but doesn't tell the whole story. 

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Re: Tip Shape
« Reply #35 on: September 03, 2018, 08:40:32 PM »


o.k. , o.k. .

put it on for the Tips Picture , but soas not to deprive you , now your started ; http://library.modelaviation.com/ma/1976/6 .

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Re: Tip Shape
« Reply #36 on: September 05, 2018, 07:18:19 PM »


o.k. , o.k. .

put it on for the Tips Picture , but soas not to deprive you , now your started ; http://library.modelaviation.com/ma/1976/6 .

Wow, Matt.  I had forgotten all about that picture of the original Moby Dick from the 1975 Nats.  That ship had curled down tips on both the wing and stab which you can sorta make out from the picture.

Funny story I love to tell about that Nats.  Jack Sheeks was judging that year and in my first flight of the flyoff (I forget how many were involved but I was sort of tail end Charley with the 64oz Nobler wing based  MD) I was embarrassed when my nerves got the best of me and I totally forgot to do the triangles, leaving my destiny to be decided solely on my second flight score (Old rules, best score of two flights used for placing).

On the second flight I made darned sure I flew those triangles so as to place as high as possible.  Banging the corners to get there attention the MB stalled dramatically in the third corner of the first one.  Never a fast learner I assumed it wouldn't do so twice and banged the third corner of the second one and, surprise, it stalled and all but crashed.  It survived and I completed my flight hoping nobody was looking. Yeah, right.

After the "show" was over I was carrying the MB off the field when Jack caught up to me, put his arm around my shoulders and said: "Ted, at first we (the judges) were sad for you when you left out those danged triangles.  After the second flight, however, we reconsidered and thought maybe you had left them out on purpose!  A pat on the back, a wink, a smile and he was gone.

Great modeler and a great guy.  One of so many we miss.

Ted
« Last Edit: September 07, 2018, 03:08:16 PM by Ted Fancher »

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Re: Tip Shape
« Reply #37 on: September 10, 2018, 07:37:56 AM »
Lots of interesting talk on tips.  I can say from experiment that the shape of the wing tips has little effect on a control line plane.  Buut- there is one wing tip shape that can make a good plane really bad.  Adding a straight tapered, swept tip is baaaddd.  See the pic.  One unique thing about this tip is that it noticeably slowed the plane in maneuvers, and you could hear the air roaring around the wing tip over the noise of the engine in a loop or corner.

The picture shows a panel with three different tips.  The double tapered tip worked fine.  The rounded tip worked fine.  The sharply swept tip with the straight trailing edge was BAD.

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Offline Chris Wilson

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Re: Tip Shape
« Reply #38 on: October 15, 2018, 02:53:58 AM »
Isn't the original question about roll axis view and not yaw axis view?
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Re: Tip Shape
« Reply #39 on: October 15, 2018, 06:11:02 AM »
Isn't the original question about roll axis view and not yaw axis view?
It was but these things tend to take on a life of their own. y1  That is a good thing as long as the original question gets answered somewhere along the way! #^
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Re: Tip Shape
« Reply #40 on: October 17, 2018, 10:30:32 PM »
It'd be an interesting thing to build a stunter with removable tips, and then just run through a bunch to see what does what to what where.
Anyone tried this?
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Re: Tip Shape
« Reply #41 on: October 17, 2018, 11:26:32 PM »
Even doing that might be hard pressed to prove something, since conditions change flight to flight. 
You'd probably have to make notes and then repeat the same procedure 2 to 3 times to be more certain, I'd suspect.
Still, it's a good idea since there isn't really more you can do to find out.
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Re: Tip Shape
« Reply #42 on: October 18, 2018, 10:07:42 AM »
Thanks a whole bunch!  Like I said, I was absent during the great technological revolution where we became obsessed with aerodynamics.  I much prefer the parabolic tip, I was afraid I was creating a problem - not good.

Ken
When I was designing in the 70's I did everything to look cool and if it was light it flew OK.  Now we have a reason for everything but unfortunately I was absent the day we wrote the instructions.  So when I have a question I look at the pictures of the newer designs and search the archives.  If I don't find the answer I ask another of those "we have been over this before" questions.

So, Does Tip Shape matter?

Not the top view.  We have plenty of discussions on that one I can find.  I am talking about the "airfoil" or what you see from the front.  All of the pictures I find seem to carve the block by extending the airfoil and rounding the end (A).  I have always tapered the tip (B) and I use 3" blocks.

Does it matter?  Am I creating a tip stall when it could be avoided?

Ken

Ken, the biggest problem with a large tip block are 1) the cost of big soft blocks of balsa, and 2)maintaining a symmetrical shape top and bottom.  Any unsymmetrical tip is likely to cause problems.  3) Carving out the insides ir tricky to do.  Molding the tip shapes works better but takes even more time.

If the wing is tapered enough to be effective(tip 45-60% of the root) the tip shape really doesn't matter much.  It can be flat, rounded or whatever.  It just has to be symmetrical on both tips.  A Flitestreak style wing tip helps noticeably on a straight wing and looks and flys even better if it is molded.
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Offline Dave Hull

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Re: Tip Shape
« Reply #43 on: October 19, 2018, 11:49:41 PM »
A quicker experiment that does not have so many issues with accuracy would be to build the wing with squared off tips, a la Twister.  Fly it and get a baseline. Then screw on some tip plates. These are very effective at reducing spanwise flow, which is what so many guys worry about.  See if you can tell the difference. The only accuracy issue would be to align both exactly to the fuselage centerline.

If you don't like the look of tip plates, you can always attach some fences for a more permanent installation. Make it in one piece and slide it on from the front. Tape it for the test. If one of these does not cause a noticeable change in the flight characteristics, then spanwise flow is a complete non-issue on a stunt plane. These are not subtle aero warts.

I do not have a plane with square tips or I would try it just for fun. Never can tell what you might learn....

Dave

PS--One thing I did not see mentioned in this thread so far was that often a wing design will have planform taper, but proportionally less taper in thickness. (ie. the tip rib has a greater thickness percentage.)  Setting aside the criteria of finding "the right amount of drag," another factor is tip stalling. You don't want to stall, but you really don't want the tips to stall first. I think the priorities therefore are:  low overall weight, thick and effective airfoil--all the way out to the tips, and avoiding secondary problems. Some of those would appear to be: air bleed thru the flap hingeline compromising the lift, especially if the left and right don't match; asymmetric wing tip shapes so that it changes the available lift left-to-right or upright-to-inverted. (I have a "trashcan Cardinal" that has a wingtip symmetry problem. If it was a better plane, I'd reshape the tip that was damaged in a crash.)

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Re: Tip Shape
« Reply #44 on: November 15, 2018, 06:46:34 AM »
My guess is that the tip shape will have a tiny (negliable?) effect on the Oswald Efficiency Ratio for the planform chosen and thus how far it deviates from a "perfect" elliptical lift distribution.

That said, I always caveat that "traditional" aerodynamics can lead you astray with a CLPA ship. The ratio of the mass of air displaced to the mass of the ship is so high that you have some really strange dynamic responses. For that reason, unless doing Bode plots makes your socks go up and down, use what works.

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