Wing planform question

[threcixty]

Being that most people around Albuquerque fly flatland, I would like to design an f3b airframe. The problem is that most of my design experience is with DS and DLG airframes. Sooo, I would like to use my ds experience and design an F3b from that. Hopefully, you guys see the irony in that!

The question is this: I want to make a plane that turns harder then anything out there without being tipstally. Since you DS guys are the best at finding the weaknesses in designs, I would like your input as to what works and what doesn't.

Here is what I have found:
1. Upturned tips, i.e. those found on the Blade, look really cool but excentuate tip stall tendencies.
2. Straight trailing edges tipstall worse then straight leading edges. Why, I am not sure.
3. Sweeping the l.e. and t.e. back on the last 10-15% of the wing seems to make a plane turn harder without stall.
4. Airfoil does make some difference, but not as much as the planform itself.

Like I said, this is what I have found. I would really appreciate any ideas as to what type planform works best at the slower speeds, like up to 100mph or so, as in F3F and F3b type tasks. I would like to bag a few wings to try out different ideas, then make molds.

Sorry to hear about the Kinetic, Spencer... I'm sure it was spectacular though!
Jim

[joe manor]

No mention of dihedral? That's important. Broad wingtips, important. Im not sure I agree with your straight trailing edge theory.

[threcixty]

As for dihedral, I have not played around with it that much. Ive built planes with 0 dihedral that seemed to tipstall a little more easily then say 2-3 degrees. I do not understand the reason for this.

The straight t.e. idea comes from pylon racers made from 1/4-3/8" solid balsa wings. The straight t.e. with a taper on the l.e. seemed to drop a wing in a hard turn more then the straight l.e. with a tapered t.e. One of the hardest turning pylon racers I made had slightly forward swept wings. Being that the airfoils on these racers were done by TLAR methods, would this be an airfoil issue or a planform issue??

Jim

[wyowindworks]

I take a risk here....

I really like planforms that use a straight LE. This planform is forward swept aerodynamically and turns very very hard. This planform will actually hyper-stall like a plank before it tip stalls.

Burt Rutan discovered that this planform has a lift distribution that is very close to an elliptical planform.

I think airfoil and planform must work together. I couldn't ever design a model without giving each ample consideration.

Here is a link to the Stinger 45 (uses the straight LE) banging some hard turns in light conditions:Tight Laps (http://www.rcgroups.com/forums/showthread.php?t=796917&highlight=Stinger)

[marfish]

The stinger looks cool and sure bangs hard turns. Who carried those?
Mar

[wyowindworks]

The stinger looks cool and sure bangs hard turns. Who carried those?
Mar

I manufacture them......Wyoming Wind Works. All the gliders for this run of 50 are claimed. I might be doing another run of 50 again in May.

I used the Rutan planform because it's very fast, stable, and turns very hard. The Stinger 45 was designed to fly in a very broad range. At 45", 15.5 oz. in 45 mph winds, it whipped up on a clean 45 oz JW in every aspect. The JW couldn't keep up on the front or the back.


Adam

[daboz]

The stinger looks cool and sure bangs hard turns. Who carried those?
Mar

Marlen I should have one at Weldon pretty soon,, :D

[wyowindworks]

Daboz, I should ship your kit on Thursday morning.

[threcixty]

So to take that Rutan planform further, here is a question. If I take a Greco Modi wing, which is a straight t.e. with a tapered l.e., similar in planform to a Thermal Eagle with the tapered tip, and change the angle of the root rib so that the t.e. is now tapered and the l.e. is straight across, would this plane turn harder without being as tipstally, or would it just require more lead in the nose to balance it?
Jim

[marfish]

cant wait 2 see it, And so sorry about your Opus. :( maybe it's a good thing that I decided at the last minute I couldn't go. :eek:

[1000MPH]

.... At 45", 15.5 oz. in 45 mph winds, it whipped up on a clean 45 oz JW in every aspect. The JW couldn't keep up on the front or the back.


Adam

That's a bold statement considering my Bee has equaled the Stinger's recorded top DS speed. :eek:
I can't wait to see one fly in person.
stu
(just jealous I don't have a stinger)

[djklein21]

Do you really want to design an F3b plane. F3b pilots are some of the most deerning and picky guys around. The bar is set really high. DS planes are good as long as they don't blow up. F3b planes need to be light, strong, fast, and thermal well.

The idea of making the plane turn harder than anything else is going against a modern trend. During an F3b speed run, you only need to make 3 turns, and the straights are 150 meters long. So most planes are launching really hard, flying heavy, with really fast straight line speed. You have to anticipate the turn more, but you go faster overall.

Are you really interested in making an F3b plane, or a just a F3X style plane?

[DT56]

text

[wyowindworks]

That's a bold statement considering my Bee has equaled the Stinger's recorded top DS speed. :eek:
I can't wait to see one fly in person.
stu
(just jealous I don't have a stinger)

Actually that's the only recorded DS speed. :)

The Stinger does have it's drawbacks at 15 oz. We will have to see how Daboz's flies. His should be 20 to 24 oz. That should give it more energy for the top turn.

[threcixty]

Do you really want to design an F3b plane. F3b pilots are some of the most deerning and picky guys around. The bar is set really high. DS planes are good as long as they don't blow up. F3b planes need to be light, strong, fast, and thermal well.

The idea of making the plane turn harder than anything else is going against a modern trend. During an F3b speed run, you only need to make 3 turns, and the straights are 150 meters long. So most planes are launching really hard, flying heavy, with really fast straight line speed. You have to anticipate the turn more, but you go faster overall.

Are you really interested in making an F3b plane, or a just a F3X style plane?
I am after a F3x plane at the moment, but have had a lot of interest in what characteristics of a sailplane cause it to turn hard, compared with those that seem to snap out of a turn. I agree that the f3b competition is really stiff out there, and am more interested in taking some hard parts that my friend designed 15 years ago and tweak it a bit, then send him a plane that does not have the bad tendencies that it used to. Granted, I am stuck with an RG15 and a double taper planform, but I think it can be tweaked a bit to make it perform the way he wanted it to back then.

In addition, I have been on an electric kick lately, and would like to make an f5b airframe soon. The end result that I am after is a hard turning plane. I'm pretty good at the going fast in a straight line designs, but the turning thing at under 150-200 I do not know much about!
Jim

[motorhead]

I like the planform on the alliaj.
http://pagesperso-orange.fr/aeromod.concept/index_eng.html
I liked it so much that I built a 100" F3f'er with a similar plan form. In my experience a strait LE results in quicker turning and a strait TE results in better strait line tracking. I also like squared off tips. I believe that they are more efficient and harder to screw up. As far as the plan form it is a balance between turning and strait line performance. For F3f I wanted a good balance and I think that I got that. The first MDS that I built had a strait LE and it did not track like I liked for DS'ing. It seemed like it wanted to turn too easily. With the strait TE it did much better.
Mike

[wyowindworks]

So to take that Rutan planform further, here is a question. If I take a Greco Modi wing, which is a straight t.e. with a tapered l.e., similar in planform to a Thermal Eagle with the tapered tip, and change the angle of the root rib so that the t.e. is now tapered and the l.e. is straight across, would this plane turn harder without being as tipstally, or would it just require more lead in the nose to balance it?
Jim

It's hard to say... There are a lot of factors to consider.....airfoil, root and tip dimensions, AR, and wing loading.

I spent over 30 hours just tweaking the DS airfoil for the Stinger 45. I'm always amazed at how easy it is to make something fly.......and, I'm equally amazed at how much work is involved in designing an airframe that performs at predetermined flight parameters.

Adam

[wyowindworks]

By the way, the Rutan planform is a compromise in almost every respect. Rutan way trying to find a planform that was easy to produce and still perform well.

I chose it for the same reasons. I had a price point and labor objective in mind, as well as, flight parameters. I concluded that this planform would work well for a model of it's size. If I was going for the absolute most efficient design, I would have chosen a different planform.

[admodesi]

I can't believe it took 13 posts before washout was mentioned!

[SpeedMaster]

I`m not sure about the straight TE comment. Bot the F3X RaceM and F5B Raketenwurm 1 uses straight TE. My own design moulded Vorlon also uses a straight TE. Both the Vorlon and the RaceM is really stable airframes with no tendensy to tipstall. Both features aggresive LE tapper on the tips. I think it`s more about the lift distribution.

[threcixty]

I can't believe it took 13 posts before washout was mentioned!
Washout is a way of fixing a problem by adding inefficiency. I have found that it works at fixing slow speed tipstalls, but not high speed accelerated tip stalls. Producing washout by transitioning airfoils does seem to work a little better, as I used more of a Phillips entry towards the tip of my 60" ds'er and it did not seem too tippy.

I could see that the straight t.e. planes would track better, due to the effective dihedral of the swept leading edge. Personally, I have not seen a difference in the handling of the two, other then I think that American planes handle a LOT better then their German counterparts, and other then the 3 piece wings with straight center sections on the German planes, they also have a straight l.e., where most of the American planes have a straight T.E. Before ya flame me on that, realize it has been 10 years since I have flown TD contests, so I am sure that this has changed.

Thanks for the imput guys!
Jim

[wyowindworks]

I can't believe it took 13 posts before washout was mentioned!

Washout can reduce tip stalling, but it comes at a cost. With washout, the the root of the wing usually has to fly at a higher AoA because the outer portion of the wing is generating less lift than possible (the tips could actually be at a negative AoA). This results is more drag.

Washout can generate less drag, but in an incredibly narrow window at lower Reynolds numbers.

Adam

[admodesi]

Washout is a way of fixing a problem by adding inefficiency. I have found that it works at fixing slow speed tipstalls, but not high speed accelerated tip stalls. Producing washout by transitioning airfoils does seem to work a little better, as I used more of a Phillips entry towards the tip of my 60" ds'er and it did not seem too tippy. Jim

Hmmm Right... everyones an expert :rolleyes: By changing an Aerofoil and quote "I used more of a Phillips entry towards the tip of my 60" ds'er " is a form of washout!

We must be reading different books on aerodynamics!

[admodesi]

posted twice bugger :censored: :rolleyes:

[wyowindworks]

Hmmm Right... everyones an expert :rolleyes: By changing an Aerofoil and quote "I used more of a Phillips entry towards the tip of my 60" ds'er " is a form of wasout!

We must be reading different books on aerodynamics!

admodesi,

Could you explain how you see the "Phillips entry" as a form of washout?

[admodesi]

admodesi,

Could you explain how you see the "Phillips entry" as a form of washout?

I'm just typing an essay to explain all of the above, but when I see someone use the term "Phillips entry" it makes me cringe every time! Its a term that many people tend to use when they think they have found the ANSWER to the question of "LIFE, THE UNIVERSE AND EVERYTHING" when they don't know the question!

EUREKA… 42!

“Standby One” Keith

[wyowindworks]

I'm just typing an essay to explain all of the above, but when I see someone use the term "Phillips entry" it makes me cringe every time! Its a term that many people tend to use when they think they have found the ANSWER to the question of "LIFE, THE UNIVERSE AND EVERYTHING" when they don't know the question!

EUREKA… 42!

“Standby One” Keith

I'm just not sure that "Phillips entry" is a form of washout. Is it?

[admodesi]

I'm just not sure that "Phillips entry" is a form of washout. Is it?You've said it again! Stop it! :o

If I've interpreted threcixty correctly by the way he applied it to his aircraft, then it is!

just wait...still typing BIG ANSWER! ....and now drawing diagrams too!

[admodesi]

The term “Phillips Entry” is named after Horatio Phillips who was an early aerodynamicist of the late 1800s. He coined the term “Phillips Entry” for the way air flowed around his "double-surface airfoils" noting that the curved aerofoils (that he had based on the shape of bird wings) he was working with generated more lift than that of his predecessors. He used his own design steam powered wind tunnel to confirm his results. In 1884 and later in 1891 he took out patents on his aerofoil designs. All these designs had the leading edge pointing down (ie a negative angle of attack at the LE). This seemed to be the key to unlocking man-carrying heavier than air flight. For this we are all very grateful Mr Phillips! (See Fig 1)

As Horatio Phillips did not define exactly what shape nor angle this was it is hard to say that anyone is using a true Phillips Entry. Therefore all “cambered” aerofoils by definition have a Phillips Entry! As the development of aerofoils continued into the next century the term Phillips Entry which only seem to define the leading edge part of the camber was dropped in favour of just “camber” for the whole aerofoil curve.

More info on Phillips:
http://www.centennialofflight.gov/essay/Evolution_of_Technology/phillips/Tech4.htm
http://www.aerospaceweb.org/question/history/q0232.shtml

NOW THE CRINGE PART AND WHY YOU SHOULD NEVER USE THE TERM PHILLIPS ENTRY EVER AGAIN!

In the 1970s there were a few model gliders like the Aquila that looked good but had some pretty average performance. They had “flat-bottomed” (another term I hate!) aerofoils (not Clark Ys) that had OK lift qualities but were very poor penetrators. In the late 70s early 80s model gliders started using these new-fangled “laminar-flow” aerofoils from Dr Richard Eppler and the like. Guys with the old Aquila types thought well these new gliders are flying so much better than ours why don’t we just modify our wings to look like these new fangled “laminar flow” jobs! So some bright spark noticed that the leading edge profile of his Aquila looked nothing like the leading edge of an Eppler 205 aerofoil and thought this must be the key to getting my glider to fly better! Just fix the leading edge! Easy! The modification performed did “seem” to improve performance a little, but the only easy way was to effectively give the bottom surface more of a ramp up to the leading edge was to add a bit of curve to the bottom of the leading edge. The end result was something that may have looked pleasing to some but certainly not the best way to improve performance. To justify the modification the term “Phillips Entry” was used to describe it. When you look at one of these mods there is a distinct looking droop on the leading edge and this is where all the confusion started and still seems to be with us today.
Now the number of “modifications” that I have seen performed to existing aerofoils to give them a so-called Phillips Entry is ridiculous. Some curve up, some down, some straight and some even s-shaped. The reality is that none of these look anything like the original 1884-91 drawings! (Fig 1)

So lets look at a modern laminar flow aerofoil, as everyone knows a good laminar flow foil has laminar flow for as far back along the upper and lower surfaces as possible for the desired performance. Most of the good ones are designed by people with a Phd after their name and access to some cool equipment and software. So when I hear that Mr Joe Bloggs in his back yard shed has bent the leading edge of his RG14 to increase performance, do you really think he has reduced the drag profile of this incredible aerofoil? The answer is no… he’s just gone and stuffed all the wonderful properties that foil was designed to have!

So why would Joe Bloggs want to modify his plane in the first place?
1. The plane he is flying is a dog and he is trying to fix it.
2. The plane he is flying is flying exactly as it is designed but he reckons he knows better!

If you have the first example, take it back for a refund, and if you are the designer of this plane try again and use it as a learning experience. The problem is, sometimes a person reckons they have the first type when they really have the second type.

So stayed tuned when I come back to post the conclusion with: Big Answer Part 2 – why adding a so-called “Phillips Entry” is really washout!

Keith

[M Scherrer]

hi

At low CL as encountered for DS I am not sure planform is anymore important in term of induced drag.
Concerning tip stalling, the less taper the happier.
All in all I am not sure this is an aerodynamic matter here.

I think planform should be chosen for tailoring bending-to-washout strutural coupling. As the number of G is huge, I guess this is the key point

- a planform with 25% of chords (roughly) perpendicular tu fuselage should help having a neutral behaviour structurally speaking. This is more or less the idea for modern f3b (due to the zoom thing)

- Forward sweep can be statically divergent (structuraly speaking), and may well increase the feelling for turning hard : the more G'sn the more bending, the more upwash at tip, hence pitch up, and more G's... etc.
On the other hand the effect of gust will be increased.

- Sweepback is statically convergent, and will provide "free" gust alleviation (as used on liners).
On top of that, it also brings some pitch damping (Cmq), and all this could be against the will for turning hard.

So maybe this is not only aerodynamics, lift & drag & PM.
This was my 2cents ;-)

matthieu

[biber]

Keith, that was a good one!

I didn't even have heared of that thing before.
Here in germany nobody uses that term nor is there any other term for it.
Same goes for tipstall and downwind turn.
Nobody has even heared of that and yet we do fine.

biber

[Avaldes]

For a nice read on the design of a modern wing planform/airfoil (Pike Perfect), give this a look:

http://www.f3j.com/perfect.htm

Philip shows his lift distributions, polars, etc. He used vortex lattice methods and Xfoil to design the wing. This is a good article.

-Aaron

[threcixty]

For a nice read on the design of a modern wing planform/airfoil (Pike Perfect), give this a look:

http://www.f3j.com/perfect.htm

Philip shows his lift distributions, polars, etc. He used vortex lattice methods and Xfoil to design the wing. This is a good article.

-Aaron
Thx Aaron.. That is a good article, the Perfect sure is a nice model!
Jim

[admodesi]

Keith, that was a good one!

I didn't even have heared of that thing before.
Here in germany nobody uses that term nor is there any other term for it.
Same goes for tipstall and downwind turn.
Nobody has even heared of that and yet we do fine.

biberThanks Biber, here is some more of my stupid ranting for you! :D

The only constant I have seen to anyone performing a Phillips Entry modification is to increase the camber by drooping the nose of the foil. See FIG 2. As can be seen from the diagrams, drooping the nose increases camber and gives the modified wing washout. As everyone should know the AOA of an aerofoil is measured along the chord-line. By definition the chord-line runs from the extreme leading edge to the extreme trailing edge of the aerofoil. By drooping the nose of the foil you effectively lower the leading edge from where the chord line is measured. If this is performed only at the wing tip then by definition you have given the wing washout!

So how can washout improve efficiency across a wide envelope? Lets have a look.

Increasing a wing's taper ratio too much over loads the tips tending towards a plan-form which will tip-stall. This is because the tips are already at an effective AOA that is already higher than the root. The fix to this situation is to add washout or reduce the taper ratio reducing the over-load and thus the tip stall. Quote: "(the tips could actually be at a negative AoA)" this will only occur if the plan-form vs geometric twist is wrong.

Decreasing the wing's taper ratio will unload the tips, requiring less or no washout, however the span-wise distribution will not be efficient if taken too far. Although the plan-form's flight characteristics will benign, the span-wise lift distribution is poor. The induced drag created at higher AOA will effectively kill the gain you have created at lower AOAs not to mention the profile drag caused by a larger tip chord. The mechanical loads on the wing-joiner or centre sections of the wing will also be high at heavy G loadings and could lead to structural failure. This is another reason taper is a good thing!

You need to find a happy medium.... and that happy medium may require a geometric twist.

So you get the most efficiency by working towards a near-perfect span-wise lift distribution curve (this curve should be as close to elliptical as possible). Now making an elliptical plan-form pretty much solves the problem, but this is not what this thread is about. The best way to give straight tapered wings this near-perfect distribution is to give the wing a taper, too much and we’ll tip stall, not enough and we’ll loose efficiency.

Lets make a wing where through calculations and experimentation you find that the plan-form is ideal, however you needed 1 deg of washout to fix the stall characteristics. If you use the same foil the entire length of the span then the lift will drop off near the tips or may even have a down-load if not flying at the wing’s ideal AOA. However if you choose an aerofoil which produces the same lift at this lower AOA then this could solve the problem. An aerofoil with more camber will produce more lift at lower AOAs. This type of wing twist and transitioning aerofoil towards the tip can even be seen on fighters like the F-18 and F-15!

Now unless you got an aeronautical degree, diploma, or similar, or have done a lot of reading about this stuff it extremely hard to get grasp of what is happening. Yes yes I know that there are more things in play than just camber, washout and plan-form, (like: Reynolds Number, span-wise flow, sweep, mechanical twist, pitching moment, etc.) but designers need to know what effect these little changes have when designing an aircraft. When designing an aircraft, I try to look at the whole and not try to just cure the symptoms.

A reference I like and I find easy to read to get a grasp of this complicated concept is Martin Simons "Model Aircraft Aerodynamics"

Keith

(comes down off the soap box again!)

[Lance Prior]

When a wing is swept the tip twists when the wing bends up and down.

Bending a swept back wing makes the tip twist to a lower angle of attack and bending a forward swept wing does the opposite.

[MarkusN]

I didn't even have heared of that thing before.
Here in germany nobody uses that term nor is there any other term for it.
But you probably have heard about TNT. Same thing really, if a bit more sophisticated by adding sweepback to the equation and mostly limited to the tip.

Same goes for tipstall and downwind turn.
Nobody has even heared of that and yet we do fine.
Now that's a bold statement to make. Both effects are discussed (and tipstalling rightly so), even if there are no standing terms for the phenomenon.

[threcixty]

Another question is comparing slow speed tipstall, i.e. downwind turn, even though I do not see why downwind is any worse then upwind, and high speed accelerated stall. An airfoil will always stall at the same angle of attack, (or so I have been told,) so are these stalls similar? Why is it that high speed stalls are so much more violent then low speed? Why is it that an airfoil does stall at the same angle of attack, regardless of speed, even though the airfoil should be more efficient at a higher speed?? Realize I am talking about faster style airfoils here.
Thx,
Jim

[Lance Prior]

Why is it that an airfoil does stall at the same angle of attack, regardless of speed, even though the airfoil should be more efficient at a higher speed?
They don’t, the wing will stall at a slightly different aoa at different speeds.

[marfish]

Doh, dbl post, sorry

[marfish]

high speed accelerated stall. Why is it that high speed stalls are so much more violent then low speed? Realize I am talking about faster style airfoils here.
Thx,
Jim

[edited for some bad theory]
For the same reason high speed car crashes are. There's a bunch more energy being released when the AOA of the wing passes the critical point of stall. Also, in my experience it seems easier to tip-stall a plane that is balanced with a more nose-heavy condition than tail-heavy. A nose heavy has partial up elevator just to keep the nose level and has to really crank to pull the nose up hard which may cause the tail to stall and slip away from you.

For me, I'm curious as to why we call it a tip stall when it is the inadequate tail group that stalls first and can't track with the main wing. [edit point] This is compounded as the tail swings out of the turn radius and causes increased alpha on the wing, sharpening the turn swinging the tail harder outward, raising the alpha more.......and walla, full wing stall.][edit: the plane will probably actually pitch forward initially until and if a complete stall occurs, when it flicks, spins, tumbles due to no direction control...] Hence, the nose heavy plane's tendency to flick and spin at relatively slow speeds. Back in the day I could never get my planes to do such nice flat spins nor ninja star spins and the only thing that I notice to have changed is the ratio of wing size to tail size. O.K., the airfoils have too. Hmmmm....
Also, if eliptical foils are better at resisting tip stall, then why does the Opus , V-2, Destiny, Blade, ad nausium flick and spin that ninja star so well when directed to do so? In my power days we needed largish rudders to do a proper flat spin and pull out of it too. Normally a large tail group anchors the model on track at the expense of speed. Just my thoughts,
Marlan
Who's not fire proof

[threcixty]

hmm.. Never thought about tailplane stall.. Maybe I am looking at it wrong, but wouldn't a tailplane stall produce a nose down attitude?? I had a tailplane stall in a Baron when I got 5" of ice on it and it pitched down violently to near vertical. Is it the tailplane or just the stabilizer and not the elevator that is stalled??
On the cg placement, I would think that moving the cg rearward makes the plane pitch around an axes farther back, which I have no idea what effects this would have other then loading the stab more during the pull and helping the elevator to become more effective.
Just thinking, no proof.
Jim

[marfish]

Increasing a wing's taper ratio too much over loads the tips tending towards a plan-form which will tip-stall. This is because the tips are already at an effective AOA that is already higher than the root. The fix to this situation is to add washout or reduce the taper ratio reducing the over-load and thus the tip stall. Quote: "(the tips could actually be at a negative AoA)" this will only occur if the plan-form vs geometric twist is wrong.

Decreasing the wing's taper ratio will unload the tips, requiring less or no washout,
Now making an elliptical plan-form pretty much solves the problem, but this is not what this thread is about. The best way to give straight tapered wings this near-perfect distribution is to give the wing a taper
However if you choose an aerofoil which produces the same lift at this lower AOA then this could solve the problem.
Keith

(comes down off the soap box again!)

Thanks for this post. It made me think about lift distribution.
I don't understand your statement of increased AOA at the tips when compared to the root in the second line. Last time I checked my tips are in line with the root. Also, I would think tapered tips need to be flying faster than the root to produce as much lift(because they're smaller) and tend to quit flying at landing speeds due to that(think wing loading).

In the last paragraph, on the thought of straight-tapered wings, you say "to give the wing a taper": did you mean sweep? and which way?

Thanks, and sorry for chopping up the post, I was only trying to use your words to summarize and to point to what I had questions about.
Marlan

[marfish]

hmm.. but wouldn't a tailplane stall produce a nose down attitude?? Baron -- pitched down violently to near vertical. Is it the tailplane or just the stabilizer and not the elevator that is stalled??
On the cg placement, I would think that moving the cg rearward makes the plane pitch around an axes farther back, which I have no idea what effects this would have other then loading the stab more during the pull and helping the elevator to become more effective.
Just thinking, no proof.
Jim

(rambling of an armchare aerodynamicist--see, I can't even spell it)
Depending on the center of lift and the CG balance. You're correct: a typical lifting airfoil will usually pitch forward around the CoL. Once the tail breaks loose, the stalled wing can spin and tumble: pretty much anywhere.
You can move the CG around but the center of lift is the pivot but even IT is AoA dependent and can drift fore and aft a bit with changes in AoA.
A nose heavy condition REQUIRES a stabilizer that is pushing down in opposition to keep the nose up such that for ordinary level flight, you're loading up the tail plane. The AOA is already several degrees deeper toward the critical moment of tail-stall than one that is neutral. It therefore has less room for additional pitch before stalling the horiz. plane.(full flying stab probably has less leeway due to no increase in airfoil camber). The obvious design parameter would be to make the wing stall before the tail, and today's tails are barely sized adequately for control.

For example, I had a nose heavy V-2(Destiny knock-off) that flew right out of my hand on her maiden but I couldn't do anything like a speed run, a loop- nothing. She was very nose heavy. It was hard to keep the nose down at speed but she stalled and flicked hard at the slightest pressure on the elevator. Once I moved the CG back, straightened up the "wash-IN" of the V-tail, removed the "wash-IN" on the wing, straightened the incidence, and repaired the fuse several times she really was a different plane- fast, predictable and banged the hardest turns. She would still flick if I wanted her to. Did I mention she started out very nose heavy? Do you get the idea that I really hate nose heavy flying?
In the '80s I had a sport/pattern plane that would nose over just above the tarmac during the flare because she was way nose heavy and full "up" was not enough to overcome the off balance during the slow approach. I learned to grease it in a little faster to avoid the pitching down moment and changing the damaged prop. every time. Would have been so simple to add a little tail weight, eh?
Mar

[wyowindworks]

deleted

[biber]

It made me think about lift distribution.
I don't understand your statement of increased AOA at the tips when compared to the root in the second line. Last time I checked my tips are in line with the root.Each little wing segment induces a certain (induced!) AoA on the ones lying next to it and vice versa.
A big chord (root) will force a bigger Cl on a smaller chord (tip) lying text to it, though all may be installed at the same incidence.
The smaller chord again will rather unload a neighboured bigger chord.
That's the mumbo jumbo of calculation load distributions is all about.

biber

[threcixty]

Each little wing segment induces a certain (induced!) AoA on the ones lying next to it and vice versa.
A big chord (root) will force a bigger Cl on a smaller chord (tip) lying text to it, though all may be installed at the same incidence.
The smaller chord again will rather unload a neighboured bigger chord.
That's the mumbo jumbo of calculation load distributions is all about.

biber
Why is it that a large chord will force a bigger Cl on a smaller chord?? I cannot wrap that one around my brain. Is it a Reynolds # thing due to the smaller chord?? I should remember from fluid dynamics, but that was a good 10 years ago!
Thanks,
Jim

[biber]

Lift at any spanwise position (or say the local lifting vortex strengh) is local Cl times the local chord.
Physical nature has the habit of trying to make the vortex strengh along the wing as even as possible.
A sudden step in vortex strenght is not possible.
Now if you take wing of infinite span, constant chord and without any washout
and bite off a chunk of it somewhere in the middle, so that one half of the chord is missing at that spanwise position,
the local lift will decrease there but will still be more than half of the previous value, though the chord is half of the previous value.
That means the local lift coefficient is higher than it was before the bite.
Think all this gedankenexperiment without any Re-issues and without any particular airfoils,
as the effect counting is not directly related to that.

biber

[threcixty]

Ok, I get it now... Out of curiosity, is that loosely based upon the old school theory of lift that states "lift is caused by lower pressure on the upper surface, and higher pressure on the lower surface"? It seems that would be the case as the lower pressure would not decrease by half if you removed half of the wing at some spanwise section. Would this translate to the flow turning theory of lift? Lol.. I just about freaked out when I overheard a flight instructor describing airfoils and lift as "A venturi with half removed, and the low pressure sucks the airplane up"!!! Stupid FAA is still requiring this incorrect theory to be taught, along with equal transit time and Newton's third law.. skipping stone.. Good times!
Jim

[threcixty]

Another question... In playing with airfoils on Profili, I was wondering about the Cl/Cd alpha polars. I know this is only for a section, and not an entire wing, but does a broader peak on this polar mean a less abrupt stall then a more peaky and sharp polar? i.e. an upside down U verses an upside down V??
Jim

[marfish]

Each little wing segment induces a certain (induced!) AoA on the ones lying next to it and vice versa.
A big chord (root) will force a bigger Cl on a smaller chord (tip) lying text to it, though all may be installed at the same incidence.
The smaller chord again will rather unload a neighboured bigger chord.
That's the mumbo jumbo of calculation load distributions is all about.

biber

Thanks for that bit of info.

I got a question about the Profili. alpha measurements: When playing with cambering the trailing edge, are the alpha angles still measured through the wing's original chord, or through the new one created by dropping the trailing edge? I.e., to keep a 0* alpha with 8* flaps is the program pitching the nose of the plane downward to keep the practical alpha at 0, or are we looking at a neutral attitude with flaps drooped?

Marlan

[biber]

I was wondering about the Cl/Cd alpha polars. I know this is only for a section, and not an entire wing, but does a broader peak on this polar mean a less abrupt stall then a more peaky and sharp polar?You can interpret it that way.
The shape of the top end of the Cl/Cd alpha polar does speak for the way, how abrupt a stall may develop.
At least with windtunnel measurements you can do that,
for calculatet polars it is to be taken with a grain of salt or two.
Especially the Cl_max value predicted by Xfoil or Profili (as a GUI front end operating Xfoil in the backround) are rather optimistic.

biber

[biber]

alpha measurements: When playing with cambering the trailing edge, are the alpha angles still measured through the wing's original chord, or through the new one created by dropping the trailing edge? I.e., to keep a 0* alpha with 8* flaps is the program pitching the nose of the plane downward to keep the practical alpha at 0, or are we looking at a neutral attitude with flaps drooped?I can't say for sure about how Profili handles this, as i use Xfoil only,
but Xfoil does leave the part forward of the hinge line unchanged, and I guess Profili does so either.
That can come in handy, since that part of the wing is generally fixed to the airplane system of reference in real life aswell.
To set the new generated airfoils TE and LE level you type in the command 'DERO' for derotation in Xfoil.
I guess Profili will have some sort of button for that somewhere.

biber

[marfish]

Each little wing segment induces a certain (induced!) AoA on the ones lying next to it and vice versa.
biber


Thanks Biber:
It seems then, that it would be reasonable to say that a forward swept wing would have less induced C/L imparted to the tips than a swept back wing?
Marlan