Wing sweep back leading edge

[Larry Wong]

It seams that I heard a long time ago that the sweep back of the leading edge has something to do with Dihedral?
Is there a general rule like thrust line to wing /stab  for the sweep angle for a stunt plane?

[Ted Fancher]

It seams that I heard a long time ago that the sweep back of the leading edge has something to do with Dihedral?
Is there a general rule like thrust line to wing /stab  for the sweep angle for a stunt plane?

Larry,

I don't think it is a significant issue from a performance perspective.  Wings with taper are more warp resistant than constant chord wings and may be slightly less affected by winds although, again, any affect is probably within the noise factor.

From a design perspective by far the most pertinent factor when dealing with any wing planform (shape from above) is the location of the MAC (average chord, for any reasonable planform) with respect to the Center of Gravity and the forces acting to change the pitch axis--primarily the tail and, to a lesser degree, the flaps.  For this reason when sweeping the leading edge you must be aware that the root of the wing will enter the fuselage further forward in order to get the MAC in the proper location.  The only significant affect of that is the more you sweep the leading edge the less length you will have for the fuel tank while keeping the engine in the proper location to balance the ship at the proper location of the MAC.

Best reason for tapering the wing of a stunt ship is the improved appearance of the final product!

Ted

[RandySmith]

And to add  to what Ted said, The swept back wing also will add stability to the plane, in winds and gusts ,the straight wing will rock and bounce more than a swept wing. This also make the tips lighter, many think that helps much.

Randy

[Howard Rush]

Sweep does the same thing as dihedral, but the dihedral effect of sweep is also a function of lift magnitude and direction.  The effect is rolling moment due to sideslip.  It is the reason the pilot got his 737 upside down last month when he mistook the rudder trim for the cockpit door lock.  Here are some results from a quick Web search. 

http://people.clarkson.edu/~pmarzocc/AE430/AE-430-7.pdf , p 12

http://www.vmihosting.com/MWS/Documents/FTM103-chapter5.pdf , figures 5.10 and 5.11 .  Figure 5.11 shows how the dihedral effect of sweep varies with lift.

http://books.google.com/books?id=AUWCXVepMKEC&pg=PA32&lpg=PA32&dq=rolling+moment+due+to+sideslip+sweep&source=bl&ots=sN2p5ddJrK&sig=M6Lak1w84-HwdMD_y_1lwwtQj7Q&hl=en&ei=QLCITtTJAonhiAKKsKmkDA&sa=X&oi=book_result&ct=result&resnum=6&ved=0CDMQ6AEwBTgK#v=onepage&q=rolling%20moment%20due%20to%20sideslip%20sweep&f=false , from p. 32 to 35.  This also explains why high-wing airplanes have less dihedral than low-wing planes.

Rolling moment due to sideslip may "add stability", but I don't think that's stability that you want when the wind shifts 40 degrees and you want to keep your tricks opposite the judges.  The more a wing is tapered, the less the dihedral effect of sweep.  It works out that rolling moment due to sideslip is roughly proportional to the sweep of a line drawn from halfway along the root chord to halfway along the tip chord. 

I think the more tapered a wing, the better it is for turbulent conditions.  As taper ratio (tip chord / root chord) decreases, the aerodynamic rolling moment due to gusts has less moment arm for a given span, but the restoring moment arm due to line tension stays the same.  I think that's the effect Randy is describing.  If you constrain a stunt plane to having a straight flap hinge line, taper, which you want, comes with sweep, which you don't.  Combat planes typically don't have flaps. They have evolved to having a very low taper ratio and a lot of forward sweep. 

[Hoss Cain]

It seams that I heard a long time ago that the sweep back of the leading edge has something to do with Dihedral?
Is there a general rule like thrust line to wing /stab  for the sweep angle for a stunt plane?

Once upon a time there was heard around the world  that 3° of sweep back in a wing, not just the LE was equal to 1° of dihedral for stability along the longitudinal axis.
Of course that was about the time Boeing was doing much research about swept wings, and it was reported that one fine engineer finally remarked that they knew an airplane really needed a wing, yet at the time they had not truly decided just what a wing really did.   

[Howard Rush]

Hoss, you didn't understand any of the above, did you?   

[john e. holliday]

Well all of us are not aero engineers with degrees and pedigrees.   I just build and fly.   Lately just having fun flying as the general public think I am one terrific flyer.

[W.D. Roland]

Whoops!
Wrong site for posting!

[Howard Rush]

Well all of us are not aero engineers with degrees and pedigrees.   I just build and fly.   Lately just having fun flying as the general public think I am one terrific flyer.

I spent an hour or so looking for stuff on the Web to answer Larry's question (well, to respond to his first sentence, actually) about the relationship between sweep and dihedral.  This required no special knowledge.  Anybody could have done the same thing with Google.  Hoss kinda pissed me off with his story about a Boeing engineer being stupid.  This is after he quoted a fact "heard around the world" which is false.   

[Steve Thomas]

I'd have thought that in a constant skidding left turn, the dihedral effect of sweep would tend to roll the model out of the circle.  Not necessarily a bad thing, and might (theoretically) let you use a bit less tip weight.

[Howard Rush]

True, but the effect varies according to where you are on the circle relative to the wind and to how strong the wind is.  Don Hutchinson's F-86 has a lot of sweep and flies great in the wind, however.

[Hoss Cain]

//SNIP//  Hoss kinda pissed me off with his story about a Boeing engineer being stupid.  This is after he quoted a fact "heard around the world" which is false.   

Mr. Rush, please accept my apology for disturbing you. I thought my post ended with displays that displayed the fact that I was not being too sincere. However the old 3 degree theory was a standard for a long time during my user-time, but a "L" of a long way from being an engineer. Therefore the "shot around the world" did, IMO, indicate just a rule-of-thumb that maybe died a long time ago.

Now back in my "afterburner days" the Boeing Engineer statement was a common joke and I have used it often in other jest. Sorry to make your day bad.
Yet I never called anyone "stupid". If the engineers are all so correct in every way, why do they have to change the Pilot's Flight Manual every other month or so. In the USAF I flew machines thirty years old and still the flight procedures changed at least each 6 months. Even the airlines had major changes fairly often. Why did they not get it right the first time? Is it related to the way that we never get a model just perfect each and every flight, or at least I don't. Always tweaking here and there.
But then I don't take things as seriously as some do. Coming up 76 years old, I doubt I ever will. 

[Howard Rush]

If the engineers are all so correct in every way, why do they have to change the Pilot's Flight Manual every other month or so.

Beats me.  Maybe I used to know, but I doubt it. I don't see many of those aero performance guys anymore, but if I see one, I'll ask him (or her).  Do you mean the AFM or the Ops Manual?

That rule of thumb applies to some airplanes flying level and upright.  Stunt planes fly inside and outside loops, where the relationship between dihedral and sweep effect changes direction.  Looking more at those references, I'm not too impressed at how understandably they explain the relationship between dihedral and sweep, nor did I find anything much better in a more extensive search.  I guess you get what you pay for.  The contradiction of the rule of thumb isn't as obvious as I thought.  I'll try to demathify it:

Take a control line stunter with dihedral, having wings attached midway up the body, and having a leadout guide at the vertical CG.  
Flying on the downwind side of the circle, it would tend to roll to the right in level flight, to the right in inside loops (adding to line tension), and to the right in outside loops (reducing line tension).  
Flying on the upwind side of the circle, it would tend to roll to the left in level flight, to the left in inside loops (reducing line tension), and to the left in outside loops (adding to line tension).

Now consider a stunter with swept-back wings attached midway up the body, with no dihedral, and having a leadout guide at the vertical CG.  
Flying on the downwind side of the circle, it would tend not to roll at all in level flight, to roll to the right in inside loops (adding to line tension), and to the left in outside loops (adding to line tension).
Flying on the upwind side of the circle, it would tend not to roll at all in level flight, to roll to the left in inside loops (reducing line tension), and to the right in outside loops (reducing line tension).  
The tendency to roll increases with lift.  This is the case with most stunters.  Sweep acts like tip weight, but relative to the air mass, rather than to inertial space.   I hope I got all those signs correct.

For folks who want to get to the nitty gritty, there's http://naca.central.cranfield.ac.uk/reports/1952/naca-report-1098.pdf .  See figure 8, which includes the interesting effect of taper. The symbols are murky, but they are all defined at the front of the report.  It just takes some tedious effort.  One conclusion I made from this report is that Brian Eather knows what he's doing.  


  

  

[Russell Bond]

You mean LE and TE sweep like this. (Modelled of Brian Eather's early Firecracker.)

[john e. holliday]

Didn't Billy Werwage also have a couple with leading edge sweep as well as the trialing edge sweep almost like Brian's???

[Ted Fancher]

Didn't Billy Werwage also have a couple with leading edge sweep as well as the trialing edge sweep almost like Brian's???

Yes, although not to so great a degree as Brian (whose later Firecrackers,alas, didn't feature the highly tapered wings and were quite ordinary in planform making them look like everyone else's modified Nobler  .

FWIW, a number of us "designers" did the double taper thing as well and abandoned it as doing so appears to exacerbate trim problems...likely due to the tethered flight factor. 

Ted

[Howard Rush]

You reckon it was the swept-forward flap hinge?  I can imagine Evil coming from that.

[Peter Ferguson]

As far as why AFM's are constantly changing. It has to do with the bookkeeping of changes either to the engine or airframe . Engines and airplanes continue to change until the end of their operational life, some are improvements , some just to keep them operating safely.. Each change may effect performance but not enough to affect the capabilities of the airplane in terms of operational performance .Nonetheless, these changes are bookkept and when they add up to a value that effects the airplanes operational performance the AFM is revised reflecting the changes. It may be the culmation of many Service Bulletins.

[Tim Wescott]

FWIW, a number of us "designers" did the double taper thing as well and abandoned it as doing so appears to exacerbate trim problems...likely due to the tethered flight factor. 

I recently re-read Paul Walker's 1979 (or 1980?) Impact article in Flying Models.  He runs down through a series of airplanes, and mentions going from swept-forward to straight hinge lines because the straight hinge line flew better.  And he still has straight hinge lines today, as near as I can figure.  So he would seem to agree (or at least have agreed just 30 years ago).

Yes, although not to so great a degree as Brian (whose later Firecrackers,alas, didn't feature the highly tapered wings and were quite ordinary in planform making them look like everyone else's modified Nobler.

Maybe it's just a sad fact that George Adrich pretty much got it right with the Nobler, and aside from a few refinements like bigger tails and higher aspect ratio, that's just what works?

[billbyles]

I recently re-read Paul Walker's 1979 (or 1980?) Impact article in Flying Models. 
<snip>

The article was in the May 1991 issue of Flying Models...I keep a copy of it on hand for the great amount of info it contains such as the trimming flow chart.

[Howard Rush]

I recently re-read Paul Walker's 1979 (or 1980?) Impact article in Flying Models.  He runs down through a series of airplanes, and mentions going from swept-forward to straight hinge lines because the straight hinge line flew better.  And he still has straight hinge lines today, as near as I can figure.  So he would seem to agree (or at least have agreed just 30 years ago).

Mr. Walker is fickle and hard to keep up with.  Here he is in 2000 with a swept-forward hinge line.

[PJ Rowland]

The article in question was  " A champ has to have impact"

In anycase the now famous quote was:

" So if you ever see me trying to build a model with a 1/4 chord swept hingeline, please break my building arm"

Here is another Rule breaker to add to Howards....  

This was one of 2 Mustangs...