Question on Dihedral (new related question)

[Phil Coopy]

As you may have seen on earlier posts, I am working on a profile Gee Bee Sportster.  I am using the Brodak P-40 wing modified to a straight leading edge.  The Brodak plan sez to put 1/2" dihedral in each wing.  My Gee Bee has the same wing to stab hight relationship as the P-40, but the engine thrust line is 1" higher. I was wondering what relationship the amount of dihedral has with the hight of the thrust line above the wing.  What would happen if I just built the wing flat? Or conversly, might I need more dihedral than Brodak calls for.

Phil

[John Stiles]

I'd go for flat...I built the .40 size P51 with dihedral, and spent the whole winter rubbing the black finish to a very bright shine.....the first time around, I lost it and had a pile of black splinters. So you see I don't have much experience in mustangs...just don't like dihedral. That's just my opinion.

[Glenn (Gravitywell) Reach]

The P-40 NEEDS that 1/2" of dihedral to fly right.  I built mine without the dihedral by accident and it does not turn outsides near as well as it turns insides.  I was told by very good builders that it absolutely needs the dihedral.  Why your P-51 crashed, probably, has nothing to do with the dihedral. 

As to the Gee Bee......if your moving the engine thrust line up on inch, I would increase the dihedral to at least 1ìnch total on either wing tip.

[John Stiles]

The P-40 NEEDS that 1/2" of dihedral to fly right.  I built mine without the dihedral by accident and it does not turn outsides near as well as it turns insides.  I was told by very good builders that it absolutely needs the dihedral.  Why your P-51 crashed, probably, has nothing to do with the dihedral. 

As to the Gee Bee......if your moving the engine thrust line up on inch, I would increase the dihedral to at least 1ìnch total on either wing tip.

I figured it was because of the ailerons, coupled with the fact that I didn't think I needed a stand-off on the inboard wing for my lines....I had one but it wasn't tall enough!

[Steve Helmick]

John...The idea of the dihederal is to put the leadout guide at the vertical C of G, or at least close enough. It sounds like your P-51 had external leadouts? Maybe a scale model or a converted R/C kit?

FWIW, I'd ask Mark Scarborough or Pat Johnston.  Steve

[Serge_Krauss]

Edit: I see Steve posted while I was typing. Since mine's a little different, I'll just post it anyway.

The dihedral is needed to get the leadout exits level with the vertical height of the c.g. This is to keep the model level. If the model's leadouts exit below the c.g., then it will roll outward and lift a bit outward in normal level flight, but inward when inverted. The latter is not good for tension, and transition between the two creates unwanted motions, which can even induce further unwanted movement about other axes. Thrust line is only indirectly related, but the fact that it's higher indicates that your model's most massive (heaviest) item, the engine, is higher, thus raising the c.g. even further than on the P-40. So I'd follow Glen's advice and keep or increase the dihedral; it's not ornamental. I have to admit that I was skeptical when I first encountered CL dihedral, but it has also been well and routinely proven out over decades by the likes of champion builders Ron Burton, and Al Rabe, himself a great CLPA champion. Their work is well documented, especially in the massive SSW forum archives, as well as in classic model magazine articles. Dihedral apparently does not destabilize the plane during our constrained, inverted flight, but it does solve an otherwise impossible trimming problem, while keeping the lines in the wing and often adding scale realism.

SK

[Howard Rush]

There's more.  You'll notice that full-scale, non-control-line, low-wing airplanes have dihedral, and full-scale, non-control-line, high-wing airplanes have negative dihedral.  That is to counteract the fuselage effect on rolling moment due to sideslip.  If you leave the wing straight on a high- or low-wing stunter and compensate for vertical CG with leadout offset, you might have a weird flying airplane.  Textbooks have a picture that explains this.  I found one online at http://books.google.com/books?id=vROcxJj4igQC&pg=PA294&lpg=PA294&dq=rolling+moment+due+to+sideslip&source=bl&ots=XcmE7mxN1q&sig=86Q_Mi78Nf7YtACWdA8SBiojvYg&hl=en&ei=XTaqSaTuPIHasAP1uMXtDw&sa=X&oi=book_result&resnum=10&ct=result#PPA297,M1 , at p. 297.  It is a pleasant coincidence that putting the wingtip in the right place for vertical CG also makes the rolling moment due to sideslip come out pretty close to the same for inside and outside maneuvers. 

[John Stiles]

John...The idea of the dihederal is to put the leadout guide at the vertical C of G, or at least close enough. It sounds like your P-51 had external leadouts? Maybe a scale model or a converted R/C kit?

FWIW, I'd ask Mark Scarborough or Pat Johnston.  Steve

You caught me...yes it was a converted RC....the lines exited the fuse above the[Foam Core] wing, and like a dummy, I put a very low standoff at the wingtip on top of the wing. When the ship took off, I knew immediately my error.....the inboard wing stayed horizontal and the outboard wing[kicked up] acted almost like a rudder, sending the plane up and over[like a wing over] that I couldn't recover from.  >> Great post Mr. Howard Rush...very informative.

[Phil Coopy]

Thanks guys,

I think I'm going to increase the dihedral a bit to compensate for the higher CG on this plane.

Phil

[Mark Scarborough]

Phil,
I would have to agree with your final assesment, I think based upon your mention of the engine being an inch higher, I would at look at total dihedral  being abotu one inch. Now once you have it all framed up, BEFORE you permenantly put the wing tips on, you can use Al Rabes method and hang it from the leadouts to see how true it hangs,, then you can put the leadout guide high or low in the tip if further adjustment is needed to get it to hang straight.
anxious to see what you come up with, always loved the old race planes!

[Phil Coopy]

Thanks for all the help on dihedral folks.  New problem for me arises.... I have never set up the  flap linkage on a profile plane with dihedral.  I can envision a lot of different possibilities, but I know you guys have the ultimate answer for the best, simple, way to couple flaps with dihedral.  Jump in here guys................

Phil

[Glenn (Gravitywell) Reach]

Hey...this is one I can answer!   Use a Lucky box.  They allow the flaps to move without binding.  You can find them at http://rsmdistribution.com/index-2.htm   

[Mark Scarborough]

Lucky box is a good solution, though I have used up to one inch of dihedral with no lucky box or anything, just the regular horn. there is not very much angular change with one inch of dihedral it will most likely not even cause ANY resistance. Pat talked me through this,, I didnt believe him either ( silly silly me ) until i did it, and hes right,, big surprise there huh,,,

[Trostle]

Lucky box is a good solution, though I have used up to one inch of dihedral with no lucky box or anything, just the regular horn. there is not very much angular change with one inch of dihedral it will most likely not even cause ANY resistance. Pat talked me through this,, I didnt believe him either ( silly silly me ) until i did it, and hes right,, big surprise there huh,,,

What Mark said  ----

Use a single horn, no lucky box. 

However, there is an important thing when you do this with a wing that has dihedral.  I hope that I can explain it without using a pencil and paper.

The horn will be attached to the flaps in the usual way.  You should use a horn that is just wide enough to insert into the flaps.  In other words, the horn wire that goes into each flap should be no more than say 1/2 or 5/8 inch from the inboard edge of each flap.  (Make sense so far?  If this is not clear, I will try to explain in another way.)

Then, with the horn located between the two flap, you will see that the horn bushings need to be set slightly above the airfoil chord line through the center section of the wing.  For example, take a 54 inch span with 1 inch dihedral in each wing tip and a horn that is 4 inches wide.  The center of the horn will need to be just less than 1/16 inch above the wing center section chord line at the wing trailing edge.  So, actually the busings which will be next to horn will be even less that that 1/16 inch dimension above the wing TE chord line.  (Again, if this does not make sense, I will try to explain in another way.)

This works.  Really.  There is enough tolerance in the hinges, there is enough flex in the way the horn mounts in the flaps, there is enough flex in the whole system to allow the flaps to travel through their full range (say plus or minus 40 degrees) without any sense of binding.  It really works.

Do not even think about using double horns on the flaps.  That will allow flexing between the flaps to the point the airplane will be difficult to trim.  There is no such thing as a flap/horn arrangement that allows too little flex from one flap to the other. A single horn is a step in the right direction.

(By the way, lucky boxes do have their place.  This is when there is a highly swept back wing/traiiling edge (or stabilizer) or in the case of the stab, there is a lot of sweep in combination with a lot of dihedral like is found on some scale models.)

Keith

[Phil Coopy]

Thanks for all the help guys,

I understand how Mark and Keith are saying to do it without the lucky box, so I'm going to do it that way. If I fit the horn into the flaps first, it should show me where the bearing will be located when in the neutral position.

The nice thing I find about the C/L community is that the guys at the top of the game are willing to take the time to help us "duffers" out.  I never found that in pattern or IMAC.

Phil

[Mark Scarborough]

Phil,
that approach, putting the  horn in the flaps, then locating the bearings is a VERY simple and good approach. I tried all kinds of cool geometric hoo doo voo doo,,, drew it in CAD laid out lines and tape,, finally did exactly what you are describing,, DUH,,,, 
best of luck with it, just be patient and accurate you will be fine,,,

[Pat Johnston]

My buddy, Mark, is right on.  One thing to say to all is that the Brodak P-40 profile was designed with 1/2" of dihedral as a SWAG sort of engineering routine.  After hanging it by the leadouts, It looks like I was close, but about 5/8" would be just right.  An 1/8" plus or minus is not a big deal, but the 5/8" figure is better.

SO!!!!!!   To all those with the Brodak P-40, install the dihedral!!!  As amazing as it flies by all reports, it flies even better with dihedral.

BTW, Mark did a lot of calculations on his Gee Bee "Y", using the Brodak P-40 wing, and placed it spot on in the fuselage to get the vertical CG neutral point just right.  We've talked about a fun event where everyone uses the P-40 wing placed in any kind of fuselage design.  The wing is one of the very best in stunt and we all could have a ball coming up with a "One-event" wing basis.  Admittedly we don't need another event, so this is just a fun exercise in day dreaming Mark and I get into.  At least this sort of plane can be flown in Profile stunt and is a lot more fun than the usual "yard dart" type so commonly seen.  Mark's Gee Bee is one of the coolest planes in the Northwest for a profile.
Pat Johnston
Mad Design Department
Skunk Works

[Bob Reeves]

This would qualify 

P-40 wing, no dihedral, not needed and the Saito 40 is the perfect power plant for that wing.

[Phil Coopy]

Bob, if  I understand what Keith Trostle said, your leadouts are very close to the CG on your plane making little ot no dihedral necessary.  But in my case the engine thrust line is a lot higher than the leadouts which makes my leadouts below the CG if I don't use dihedral, so I will need it to make the plane fly level.

Phil

[Bob Reeves]

Bob, if  I understand what Keith Trostle said, your leadouts are very close to the CG on your plane making little ot no dihedral necessary.  But in my case the engine thrust line is a lot higher than the leadouts which makes my leadouts below the CG if I don't use dihedral, so I will need it to make the plane fly level.

Phil

Yes you are correct.. The wing in the profile Shoestring is very close to the vertical CG and why it doesn't need dihedral.. In your case I would follow the advice given.. Keith, Pat and others know what they are talking about and have already given you great answers. I just posted because I wanted to show off what I did with a P-40 wing 

[Pat Johnston]

Bob,
Yours looks very cool.  It is great to see others using the P-40 wing in different layouts.  This gets back to the concept of having a "One-design" event at least as far as the wing goes.  The fuselage is there simply to hold on to the engine, tail, etc.  Well, maybe a little more than that, but you all get the concept.  Of course, your Shoestring is a wonderful example of a super design approach.  Bet it flies very well???
Another plane which is possible is one of the versions of the ME-109 which had the rounded tips.  Very close to the P-40 shape so it will be just fine.  Anyone with other thoughts?  Perhaps a Brown Miss Los Angeles? Ike and Mike racers?....
Pat Johnston
Dream Works Aircraft Design Division
Skunk Works

[Randy Powell]

>>This is when there is a highly swept back wing/traiiling edge (or stabilizer) or in the case of the stab, there is a lot of sweep in combination with a lot of dihedral like is found on some scale models.)<<

Or swept forward.

Bob,

I love the Shoestring. But your wing is mounted a lot higher in the fuselage than the P-40, so no dihedral needed. Sure that that layout. I may steal it.   

[Derek Moran]

There's more.  You'll notice that full-scale, non-control-line, low-wing airplanes have dihedral, and full-scale, non-control-line, high-wing airplanes have negative dihedral.  That is to counteract the fuselage effect on rolling moment due to sideslip.  If you leave the wing straight on a high- or low-wing stunter and compensate for vertical CG with leadout offset, you might have a weird flying airplane.  Textbooks have a picture that explains this.  I found one online at http://books.google.com/books?id=vROcxJj4igQC&pg=PA294&lpg=PA294&dq=rolling+moment+due+to+sideslip&source=bl&ots=XcmE7mxN1q&sig=86Q_Mi78Nf7YtACWdA8SBiojvYg&hl=en&ei=XTaqSaTuPIHasAP1uMXtDw&sa=X&oi=book_result&resnum=10&ct=result#PPA297,M1 , at p. 297.  It is a pleasant coincidence that putting the wingtip in the right place for vertical CG also makes the rolling moment due to sideslip come out pretty close to the same for inside and outside maneuvers. 

Oh, great.  Does this mean my full-stunt Piper Cub is going to need anhedral?
Derek

[Bob Reeves]

>>This is when there is a highly swept back wing/traiiling edge (or stabilizer) or in the case of the stab, there is a lot of sweep in combination with a lot of dihedral like is found on some scale models.)<<

Or swept forward.

Bob,

I love the Shoestring. But your wing is mounted a lot higher in the fuselage than the P-40, so no dihedral needed. Sure that that layout. I may steal it.   

Steal away.. If you would like I have the fuselage in an AutoCAD file. Geoff Goodworth sent me a CAD file of the Midwest Shoestring, I pulled the fuselage and blew it up to match the P-40 numbers.. Pat sent me the wing cut-out for the P-40 so the fuselage drawing is complete with the proper cut-out for a P-40 wing..

Speaking of wing placement, actually the wing either needs to be moved down 1/4 inch or the engine needs to be moved up a tad. The leadouts are slightly above the CG, not enough to cause as much roll as the P-40 without dihedral but not perfect either. I can live with it on this one but if I build another I will probably lower the wing.

[BillP]

Oh, great.  Does this mean my full-stunt Piper Cub is going to need anhedral?
Derek

What planes use anhedral? I'd never build a full scale with anhedral and expect it to fly right side up easy...but they would fly inverted easier.  Never heard of diehedral doing anything but help the plane fly level due to pressure differentials. The two experimental planes I built had dihedral but they weren't aerobatic. All of the rc and cl planes I build without it.

[Serge_Krauss]

In CL planes, the line tension provides a restoring force component in roll (and yaw). The lines' relationship to c.g. is the dominant factor. I would not expect a dihedralled plane to be particularly stable in inverted free flight. Anhedral is sometimes used to correct adverse affects of extreme wing sweep, which brings its own dihedral affect. Planes are not simple, especially controlliners, with their extra asymmetries, but once in a while there is an advantage to being on the end of those lines.

Edit - just to note that the Wrights used anhedral for an entirely different reason on their marginally stable, but control oriented "Flyer". They had troubles with gusts upsetting the plane in their intentionally low-level tests, causing them to veer away from the wind and snag wings at higher than preferred speeds and angles, sometimes causing damaging flips. The anhedral banked their wing into the gusts for, at worst, only minor ground loops. Now you see it on some jets (e.g. 'Harrier').

Edit 2: spelling

SK

[Howard Rush]

My post above was too crude, as was politely pointed out.  As that picture shows, low wings have the same effect as negative dihedral; high wings have the same effect as positive dihedral.  Cessnas or Cubs probably need more dihedral effect than just the fuselage placement provides, so they also have some positive dihedral, but not as much as low-wing airplanes made for the same application.  Kit Gerhart's low-wing 1/2A free flight had a lot of positive dihedral.  Some high-wing airplanes do have negative dihedral: the C-5 and C-141, do, as I remember, and maybe the F-8U and A-7. 

As Serge said, wing sweep also has a dihedral effect, but it is proportional to lift.  On a stunt plane, either dihedral or wing placement on the fuselage will do one thing in inside maneuvers and the opposite in outside maneuvers.  Sweep does the same thing both ways.  For unflapped airplanes, sweep is probably bad for control line planes: it exacerbates the effect of wind.  Forward sweep is a virtue for combat planes.  It lets you use more of the circle.  Stunt planes have flaps.   I don't know how rolling moment due to sideslip changes with flap deflection, but I suspect something perverted happens with much forward sweep of the hinge line.