How Much Dihedral is needed?

[hoppy]

I'm building a house of balsa Spacewalker. It has a 47" wingspan and calls for the dihedral to measure 2 to 2 1/2" at each wingtip. On the drawings, that looks like a lot of dihedral for what I'd like to be a nice aerobatic flyer.

How much dihedral is needed? Can it be reduced without making the plane a flying nightmare?

It will be electric powered - 1:1 power ratio or more.

Thanks
hoppy

[JetPlaneFlyer]

If the model has ailerons and assuming this is not your first RC model then you should be able to remove all the dihedral without any problem. Ideally may want to leave in just a shade to compensate for the low wing position, maybe half inch per side, but in practice i dont think you have any problem at all with a dead flat wing.

Steve

[slipstick]

Unfortunately Spacewalkers (and many other low wing aricraft) look really stupid with no dihedral. I'd leave at least an inch under each wing. As well as looking better you'll find it turns better too.

Steve

[JetPlaneFlyer]

I don’t want to be argumentative but I've flown a low wingers with no dihedral and, using ailerons, they turn just fine. If you want to be able to turn using rudder then you will need plenty of dihedral, but I don’t think this is the issue here. Just look at any pattern aerobatic RC model, they all have zero dihedral and they turn rather well.

The aesthetic qualities of a model with little dihedral is of course purely subjective, personally I think they look ok.

Steve

[vintage1]

I've flown the simulated Spacewalker on RealFlight G2 a lot..it turns on rudder OR aileron. Nice plane. Best spinner in the suite..

Put in whatever you like, but aesthetically it should have some. You need about 7 degrees to not need ailerons..sounds a lot less than that is specified.

I'd put in SOME to counteract the low drag and high thrust line. Otherwise it may end up mildly unstable in roll.

If the real thing flies anything like the simulator, its going to be a total hoot. Whilst not pattern or 3D,. with plenty of power it will do the rest with ease.

Inverted flat spins is something to go for ;)

[rofujiyama]

Dihedral on a low wing sport/aerobatic model, with a thrust line set for propellor ground clearance works very well in flight as well as on the ground when you are taxing ( inadvertant tip rash from crosswinds) with belly mounted landing gear.
I have models with the tips reaching or a little past the thrust line of the engine and they still will hold a decent Knife edge without, roll coupling. I like dihedral on low wingers. ;)

[Ollie]

"On the drawings, that looks like a lot of dihedral for what I'd like to be a nice aerobatic flyer."

Fix your point of view.

[pmackenzie]

If the model has ailerons and assuming this is not your first RC model then you should be able to remove all the dihedral without any problem. Ideally may want to leave in just a shade to compensate for the low wing position, maybe half inch per side, but in practice i dont think you have any problem at all with a dead flat wing.

Steve

A flat wing on a low wing plane will have a lot of adverse roll with rudder input.
Quite unpleasant.
It also looks goofy.

Pat MacKenzie

[vintage1]

Dihedral on a low wing sport/aerobatic model, with a thrust line set for propellor ground clearance works very well in flight as well as on the ground when you are taxing ( inadvertant tip rash from crosswinds) with belly mounted landing gear.
I have models with the tips reaching or a little past the thrust line of the engine and they still will hold a decent Knife edge without, roll coupling. I like dihedral on low wingers. ;)

I think you have it in a nutshell. Ok a midwing pattern plan zero dihedral..or even a high wing plane, but a low wing? No, you need a bit to neutralise all that low down drag.

[slipstick]

Just look at any pattern aerobatic RC model, they all have zero dihedral and they turn rather well.
I have looked at aerobatic pattern planes. Not one of the modern ones has a low wing, they are all mid wing, basically thrust, wing and tail all in line and all at 0 degrees. In short very little like a Spacewalker with the thrust line well above the wing ;).

On the couple of occasions when I tried taking all the dihedral out of a low wing plane I got rotten adverse yaw with ailerons and ended up putting some dihedral back in.....must have just been unlucky I guess ;).

Steve

[JetPlaneFlyer]

Steve,
I’ve got to admit it's quite a few years ago when i last flown an R/C pattern model but at the time the fashion was low wing with no dihedral other than that given by the taper of the wing thickness (top surface flat) and they went ok as far as I could tell... Not that I’ve ever been at a standard to compete.

I do however agree that, at least in theory, a low winger may a little dihedral to counter the destabilising low wing effect... but not much will be required.

BTW, I don’t think dihedral, or lack thereof, should have any direct effect on adverse yaw with aileron input ... Adverse roll with rudder quite possibly.

Steve

[JetPlaneFlyer]

Ahh... I was forgetting about my fathers 1/4 scale Sirocco that i flew only a couple of years ago.

The Sirocco is a low wing with absolutely zero dihedral... it looks great and flys perfectly:http://www.airliners.net/open.file/0868817/M/

The fact that it has no unusual handling traits is probably because of the stabilising dihedral like 'fin effect' as result of the large and high set fin... But this goes to demonstrate that low wing and zero dihedral can work just fine.

[hoppy]

Thanks for the inputs guys--
Looking at the plane, thrust angle and dihedral, if build according to plans, the wing tips will be about 3/4" above the thrust line.

Sooooo, build according to plans (change my point of view as Ollie remarked)?
My concern was that knife edge would be a problem but I guess the concensus is that it won't be.

[Al M]

I fly similar sized low wingers with about 1/2 degree per panel rounded up to the nearest convenient dimension with no problems. It makes it more pleasant. Pattern ships don't need to be fun to fly. They are not built for that purpose today. If they turn out to be fun they are great.

[hoppy]

Hi Al,
Is a panel the whole left wing for example or the structure between each rib?

[vintage1]

I find the nearest convenient dimension is often the 6th ;)

[Daniel7866]

A flat wing on a low wing plane will have a lot of adverse roll with rudder input.
Quite unpleasant.
It also looks goofy.

Pat MacKenzie

I did that mistake when building my 1/4 CAP10.
3° in place that 6° for the full size: adverse roll :eek: .

Daniel

[Al M]

The whole panel. ! degree would work well also.

[harveyweewax]

I find the nearest convenient dimension is often the 6th ;)

I'm more fond of the 5th dimension myself :p

Regards,

Sven

[hoppy]

Checked the wing dimensions and the SpaceWalker drawings call for 6 deg per side. 2.5" for 23.5" wing panel. 12deg total.

Daniel, is that the same as your CAP?

[vintage1]

6 degree a side is almost enough to not need ailerons..I reckon 7 degrees for a low winger..

what's the scale dihedral?

I'd go fer that!

[hoppy]

Looks about the same - I'll have to calculate.
Roughly 6.5 deg - hmmmm, just what House of Balsa used.

Good idea Vinny, I think that's a keeper.

[vintage1]

What a sweet little plane to be sure.

If I could get a better 3-view that'd make a dead easy scale subject to do a really good kit on.

Bit like the Druine Turbulent, only bigger.

[Daniel7866]

Checked the wing dimensions and the SpaceWalker drawings call for 6 deg per side. 2.5" for 23.5" wing panel. 12deg total.

Daniel, is that the same as your CAP?

From CAP AVIATION original documentation the angle is 5°36' per side.
You can find this doc on www.rc-scale.com , download page (http://translate.google.com/translate?u=http%3A//www.rc-scale.com/index.html&langpair=fr|en&hl=de&ie=UTF-8&oe=UTF-8&prev=/language_tools).
Mine was build from rc-scale enlarged plan and I did somewhere a mistake since I put a small 3° on each panel. What I can say you is that with full rudder (~30°) from horizontal flight opposite roll occurs.
Best to stick to original dihedral ;) .

Daniel

[hoppy]

Thanks guys -
You saved me from making a big mistake.

[vintage1]

The invoice is in the post ;)

[hoppy]

The check is in the mail.:)

[Work in Progress]

The Yak-50 and Yak-52 are low wingers with very little dihedral - the wing tapes in thickness, is flat across the top surface, so there's about one degree of dihedral on the centre of the thickness. They fly very well.
The Spitfire is a low winger with around 7 degrees of dihedral. That flies very well too. It has the advantage that you can probably fly it home if someone puts a bullet through your aileron linkage, which you definitely wouldn't be able to in the Yak. In that, you can go from stop to stop on the rudder at cruise speed and it doesn't give you any roll at all.
Both types are among the best-handling machines of their type, both a joy to fly and capable of putting on a show that will delight the pilot and onlookers alike. The difference is that the Spitfire is aerobatic as a side function of its design, whereas the Yak-50 was designed to win the World Aerobatics Championship.

Completely flat wings, a la Scirocco, always look droopy to me, and I'd rather have a smidge of positive roll-yaw coupling than none at all. I actually prefer the handling that way. But it's a question of personal taste.

[hoppy]

The completed SpaceWalker - Thanks again guys.

[vintage1]

Sweet!

Watch out for tropical storms tho..

[hoppy]

Ya, we got the generator ready and the 5 gal gas cans filled up. Last year was a good one and the gas ended up in the lawnmower, car and boat. Hope I can say the same thing next year.

[dead]

Iwould like to throw another, related question in the ring. I have a 10 E chipmunk (goldberg models) and Have been messing around with the dihedral, wing insidence, as well as other attributes. I had a lot of coupling with the stock dihedral, so I flattened it out. Now I get a left roll with full right rudder. My first question is WHY does that occur?

My next question comes from the fact that this plane goes hard nose down when I input rudder. I can't even get in a good knife edge without doing a sideways outside loop. Any thoughts there? The CG is dead on by the way....

Thanks,
Rob

[vintage1]

If you have negative roll/yaw coupling it will indeed bank the other way..

Typically if its a slow wing model, the yaw will give you a sideslip that will push the fuselage over the wrong way. A high CG can make this an unstable situation as well.

Those of us who fly RET models with lots of dihedral INVERTED are fairly well used to it. You use OPPOSITE rudder when upside down!!

I am not so sure about the 'whys and wherefores' of nose down coupling from rudder..I know it happens to me though. Could be that you have a forward CG with net downforce on the tail, and sideslipping blanks part of that leading to a nose down moment.

[Brandano]

I suppose there could be several reasons for the plane nosing down when rudder is applied. I think the most macroscopic would be partial blanking of the horizontal tail surfaces by the fuselage, while a small component might be the lift produced by the rudder if the hinge line isn't exactly perpendicular to the line of flight

[vintage1]

I suppose there could be several reasons for the plane nosing down when rudder is applied. I think the most macroscopic would be partial blanking of the horizontal tail surfaces by the fuselage, while a small component might be the lift produced by the rudder if the hinge line isn't exactly perpendicular to the line of flight


yeah..Id forgotten about that..models with balance sections at the top of the rudder are equivalent to a forward raked hinge line.

It's certainly different plane to plane.

[dead]

Thanks for the thoughts so far. The tail section being not vertical to thrust was my first thought as well. I even changed that aspect on this CHipmunk with little changed. I also have a 10 E Hyperion Cap with the same bad habbit. Neither will knife edge without a lot of up elevator to hold it straight. Surely there must be something else going on other than the fuse blocking the air? If that were the case, would this not be a common problem for every model?

Rob

[rofujiyama]

http://www.rcgroups.com/forums/showthread.php?t=702833

Hi I have a post in this thread. It deals with trimming a plane for aerobatics, that I found works for the planes I have built.
I had a friend that would really ring out a small super chipmunk, but when I finally got one , I found out it exhibited the same characteristics you described. Since then I tend to shy away from planes that have the rudder only on the top of the fuselage,"If "I want it to be fully aerobatic, with the least coupling of control surfaces.
There are many factors involved in getting a plane to fly as you imagine , but thats part of the fun of flying different styles of aircraft.
For a quick fix to your planes, a good dose of down thrust should cure the pitching to the belly in knife edgfe flight, about 2-4 degrees , but then you will probably have to retrim all the other control surfaces and the wing incidence.
Alot of the "foamies guys" , have been designing profile models with very similar fuselage side areas, above and below the wing and/or thrustline to try to minimize the effects the body outlines have on trimming , and have been very successful, to me.
Well hope the link and note here helps a little.
Good luck.
Roland

[dead]

Thanks Roland,
Your info is very helpful. I agree that working with these planes to get them to fly right is half the fun! I am very willing to do the work, but I have been stumped on this one, and am still a bit confused. First, I am not understanding why the rudder on top on a low wing plane causes a pitch to belly when rudder is applied. The pitch to belly must have something to do with an "Up Draft" on the horizontal Stab assuming the elevator is neutral?? If that is the case, why does that only occur when rudder is applied? Second, I have a Hyperion 10 E Cap that has a rudder above and below the horizontal stab, and that too pitches to below when rudder is applied. Can you give me a little moe explaination as to what, aerodynamically speaking, is actually at play here? As they say, "knowing is half the battle". Thanks in advance for your help!!!

Rob

[dead]

One more question, In the other thread I saw some argument over the semantics of thrust. When you tell me down thrust, do you mean to point the front of the motor down? If so, Both of my planes already have a good bit. Also, if the plane is pitch to belly (nose down attitude), then wouldn't I want to point the motor up to counter act?

ROb

[rofujiyama]

Hey Rob,
In regards to the top rudder setup, what I have noticed is that it will yaw and bank/roll the airplane, as opposed to having a yaw only response to rudder in a low wing plane with some dihedral.
The pitching to the belly in Knife edge mode with applied rudder, I just add downthrust(pointing the motor down on a tractor setup) by habit. But if I were to surmise the reason , I would have you set your plane on Knife edge on table and looking at it from the front, (1) the lifting surface is now the fuselage side and the rudder now becomes the angle of attack control surface, (2) generally speaking, the wing is just along for the ride now, BUT its frontal area is still there and now addding drag without contributing to lift on one side of the lifting surface( the fuselage ) keeping in mind, that the rudder may not be centered also, in respect to the center of the lifting surface ,viewed from the front.(whew,I am not good at typing ,lol) (3) OK, now, if you have your wing and horizontal stab and ele.,trimmed for level flight at full throttle, conceivably it should be able to fly straight ahead, But now you have to try to balance out the Drag of the wing and or undercarriage when you are in Knife edge flight, as compared to the Drag of the fuselage in the middle of the wing in level flight.
Ok, now with that in mind I think the Drag of the wing and under carriage will tend to pitch the model to the belly in Knife edge, unless you do something to compensate for it, in a low wing , or even a mid wing model( because, generally speaking , the rudder will still have more volume(?), offset to one side of of the lifting surface and midwing section, in most scale planes that I know of.)
SO,, lol,,whew,,,for me,,, I use the motive force of the engine, applying downthrust in relation to the wing so i can have the wing see a positive angle of attack in relation to the motive force to counter act the Drag of the wing, in Knife edge flight and have the wing lift the body to the canopy to overcome the diving to the belly(pitching).
BUT, now it may be too much for the trim you had before, for level flight :eek: Soooo, the tuning goes on , till I burn out ,lol :)
Well If I had to anaylize how I trim, my planes, I think I could live with this. :D

[dead]

Thanks Again Roland!
As I have been studying this, I have come up with one idea that may help in trimming. If I set all of my control surfaces to neutral, and then fly the plane (without rimming for level flight) to high altitude and then into power off vertical dive, I should be able to trim the wing incidence to hold straight down path. Then, I can adjust motor thrust to allow for close to level flight with rudder input as well as close to level flight with now rudder. I will assume that there will be a comprimise here where the plane will likely rise somewhat during level flight and dive somewhat during rudder input. But I wonder what you think (in theory) of this procedure?

Rob

[rofujiyama]

Hi Rob
Took me awhile to consider your idea.
There are other approaches,other then mine, for trimming an airplane, however I have found that if I can achieve the parameters I have described I have ended up with a very well designed and constructed aircraft.
It will be an aircraft that will go were i point it and not be sensitive to CG placement(or have a broad range of CG that I can fly with)
It should be able to fly any sequence of manuevers(with practice and a good caller,lol)
It should be able to let you handle any wind situations that may arrive, wether it be crosswinds, thermal ativity(gusting) or high winds, etc.
In other words, what I descibed, is MY baseline for a good aerobatic model, if it can do the things I describe ,I can concentrate more on flying ,then on any qwirky behavior that may exquiste in aircraft not designed for full on aerobatic manuevers.
P.S. Once I achieve my parameters, I havent had a problem with up or down lines, So I am not sure about tuning, in that respect. :D
Time to Fly Rob. :)

[macboffin]

If you have negative roll/yaw coupling it will indeed bank the other way..

Typically if its a slow wing model, the yaw will give you a sideslip that will push the fuselage over the wrong way. A high CG can make this an unstable situation as well.

Those of us who fly RET models with lots of dihedral INVERTED are fairly well used to it. You use OPPOSITE rudder when upside down!!

I am not so sure about the 'whys and wherefores' of nose down coupling from rudder..I know it happens to me though. Could be that you have a forward CG with net downforce on the tail, and sideslipping blanks part of that leading to a nose down moment. I agree about blanking part of the tail

[JetPlaneFlyer]

I agree about blanking part of the tail

Blanking the tail is an interesting idea... However, here's a thought:... Whether this blanking caused an 'up' or 'down' pitch would depend on if the tail was 'lifting' upward or downward. Many automatically assume that the tail 'lifts' downward but that is not always the case. I'm guessing that the model in question will have a symmetrical airfoil, if so then airfoil pitching moment is not and issue so you can easily see if the tail 'lifts' up or down by looking at the position of the CG relative to the aerodynamic centre of the wing (25% chord, or more precisely 25% MAC)... If the CG is ahead of 25% MAC then for trim in level flight the tail must 'lift' downward. If however the CG is behind the 25% mark then the tail, in level flight, must be contributing upward lift. In my experience most RC models have a CG close to, or just behind 25% MAC.
I guess for a model to have minimum reaction to horizontal stab blanking caused by yaw, that you would be looking for the tail, in level flight, to be contributing very little lift either upward or downward. This would require that the model was balanced at 25% MAC, at least for a symmetrical airfoil. Things are more complex with a cambered foil that has pitching moment to consider.
Perhaps then Dead should look at his CG location; is it possibly too far forward? Too far forward CG (and resulting excessive static margin) may also explain why the model needs lots of downthrust.

Steve

[dead]

I'll try this when I can. They ARE symmetric airfoils, or at least simi-symmetric for sue. Unfortunately, neither of the models in quetion are flyable at this point. But my Ultimate E Bipe is flying and also suffers some (not nearly as bad as the chpimink or the cap) from nose down with rudder input. I will play with getting the cg set so that my elevator is exactly level with the horizontal stab for level flight. I assmue that is what Steve means to do? I'll test and see what happens.

Rob

[rofujiyama]

Hi guys, I have been away from pattern flying for quite some time and it seems the state of the art has past me by ,lol. However with the current trend, to zero thrust angles on the engine/motor (lol) is held by the Oxalys and Christophe Pasant-Le-Roux. Mr Flash told me the "canaliser " was used to redirect air over the tail surfaces. However, it sure seems to me that having that fin assembly on top of the fuselage, being finely tuned , would sure help offset the drag I talked about in Knife edge, and negate the use of engine thrust angles, Used in the early 90's, to make ones plane fly true , with no pitching in knife edge flight. Maybe I will get one, lol. Sure seems to work for him ,lol. :D ;)
Roland

[JetPlaneFlyer]

Rob,
I'm working very much on theory here as I've not really experienced this problem...
It's not so much about getting the elevator level, more about getting the model to balance on (or even aft of) the wing's aerodynamic centre. This way if the tail is blanked due to yaw the nose should remain level or even pitch up.

Bottom line: try moving the CG back, but do it in small steps because too far aft CG can make the model very tricky to fly.

Steve

[RCAV8R13]

Too much dihedral will cause adverse yaw and all the differential aileron you can get will not cure it.
RCA

[dead]

I havfe moved CG Back and have plane trimmed to fly level at full throtle with elevator flat. I can knife edge, but still have to add in up elevator. It is definitely improved however with good cg and flat elevator (No up trim). Wonder if I should try moving cg further back and adding down elevator trim?

[JetPlaneFlyer]

Wonder if I should try moving cg further back and adding down elevator trim?

Yes, providing the model is not becoming difficult to control in pitch then you can move the CG back further... but do it in small steps!

Where is the CG as a percentage of the wing chord? If you can give me the geometry of the model I could work out where the CG should be (need span, root chord, tip chord, LE sweep, same data for horizontal tail, plus distance between LE of wing and LE of tail)

[dead]

Here is some of the info. I will try to get rest today or tomorrow. Thanks. Also, the cg right now is about right under the main spar that runs throught the entire length of the wing. Maybe right on the back edge of spar. One thing I noticed during trials is that when the cg was too far forward I had to put in up trim on elevator. It fly pretty well, but on landing approach it would tend to balloon on me and make landing very tricky. Now it is MUCH EASIER to bring it in....

[dead]

Here is stock info:

Wing Span 930 mm/36.6"
Wing Type Semi-Symmetrical One-piece, Removable
Length 800 mm/31.5"
Wing Area 14.7 dm2/229 sq inch
Kit weight 280g/10oz
Flying Weight 480-580g/17-22oz

power Hacker A20-20L Brushless Motor & Hacker X-20 Brushless ESC

MAIN WING:
Span: 36.6"
Root Chord: 7 3/8"
Tip Chord: 4 3/4"
LE Sweep: 3/4" to 1"
Trailing Edge Sweep: 1 1/2"

Distance from LE of Wing to LE of horizontal Stab: 16 1/2"

HORIZONTAL STAB:

Span: 13 3/4"
Root Chord: 5"
Tip Chord: 3"
LE Sweep: 1/2""
Trailing Edge Sweep: 1 1/4"