rabraaten

I have a Hangar 9 Arrow, well.... the back half anyways. I replaced the airframe when the plane went in but saved all of the pieces for ...the future. As the budget is a bit limited the first Arrow will be the second plane. My winter plan is to rebuild the plane into something, different....better. A couple of aerodynamic questions have come to mind.

1) I'm planning on dropping the wing down, shoulder, mid or perhaps even a low configuration. If I keep the wing itself the same and the position fore and aft the same will the CG be located in the same place? I do plan on keeping the original fuselage length as well, engine mount and everything.

2) The wing currently has 1 3/4 inches of dihedral over the 62 inch span. I'm thinking of taking it down to zero. Is there any reason not to?

3) The wing is described as semi symmetrical. Would I get more of a symmetrical effect by increasing the angle of incidence very slightly?

4) I would like to get a bit more authority out of my control surfaces, the tail feathers especially. Plan is to cut the rudder hinge straight instead of on the angle its at now and increase the length by and inch or so of both rudder and elevator. Will I get any more authority if I don't increase the throw, only the size of the surface?

I think that just about covers it for now... Any and all thoughts are greatly appreciated.

Ryan

rabraaten

Oops.... In question 3) I meant to decrease the angle of incidence, thereby making the wing look more equal to the air upside down as it does right side up...

wellss

1. yes
2. spiral stability, it will tend to tighten bank in turns unless you make the fin bigger. You will lose any yaw/roll coupling which may be a good thing, depending on what you want
3. nah, doesn't matter. Regardless, the wing has to go through the same angle of attack change for inverted flight. It just won't be as snappy inverted, if at all.
4. yep, if you take the dihedral out, making the fin taller/larger is a good idea.

moving the cg back a little will make the controls more sensitive and help the inverted flight trim. I would add surface area to the tailfeathers, so as to maintain stability.

rabraaten

Thanks Wellss, so making the control surfaces larger is effective without changing throw? I think that I may be at or near the limit of my servos for mechanical advantage at higher speeds. Also you would increase the area of the static components as well, the horizontal stab and the vertical.

I'm thinking that as this plane started its life as a trainer that flying wires on the tail might be prudent given that I'll be putting more force on them than they appear they were designed for. Does changing the profile of the tail feathers have much affect on aerodynamics?

wellss

Sweep, Taper and aspect ratio all affect aerodynamics. In general, you want a taper of about 0.5 ( tip chord / root chord ). You want as high an aspect ratio as possible, within structural limits and no sweep if possible. Adding thickness to your surfaces might help with stiffness. Hinging your control surfaces at the 1/4 chord will aerodynamically balance them and prevent flutter, as on many full size planes ( and take some load off the servo ). Check out some museums for photo examples...

da Rock

Yes, if the planform of your design doesn't change, the moments won't change either. There is an excellent website that does all the math for you to figure CGs. It uses the aerodynamic formulas that're basic to the problem and have been used forever. It's geistware.com and their CG application is both simple and quick. The 9 measurements are very easy to do, if you have a yardstick.

The amount of dihedral is worth looking at, depending on what you want for roll stability out of your new design. If that much was used for a highwing, then it, plus the dihedral effect of the highwing, obviously gave the original layout a fair amount of positive roll stability. If you move to a low wing layout, the low wing position actually causes a negative roll stability. Almost every full scale low wing flying has some dihedral to at least cancel out some of the negative roll stability. There are amounts published to suggest how much is needed for what. It's worth keeping a bit if you go low wing. I don't remember how much is needed to get a neutral effect, but it's worth looking up.

Changing your AOI will only cause more drag from the fuselage at the design speed of the original. No matter what the AOI, at any speed the airplane flies, the wing will be at whatever AOA it needs to hold the airplane at your chosen altitude at the speed you're holding. So no matter what you do, the wing sorts out where it wants to be, and everything else winds up flying at that AOA. And if all that stuff is not lined up to the airflow it creates more drag. Designers figure out what weight will be carried and at what speed. They then look up what the wing they've laid out will choose for an AOA to carry that load. They then set the AOI so that the wing will carry the rest of the airplane cleanly. So don't change your AOI unless you've seen something that suggests to you it was wrong on the original layout.

You should get more authority and you probably will also get more efficiency. You might or might not get more servo load. As for getting more aerodynamic balance, don't plan on it. Aerodynamic balance of a surface has almost nothing to do with where they are hinged relative to the stab/elevator chord. Flutter can be affected by how firmly they are rigged and how strongly they are held by the servo and a few other minor details. You could add area forward the hingeline for example. Or place balance weights forward the hingeline. Flutter is also affected by the mass of the surface that isn't counterbalanced so keep the elevators light. Make the stab stronger if you're worried about folding it. Matter of fact, making a new stab from scratch is dead simple and easy and quick. And making it bullet proof is dead simple if you making it yourself. Use a spruce doubler inside the balsa LE, or simply a spruce LE. Use the same idea for the stabilizer's TE. No need for extra thickness then. But extra thickness is an excellent way to make a stronger span no matter what surface.

If you're worried about your servo strength, then think about the rigging.
Connect the pushrod to the close in hole on the servo arm. Connect that pushrod to the outer most hole on the surface's control horn. If that doesn't give the deflection needed, first move the horn connection in a hole. Try to keep the connection on the servo arm shorter than the connection on the horn.

da Rock

About the tail.........

Generally speaking, if you move the hingeline forward, increasing the percentage of elevator to stabilizer, a couple of things usually happen.

The elevator deflection is reduced to get equal results. This sometimes gives less drag, but since either planform must produce the same lift to create equal pitch control, the difference in drag is not much at all.
The weight of the elevator and the load on the servo increase. If you've got the rigging done right, this won't matter much at all. And the difference in weight will be not much at all.
There might be an increased possibility of flutter. With our models, if you've got tight rigging and close to an adequate servo, this won't matter.
Aerodynamic balance comes from things other than the location of the hingeline.

Rigging..................

BMatthews

Re2- If you remove dihedral then you should make the fin and rudder area SMALLER to avoid the tightening of the turn. I know this seems backwards but the whole explanation of spiral stability is also "backwards" to a degree. Too large a fin area produces a tendency to wind into the turn. Too small lets the model sideslip a lot and suffer from a wandering tail. Fin area is definetly a case of being not too small and not too large just like the baby bear's porridge....

Re4- same thing as 2 for the same reasons.

Moving the CG back will help the pitch be more sensitive and from what I've experienced also sharpen the rudder a little but not as much as the elevators. It didn't do a darn thing one way or the other for ailerons.

wellss

That would be true if the wing had dihedral. But, since he's taking that out...
Actually, the area of the fin won't matter. It will produce whatever lift it needs to in order to stabilize at some yaw angle. You are right that a smaller area = larger yaw angle. What I was really getting at, was that the center of the fin area should be moved higher above the CG, so it produces a roll moment out of the turn, to compensate for the missing dihedral.

da Rock

Actually, it is true as stated. With the removal of the dihedral, less fin/rudder is needed and desired.

And the area of the fin does matter in every situation and application and modification. The yaw stability provided by a fin/rudder is more a function of it's area than is the pitch stability of a horizontal stab/elevator. It was discovered long ago the fin/rudder size was critical and they labeled the antics of an inadequate vertical tail as "Dutch roll". And labeled the results of too large a fin/rudder as "unrecoverable spin." When they were inadequately sized they were unable to find and hold the "magic" angle of attack. They didn't find that "larger yaw angle" and hold it. The horizontal tail works as you suggest, by producing whatever lift it needs by changing the pitch of the aircraft, but the vertical tail isn't working in downwash, nor working to hold a wing against the forces a wing produces. And the vertical tail has to work against the oscillations that a wing can produce when it yaws. The vertical tail has to more exactly match the yaw it's controlling because it can fairly easily overwhelm and create a spin that it then can't overcome. A horizontal tail works as you describe to control constant state pitch, but the vertical tail does not do the job well unless it's the appropriate area. The too small vertical tail will overshoot the appropriate AOA and start yaw oscillation. The too large vertical tail will contribute to unwanted yaw that leads to a spin that it then won't help recover.

BMatthews

That sums it up nicely Rock.

Wells, a bunch of years back I wanted to experiment with this myself so I modified an aileron glider I had with a larger than normal vertical area. The first flight was all but uncontrollable. It was like trying to balance a broom handle in your palm. Constant corrections were required to pick up a wing to both sides and if left alone the model would very quickly falloff into a steep spiral dive. Clipping off bits of area quickly tamed this and after two or three chops of about 3 to 4% area it was down to where it was a delight to fly and would hold all but the steepest banks with no "top aileron" being required. Going further resulted in poor yaw damping and the first hints of dutch roll and a tail that "hung low" in the turns unless some pro turn rudder was used to lift it back into line. So I glued the last bit back on and doctored up the covering and called it a day. It was a lovely model to fly from that time on until I sold it to a buddy that still loves it to this day some 15 years after.

We also see a lot of this in my free flight models. Old timers are especially prone to this trouble. The original Comet Clipper with V dihedral requires that you have a nice big engine cowl to counter the too high value of the vertical tail volume coefficient. They got around this in the Clipper II by leaving the center angle alone and ADDING tip dihedral. This countered the tail volume and produced a much easier to trim model. But another way to fix it would have been to reduce the fin size. But at the time I suspect they wanted more dihedral to tame the steeper climbs produced by the new hotter engines of the day.

rmh

The fin size is difficult to guesstimate
On many so called aerobatic models -- the modeler would often make the rudder and fin larger-- to make th thing "knife edge better .
but strangely they did not improve .
If you tip the model on it's side - you see that the percentage of side area ( lifting area ) AFT the cg is now much further aft . this will as the model transitions from level to knife - automaticaly , allow/ cause the nose to drop.
On the best , latest aerobatic stuf for slow speeds - the entire side are is greatly increased , with only enough rudder /fin to allow easily controlled pitch and yaw during knife or upright flying.
these models roll with minimal inputs and tho some may doubt it --they are incredibly stable in all axis - with no dihedral
The models look odd to those used to traditional layouts but they are far better aerobatic performers.

Ed_Moorman

My Feedback: (1)

I have designed and built a bunch of airplanes and probably made every aerodynamic mistake there is.

I can assure you that if you take all the dihedral out of a low wing airplane, unless it has sweep back, it will have reverse yaw-roll coupling. Right rudder will produce a left roll.
High wing location, dihedral and sweep back will produce roll in the direction of rudder.
Low wing location, anhedral and sweep forward will produce roll opposite to the rudder.
Mid wing location, flat wing and no sweep produce no roll with rudder. Yes, I have tried all of these, including forward sweep.

A fin & rudder too large will produce spiral instability. Your plane will tend to increase the bank through a turn. This is opposite to a high wing trainer, for example that tends to roll out in a turn.

I designed a plane once back in 1975, flat, low wing. In knife edge it wanted to roll over on its back, opposite to rudder. I thought I'd try the rudder area thing, you know move the rudder area down so it would act like an aileron to counter the roll. It didn't work so I added more low area and a sub fin. Still didn't work. I added so much I got into spiral instability where the plane would tighten up the bank and turn. It's not deadly if you are a good pilot, but a beginner might get into a "death spiral" where you pull up elevator and it just tightens up the spiral. You must roll out first, then pull. Anyway, in my opinion, changing the location of the rudder area doesn't work worth crap. The rolling force from the wing location is just too great. I finally sawed the wing on my plane apart and changed the dihedral.

For the same reasons, I use anhedral in my high wing planes.

You can tune the wing by using downward or upward end plates. These act as effective anhedral or dihedral.

rabraaten

Well, after far more information than I expected here's where my brain is now. After looking at the wing and plane I think I am going to leave the dihedral in as it is. When I straighten the rudder hinge I am going to lose some of the static area of my vert stab. I will brace the stab so it will not move. The straightening process will, with slight addition, give me the desired increase in rudder authority I think I want today.

I think I am going to go with a low wing configuration as I have to repair my aileron torque rods and the surrounding wing anyways. I'll just switch the rods left for right so they stick up instead of down, Then mount the servo in the middle of the top of the wing. I don't suppose that any of the mods I've been thinking about will do anything at all to improve the knife edge characteristics of the plane but hopefully I'll at least get a decent four point roll out of it.

Seems that with all of the aerodynamics that are involved with the original design and with the 'updates' I'll be 'engineering' into it, it looks like the maiden is just gonna be plane fun. I guess it all boils down to how adventurous I actually feel when it comes time to build and how much my brain is actually working with a hobby knife and glue in my hands. After all.... its not rocket science.

da Rock

Keeping the dihedral while moving the wing from highwing to low wing is probably a good idea. Having dihedral on a low wing biases the design toward neutral roll stability. Having the same dihedral on a high wing simply increases the dihedral effect and increases the roll stability. You're going from high roll stability toward neutral. Gotta see some change. And for the better if you're wanting less trainer like performance.

Turning the airplane into a low wing is probably going to significantly alter it's knife edge handling from what you were seeing before.

Changing stuff makes the airplane more personal. No matter what, you're going to learn lot's more from this than you'd learn just slapping together a new ARF.

rabraaten

Okay... Maybe one more quick question... I was reading some of the responses to the changing of the Kobra tail fin and got to thinking. Is it necesssary to change the rudder hinge angle at all or will dropping the wing potentially change my knife edge characteristics more? The description about the aileron effect of a rudder in knife edge made a lot of sense to me and makes me think that increasing the length of the rudder as it gets closer to the roll centre may give me some of the authority I am looking for.....thoughts?

BMatthews

Knife edge has it's whole own set of charactaristics. You're moving the wing and making a new fuselage. That's going to totally change how it knife edges. So will altering the tail and almost everything else about this conversion. To worry much about the hinge line angle is a little minor. You'll need to look at the overall new look of the design and do a "best guess" of how you want your hinge line to be.

But generally for knife end flying anything but a vertical hinge line is not a good idea. Beyond that you're worrying about small things that only a contest pattern model designer should be obsessing over.