Dihedral question
 

I'm in the planning stage for an SPA plane for next season, looking at a 1969-vintage design which (naturally) calls for a good deal of dihedral. I'd like some thoughts on the performance and handling implications of modifying the amount of dihedral, as shown in the attached diagram.
Thanks!
[8D]
PJ

RE: Dihedral question
 

I am interested also. I am looking at the planes for my db.(in the box)an wondering the same thing.
I keep asking why any is needed. Seems to me its counter productive when the airplane should be zero zero.
Falcon
Also building for SPA.

RE: Dihedral question
 

I learned something from Steve Byrum a few weeks ago about dyhedral. When building a Curare with an Anhedral stab, the dyhedral is necessary if not the airplane tends to have coupling issues. He said after building three or four of them following the instructions did help.:D

DP#3

RE: Dihedral question
 

With the center of pressure on the rudder above the aircraft CG there is an adverse roll induced by rudder deflection. That is, right rudder puts a force on the tail which pushes to the left. But, it pushes above the centerline of the fuselage so that there is an induced roll to the left. Right rudder > left roll and vice versa. Dihedral, on the other hand, induces a proverse roll. In the case of a yaw to the right, the dihedral induces a right roll. So, the idea is to balance the adverse roll induced by the rudder with the proverse roll caused by the dihedral. If the two are matched correctly, you get a pure yaw with no roll either way. That is the goal. There are clear interactions between the amount of dihedral and the design of the rudder. Also, as mentioned in another post, there can be an interaction with the anhedral in the stab (if any).

Jeff

RE: Dihedral question
 

Thanks, Jeff - I understand that phenomenon from sport and trainer models.
I was curious about the tradeoff with inverted maneuvers in a pattern aircraft (as if my flying skills would make that noticeable).
However, in reviewing a number of pattern designs thru around 1980, I find that most incorporate some dihedral.
Another factoid I discovered had to do with tapered wings and their tendency toward tip stall. Since a pattern plane doesn't want to use washout to counter tip stall, the early designers did two things: made the wingtip leading edge slightly more blunt, and/or used a slightly thicker airfoil towards the tip to add lift. For example, a NACA 0015 enlarged to a NACA 0017 at the tip. The old magazine articles from the late '60's through early '80's are quite informative.
:)
PJ

RE: Dihedral question
 

Greetings all,

Apropos of the above, I've always wondered why in all designs I've seen, tailplanes use thinner sections than wings. Does anyone have an explanation for this?

My guess is that, since the airflow is already deflected by the wing, the effective angle of attack on the tailplane remains within a narrower range than that on the wing. So a thicker section would only add drag, and offer nothing in return. But this is only my own guess. I have never read about this or seen it discussed anywhere. I'd be glad to hear from anyone who knows for sure.

Best regards,

George

RE: Dihedral question
 

Greetings all,

Apropos of the above, I've always wondered why in all designs I've seen, tailplanes use thinner sections than wings. Does anyone have an explanation for this?

My guess is that, since the airflow is already deflected by the wing, the effective angle of attack on the tailplane remains within a narrower range than that on the wing. So a thicker section would only add drag, and offer nothing in return. But this is only my own guess. I have never read about this or seen it discussed anywhere. I'd be glad to hear from anyone who knows for sure.

Best regards,

George

RE: Dihedral question
 

You are right George and this is known by aiplane designers of course.
See the picture I did make a few month's ago.
Even modelers not always want believe this, but the design of the pattern plane is depending of this phenomenon.

The "cotton thread downwash" is showing downwas still also below and after elevator bending upwards as result of up deflection of the elevator.
Glidepath is horizontal and the Taurus is flying at highest possible angle of attack.

Cees

RE: Dihedral question
 

PJ,

See the pictures I did make a few years ago. One is photoshopped of course.
Reducing dihedral doesn't have any profit IMO only risks will be more in windy circumstances.
My planes always do have the normal dihedral.

Cees

RE: Dihedral question
 

George,

not being an aerodynamicist, I'm going to venture some ideas.

At least in single engine nose mounted model airplanes, I think the concept is that the propwash airstream flowing over the stab is more turbulent than that flowing over the wing due to the increased distance of the tailplane compared to the wing. A thinner airfoil aids in keeping air speed up thereby retaining stab and elevator effectiveness.

Changing airspeed flowing over a wing causes the center of lift (CL) to move back and forth with respect to the CG of the airplane. I believe in general, as airspeed (i.e., increased power) is increased, the CL moves aft and vice versa. This CL movement over the wing causes the model to pitch up and down and is what necessitates the stabilizer which is aptly named and counteracts the pitching movement

I always found it interesting how Bridi went from rather thin airfoils on the Dirty Birdy (particularly on the stab) to much thicker airfoils on the UFO. I can only venture that these changes were made in an effort to obtain a model which had a lower critical stall speed and was able to be flown at a slower speed. Although I haven't flown either of them, I wouldn't be surprised if the DB needs to be flown faster than the UFO in order for the two airplanes to exhibit similar behavior.

My thinking (and I think some others too) was that Joe increased the stab airfoil thickness to cause it to stall after the wing in slow speed pattern maneuvers. Presumably the model didn't need that high an airspeed over the stab in order for it to remain effective.

It seems that in full scale aircraft, stabilizer theory with respect to stalling conditions and recovery means is not quite fully understood. I find this amazing considering the airplanes that we have built and fly and the fact that it has been over 100 years since we first took to the airs. Stabilators which are often thought of as a more modern device compared to fixed stab with movable elevator (we see stabilators in modern jet aircraft) are actually older. The wright brothers initially used a canard stabilator in their experiments before finally changing that to the more conventional fixed stab/movable elevator.

I guess not much of this relates directly to dihedral though. :eek:

David.

RE: Dihedral question
 

Nicely put Jeff.

There has been some discussion on the subject at least in two or three other threads, one of them somewhat current. The theme is generally on the interaction and requirement of increased wing dihedral in models which have pronounced stab anhedral. Models with significant anhedral such as the Curare and the Tiporare have been discovered to require large amounts of wing dihedral for a more neutral pattern plane. On the other hand, models such as the Aurora (and several other Japanese designs) which have zero stab anhedral (or dihedral for that matter) and a more concentric wing position (i.e., placed closer to or on the thrust line) tend to have very little wing dihedral with a frontal design resembling that of the third diagram in PJ's post. Given the more symmetric frontal and lateral planform of the model, the airplane is presumably more neutral in either upward or inverted flight.

It has been mentioned in construction articles of models designed around the same time as the Curare (e.g., the Deception) that stab anhedral is to be avoided at all costs in a model designed around a flat top wing (i.e., the second drawing in PJ's sketch). No doubt a Deception with the typical flat top wing (as many American classics had at the time) would fly rather erratically with a stab built with considerable anhedral.

In short, how much dihedral is introduced in the construction of an SPA wing is dictated by the overall planform and moments of the model. Reducing dihedral might result in a more neutral model with similar flight characteristics whether upright or inverted but the model might also be inherently more unstable. Building a Curare wing with a flat top while retaining the pronounced stab anhedral turns out to not be such a good idea as tested and discovered by Steve.

PJ, what model do you have in mind for SPA?

David.

RE: Dihedral question
 

No, not much to do with dihedral, David, but another factor to be considered. Andy Lennon's R/C Model Aircraft Design speaks to horizontal tail airfoils in this way.
"Since the horizontal tail surface has to provide lift - both up and down - symmetrical airfoils such as Eppler E168 are recommended."
That is a roughly 12.5% airfoil - confirming George's hunch that a thinner airfoil is suggested for the horizontal stab. Lennon does not advocate flat plate (aka slab) airfoils in that they are less effective than symmetrical airfoils.
Jim Kirkland's Intruder made use of a "diamond" airfoil section. This is what he had to say:
"For a given stab section, a rounded leading edge decreases elevator sensitivity around neutral, while a sharper leading edge increases sensitivity around neutral. Stabilizer thickness governs the elevator's sensitivity away from the neutral zone, contributes to tail drag coefficient, and is a factor in determining the amount of elevator deflection requirements. The curvature of the airfoil did not seem to matter very much!
"So why bother to jig-up and build a symmetrical airfoil stab, when the airfoil curvature played so little a part in the overall performance? ... The result ... became the diamond stab section used first on the Triton, and further refined on the Intruder.
"Ed Kazmirski used a diamond stab section on the Taurus, and I suspect for the same reason that I finally selected such a section for use on the Triton."
At the risk of thread-creep, I'll scan and post how a diamond-section stab is built...
[8D]
PJ

RE: Dihedral question
 

Another thing to consider is that your COG is above the wing in a low wing airplane and the the garavitaional pull on that COG is always straight down (and during a loop the centrifugal force acts perpendicular to the flightpath).

What's this mean?

It means in a low wing airplane at high angles of attack the force acting through the COG moves further back along the wing. Adding dihedral means the wingtips move backwards at high angles of attack to help with this.

This is just another reason to consider along with the many other ones listed above. Dihedral isn't there for any one reason, but roll coupling is usually the one we use dihedral to fine tune.

cheers

RE: Dihedral question
 

Thanks, guys. I'm pretty well sold on holding the model close to the original design - IM's Corsair MkII, circa 1969, kitted briefly by World Engines in 1970. Things like motor mounts will be tweaked, but I'll try and stay true to the planform.

I mentioned Ed Kazmirski's and Jim Kirkland's use of the "diamond" stab section - this is a low-res scan of what that looks like:

:)
PJ

[btw - I loved your empirical approach to downwash, Cees!]

RE: Dihedral question
 

Thge 'Diamond' stabiliser section is much older than Kirkland or Kazmirski, and goes back beyond the 'Smog Hog'. It was, originally, a F/F way of making a symmetric, stiff tailplane simply and lightly. It was only after R/C was fitted to the elevator nailed on the T/E that modellers started to ascribe several other attributes to the section, though I cannot tell any difference between the usual sections used on various models. Not in flight, any way.
Evan, WB #12.

RE: Dihedral question
 

I think that was Kirkland's point in the '71 construction article, Evan:

Not so much about enhanced performance, but simplified construction (although I've seen some apocryphal attributions to performance here n' there).

:D
PJ

RE: Dihedral question
 

Quite so, PJ, I was trying to make the point that the idea wasn't Kirklands, or Kazmirski's. The Astro Hog uses it, and Smog Hog, and who knows what before that? I have Matt's 'Superstar 2' and it uses the same section too, so the influence wasn't just American, either. But whoever did invent it must have had those amateur model aerodynamisits well to the fore in mind...:D
Evan, WB #12.

RE: Dihedral question
 

We have played with increasing the percentage thickness of the tail group so as to ensure that the wing would stall before the tail in an effort to increase the predictability of spins and snaps.

I can say this works, and works well, but how much of a difference it makes from a 'typical' thickness tailgroup I couldn't say.

RE: Dihedral question
 

Rendegade,

Have attention for this picture, the angles are better visible.

Even the Taurus does fly with an angle of attack of 10 degrees (light/dark blue) at (controlled!) low airspeed the AOA of the stabilizer in the down wash is less than 2 degrees (orange/green). We also see the flight path indicator and later the development of the vortex of the wing tip.

There are a lot of posts about this, read this for example.

http://www.rcuniverse.com/forum/fb.asp?m=8319781


I wouldn't worry about any 'downwash' off the main wing either. I have somewhere a publication where the chief aerodynamicist for BAC (who have the time, money, requirement, tools etc to find out) states that the local change in the freestream direction (downwash) due to the lift produced by the wing can extend as far back as 1.25 chords behind the trailing edge of the wing. If a 100 ton airplane, operating at speeds, AOA's and lift coefficients unapproachable by any model airplane, and influencing cubic acres of air can get the downwash to extend as far as 1.25 chords behind the wing, I would go so far as to say that no normal (wing at front, tail at back) model airplane will have any downwash effect that could possibly influence any tailplane. He also stated that this wash actually reduces the lift generated by the wing (like the wing was 'climbing a hill' all the time). I think you can forget any 'downwash' calculations with wing/tail setting angles.
Evan, WB #12.


I do show that 1,25 short distance in the photograph.

When these kind of theories are right, than we have to believe the stabilizer would have to function under an AOA of 10 degrees? Of course not and we see for the Taurus the AOA of the stab is less than 1/5 th of that of the wing, so even a flat plate often can do the job.

For me these are facts of dowh wash I count with, not only for my gliders but also in the design of the Simla I am working on these days.

Cees

RE: Dihedral question
 

PJ,

Dihedral and pattern ships.

The Taurus is characteristic for any low winger and a source of information how they did fly their pattern planes in the past. Scratch build, no plans as useable and a lot of fine tuning while flying the plane. Months of work to count with all variables and interactions between them.

One of the variables is, wing position, wing setting in the fuselage and dihedral.
My example picture 1 I did change the horizontal position and the angle of the wing saddle as normally. I also did change the dihedral, see picture 2. You can see the coupled halves.

This was the only way to adjust a pattern plane because the Taurus is of the period of the Slow Roll. The Cream Puff of Vic Husak did have the adjustable wing center section for example so a development of 1964 / 1965 I think.

Cees

RE: Dihedral question
 

Hello PJ
You never said which airplane. Some of us will remember whether a certain old design was just right as-was!
Regards,
Dean Pappas

RE: Dihedral question
 

This is what I posted earlier, Dean:

Not modeled very often, but it's an attractive plane. Kind of on the cusp between "old school" pattern designs and the "competitive" designs people are flying in SPA today.
;)
PJ