Trainer selection: dihedral angle

Cz10 wrote:Gee, perfesser, why is it helpful in the absence of ailerons?

Ailerons aid the pilot in flying "straight and level" as well as flying off to the side, correct?

If they are lacking, doing so requires cooridinted elevator and rudder inputs to perform these functions such as in two channel configuration.

(If you've ever flown two channel you know this for a fact)

Well, they way I understand it is without dihedral, rudder only input will just cause the plane to skid sideways and not turn, at least very well. With dihedral, as the yaw (turn) axis changes relative to the direction of flight, the pressure under the leading wing causes the plane to bank and turn more effectively without the benifit of ailerons. Elevator is to counteract the tendancy for the pitch (nose up/down) to change (nose own) and keep the airspeed under control.

cz:

The whole objective being a smooth path of flight. Jerky and spazmatic flight looks (and feels) crude to both passengers and observers.

I learned to fly RC with three channels and later switched to four when throttles became available first as sliders placed in engine exhausts as in CL "carrier landing" competitions and later RC applications.

Seems the importance of dihedral angle been evident for quite some time. The Wright Brothers discovered this in 1903 at Kill Devil Hill in NC when they manually "warped" their wings by pulling on ropes.

THREE channels??? Thems modern times! I started with one, with the control powered by a wound up rubber band and tubes in the radios.

Cz10 wrote: .....With dihedral, as the yaw (turn) axis changes relative to the direction of flight, the pressure under the leading wing causes the plane to bank and turn more effectively without the benifit of ailerons. Elevator is to counteract the tendancy for the pitch (nose up/down) to change (nose own) and keep the airspeed under control.

I agree.

One way to visually demonstrate this is to fold a 3x5 card along a centerline drawn from top to bottom giving it the appearance of a wing. Grasp the card with thumb and forefinger at each end of the fold line and hold it level in front of you so the angle of attack is zero, i.e., you're looking only at the edge of the card -- it looks like a plane flying directly toward you. Without changing the angle of attack, rotate your hand clockwise, changing the yaw axis only. Keep in mind that the plane is still flying directly toward you, but it is not pointed directly at you, i.e., it's in a skid. Notice the angle of attack on the leading wing (or right side as you're looking at it). The AOA of the leading wing has increased and the AOA of the trailing wing has decreased. At this point, the plane will begin to bank toward your left hand due to the increased lift generated by the greater angle of attack on the right side and the rest is history.

If you do the same exercise with a flat card (no dihedral), you will see that there is no differential in the AOA of each wing during yaw and no dihedral effect causing the plane to roll.

Next homework is why wings with dihedral are self stablizing and why freeflight competitors use planes with polyhedral, a flat center section and tip dihedral sections.

andrew

[quote="andrew"]

Cz10 wrote:

Next homework is why wings with dihedral are self stablizing and why freeflight competitors use planes with polyhedral, a flat center section and tip dihedral sections.

andrew

andrew:

Wings with dihedral are self-stablizing because the weight of the fuselage causes it to center.

Poly(many)Dihedral wings are used on free-flights because the flat center sections reduces undersired drag ergo greater lift allowing greater duration of flight..

SuperDave wrote:
Wings with dihedral are self-stablizing because the weight of the fuselage causes it to center.

Poly(many)Dihedral wings are used on free-flights because the flat center sections reduces undersired drag ergo greater lift allowing greater duration of flight..

The pendulum effect does come into play, particularly with high wing and parasol designs. It is likely less of a factor with midwing and low wing planes. I think it can be better explained using actual and effective lift vectors (the wing orientation is shown in blue).



The actual lift generated by a wing panel will be measured at 90 degrees to the panel as it moves through the air. The direction of lift is shown by the red vectors and the amount of lift is proportional to the vector length. The effective lift is measured perpendicular to the ground, as shown by the green vectors. As long as the wing is not parallel to the ground the effective lift will always be less than the actual lift generated by the panel. The lifting force (green) is a composite vector and can be calculated with a little trigonometry. As the plane banks, the effective lift of the lower wing approaches the actual lift and the effective lift of the high wing decreases, as shown by the differing lengths of the green vectors. Higher lift on the downwing side raises that wing until the effective lift for each wing equalizes. There may be some oscillation until the plane reaches a steady state.

With a flat wing, effective lift on both sides remains the same regardless of the orientation to the ground.

SuperDave look at this for dihedral angle.
http://www.centennialofflight.gov/essay/Theories_of_Flight/Stability_II/TH27G8.htm

I'm not sure that I'm totally correct on this, but my take on polyhedral is that it allows a greater degree of stability AND, (in the case of long-winged R/C planes) aileron-like banking ability in a model without making the builder sacrifice a bunch of lift with radical dihedral in the wing. Figure that, located out farther out on the wing panel, it commands a little more "aerodynamic leverage".

I "accidentally" performed a practical experiment in this area in the 1980's ( note the Cool Shirt !) when I decided to go with straight dihedral in a Midwest Lil' T sailplane I was building. I wanted it to "look more like a real sailplane", so I built the outter panels flat, with the recommended dihedral angle at the center joint.

It worked...at least looks-wise. The Lil' T had the lines of a real sailplane (in MY eyes)...with one catch...IT STEERED LIKE A DUMP TRUCK ON ICE !



The tiny rudder was designed to accommodate full-deflection, non-proportional tone and pulse radios of the time (an early 1960's design). It's diminutive surface area would have been aided by the polyhedral's assistance in banking those long wings. It still flew, and I thought it was beautiful in the air.

However, I had to give it plenty of lead in the turns and NEVER let myself get caught slow and low to the ground in a bank, due to it's anemic roll response. The Lil' T was damaged several times as it slid into the ground with a full rudder trying to level it against it's turn to landing...a smaller version of that tragic full-scale B-52 crash on Youtube.

Lil' T met it's end from a "New" pack of drycell transmitter batteries that were dead in the package when I bought them!

I've got another Lil' T project on the back-burner...and it WILL have "double-bent" wings ! (AND good batteries!!!!!!!!)

And speaking of vectoring, Boeing 757's and some other commerical aircraft feature up turned wing tips. As, in flight, air flows off wing tips and forms a circular vector which creates drag reducing fuel efficiency.

Solution, upturned wing tips that slice through the vectors breaking them up. Result: greater fuel efficiency/lower emissions.

(BTW, that does NOT mean your next airfare will go down. All things considered they're not that bad now)

(Another + for Andrew)

Kim wrote:I'm not sure that I'm totally correct on this, but my take on polyhedral is that it allows a greater degree of stability AND, (in the case of long-winged R/C planes) aileron-like banking ability in a model without making the builder sacrifice a bunch of lift with radical dihedral in the wing. Figure that, located out farther out on the wing panel, it commands a little more "aerodynamic leverage".

That is exactly my take on the matter. The flat center panel allows most of the actual lift to be used as vertical lift and the tip sections may be smaller due to the "leverage factor".

Landings must have been tough on that powerpod. Nice shirt, BTW.

When I looked at your "location" change, my initial reaction was, "Whaaat?". But then it made sense.

SuperDave wrote:And speaking of vectoring, Boeing 757's and some other commerical aircraft feature up turned wing tips. As, in flight, air flows off wing tips and forms a circular vector which creates drag reducing fuel efficiency.

I had the chance to see Boeing's new Dreamliner at Oshkosh this summer -- it is indeed a beautiful airplane.

When it made a couple of showcase passes before landing, one of the most apparent features was the amount of dihedral. But, the wings were not at all straight -- the had a considerable amount of bow all the way out to the tips (with winglets, BTW). When it landed, the tips came down, and down, and down until the wing was flat. I didn't know how much it flexed, but folks all around us were commenting on the same thing.

On takeoff, the wings flexed up to flight position, then it took off. For those of you that remember watching the takeoff of the Rutan Voyager with it's wing flex, the Dreamliner about the same.

When I got back I looked it up. Standard flight flex is 10 feet; testing has been done to 26 feet without failure.

"Landings must have been tough on that powerpod. Nice shirt, BTW.

When I looked at your "location" change, my initial reaction was, "Whaaat?". But then it made sense."

Hey Andrew !

The power pod was my home-made response to anyone who might assert that sailplanes are boring! When the fuel was exhausted, I'd pull the elevator trim back to full up, putting tension on a cable and pin that ran to a dowel in the front of the pod. A quick blip of "Up", and the pin was pulled, releasing the pod, and hopefully deploying the homemade dacron parachute within.



The release worked 100% of the time...the chute, however had about an 80% success rate! When, at altitude, the parachute decided to stay tucked in it's little falling hangar, things could get a little exciting for me and my friends as we tried to track the plummeting engine !

Never had any injuries, but there WERE extended searches for the engine and it's pod! Another advantage of the engine's location was that the sailplane stayed "High-Start Clean"...right up to it's last flight !

Yeah, I don't normally get political without provocation, but the note about my location was just some venting of frustration with the continuing, hopeless corruption of my beloved home state of Illinois.

The shirt WAS a babe magnet...I needed it to offset the effect of my Nerd Hobbies !

but how did you flap the wings in?
i can't see any in the picture.

GermanBeez wrote:but how did you flap the wings in?
i can't see any in the picture.

Not sure I understand your question, Beez.

Kim wrote:

GermanBeez wrote:but how did you flap the wings in?
i can't see any in the picture.

Not sure I understand your question, Beez.

I think he think that the whole fuselauge was attached to the parachute and the wings "flapped" down.
But Beez that was only the engine pod from the bottom.
The pod falls off and the glider glides down after eventually.

aaaahh, now i get it. the glider is taken up by the pod, and dropped, while the pod glides
down with its parachute! that's VERY cool!

GermanBeez wrote:aaaahh, now i get it. the glider is taken up by the pod, and dropped, while the pod glides
down with its parachute! that's VERY cool!

I think that's it Dave.

AS a side note, IMO this discussion of dihedral angle is one of the finest that I've read here. It typifies the essense of a fine inter-active community dedicated to a common interest.

Although I originated this thread I claim no credit for what has followed; that credit may be rightly claimed buy the contributers of which there have been many.

KAMPAI!!!

I think thi is what happens when a subject does not veer off course.
You can claim credit because when a country is formed the person who decided to form
It gets most of the credit.

The essense of good leadship lies, not in single person, but the greater community that suppots that leader; George Washington here in the US is a fine example.

Here on the forum I believe it is the duty of those senior and experienced to inspire others with thoughtful ideas and topics; our passion is the better for it.

Kim --

That powerpod ejection and chute is very cool. Was it difficult to start in that position?

andrew

I have a stand made from plastic pipe that suspended belly of the plane pretty much out in the open. It IS a different set-up though, and deserves extra caution and testing to make sure you don't launch a screaming .049 at your crotch!

Initially, I DID have a couple of accidental releases under power, but the plane was airborne, and the engine was happy just to tear into it's chute like a terrier on a rat !