highfly3D

Hi all,

I was debating with my friend that an aerobatic plane with a symmetrical aerofoil 0 degree wing & tail incidence the airplane will fly.. but his point is you need an up elevator to keep the plane in the air.... please give your views on this.

Thanks,
High

Lnewqban

All the wing with symmetrical airfoil needs to fly is some speed and some angle of attack (respect to the incoming air).

To keep the wing stable and at that AOA, the stab is needed.

Symmetrical airfoils don't produce lift at zero degrees of AOA.

BMatthews

With a 0-0 setup it will fly but with some amount of up trim needed.  As the CG is shifted back more towards the model's nuetral point the amount of up trim will become less and less.   When the CG is at the neutral point then the amount of trim will be zero and the model will be neutrally stable.  At that point it will be truly a 0-0 flying model.  BUT.. it would be flying with a positive angle of attack as noted by Lnewqban.

Note that neutrally stable does not mean UNstable.  The model will fly just fine.  But it will need constant attention from the pilot to correct for any disturbances caused by turbulence.

Now when you add some up trim it could correctly be said that the model is no longer truly 0-0.  And that would be right.   So in a way you're both right.  It just depends on the CG location.  And to some extent where on the model the wing is located.  A high wing model will produce a high drag center and that acts like positive incidence as well when it comes to the model self stabilizing.  Similarly a low wing with big "grass" wheels can have so much low centered drag that it needs to have some positive incidence in the wing to achieve an effectively 0-0 setup.

highfly3D

Remember the ultra hots by Dan Santich this plane i think this plane had the 0-0 incidence.

BMatthews

Yes, and on top of that one the normal setup for all of the hot 3D fun fly flat foamies is an honest 0-0-0 setup as well.  But again, if the CG is located forward from the neutral point by very much some up trim will have to be used.   It may not be much but it would be there to allow the model to fly hands off in level flight at a moderate cruise speed.  And up trim is merely an alternative to built in postive and negative incidence angles.  Or as the terms are used these days in modeling some decalage angle or longitudinal dihedral.  This is why 0-0-0 setups built into the model are only seen on models where the owner is expected to trim the CG location back enough that it is very close to the neutral point and the amount of angular difference between wing and stabilizier needed will be so small that. it is most easily provided by a click or two of up trim or a turn or two of the elevator clevis.

ARUP

I set up my CL planes 0-0-0 and push the c.g. back until twitchy then bump fwd a 'smidge'. Not technical speak but works. Varying the wing AOA is what causes ascent and descent. I don't fly 3-d or aerobatic RC but the principal is similar.

Lone Star Charles

I use "twitchy", "bump", and "smidge" all the time in my technical speak.  Of course, I also use the TLAR method of design.  Let's go flying!

BMatthews

Arup, on a control line model you'll automatically and unconciously select the amount of up elevator needed to fly level in any event  So your model will not truly be flying at an actual 0-0-0.  It'll be 0-0-x due to the slight up trim needed to hold level flight.  Only during the vertical parts of squares and wingovers will it truly be 0-0-0 when the model is pointed straight up or down.

thailazer

BMatt & Lnewqban..... Good posts! Here's an additional question: My Tiger 2 with fully sym wing flys fast just great with little trim. When I start doing landings (i.e. flying slower) I normally dial in 4 clicks of up trim. Is this because the horizontal stab is less effective at lower air speeds, or simply that I have to increase the angle of attack to fly slower?

pimmnz

Yep, lift is proportional to angle and airspeed, you can go slower, and higher angle, until you reach the stall angle, then you go faster, but down, unless the bottom edge of the air is a bit close.
Evan, WB #12.

rmh

forthose who actually fly models : I have a UMX300 (a tiny one ounce 6channel radio electric model)
This model has flat foam wing with huge ailerons. I programmed the tx (DX8) to provide
A- flaps for landing /take off (flaps are actually the ailerons)
B flaps coupled with elevators (upflap with down el /down flap with up el.)
Now I can watch the SPEED differences required to stay aloft when the wing is either FLAT/ Symmetrical or cambered.
There is a huge difference in speed and lift.-with only a few degrees of camber
However changing power, creates instant climb or descent. These silly little models show instantly the differences in wing configuration - power requirements etc. and all in a 30x50 ft room!

ARUP

Thanks for clarifying the point I was trying to make. Changing the elevator causes a change of stab-elevator combo incidence as a system which then changes AOA of wing.

Lnewqban

Yes, the tail is forcing the wing to adopt a higher AOA, so the coefficient of lift increases for reduced airflow and lift force remains about the same to sustain horizontal flight.

When you dial in 4 clicks of up trim, you are modifying the camber and the AOA of the stab-elevator surface.
By doing that, you are increasing the down lift of the tail, even for the reduced speed.
The down lift pitches the nose of the plane up enough to increase the AOA of the wing as much as the reduced speed demands.

Remember that lift is proportional to AOA and to the square of the air velocity:

http://en.wikipedia.org/wiki/Lift_coefficient

rmh

lift is porportional to the square of the air velocity? Could you clear that up?

flyncajun

http://hebertcompetitiondesigns.com/triangulation.aspx

Lnewqban

Grammatically incorrect?

rmh

A little

MTK

On your CL model, if your prop is turning to the outside of the circle and your thrust angle is truly 0, you would need rudder offset to keep the lines from going slack. Your wing, stab and engine/motor might be zero, but the model still needs to fly in constant slip

thailazer

CL models always want to go outbound due to centripetal forces. I never had any rudder offset on my models years ago and doubt you would ever need any.

But back to the original thread. Good stuff here. Anyone want to post a good curve of parasitic (form) drag and induced drag versus AOA? Always good to know how one gets "behind the power curve".

rmh

There are a jillion web sites which address that stuff ad infinitum
How about practical demonstrations of the question - as it affects models?
When we flew Stunt - in the 1950/60's we always included a little rudder -in case something went wrong (power loss) and the lines lost tension
The "bucket on a string" physics usually did the job .

Ben Lanterman

Well - I have always thought it is better to try to raise the level of understanding when it comes to models and aerodynamics than to lower the level. The forum is called "Aerodynamics" after all. I have a badly bent heart (7 bypass total, valves, etc. ) and when I have a question about heart health I don't want an answer like - well it just beats you know!

It is OK to give a good answer and then if necessary move the level of science up or down until understanding is arrived at.

"lift is porportional to the square of the air velocity"

The example given in the figures several answers above is terrific in it's simplicity. It is only a little bit of math. It won't hurt. The lift equation is

L = .5 * Cl * r * V^2 * A

L = total wing lift
Cl = wing lift coefficient
r = air density
V = air velocity
A = wing area

So write it as

L is proportional to V^2 or....

the lift of a wing is proportional to the square of the air velocity (relative to the wing).

When you go twice as fast you get four times the lift out of the wing.

Ben

Lnewqban

rmh

OK - double the speed - the lift goes up four times. Simple and concise.
Sorry if my approach to aeronautics is based in application for the novice modeller.
I personally thought the info here was to be constructive/ easily understandable to a model builder flyer .
I personally see no advantage in presenting math to layman. most flyer I know don't speak math.
With one notable exception who is world renown in physics. (Prof Robert Beck Clark) and he NEVER explains flight using tech terms -especially to neophytes
Having past experience in presenting technical stuff to juries - -I got broken of that approach long ago.
For the modeller - the fact that a little speed increase OR decrease -creates a BIG change in lift -is worth knowing
Realistically we fly in a fairly small, low speed range - but the rule ,of course is still valid.
So basically control the speed -other than wingloading it is the largest factor in providing lift.
Unless the AOA is a larger part of the physics -which are involved in flight.

Ben Lanterman

I have never assumed the novice builder is without math or physics skills - only about aerodynamics in particular. I belong to a club that is filled with engineers and tech guys who have no trouble with the math or physics. I always ssume that is the case until the questioner says they can't understand what you are saying. I would think guys at a keyboard have elementary math skills.

This isn't presenting things to a jury (I would hate to be on a jury and felt that I was being talked down to) - it's simply talking aerodynamics. It's a lot more fun when you really understand the details about what is going on. Questions like this one using symmetrical airfoils, factors in real airfoil design (not just drawing curves), etc., are fun and just not that hard to understand. The thing is that aerodynamics isn't magic or wild guessing. It is based in science which most of the guys that post here have had some of.

Why not present it that way? We have disagreed on this before - I can't stand your approach. I just realized why I stopped answering questions months ago.

Ben

rmh

You don't have to like my approach-however -based on conducting seminars to model groups and running /conducting mechanical training schools for machinery technicians and preparing and giving testimony to juries - A tech approach tends to "loose the audience". We never talked down to these groups or a jury - that is a sure fire recipe for alienation. We talked to them in conversational English. And notwithstanding your opinion, all the information required can be passed on in this manner.

You are fortunate in having a tech savvy club. My observations are that this is not a typical scenario amongt most modelers.
They are of good intelligence but by and large they are not into tech speak.
Tech speak is great for those who prefer it and it is a excellent method used by professional groups to rapidly exchange precise data -