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AOA - what should it be on a symmetric wing? - 4/1/2008 11:18:05 PM   
Robert_Ellis


 

Posts: 23
Joined: 12/27/2006
From: , CA, USA
Status: offline 		I have a 90 size edge that I did the maiden on today and it was a handful - extremely pitch sensitive. I had set the CD in the middle of the manufacturers range. I had programmed low rates on it with expo but it was very sensitive. So I am guessing that either they are wrong on the CG or I am wondering if the angle of attack could be off. The AOA is easily adjustable on the plane. I know very little about aerodynamics so I am hoping someone here can give me some guidance.

At the very least, given that the AOL is adjustable, what should it be?

Thanks in advance.

- Rob
       Post #: 1

RE: AOA - what should it be on a symmetric wing? - 4/1/2008 11:51:45 PM   
crasherboy


 

Posts: 112
Joined: 10/21/2006
From: Bryant Pond, ME, USA
Status: offline 		Please understand that I am by no means an expert, but I will offer my idea on this. I would say not more than 1 or 2 degrees. This being an aerobatic model if you has to much AOA in normal flight,what about inverted flight? There are others here that must likely will have a better idea on this model,but I would think that you have a balance problem or engine thrust problem . Are you sure to that you do not have to much travel on your control surface? Are the linkage on your elevator tight? Just a few thoughts. I would be interested in the outcome of this.

_____________________________

Do unto others as you would be done unto,well most of the time anyway!

(in reply to Robert_Ellis)
       Post #: 2

RE: AOA - what should it be on a symmetric wing? - 4/2/2008 12:10:39 AM   
da Rock



Posts: 6641
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		You've got 2 things that cause most of the pitch sensitivity problems we have. CG and elevator throw.

Double check your CG. Make sure of it's location by running the airplane's measurements through something like geistware. It's simple and quick. All it takes is a yardstick. Then check that the model balances where it should be. It's happened before that the mfg's setup info was a misprint or a copy from another model they sell.

Next thing is your elevator throws. If the airplane is 3D capable, the throws might be for modelers who're experienced at it. And the high rates and full throws are quite sensitive then. And often times, the low rate throws are still what most modelers who don't fly 3D would consider FRANTIC. And checkout the throws you get on the elevator with the TX set for low rates on elevator.

We found a TX at the field last year that had dual rate switches for aileron and elevator. Turned out something had happened to this TX and the elevator dual rate switch wasn't working anymore, but the aileron D/R was working both surfaces. Strange but it showed up when we actually started flipping switches and measuring throws. The guy was flying with elevator full rate when he thought he'd flipped to low rates, and he was sure the airplane was dangerously tail heavy.

A couple of years back a guy with a Futaba TX had setup EXPO and had put in +40%. Futaba's actually want -40% to give you the kind of expo most people want. The +40% turned the airplane into a hairtrigger sucker. He also thought his CG was too far back.

What is the model? Mfg? And what throws did they suggest.

< Message edited by da Rock -- 4/2/2008 12:13:43 AM >

(in reply to crasherboy)
       Post #: 3

RE: AOA - what should it be on a symmetric wing? - 4/2/2008 12:35:17 AM   
gboulton



Posts: 1862
Joined: 5/28/2005
From: La Vergne, TN, USA
Status: offline 		Robert,

Hopefully you'll take this not as a "slam", but as enlightenment.

I believe you're using the term "Angle of Attack" incorrectly. I THINK you MIGHT mean Angle of Incidence, but I'm not sure...so, I'll put the question to you after a brief explanation.

Angle of Attack is the angle the wing makes with the "relative wind"...the oncoming "wind" made by the airplane as it moves through the air. Think abouot a plane that is moving straight ahead, in level flight, neither gaining nor losing altitude. If it's doing so with its nose up high, that's a "high an angle of attack". If it's doing so with its nose down lower, it's a "low angle of attack".

Angle of Attack is ENTIRELY dependent upon (and is in fact DEFINED by) the angle the wing makes with ONCOMING AIR. In short, for there to BE an angle of attack, the plane must be moving. So...it's not likely to be something you can adjust on the ground....since it doesn't exist. *heh* Now, in FLIGHT it is, certainly, VERY easy to adjust...just move the elevator up or down.

Angle of incidence (or wing incidence) on the other hand is the angle of a wing's chord (well, technically its lift line, but close enough for a symmetrical wing) makes with the longitudinal axis of the plane. In other words, if the plane's fuselage was sitting perfectly level, what angle is the chord of the wing at in relation. This, obviously, is precisely the sort of thing one can adjust on the ground.

Now...while it's accurate to say it'll make NO difference, it's not LIKELY that the wing incidence is causing your issues, unless it's WAY WAY off. For all PRACTICAL purposes, its an angle that doesn't matter a whole lot in flight. Its most notable area of impact would be during take off.

da Rock makes excellent points...a pitch sensitive aircraft is USUALLY due to either a CG that is too far rearward (depending on the model, even a 1/4" can make a notable difference", or too much elevator travel. As he pointed out, since you did not specify the model type, size, or the throws you're using, it's hard to say which of those might be the issue.

_____________________________

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(in reply to da Rock)
       Post #: 4

RE: AOA - what should it be on a symmetric wing? - 4/2/2008 12:35:58 AM   
Campgems


 

Posts: 1910
Joined: 2/18/2006
From: Arroyo Grande, CA, USA
Status: offline 		Some of those planes are really sensitive. A quarter inch of elevator movement may be more than enough for simply flying around. For full 3D what you have is probably what you want. Before changing incidence of the wing, have someone at the field who has experience with that type aircraft (and is a good pilot) take it up and check it out. If you think about it, changing the incidence isn't going to do that much for you. The biggest change would the handeling between full power and low speed from the thrust and thrust vector of the motor. The wing will be near zero incidence for level flight. The elevator will determine the AOA.

On my first trainer, the incidence of the wing was about three degrees off the thrust line and five degrees off the incidence of the horz stab. You could trim it out for level flight and it handled very well until you changed throttle setting. If you set the trim up at mid range, the punched it, it went for the stars and if you cut down to an idle, it dove for the ground. At mid range though the response to the elevator was good. Before I totaled it out, I was flying with a stack of popsicle sticks under the TE of the wing and that climbing and diving were greatly reduced, but not eliminated. There was no difference in the handeling as a result of elevator change, still predictable and smooth.

I have a little first generation quickee 500 that you need almost no movement of the controls to make radical changes in pitch. My instructor at the time flew it and just about lost it on take off. He kicked in a little right aileron and imeaditly rolled hard right and he caught it a 90 degrees but it then rolled 90 to the left. All of this was at about two feet elevation. That plane is touchy to the extreme and the controls have very little movement. The ailerons are 3/4" tapered stock and the movement is only about 1/8" on low rate and 3/16" on high.

I have a Ruperts Dad, full semitracal wing and a lot of controls athority. I have had that plane, when I got sloppy on the sticks, do a 90 degree to straight up on take off and when I caught it, back to level all in less that two feet off the deck. Anything but very smooth and gentle movements of the stick produces some wild changes. Bad for the nerves, but if you are doing 3D stuff exactly what you want. Needless to say I'm still flying that at low throws and 60% expo.

You may want to put the CG further ahead until you get better aquainted with the plane. This will dampend some of the wild moves, but you have a plane with a lot of athority in all of the control surfaces, so the real answer is going to be easy does it on the sticks. limited full throw movement and a lot of expo to start.

Don


(in reply to Robert_Ellis)
       Post #: 5

RE: AOA - what should it be on a symmetric wing? - 4/2/2008 2:57:25 AM   
Robert_Ellis


 

Posts: 23
Joined: 12/27/2006
From: , CA, USA
Status: offline 		Thanks everyone. Yes I meant angle of incidence - thanks for correcting me. I guess I was under the impression that the incidence relative to the horizontal stab would have an effect on it's handling. I never got to test inverted flight, it was so sensitive, I got it trimmed and one of the real experienced guys flew it for me and landed it. He trains new pilots and flies a 50cc CAP (set up for pattern not 3D). He said it was very sensitive to elevator - so it wasn't just me.

I have been flying a 4star 60 and over the months I have greatly increased the control throws - the elevator has about +7/8" and -7/8 throw - with less surface area than the edge.

The origin of the ARF model is unknown - I have the limited build instructions and there is absolutely no branding or company name. I bought it from someone here about a year ago. The wings have adjustable alignment pins so that you can adjust the angle of incidence, but I left it as is.

I will run that CG program and see where it comes out to - and will certainly move it forward until I get used to it. I had all the rates tied in to switch D and with -expo (Futaba). Here's what I have...

BTW, it came back trimmed out with 1/8" up elevator.

Aileron low rate:
D/R=50%
Expo=-60%
throw=+5/8" to -5/8"

Elev low rate:
D/R=50%
Expo=-60%
throw=+1" to -1"

Aileron high rate:
D/R=100%
Expo=-60%
throw=+3/4" to -3/4"

Elev high rate:
D/R=100%
Expo=-50%
throw=+1-7/8" to -1-7/8"

- Rob

(in reply to Campgems)
       Post #: 6

RE: AOA - what should it be on a symmetric wing? - 4/2/2008 3:07:17 AM   
AERORICH


 

Posts: 291
Joined: 4/21/2006
From: Scappoose, OR, USA
Status: offline 		Hi Robert: I just reviewed my Andy Lennon's book, "Basics of R/C Model Aircraft Design", and found on page14 under the Heading, "Solution No. 1, the discussion concerning a symmetrical airfoil cord position in relation to the fuselage longitudinal centerline. The wing cord should be parallel to the longitudinal centerline. This gives the wing the same AOA in the upright and inverted flight position. Page 27 shows a rib with all of the CG information you need to correctly balance your plane. After finishing your corrections, do a good research on the flight control throws for low and high rate settings, then on the low rate setting, try another test flight, and evaluate. It may be necessary to decrease the low rate settings in order to tame the flight path until advancing to the high rate position. I have found Andy's manual very helpful in solving several of my models flight problems. Get back to us what you find corrected the negative flight characteristics.

Rich S.

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RE: AOA - what should it be on a symmetric wing? - 4/2/2008 3:36:18 AM   
Robert_Ellis


 

Posts: 23
Joined: 12/27/2006
From: , CA, USA
Status: offline 		Thanks Rich. I did a check and the wing is parallel to the horizontal stab and _appears_ to be parallel to the fuse - it is hard to determine what the true centerline of the fuse is. The bottom has a constant taper, but the top is low in front of the cockpit, high at the back of the cockpit and tapers down rapidly to the tail. So how do you determine what is the true apparent centerline with that kind of profile?

Anway given that it is parallel to the horiz. stab and looks right, I will leave it like it is. I am going to calculate the CG and move accordingly and just slow down the rates until I get comfortable with it (besides, the fun high rates are only a switch away to play with and then set it low to land).

Thanks.

(in reply to AERORICH)
       Post #: 8

RE: AOA - what should it be on a symmetric wing? - 4/2/2008 6:46:04 AM   
Robert_Ellis


 

Posts: 23
Joined: 12/27/2006
From: , CA, USA
Status: offline 		Da Rock - I checked out geistware - it looked pretty cool. I went to use the CG calculator and it needs the %MAC (Mean aerodynamic cord). The calculator they have for %MAC requires the CG. I am not sure what the MAC is and even less sure what %MAC is and even less sure how to calculate them. Sad, I know. I'm a photographer. Hints? Help?

Also in their diagram of the fuse, wing and stab, they seem to use wing measurements (and horiz stab) that derive from the centerline of the fuse - a theoretical point where the leading and trailing edges of the L and R wing would intersect. Should I be doing the same or use the dimensions of the physical wing?

Dazed and confused.

- Rob

(in reply to Robert_Ellis)
       Post #: 9

RE: AOA - what should it be on a symmetric wing? - 4/2/2008 7:39:29 AM   
Campgems


 

Posts: 1910
Joined: 2/18/2006
From: Arroyo Grande, CA, USA
Status: offline 		Rob, the centerline of the fuselage is arbratery. It is usually refered to as the datum line. Say your plans, this is just an example, call for wing incidence of +1 and the horzontal stab as 0 and the engine down at -1. If you drew an arbartary line at a 20 degree slope through the fuselage front to back, the woud be at -20, the wing at -19 and the motor at -21. The relationship between the three points is the same. What is important is their relationship to each other, not to the center line of the plane.

Usually the datum line is from the center of the motor at the fire wall to the cord line of the horzional stab. Remember your geometry class. With two parallel line and an intersecting line, the intersecting line crosses both parallel lines at the same angle, so that means your "center line" could be 10" above the model or 10" below and you angle relationship wouldn't change.

On you elevator throw, I would set the low rates at 3/4" or maybe less and the high rates around the 1" mark to start. This will give you a less agressive plane. The design of the elevator has a tremendous amount of movable area to fixed, so it will have a bunch more athority than your 4* elevator. The fact that it is balanced, will allow a much faster servo movement as there will be less resistance to movement.

For a ball park CG, check some of the other Edge planes. http://www.performancercaircraft.com/images/products/edge2/edge3manual.pdf
This is the manual for a 30% Edge 540. that has an 85" wing span. It calls for the CG to be 5 to 6" behind the LE at the body. I expect that if you scale it to your wing that would give you a CG that would be very close to correct. You may also not that near the end of the manual is a printable throw gage. I would suggest printing that out and setting up your throws and expo per their recomendations.

You may want to search for some of the other Edge manuals and see what their recomendations are. Not the foamy electrics, but at least a 60 size and up.

Don

(in reply to Robert_Ellis)
       Post #: 10

RE: AOA - what should it be on a symmetric wing? - 4/2/2008 11:41:02 AM   
da Rock



Posts: 6641
Joined: 10/11/2005
From: western, NC, USA
Status: offline
quote:

ORIGINAL: Robert_Ellis

Da Rock - I checked out geistware - it looked pretty cool. I went to use the CG calculator and it needs the %MAC (Mean aerodynamic cord). The calculator they have for %MAC requires the CG. I am not sure what the MAC is and even less sure what %MAC is and even less sure how to calculate them. Sad, I know. I'm a photographer. Hints? Help?

Also in their diagram of the fuse, wing and stab, they seem to use wing measurements (and horiz stab) that derive from the centerline of the fuse - a theoretical point where the leading and trailing edges of the L and R wing would intersect. Should I be doing the same or use the dimensions of the physical wing?

Dazed and confused.

- Rob



Rob,
a couple of things.
Geistware actually only requires 9 measurements, no decisions other than one. They do all the figuring for you. Well, except for one decision, but more on that in a minute.

The light purple box has 9 blanks to fill in. Theblue arrows in the attached picture show how to sort out each of the measurements. The picture shows blue arrows pointing at measurement "A". The big blue arrow on the left shows "A" and you can see it's a "Root Chord". The big blue arron on the right shows where on the model you'll find the "A" root chord. The small lighter blue arrow in between the big ones shows you where to fill in that measurement. You measure that on the model and you've found what your model's Wing Root Chord is. Now, do the remaining 8 measurements and you've got one more detail to go.

The one detail left is where you make a decision. They want you to tell them your Desired Static Margin. That is aerodynamic jargon for where you want the CG to be. If you look at their small print you'll see that they describe a 5% SM as being "twitchy" and 20% as being "mushy". And that they recommend you work out a CG range of 10%-15%. Basically they're providing you a way to decide on your own preference for the CG range they'll figure for you.

If you were to shift weight around in your model so that the CG winds up where the 5% SM computations suggest, you'd have a model with very quick response to the elevator. The CG would be back about as far as most people are comfortable with. OK, most people would actually be unconfortable with. If you were to balance your model where the 20% computation says, the model would respond "mushy". Truth is, that's where a lot of people have their CGs after they take the mfg's suggested CG range, choose the most forward location of that range and then add a little bit more "to make it really safe". They wind up with an airplane that actually has reduced elevator effectiveness, but that's another story.

What you do is run the program twice. Once with an aft SM value like 10%, and once with a forward SM like 20%. You run it by filling in that 10th box with say "10" and then clicking on the button the red arrow points to.

It'll run so fast you'll wonder if your click did anything or if you'd not clicked hard enough. You'll have to scroll down to see the computations. Look for where it says the CG will be. It'll tell you in percentages and also give you a specific measurement for it. That measurement is what you're after. Write it down and go back up to change the SM input from 10 to 20. Click the button again and you've got a 2nd measurement. The two measurements are a very safe CG range. And it only took a yardstick and 4-5 minutes.

It's absolutely reliable. The formulas and computations have proven to be perfectly reliable. And you don't have to compute a thing. And you don't have to rely on anyone to get irrefutable information. Not on flying field experts who have no clue that pitch stability actually is a function of the tail, not on Chinese workers who might have pulled the CG page from another model's completed manual, not from anyone.

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RE: AOA - what should it be on a symmetric wing? - 4/2/2008 11:51:56 AM   
da Rock



Posts: 6641
Joined: 10/11/2005
From: western, NC, USA
Status: offline
quote:

ORIGINAL: Robert_Ellis

Also in their diagram of the fuse, wing and stab, they seem to use wing measurements (and horiz stab) that derive from the centerline of the fuse - a theoretical point where the leading and trailing edges of the L and R wing would intersect. Should I be doing the same or use the dimensions of the physical wing?

Dazed and confused.

- Rob


They are using projected points on the centerline. And you should estimate on your model where those points would be. It's actually not extremely critical so estimated points are good enough. It's also pretty easy to eyeball, and if you do have a yardstick, it helps as a straightedge to show you where the LE for example points. Just hold it along the LE or TE and make an educated guess where that straightedge crosses the CL of the fuselage.

BTW, the same rough estimate of the CL of the fuselage for AOI works adequately too.

(in reply to Robert_Ellis)
       Post #: 12

RE: AOA - what should it be on a symmetric wing? - 4/2/2008 4:49:33 PM   
Robert_Ellis


 

Posts: 23
Joined: 12/27/2006
From: , CA, USA
Status: offline 		I get it now. You all have been extremely helpful. This is such valuable information - thanks for taking the time to explain it to me. When I get some time later today I will run thru the #s and crank it out. This has been a very enlightening process - not *quite* ready to design my own aircraft, but have a much better understanding of the relationships involved. THANKS.

(in reply to da Rock)