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biplane wing incidenct

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Old 06-25-2014, 11:13 AM
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michael wood
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Default biplane wing incidenct

I have an old Dymond 72 inch biplane the horizontal stab is set at 0 using a bubble level the bottom wing is at +4 degrees and the top wing is at +2 degrees that would mean the top wing is at a negative 2 degrees is that correct? also if I take the reading in the middle of the top wing it reads 0 degrees so I took the readings on both sides just over to the sides of the center to where the incidence meter would connect. and that gave me a reading of +2 degrees on the top wing is that how I should have done it on the sides of the center or in the middle of the top wing.
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Old 06-25-2014, 12:56 PM
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Rodney
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Yes, in many tests I've made (some dozen or more on many different bipes) all flew best when the upper wing was at a lower angle of attack than the lower wing. Any where from 1.5 to 3 degrees seemed to work best depending on the plane. Now these were all bipes with the upper wing forward of the lower wing to some degree. I do not know if having the lower wing positioned forward of the upper wing would make a difference or not. Also, all my bipes seemed to like about 2 to 4 degrees down thrust and about 3 degrees right thrust.
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Old 06-25-2014, 03:45 PM
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michael wood
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my top wing is forward of the bottom wing so I guess iam alright. Now to check the engine thrust line. Rodney I flew this plane last week end and it was balanced per the instructions and what a hand full it was it seemed like it was tail heavy. I chopped the throttle and managed to land it some what but when I gave it throttle it would climb like a rocket. This is what made me think of the wing incidence. Any suggestions on hoe to balance this biplane?
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Old 06-25-2014, 04:50 PM
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dbacque
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Originally Posted by michael wood View Post
I have an old Dymond 72 inch biplane the horizontal stab is set at 0 using a bubble level the bottom wing is at +4 degrees and the top wing is at +2 degrees that would mean the top wing is at a negative 2 degrees is that correct? also if I take the reading in the middle of the top wing it reads 0 degrees so I took the readings on both sides just over to the sides of the center to where the incidence meter would connect. and that gave me a reading of +2 degrees on the top wing is that how I should have done it on the sides of the center or in the middle of the top wing.
Yes, the stagger makes a difference. While I don't have first hand experience with anything other than positive stagger (top wing farther forward), I have a friend who is an aero PhD candidate and modeler that explained it to me this way:

On a biplane, you want the forward most wing to stall first, this means lowest incidence on the forward wing, usually one degree or so. So when the forward wing stalls and lift drops, the coefficient of lift moves forward to a nose heavy configuration. The nose will drop, airspeed will increase and the plane will recover from the stall. If the aft wing stalls first, the center of lift moves forward, creating a tail heavy situation. At that point, the tail will drop and the increased AOA will cause the forward wing will stall creating a total loss of lift and a very nasty stall/snap/spin situation.

On the few bipes I've built, lower incidence on the leading wing has worked well and created gentle stall characteristics. This guy knows more aero junk than 99.99% of us, I trust him. And my bipes flew great. Until other issues caused problems. Oops!

Dave

Last edited by dbacque; 06-25-2014 at 04:54 PM.
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Old 06-25-2014, 05:09 PM
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michael wood
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top wing set at +2 degrees and the bottom wing set at +4 degrees Now I have to figure out how to balance it The instruction say 86 millimeter back from the leading edge on the lower wing and its nose heavy but when I flew it seemed tail heavy and real pitch sensitive I give it throttle and the tail would drop maybe by changing the wing incidence to what I have now it might tame it down. My 120 size Tiger moth flies real nice but this one has me stumped
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Old 06-25-2014, 11:42 PM
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JohnBuckner
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You need to describe the wing configuration the only thing you have established is that it has positive stagger. Are the wings straight leading and trailing edges if not are they tapered leading and or trailing edges and is there any sweep both or just the top wing swept.

One method of balancing is to select your target CG and my target would be 25% then balance the airplane at 25% of the projected wing plan. In other words the projected wing plan is the shadow you see as if a light was placed high above the airplane directly above and centered.

John

Last edited by JohnBuckner; 06-25-2014 at 11:44 PM.
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Old 06-26-2014, 06:41 AM
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Rodney
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First, make sure you have some down thrust in the engine. Second, start out with the CG set at about 25% of the distance between the leading edge of the upper wing and trailing edge of the lower wing when viewed from directly above or below the plane.
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Old 06-26-2014, 02:38 PM
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michael wood
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the wing cord is 11 inches and the top and bottom wing is swept back about 2 inches the instructions say the C.G. is 86 millimeters back from the leading edge of the bottom wing I have it set there and it is nose heavy a little
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Old 06-26-2014, 03:44 PM
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[QUOTE=michael wood; the instructions say the C.G. is 86 millimeters back from the leading edge of the bottom wing I have it set there and it is nose heavy a little[/QUOTE]


If the airplane sit level at 86 mm then it is balanced there if it is nose down then it is not balanced there but some unknown point forward.

John
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Old 06-26-2014, 04:52 PM
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So what Michael needs to do is to measure the chord of the combined top and bottom wing, then measure back 25% of that measurement and balance there ?

If plans call to balance at 86 mm., the chord should be 344 mm. ? (86 X 100 / 25)
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Old 06-26-2014, 07:13 PM
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michael wood
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I like to start with the plane a little nose heavy and adjust from there depending on how much up trim I have to put into the elevator for hands off straight and level flight. A little history on this airplane I flew this airplane last week and it took off fine but once airborne it was a hand full to fly it acted like it was tail heavy and I had to kill the engine to land it if you call it a landing if I gave it power the tail would drop and it would climb nose high I lowered the throttle to idle and the nose would drop so this is why I asked about a biplanes wing incidence thinking that it was off. When I took the readings with an incidence meter the airplane sitting level and a bubble level on the hor stab (bubble centered) the readings were lower wing +4 and the top wing was +4 I guess I should have explained it a little better in the beginning> I checked the thrust angle and its roughly about 3 degrees down and it has built into the mount right thrust so I hope that by changing the the top wing incidence buy 2 degrees negative it will help wont know until next week I want to thank all of you for sharing your vast knowledge with me because I don't know that much about biplane I going to try and post the balance diagram of the instruction for the balancing of this airplane thanks michael
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Old 06-27-2014, 12:22 AM
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jeffo
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As a general rule I'll go with -
Stab-0*
bottom wing- -2
top wing-0*/-1*
Many opinions on this one,Keep in mind that if you have a flat bottom air foil,your plane wants to climb.If your plane has +4/+4 on wings I'm not surprised she acts tail heavy.
On BUSA kits they generally go with bot.wing 0*,top+1/+2,stab 0* You then find out you need a lot of down trim.You then have to reset the stab plus a degree or two>jeffo
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Old 06-27-2014, 12:50 AM
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Dave,

Hello from a displaced fellow Houstonian.

I need clarification on the statement made by your PhD candidate friend ...

While I agree with the statement and understand the concept the you want the forward wing to stall first, I am confused on the how. A lower incidence on the upper wing will (in my mind) cause it to stall later. Yes? One thing I have not accounted for, of course, is affects on the air flow direction over the lower wing created by the upper.

Bedford
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Old 06-28-2014, 03:22 PM
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michael wood
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here is the photo of the balance points I have it balanced at 86mm slightly nose low is this right
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Old 06-29-2014, 12:10 PM
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Are these wings swept back? What is the chord on both wings.jeffo
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Old 06-29-2014, 08:18 PM
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michael wood
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the wing cord on both wings are 11 inches and the top and bottom wing is swept back about 2 inches back from the leading edge
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Old 06-29-2014, 08:51 PM
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hopefully these pictures will help
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Old 06-29-2014, 08:54 PM
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michael wood
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pictures of my 120 tiger moth
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Old 07-02-2014, 07:20 AM
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At what location did you balance. On a swept back wing you should balance at the fuselage as opposed to the wing tip. If you balanced at the wing tip and it was nose heavy, it is probably tail heavy if you balance it next to the fuse. Need to find out where you are supposed to balance it.
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Old 07-02-2014, 01:26 PM
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Mike,
Your projected wing is 13",you should roughly be 25%.The 86mm would apply to top wing,not bottom.Jeffo
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Old 07-03-2014, 03:18 AM
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michael wood
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I balanced it 1/2 inch out from the fuselage. Jeffo the diagram is showing 195mm on the top wing, 109mm from the leading edge of the top wing to the bottom wing and then 86mm to the C.G. on the bottom wing. so if I use the 195mm on the top wing for the C.G. it would be right
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Old 07-03-2014, 05:44 AM
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I think the +4/+2 wing incidence is your problem. I would reset the wings to 0/+1. I once had a Christen Eagle bipe with the same problem. Check out this article: http://www.modelairplanenews.com/blo...-to-fly-right/
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Old 07-03-2014, 02:02 PM
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Mike'
195mm is almost 8",that does not sound right.Even with the sweep of the wing,your over 50%.Can you contact the manufacture to express your concern about the C.G.?I'll be honest with you,if it were me I'd set it up 4" from the leading edge.Jeffo
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Old 07-05-2014, 05:21 AM
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I found the a set of instructions for the Great Planes Tiger Moth 60 it has the same spec as the Dymond Tiger Moth. I would never have thought to balance the plane upside down and it shows the C.G. at 2 3/4 inches from the leading edge of bottom wing next to the fuselage. Is it common practice the balance A staggered wing biplane upside down? I have looked at the links for balancing a staggered wing and from the top wing looking down that you would take the wing cord from the top wing plus what is showing form the bottom wing past the top wing and add that to the top wing and that would give you the total wing cord and divide that by 4 to give you 25% of the wing cord is that right?
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Old 07-05-2014, 12:25 PM
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FYI - I got this gem from the Toni clark web site https://toni-clark.com/english/galer...ni/gtmmani.htm
Keep in mind these are 21% and 30% very accurate scale model airplanes:

* The unusual rigging angles of our Tiger Moth:
"There are those "experts" that, according to their aerodynamic theories, cannot accept the Tigers rigging angles of +2,5 degrees for both wings and +4 degrees on the tailplane. On paper this gives a negative rigging angle difference of 1,5 degrees. This set up is simply not correct they say, with the result that "a small amount of modifying" results in a quick sale after the first flight... The air around us has no idea about such erroneous thinking and will not alter anything to accommodate such geometrical nonsense.

According to Isaac Newton, a wing to produce lift must divert air downwards after it passes the trailing edge. This downward deflected airflow of course strikes the tailplanes from above, hereby producing a positive rigging angle when airborne.
The Tigers tailplane positive angle of attack completely obviates the use of the well known "pregnant duck" title when airborne. In flight the nose is inclined slightly downwards, moving the tailplane partly above and out of the upper wings destabilizing downdraft. This effect is almost identical to that of a TEE tailplane configuration, allowing the use of a smaller tailplane, or alternatively gaining more in flight stability."

Lol. Confused yet? Since the Original Tiger Moth tail fly's higher than the nose - the adjusted effect of the wings incidence is a negative number! They achieved this by the +4 deg. on the horizontal stab.

Don't be afraid to experiment for yourself - vary the wings -2-3, +2-3 degree's and/or with varying degree's of washout to see how your plane fly's. If the horizontal stab can be removed - you can shim the leading/trailing edge up or down also. Keep a log and jot down the results. As long as your wing is not twisted drastically and induces a severe roll condition and no tail-heavy condition exists, you should be able to find out what works the best for your plane and yourself, safely and have a bit of fun also.

Whew! Hope I haven't muddled things up for ya! Tiger Moths are one of the best biplanes ever designed and I know she will fly just beautifully for you.

All the best to you and your nice Tiger Moth.
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