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Old 10-23-2011, 04:02 PM   #1
PeterL
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G'Day from "Downunder" where flying upside down is normal.
I'm a "newbie". Have been visiting as a guest for a while and learning heaps. Time for a question, so I have registered.
I am seriously contemplating building a Biplane. My 1946 Aeronca 11AC is great fun but I like the idea of LAROASA (learned that term here) and the Aeronca is older than I am, so I treat it gently.


I am very taken with the Acrosport II and have bought a set of plans to study. I notice, by comparison to the Pitts family, that the Acrosport upper wing is not swept back.


I assume there is a reason for the sweep on the Pitts and not on the Acrosport. I aslo assume there is some difference in handling between the two types because of the sweep. Can anyone educate me?


Cheers Peter L


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Old 10-23-2011, 07:37 PM   #2
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My understanding of the sweep is to help snap rolls. When you yaw the plane, the inside wing is trailing off and so more likely to stall where as the outside wing is going faster so more likely to generate more lift so it makes the snap go quicker. The downside to the sweep is that if you are not straight when you stall, you are more likely to enter a spin, so the plane is less forgiving. I am sure others will correct or better explain this, but that is the way it was explained to me many years ago.



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Old 10-23-2011, 08:08 PM   #3
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Welcome aboard and thanks for signing up.

The Pitts not only has a swept upper wing it also has different airfoils on the upper and lower wings. The airfoil on the lower wing is supposed to stall at a higher angle of attack than the upper (forward) wing. So the wings can be rigged at the same angle of attack and the forward wing will stall first whether upright or inverted. When the sweep is added to the upper wing it will not only stall first but if stalled in a yaw will start rotation more quickly than a non swept wing. The theory seems to work as many years of acro pilots and designers will attest to. [img]smileys/smiley1.gif[/img]

The AcroSport II is a fine airplane and a little easier to land and take off than the Pitts. The other side of that is that it is not quite as responsive as the Pitts. It may not snap roll quite as good as a Pitts but it will snap roll cleanly.


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Old 10-23-2011, 09:16 PM   #4
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You can do a nice safe loop or roll with that Aeronca.
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Old 10-23-2011, 09:57 PM   #5
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In one of the few but valued conversations I had with Curtis Pitts I asked why he swept the top wing in the first place. His answer was simple. He couldn't get in or out with a straight wing.
Does help with the snaps though.
If you are familiar with the Curtis Pitts designed Samson you will note that it has a straight top wing. Doesn't seem to hold it back any.

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Old 10-23-2011, 10:39 PM   #6
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As previously stated, the number one reason for top wing sweep is access and visibility, but as you can see on other designs there are other ways to get access and visibility, usually by increasing the stagger. Also as previously stated, it helps with the snap rolls. There is also another advantage: effective dihedral. You can't put much dihedral on an unlimited class aircraft as it works against you when you are trying to roll or fly upside down. Wing sweep helps to get the roll to follow yaw. Yaw into a turn and the planes rolls into the turn. Makes for a much nicer plane to live with on a cross country. Unlike dihedral, sweep doesn't work against you inverted. Of course with symmetrical ailerons, the nice feel of that coupling goes away to a degree since they can generate adverse yaw when rolling. Not an issue though and kind of fun once you learn to use your feet.
At some point designers just give up on feel and make the control surfaces stupid large and the control forces extremely light. Many of the monoplanes fall into that category. No harmony, but light enough you pretty much don't care.
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Old 10-23-2011, 11:34 PM   #7
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" His answer was simple. He couldn't get in or out with a straight wing."

Well there you are. So much for collecting general information over the years.... Just goes to show that one should take things learned at the local airport and the internet with a grain of salt.


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Old 10-24-2011, 09:02 AM   #8
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Thanks to all for the education.
It would appear that the straight wing would have been a deliberate choice by Paul Poberezny in line with the goal of a docile but aerobatic biplane.
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Old 10-24-2011, 12:03 PM   #9
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It would appear that the straight wing would have been a deliberate choice by Paul Poberezny in line with the goal of a docile but aerobatic biplane.
Yes, that is probably the main reason for the straight wing. However the Acro Sport like the EAA Biplane was originally designed to be build by young people in high school industrial art programs, so I believe that ease of construction was an important factor in the design of the aircraft. And a straight wing is simpler and easier to build than a swept one.

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Old 10-25-2011, 02:06 AM   #10
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"And a straight wing is simpler and easier to build than a swept one."

Hmmm. How do they deal with the angle between the ribs and the spar on a swept wing?


Are there wedge shaped packing pieces between the ribs and spars? Or are the ribs built with an angle in the spar cutout?
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Old 01-28-2015, 12:50 AM   #11
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Quote:
Originally Posted by PeterL View Post
"And a straight wing is simpler and easier to build than a swept one."

Hmmm. How do they deal with the angle between the ribs and the spar on a swept wing?


Are there wedge shaped packing pieces between the ribs and spars? Or are the ribs built with an angle in the spar cutout?
Hmmm. Not sure I grok the question right. The sweep in the top wing is in the center section not in the wings. The wing spars are straight. They come together into a set of blocks cut to the sweep and faired to the airfoil in the center section, then glued together and topped with plywood as essentially a one piece wing.
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Old 01-28-2015, 02:59 PM   #12
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"Are there wedge shaped packing pieces between the ribs and spars? Or are the ribs built with an angle in the spar cutout?"
We used a sanding stick to cut the spar angle in the rib opening and also used corner blocks cut to the correct angle instead of 90 degrees.
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Old 01-28-2015, 03:27 PM   #13
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There must be something more than ease of entry, to explain the delightful flying qualities of a Jungmann, Stampe, and Pitts.
Perhaps Kevin can enlighten us?
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Old 01-28-2015, 05:15 PM   #14
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Quote:
Originally Posted by planebuilder View Post
"Are there wedge shaped packing pieces between the ribs and spars? Or are the ribs built with an angle in the spar cutout?"
We used a sanding stick to cut the spar angle in the rib opening and also used corner blocks cut to the correct angle instead of 90 degrees.
Doh! Now I get it. Great explanation plane builder.

Dan
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Old 01-28-2015, 05:16 PM   #15
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There must be something more than ease of entry, to explain the delightful flying qualities of a Jungmann, Stampe, and Pitts.
Perhaps Kevin can enlighten us?
Don't forget the beautifully handing Marquart MA-5 Charger. Both wings are swept at 10 degrees. She snaps rolls really well

-Glenn
1004042_521070397942423_486069214_n.jpg  
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Old 01-28-2015, 05:25 PM   #16
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I had learned that during a snap roll on a swept wing aircraft, there is a yaw induced lengthening of the outer wing which would also have a shortening effect on the inside wing thus increasing the lift differential between the left and right wings which further accelerates the snap.
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Old 01-28-2015, 05:43 PM   #17
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I wonder how center of lift and CG is calculated. I suppose it's straight arithmetic? I guess we look at CL of the airfoil then calculate the CL of the wing as a function of that percentage forward of a chosen datum (not THE datum) and that percentage aft of it? Some smart guys probably worked all that out in the 1920s. I guess it becomes math when you add in yaw? My degree was in philosophy so it's my job to ask questions; the answers to which nobody really cares about.

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Old 01-28-2015, 07:41 PM   #18
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Glen...that picture really shows the sweep of the wing well. A real eye pleaser. I'm curious how a swept wing biplane vs a straight wing biplane of same basic design and weight effects entry speeds, stall and such. Wasn't there some Pitts "magic" about have a straight lower wing and a swept upper? Vs both straight or both swept?


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Old 01-28-2015, 08:58 PM   #19
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Quote:
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I wonder how center of lift and CG is calculated. I suppose it's straight arithmetic? I guess we look at CL of the airfoil then calculate the CL of the wing as a function of that percentage forward of a chosen datum (not THE datum) and that percentage aft of it? Some smart guys probably worked all that out in the 1920s. I guess it becomes math when you add in yaw? My degree was in philosophy so it's my job to ask questions; the answers to which nobody really cares about.

Dan


Dan,
Were you wondering about how to calculate the CG on a swept wing?

This example shows the CG at 25% MAC on a swept wing. The location is a geometric construct.

-Glenn
swept_cg.jpg  
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Old 01-28-2015, 09:01 PM   #20
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Quote:
Originally Posted by Lotahp1 View Post
Glen...that picture really shows the sweep of the wing well. A real eye pleaser. I'm curious how a swept wing biplane vs a straight wing biplane of same basic design and weight effects entry speeds, stall and such. Wasn't there some Pitts "magic" about have a straight lower wing and a swept upper? Vs both straight or both swept?


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Skybolt, Pitts and Great Lakes have a straight lower and swept upper.

Jungmann, Jungmeister and Charger have both wings swept.

Why one planform over the other, I can't say. I am interested in knowing though

-Glenn
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Old 01-28-2015, 09:24 PM   #21
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I seem to remember a quote attributed to Curtis in response to being asked why he swept the top wing. His answer was something like "So I could get into the cockpit."
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Old 01-28-2015, 10:15 PM   #22
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Quote:
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Dan,
Were you wondering about how to calculate the CG on a swept wing?

This example shows the CG at 25% MAC on a swept wing. The location is a geometric construct.

-Glenn
Thanks Glenn. That's a great diagram.
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Old 01-29-2015, 01:43 AM   #23
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Glenn,

That diagram helps me quite a bit, as I have a model airplane that I need to locate the mean cord on a tapered swept wing. What your diagram does not show is how to average the constant cord swept sections with the straight center section. I know how to do it, but I don't have any diagrams of it to share.

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Old 01-29-2015, 02:32 AM   #24
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Here is a link to an online CG calculator
online-calculator.jpg  
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Old 01-29-2015, 11:41 AM   #25
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Quote:
Originally Posted by freerangequark View Post
Skybolt, Pitts and Great Lakes have a straight lower and swept upper.

Jungmann, Jungmeister and Charger have both wings swept.

Why one planform over the other, I can't say. I am interested in knowing though

-Glenn
The Great Lakes originally had a straight upper wing. The Cirrus powered version was tail heavy and it was difficult to recover from a flat spin. Sweeping the upper wing aft fixed the spin issue and also improved the snap roll. As the article on the GL history stated, the bandaid fix turned the plane into a first class aerobatic plane.


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