pmisuinas
Jan 31, 2009, 11:07 AM
Yes, it does say “Build Log” but it’s almost an “As Built Log”…I have 3 projects I was brave enough to put on the web but think I must have a mild case of model ADD because I start thinking about new projects until it becomes too much to ignore—so another one gets started. All of them eventually get finished, but just not in the same order as they got started :D This is especially so when I reach the covering stage—I just hate to cover and too often that gives me that extra “incentive” to start something else…so to avoid another part way effort I vowed not to post this one until it was far enough along that I was sure I would finish it in a reasonable amount of time…
But enough on that, back to the Cutting Edge. This is a sport aerobatic biplane that was designed in 1991 and published in 1992. It looked so sharp I had to order the plans, and it is one of several I have in a stack—and one of several that finally called to me so loudly I had to build it. With a 64” wingspan it qualifies as a Giant model, hence the posting here (although I did consider Glow to Electric Conversions—but that is really more for ARFs). The original was an all wood, fully sheeted model with lots of plywood doublers. That added up to a 12 pound behemoth designed for a 1.08 2-cycle fuel gulper. I am now basically all electric, so felt compelled to see what I could do to lighten it up. So here is my composite version (balsa, a little ply, Depron, foam, and paper) (what—you thought composite meant it had to be carbon fiber or Kevlar, or boron? Then you don’t know what the definition of composite is, so go look that up in your Funk & Wagnel’s [those of you in my generation will know the reference :p ). But I do have some carbon fiber tubes in the bottom wing, so not all outside the accepted meaning.
For the best description, let me just use the designer notes:
Brian Reed’s notes that came with the plans for the Cutting Edge:
I was quite pleased to receive your letter expressing your interest in knowing more about my Cutting Edge biplane design. The plans and instructions do a pretty good job of guiding you though construction so what follows is some background and design details to help you understand my design goals and the aircraft’s unique abilities. While I’m pretty well versed in aero-theory and such, I tend to be more ‘style’ motivated in my designing. And, while I’m sure there are optimum numbers for everything, if all designers used these numbers, all aircraft would look the same so, I put creative originality ahead of numerical perfection.
The Cutting Edge started out as Tricksy, a .28 powered, 36” span biplane I designed for a friend in 1987 while stationed at Hunter Army Airfield in Savannah, GA. This little airplane was capable of some very unique maneuvers and had a roll rate that had to be seen to be believed! Even so, it was quite easy to handle and I let anyone that wanted to try it do so…aileron rates on low of course. I’m not sure if it’s still around (note this was written in 1992) or not as I gave it to the gentleman I’d designed it for when I left Savannah.
In the spring of ’92 it was again time for a new airplane and I decided to draw up plans for a quarter scale Sukhoi SU-26. Space limitations killed that idea (wouldn’t fit in my station wagon) but a biplane only needed a 60” span to get into the “Big” meets so I again turned to Tricksy. Let’s start with the wings first s they’re what makes a biplane work.
The 64” span was chosen as a compromise between what I needed (60” minimum) and the rib spacing I wanted. One less rib bay per side and the span would have been only 56”. I considered reversed stagger (bottom wing ahead of the top) too, at first, as I wanted to mount retracts in the bottom wing, ahead of the CG. This was scrapped, though, because it put the top wing’s mounting pylon back into the canopy area. Top wing sweep is used for stability: it adds “effective” dihedral without some of the crosswind problems of actual dihedral. Actually, 42 degrees total sweep is more than is necessary, but more looks better, has less drag, and quickens up the roll response. A general rule for biplane wing separation is one chord width. This keeps the tip and TE vortices from interfering with each other and the tail. Actual air compression doesn’t occur until near-supersonic speeds so it’s not even a factor.
Proper wing incidence is very important to biplane design and, while quite simple, can also cause headaches. Think of it this way: it’s okay to have the airfoil centerlines parallel or slightly apart (LEs aimed away from each other) but try to keep the centerlines from converging ahead of the aircraft (LEs slanted towards each other). This can lead to erratic flight and a pitching oscillation that may or may not be recoverable. Consider washout, also, when setting wing incidence angles. IE: The bottom wing’s incidence at zero and the top wing’s center section positive with the tips washed out to zero.
The plans show both wings set at zero degrees and the Edge flies fine that way. I’ve shimmed the original’s top wing slightly positive, though, and gained some straight line stability. Here’s why: in straight and level flight with the bottom wing at zero and the top wing slightly positive (1 - 1 1/2 degrees), most of the work is being done by the top wing. In other words, most of the aircraft’s weight is being carried by half of the available wing area, doubling the effective wing loading and increasing straight line stability and resistance to crosswinds and turbulence. As up elevator is applied the bottom wing starts working, going positive, relative to the airflow. The wing loading is halved and maneuvering is crisp and predictable.
“Ahh! that’s fine, but what about inverted flight?” you ask. The same rules apply except it takes some extra down elevator to hold inverted flight. But, when you fly the Edge, you’ll notice that it takes very little down elevator to stay level, inverted. This is due to the airfoil selection which I’ll explain next.
I designed a semi-symmetrical airfoil especially for this airplane. It’s dual cambered (top Max Camber Point at 33%, bottom MCP at 29%) for good lift and excellent performance both upright and inverted. Here’s how it works--right side up it behaves just like any other semi-symmetrical section producing good lift and a proportionate amount of drag. Turn it upside down, though, and things change. Since the airfoil’s bottom MCP is ahead of the top’s, the aircraft is now “more” tail-heavy than when upright, increasing the effectiveness of the elevators so you’ll need less down input to maintain level, inverted flight. And, since the bottom MCP is closer to the wing’s LE, the LE’s shape is more equal top and bottom (actually a 7/8” ellipse) giving an increased flying (angle of attack) range. This means nice, tight inverted maneuvers without unexpected stalling.
CG is easy too. Locate the bottom wing’s MCP and mark it. Then, locate the top wing’s center section and tip MCPs, average them together and mark that. Now just average the top and bottom marks and you’ve got it. I’m sure there are all kinds of formulas and “crossed line” drawings for finding correct CGs, but this method works on just about any wing-and-tail aircraft (deltas and flying wings being two of the exceptions). That’s about it for wings. Pretty simple, huh!
Vertical and horizontal tail areas were chosen as what looked right to balance the design visually. The bigger the aircraft, the less area (% wise) you can get away with, but you can still get carried away.
Tail incidence is easy too, but still needs careful attention. The horizontal stabilizer can be set at zero degrees on just about any smaller aircraft but, as models get larger we start to see the effects of downwash created by the wings producing lift. Air deflected off of the bottom of the wings moves back and down over the tail causing the airplane’s nose to pitch up. To counter this, we set the tail’s incidence slightly positive (LE up 1 - 2 degrees) placing it inline (parallel) to the relative airflow (washing down off the wings). Here’s where I cheated a little bit. I set the tail incidence to zero and tapered the elevator so that its bottom is parallel to the stab with the top tapering down to meet it. I think it’s easier to build it to zero, and cheat a little bit like this, than to have to mess around with degrees and fractions, getting confused and messing it up.
On to the fuselage. Length and shape were chosen for what looked right and what I needed to fit the radio and engine installation. Nose and tail moments were also an artistic decision but the general rules still apply--short nose and tail makes for a snappy airplane, longer ones give it more stability and smoother, more graceful maneuvers. With the Cutting Edge I tried to strike a balance. The long nose makes it look fast (which it is!) and the relatively short tail keeps the aerobatics coming.
The canopy design is similar to one I’ve seen on some pylon racers. I chose it for its simplicity (can be cut from a flat sheet of clear butyrate) and “mean” looking presence that adds to the Edge’s fast look.
The top wing mounting pylon is of built-up construction and incorporates a rather thick, symmetrical airfoil section for low drag (no cabane struts) and enhanced knife edge capabilities--hence the name. It uses 1/8” ply ribs, 1/4” spruce spars, heavy balsa leading and trailing edges and is fully sheeted. The airfoil’s thickness also lends some lateral support to the top wing but don’t even consider flying this one without the wing struts! The pylon’s forward slant (sweep) was also just for looks and I even added a trim stripe to enhance the effect.
In flight, the pylon does a couple of things. Straight and level flight is quite fast with excellent directional stability. Knife-edge flight is also very easy with knife turn-arounds and consecutive knife loops coming with practice. One negative aspect of the pylon is that it adds proverse roll with the application of rudder. This is because it’s only producing lift on one side of the fuselage centerline at positive angles of attack (yaw). My transmitter allows me to mix in some adverse aileron correction with rudder, allowing me to fly around “sharp” indefinitely.
Half-throttle knife edge flight is also possible and if you get it too slow, it’ll start “bucking”, stalling the pylon. This will continue until you add power or become mesmerized and crash.
For power, I’m using an OS 1.08 with the stock muffler. I use APC 14x8 props and a 24 oz tank is good for about 15 minutes. Torque rolls and hovering flight on take off are always attention grabbers but it’s not completely vertical. A little more power wouldn’t hurt, but I wouldn’t go over about a 1.5ci 2-stroke. The aircraft is traveling at over 100mph now! Larger models have a VNE (Velocity Never Exceed) just like their full sized counterparts and too much speed can lead to destructive flutter and the loss of an airplane and/or other personal property. More is better--to a point!
Well, there it is! And remember, you asked for it.
But enough on that, back to the Cutting Edge. This is a sport aerobatic biplane that was designed in 1991 and published in 1992. It looked so sharp I had to order the plans, and it is one of several I have in a stack—and one of several that finally called to me so loudly I had to build it. With a 64” wingspan it qualifies as a Giant model, hence the posting here (although I did consider Glow to Electric Conversions—but that is really more for ARFs). The original was an all wood, fully sheeted model with lots of plywood doublers. That added up to a 12 pound behemoth designed for a 1.08 2-cycle fuel gulper. I am now basically all electric, so felt compelled to see what I could do to lighten it up. So here is my composite version (balsa, a little ply, Depron, foam, and paper) (what—you thought composite meant it had to be carbon fiber or Kevlar, or boron? Then you don’t know what the definition of composite is, so go look that up in your Funk & Wagnel’s [those of you in my generation will know the reference :p ). But I do have some carbon fiber tubes in the bottom wing, so not all outside the accepted meaning.
For the best description, let me just use the designer notes:
Brian Reed’s notes that came with the plans for the Cutting Edge:
I was quite pleased to receive your letter expressing your interest in knowing more about my Cutting Edge biplane design. The plans and instructions do a pretty good job of guiding you though construction so what follows is some background and design details to help you understand my design goals and the aircraft’s unique abilities. While I’m pretty well versed in aero-theory and such, I tend to be more ‘style’ motivated in my designing. And, while I’m sure there are optimum numbers for everything, if all designers used these numbers, all aircraft would look the same so, I put creative originality ahead of numerical perfection.
The Cutting Edge started out as Tricksy, a .28 powered, 36” span biplane I designed for a friend in 1987 while stationed at Hunter Army Airfield in Savannah, GA. This little airplane was capable of some very unique maneuvers and had a roll rate that had to be seen to be believed! Even so, it was quite easy to handle and I let anyone that wanted to try it do so…aileron rates on low of course. I’m not sure if it’s still around (note this was written in 1992) or not as I gave it to the gentleman I’d designed it for when I left Savannah.
In the spring of ’92 it was again time for a new airplane and I decided to draw up plans for a quarter scale Sukhoi SU-26. Space limitations killed that idea (wouldn’t fit in my station wagon) but a biplane only needed a 60” span to get into the “Big” meets so I again turned to Tricksy. Let’s start with the wings first s they’re what makes a biplane work.
The 64” span was chosen as a compromise between what I needed (60” minimum) and the rib spacing I wanted. One less rib bay per side and the span would have been only 56”. I considered reversed stagger (bottom wing ahead of the top) too, at first, as I wanted to mount retracts in the bottom wing, ahead of the CG. This was scrapped, though, because it put the top wing’s mounting pylon back into the canopy area. Top wing sweep is used for stability: it adds “effective” dihedral without some of the crosswind problems of actual dihedral. Actually, 42 degrees total sweep is more than is necessary, but more looks better, has less drag, and quickens up the roll response. A general rule for biplane wing separation is one chord width. This keeps the tip and TE vortices from interfering with each other and the tail. Actual air compression doesn’t occur until near-supersonic speeds so it’s not even a factor.
Proper wing incidence is very important to biplane design and, while quite simple, can also cause headaches. Think of it this way: it’s okay to have the airfoil centerlines parallel or slightly apart (LEs aimed away from each other) but try to keep the centerlines from converging ahead of the aircraft (LEs slanted towards each other). This can lead to erratic flight and a pitching oscillation that may or may not be recoverable. Consider washout, also, when setting wing incidence angles. IE: The bottom wing’s incidence at zero and the top wing’s center section positive with the tips washed out to zero.
The plans show both wings set at zero degrees and the Edge flies fine that way. I’ve shimmed the original’s top wing slightly positive, though, and gained some straight line stability. Here’s why: in straight and level flight with the bottom wing at zero and the top wing slightly positive (1 - 1 1/2 degrees), most of the work is being done by the top wing. In other words, most of the aircraft’s weight is being carried by half of the available wing area, doubling the effective wing loading and increasing straight line stability and resistance to crosswinds and turbulence. As up elevator is applied the bottom wing starts working, going positive, relative to the airflow. The wing loading is halved and maneuvering is crisp and predictable.
“Ahh! that’s fine, but what about inverted flight?” you ask. The same rules apply except it takes some extra down elevator to hold inverted flight. But, when you fly the Edge, you’ll notice that it takes very little down elevator to stay level, inverted. This is due to the airfoil selection which I’ll explain next.
I designed a semi-symmetrical airfoil especially for this airplane. It’s dual cambered (top Max Camber Point at 33%, bottom MCP at 29%) for good lift and excellent performance both upright and inverted. Here’s how it works--right side up it behaves just like any other semi-symmetrical section producing good lift and a proportionate amount of drag. Turn it upside down, though, and things change. Since the airfoil’s bottom MCP is ahead of the top’s, the aircraft is now “more” tail-heavy than when upright, increasing the effectiveness of the elevators so you’ll need less down input to maintain level, inverted flight. And, since the bottom MCP is closer to the wing’s LE, the LE’s shape is more equal top and bottom (actually a 7/8” ellipse) giving an increased flying (angle of attack) range. This means nice, tight inverted maneuvers without unexpected stalling.
CG is easy too. Locate the bottom wing’s MCP and mark it. Then, locate the top wing’s center section and tip MCPs, average them together and mark that. Now just average the top and bottom marks and you’ve got it. I’m sure there are all kinds of formulas and “crossed line” drawings for finding correct CGs, but this method works on just about any wing-and-tail aircraft (deltas and flying wings being two of the exceptions). That’s about it for wings. Pretty simple, huh!
Vertical and horizontal tail areas were chosen as what looked right to balance the design visually. The bigger the aircraft, the less area (% wise) you can get away with, but you can still get carried away.
Tail incidence is easy too, but still needs careful attention. The horizontal stabilizer can be set at zero degrees on just about any smaller aircraft but, as models get larger we start to see the effects of downwash created by the wings producing lift. Air deflected off of the bottom of the wings moves back and down over the tail causing the airplane’s nose to pitch up. To counter this, we set the tail’s incidence slightly positive (LE up 1 - 2 degrees) placing it inline (parallel) to the relative airflow (washing down off the wings). Here’s where I cheated a little bit. I set the tail incidence to zero and tapered the elevator so that its bottom is parallel to the stab with the top tapering down to meet it. I think it’s easier to build it to zero, and cheat a little bit like this, than to have to mess around with degrees and fractions, getting confused and messing it up.
On to the fuselage. Length and shape were chosen for what looked right and what I needed to fit the radio and engine installation. Nose and tail moments were also an artistic decision but the general rules still apply--short nose and tail makes for a snappy airplane, longer ones give it more stability and smoother, more graceful maneuvers. With the Cutting Edge I tried to strike a balance. The long nose makes it look fast (which it is!) and the relatively short tail keeps the aerobatics coming.
The canopy design is similar to one I’ve seen on some pylon racers. I chose it for its simplicity (can be cut from a flat sheet of clear butyrate) and “mean” looking presence that adds to the Edge’s fast look.
The top wing mounting pylon is of built-up construction and incorporates a rather thick, symmetrical airfoil section for low drag (no cabane struts) and enhanced knife edge capabilities--hence the name. It uses 1/8” ply ribs, 1/4” spruce spars, heavy balsa leading and trailing edges and is fully sheeted. The airfoil’s thickness also lends some lateral support to the top wing but don’t even consider flying this one without the wing struts! The pylon’s forward slant (sweep) was also just for looks and I even added a trim stripe to enhance the effect.
In flight, the pylon does a couple of things. Straight and level flight is quite fast with excellent directional stability. Knife-edge flight is also very easy with knife turn-arounds and consecutive knife loops coming with practice. One negative aspect of the pylon is that it adds proverse roll with the application of rudder. This is because it’s only producing lift on one side of the fuselage centerline at positive angles of attack (yaw). My transmitter allows me to mix in some adverse aileron correction with rudder, allowing me to fly around “sharp” indefinitely.
Half-throttle knife edge flight is also possible and if you get it too slow, it’ll start “bucking”, stalling the pylon. This will continue until you add power or become mesmerized and crash.
For power, I’m using an OS 1.08 with the stock muffler. I use APC 14x8 props and a 24 oz tank is good for about 15 minutes. Torque rolls and hovering flight on take off are always attention grabbers but it’s not completely vertical. A little more power wouldn’t hurt, but I wouldn’t go over about a 1.5ci 2-stroke. The aircraft is traveling at over 100mph now! Larger models have a VNE (Velocity Never Exceed) just like their full sized counterparts and too much speed can lead to destructive flutter and the loss of an airplane and/or other personal property. More is better--to a point!
Well, there it is! And remember, you asked for it.