That's a good question, and Bob Hunt has a Stunt News article written by Igor explaining it. Either you or I could use XFoil to figure out the aero part. I'll see if I can do it tomorrow. Then there's the wing weight, but all that is calculable. If it's for a stunt plane, there are a couple other considerations, both of which favor the thicker wing for stunt:
The thinner wing will be slapped around by turbulence more than the thicker wing because of its lighter wing loading.
The thinner wing will have a higher ratio of drag in maneuvers to drag in level flight than the thicker wing.
For combat planes, which have different design requirements than stunt planes, the optimal thickness seems to be about halfway between those.
The thinner wing will be slapped around by turbulence more than the thicker wing because of its lighter wing loading.
A lot of that is wrong. Ted knows a lot about stunt, but he has a habit of making up theory to explain it.Expand?
A lot of that is wrong. Ted knows a lot about stunt, but he has a habit of making up theory to explain it.
A lot of that is wrong. Well, Howard, I guess if you say it's wrong it must be since, as you frequently point out, you're smarter than the rest of us all put together.
Ted knows a lot about stunt, Thanks, I guess, for the back handed compliment.
but he has a habit of making up theory to explain it. This--if I may quote a much smarter man-"is [just plain] wrong."
For stunt. I'm interested in reading the article by Igor the Brilliant. Do you recall the date of the SN issue?
This question came up when I flew a Teosawki for the first time. (I know you object to planes of this type, and your objections were addressed in the Sakitumi: scratch or kit built with stunt wing construction replacing the combat-style wing, a very attractive fuselage shape and conventional finish---all weighing in at about the same weight as a Teosawki, due to the use of 4 lb wood. There is a full-fuselage version in the queue with the same weight goal, thus eliminating the objection to profiles).
The first time I flew the Teo, I was amazed that such a thin wing could fly so well. All the thin wings I had flown before had much higher wing loadings, and I believed that thinner wings can't fly stunt at all. The feel of the Teo was different from Flight Streaks and Skyrays (I had trouble getting several Skyrays to track in level flight*): graceful and effortless are the best words I can think of to describe it. The very large TVC probably had a lot to do with it, as well as the larger overall size of the Teo and its engine (LA46). And of course these are subjective impressions, so do not constitute evidence.
*Possibly a neurological limitation on my part since inverted level was even less stable than upright level.
I became curious whether the thin, double-tapered Teo wing was part of the successful formula, or whether a scaled-to-match total lift 18% airfoil would perform as well (or maybe better). Also constant chord or tapered. I had no idea how to calculate the area required for an 18 percenter to generate equivalent lift. Perhaps XFoil can answer that question.
I guess this is the final rock v. hard place for flapless planes. The light loading is needed for cornering performance, but leaves the plane vulnerable to wind and turbulence. Not that this is anything like news! I'm just attracted to flapless planes for some unknown reason (nostalgia?) and ways to optimitize their performance interest me.
Your comments about refinishing remind me of the old joke about a guy who is banging his head against the side of a building. A woman walks up to him and says, "Sir, why are you banging your head against the side of a building?" The guy replies, "Because it feels so good when I stop." Think of how good it will feel to have your newly refinished beauty ready to show off. Will you grace us with pictures? How about taking bets on base color. $10 on orange for me.
This morning James Aron of my airplane painters' support group recommended Isocyanate Death Paint clear coat as a way out of one problem I'm having with the finish. If I survive spraying that, I'll eventually post a picture. Orange is involved, although I keep sanding through the orange.
I still haven't answered your original question.
.....More importantly, two other factors play at least as big a role. One thing I've found over the years that the quickest, lightest, easiest way to improve performance is to add 10-15% to the span. Putting 3 in wide tips on a 48 in. plane will make a dramatic difference in performance. With a fairly highly tapered wing the motor doesn't even notice the extra area, but the plane does everything better.....
If you have two flapless wings that both produce the same maximum lift (I assume that would be in a square corner), one has an 18% airfoil and less wing area, the other a 14% airfoil (same approximate shape, just thinner) and more wing area, would they both generate the same total drag (including all categories of drag that apply to a wing), or would one produce more drag than the other?
Thanks.
This is from Ted Fancher and does at least address some of the questions here:
As I mentioned in the Doctor article in Stunt News, flaps pretty much only do a few things. First and most importantly, by changing the airfoil from an uncambered section to a cambered one they inherently increase the lift per unit of total area (assuming the gap between the main plane and the flap isn't so excessive as to allow the value of the flaps to be largely defeated); second, they increase the induced drag and, therefore, the total drag (induced plus all others ); third, they produce a negative pitching moment (the cambered wing which results from deflected flaps has a natural desire to rotate about itself in the direction opposite to the camber).
...
( Redacted for brevity) CS
Ted
Well, given a symmetrical section on a flapless wing, the wing area would be the same. You'll find the slope of the Cl vs AoA for the 0014 and 0018 airfoils are essentially the same. So for a given angle of attack the thickness won't affect the lift coefficient, and thus the required area would be unchanged.
I can't be sure exactly what Kim meant, but I think he meant to start out with two wings, of different thickness, with area adjusted so that the lift right at the onset of stall is the same. Not (I hope) that he thinks that the CL vs. pitch slope is different from one to the other.
If that's the case then the thicker wing could have less area. And then his question kind of makes sense, except for the part where he's ignoring everything else that's going on.
It increases the incidence of the tail. It also creates a pitching moment contribution from the fuselage as show by Munk and later work by Multhropp. Further, it reduces the apparent mass effects of the displaced air as the aircraft rotates. This change in downwash angle can be remarkably high. So given the tail volume coefficient and horizontal tail efficiency ( which can be greater than unity on a stunt ship due to the propeller slipstream) the flap deflection's effect on the tail's positive contribution to the aircrafts overall pitching moment combined with the introduction of a positive pitching moment from the fuselage - can be higher than the wing's increase in negative moment due to the increase in camber.
For a stunt plane, the sign of pitching moment from flaps (for the whole airplane) depends on the configuration. A local guy told me that his elevator linkage disconnected during a wingover. He attempted to recover by giving full control, first one way and then the other. The airplane went straight into the ground. A change in CG or tail geometry could have caused it to turn either the same direction the elevator would have turned or the other direction.
If you have two flapless wings that both produce the same maximum lift (I assume that would be in a square corner), one has an 18% airfoil and less wing area, the other a 14% airfoil (same approximate shape, just thinner) and more wing area, would they both generate the same total drag (including all categories of drag that apply to a wing), or would one produce more drag than the other?
Thanks.
Ahhh, but now we are in "stick free response" which differs from ""stick fixed response". Different things.
With the elevator free to seek it's happy place the aircraft response is changed and the aircraft is free to find its neutral point. Think about it...free elevator- you defelct the flaps down, the downwash will tend to push the free-floating elevator down! In a very loose sense, it acts like a trim tab for the stab in that condition. When a trim tab goes down , the hinged control surface moves in the opposite direction. Same with an airplane where the CG is the "hinge".
Some of the early CL models used flaps instead of elevator to try to dodge Jim Walker's patents. They weren't very popular. From that I'm going to presume that they worked, but not terribly well.
Wake turbulence is a function of induced drag.
it is the prop that causes a huge amount of wake turbulence
It is easy to extend (beside the fact that induced drag is function of lift and prop makes part of the lift and therefore directly makes wake turbulence) ... the induced drag is function of speed ^2 ... and therefore of available power ... so if the large prop can recharge energy lost in that maneuver, it is directly source of that turbulence :- ))) ... not to mention that if model slows down in that maneuver, it will come back later, when the turbulence is little slower
and if someone does not believe that statement with flaps, I recommend to try flapeless indoors with props = 1/3 of span flying in dead calm gym ... I never had problems with wake turbulence :- ))))
Howard said "Wake turbulence is a function of induced drag. For a given speed and loop radius, a plane with a lighter span loading will have less wake turbulence, with or without flaps."
HI Howard
Paul Walker told me it is the prop that causes a huge amount of wake turbulence, do you have any info to share about this?
I forgot about the prop when I said that. I reckon the prop wake could be calculated-- at least how much energy it puts into the air in a corner.
He has said that he has less problem with an 11.3"-diameter prop in still air than with a bigger prop. My guess as to the mechanism was that the higher speed of the small prop's wake blows its wake toward the outside of the maneuver. This wouldn't affect me, but may be of benefit to you guys who put your consecutive maneuvers in the same place. Igor probably has a better explanation. I wonder if prop size has the same effect with electric power.
It is interesting, We had a talk about this once ,and Paul said that he flew his ship with a very tiny prop one time, maybe a 9 or 10 inch diameter in dead calm, he said that he could just stand flat footed and do consecutive maneuvers, over and over without any problems from turbulence, this is one thing that convinced him to go to the smaller props, of course that is seemingly out the window now, he is using much larger diameter ones.
Randy
First, the effect of tip losses are less on a larger prop ( at the same thrust as a smaller one) because either the pitch is less or the angular velocity is reduced or a combination of both. (For example, we could go from a 10" to an 11" and reduce the pitch, or we can use an 11" at the same pitch but a lower angular velocity.) In other words, the percentage of tip loss to to total losses decreases with diameter up to a point.
Second, forget al that jazz about Bernoulli's principle and airfoils people argue about. That's not what causes lift or propeller thrust. What makes a subsonic airplane fly or a wing work is the air's viscosity. Now, figuring in the viscosity we see how the propeller will drag the air along creating a spiral slipstream. The rotational part of the air's change in momentum consumes power but does not contribute to thrust. So, a slower rotating prop will lose less to rotational losses.
Every prop has some optimum, so expression that larger prop is more efficient is someow "not so accurate" :- ))) it could be better ans also worse, depending on conditions.
My example shows you fact that since we fly our models with relatively large props prepared to make lot of thrust when necessary (when slowed), so somewhere at very high advance ratio, means on descending part in attached picture, larger prop simply works with worse efficiency becuase it will push it to even higher advance ratio.
The failing that we often have with this discussion is that people are used to assuming that "efficient" = "better" when in stunt it's arguably the opposite.
I gave up trying to have sensible discussions about power, thrust, and props on SSW in about 2003! It's impossible because there are so many ingrained notions about it, and nobody likes the actual definition of power.
Brett
He he, I hear ya.
a propeller's Power Coefficient, Thrust Coefficient, Speed-Power Coefficient and Activity Factor are sometimes hard things to grasp, but essential to understanding and predicting propeller performance. When you get into dimensionless numbers with the 5th root...takes a few beers to absorb it all. That, and the formulas fall apart at low velocities, because they predict infinite thrust when the velocity is zero. We know that ain't true.
Actually, the Speed-Power Coefficient is the one that can predict how diameter affects performance, but the math is probably beyond the nerd-o-meter quotient for a forum such as this so I've stayed away from it and tried to simplify it and talk in terms of force, which Newton showed us is the time rate change of momentum.
If anyone is really interested though, a great resource is the Hamilton Standard Rept. PDB 6101A, Generalized Method of Propeller Performance Estimation
There's a famous chart within it of Power Coefficient as a function of Advance Ratio that ties it all together. Genius.
It's from the 60's and my well-worn original copy was handed down to me by a mentor. One thing to note: when using it you need to take a step back and reduce efficiency for a model's prop by about 20 percent because of the thickness of our props compared to full-size aircraft. That, and we use fixed-pitch props which makes it a little more complicated. Not overly so, but easy to forget sometimes as you bash the equations.
Thankfully, we design jets and fans these day, making thrust calcs a lot easier! y1
The problem we always seem to run into is that even if we knew the performance and could predict it perfectly in every regard in an engineering sense, no one has ever defined why one prop is bette" than another in terms of getting better stunt scores. Plenty of obvious potential measures of goodness are demonstrably not the correct ones, but that doesn't seem to change anybody's mind. Static thrust and diameter being the two most obvious candidates.
BTW, Chuck I was not particularly referring to you above! Just a general comment on this sort of thread.
Brett
Wing root chord | 11.00 |
Wing tip chord | 6.00 |
Wingspan (in.) | 39.50 |
MAC (in.) | 8.50 |
Area (sq. in.) | 335.75 |
A/R | 4.65 |
Stab root chord | 2 7/8 |
Stab tip chord | 1 1/2 |
Elevator root chord | 2 1/4 |
Elevator tip chord | 1 1/4 |
Span | 15.2500 |
Area | 60.0469 |
Elevator/Stab % | 44% |
Stab/Wing % | 18% |
Spinner backplate to LE | 7 3/16 |
Cylinder centerline to LE | 5 5/8 |
Wing TE to Stab LE | 9 5/8 |
Motor weight (oz.) | 6.7 |
Tails needn’t be as large as for flapped ships since they don’t need to overcome the negative pitching moment of the cambered wing (deflected flaps) in maneuvers. Fifteen percent of the wing’s area is probably a minimum and anything over 20% probably more than necessary. Either will provide adequate stability margin for an easy handling ship. I think the current practice of low aspect ratio tails for stability in turns is probably still valid. I’d start with about 4.5 to one.
Modestly higher aspect ratios (for the wing) are probably desirable, say 5.2 to 5.5 to one. The greater the aspect ratio (span/average chord) the less the angle of attack necessary for a given lift requirement. This would mean; 1. a smaller wing area is necessary for a given weight, and; 2. the reduced necessary angle of attack required in hard corners would result in less of the theoretical need for body angles in excess of the track change desired, i.e. it wouldn’t seem to have turned “inside” the desired track.
The wing should be tapered so as to approximate the “ideal” elliptical lift distribution. A simple taper should suffice with probably a minimum of a 75% tip/root ratio and a minimum of 65-70%. Chords at the tip of less than nine inches or so are likely to be less effective than they should be. I’m guessing that the taper should be distributed between the leading and trailing edges so as to result in a quarter chord line perpendicular to the fuse. I’m really open on this and would be interested in divergent points of view. I doubt very much that a swept forward quarter chord is desirable but a slightly swept one could be stabilizing in roll.
... a sharp trailing edge...
... and nobody likes the actual definition of power.
The failing that we often have with this discussion is that people are used to assuming that "efficient" = "better" when in stunt it's arguably the opposite.
I gave up trying to have sensible discussions about power, thrust, and props on SSW in about 2003! It's impossible because there are so many ingrained notions about it, and nobody likes the actual definition of power.
Brett