Straight and Level Wing (Slow Rat Content)

[Motorman]

I'm posting this here because I wanted some advise about aerodynamics.

Standard plane. My Johnny Reb left over from the old 21 Nova Rossi big Goodyear days. Flew very well and won a few races before the competition got the GO25 engines working. Fitted here with GO25 but had mushed out motor mounts and big draggy fuselage with two wheel gear so, time to build a dedicated slow rat plane.




1) Somebody once told me if you're only going to fly straight and level then make the high point 33% on the top and 25% on the bottom and it will go faster (less drag). How you figure that works?

2) A fully symmetrical airfoil will have to fly at a positive angle of attack to provide lift. Depending on wing loading you would want X amount more hump above the chord line to fly at zero angle of attack, true or false?

3) My event has a 1" minimum wing thickness rule and 300 Sq. In. minimum rule. To get around the frontal area drag I made a wing with almost 1:1 aspect ratio and the above mentioned features see pic. How do you think it worked?

The Lawn Dart


MM

    

[Paul Smith]

I have never liked the ultra-low aspect ratio.  Among other things, the tip weight has almost no leverage, so you need A LOT more.  Leadout guides are not far enough inboard for good handling.

Speed classes like F2A use very high aspect ratio with good results.  I see it their way.  

[bob whitney]

the problem with H/A is that it takes too long to slow down where a shorter wing will fall out of the sky ,assuming that u are using a shut off

[Motorman]

Hmm thought this thread would get more mileage. Anyway the stubby winged plane is slow because it plows through the air nose up.

At first I thought the chord was to long for the thickness so it was acting like a flat plate. Now I realize too much lift pressure is slipping off the tips so there's not enough left to lift the plane. It has more frontal area drag and induced drag than the high aspect ratio wing ever could.

So Bobby, what do you think the prefect aspect ratio is for Slow rat? My guess is it might look something close to a flying clown.


MM  

[Kevin Ferguson]

Hopefully this won't turn into a Bernoulli etc. pissing match, but a wing generates lift by imparting downward momentum (mass x velocity) to the air.

  For a given lift force, a long span accelerates a large mass to a modest velocity, and a short span accelerates a modest mass to a proportionally higher velocity, so mass (per time) x velocity is the same.

The power required is not M x V, but M x V _squared_, though, so long span needs less power, or you can go faster on the available power.

But at high speeds, there is less advantage to the long span, because the speed gives a shorter span a higher mass flow to act upon.  Also, if the rules require a minimum thickness, then a short chord makes for a fatter wing, percentage wise, which is not great for speed.

There is also a weight penalty for longer spans that will survive at high speeds, and that weight has to be supported with drag inducing lift.

Finally, at model speeds and scale, a wider chord can yield  a more favorable Reynolds number, so a model wants lower AR wings than full scale aircraft.

So there are trade offs, and these change with speed, and speed changes depending on how you make the trade offs.

So it is not easy to determine the optimum, but it is not super high or super low AR.  Look at the fast designs to get an idea what works well.

I agree that a fully symmetric airfoil seems silly on a racer, especially if there is a minimum thickness requirement.  I have no idea what the optimal airfoil looks like except that it will make a gentle transition to a 10  degree (+/-) trailing edge.  This is near optimal for pressure recovery.

[Motorman]

Why does a short span/ long chord accelerate the air to a higher velocity when the wing thickness is the same as the long span/ short chord?

Wouldn't the wing thickness have to be thicker to make the same lift as the long span via the Bernoulli principle? But it's the same thickness only shorter span so, less hump equals less lift. That's one reason it flys with more AOA right?

I'll calculate on that 10 degrees with a 33% high point and see what aspect ratio that gives me.

MM

[Andrew Tinsley]

I don't know the answers to the questions. But intuitively one would agree that a symmetrical section must have a higher angle of attack than say a flat bottomed section and hence the drag should be lower. It is a question that I have often mulled over, but never tried in practice.
There was a German design of speed model that had a relatively low aspect ratio and a large tail area. This would be in the late 60s time frame. There were supposedly good reasons for the low aspect ratio and this was to do with drag reduction. The theory was supported by some real wind tunnel testing, which supposedly confirmed the drag reduction. The model was called the Sluppi if I can remember correctly.
I intend to do some vintage speed and I was thinking of trying this model as a relatively easy flier, for someone that has not done speed for 50 years. The A/R was probably 3.5 to 1, so nothing as extreme as the plane cited above.

Andrew.

[Paul Smith]

I have never considered the 1" thinkness to be a significant limitation.  Other than solid carbon fibre, it's next to impossible to make a 300 square inch wing any thinner and still have enough strength and rigidity.

40" is the optimum wingspan, based on 36" stock with 2" tips.

[Kevin Ferguson]

Why does a short span/ long chord accelerate the air to a higher velocity when the wing thickness is the same as the long span/ short chord?

Wouldn't the wing thickness have to be thicker to make the same lift as the long span via the Bernoulli principle? But it's the same thickness only shorter span so, less hump equals less lift. That's one reason it flys with more AOA right?

Yes, it needs more AOA to produce the lift.   

You might want to explore beyond Bernoulli's law in your understanding of how wings produce lift.  Much of what has been published is misleading at best, and often totally wrong, even FAA study materials and test questions.

[bob whitney]

to me ,the Tex group (Bill Lee and the Biscuits & Gravy team) have about the best all around slow rat ,they post on here ,and will probably come back with the measurements. i think i have a small 3 view i can copy and email u if u like

[Motorman]

Bob, is that the one with 26" span 15" root and 8" tip? That wing would fit nicely into the Lawn Darts fuselage although I try not to copy anyone too much.

Paul, I hear 'ya. Funny thing the 10 degree trailing edge with 33% high point works out to 8.25 chord which puts the span at 40" for 300 squares. So, that looks like the highest you should go on aspect ratio. Works out perfect for 36" balsa.

Anyway, the stab got completely blanked out somehow and there's no way I could fly it in a crowd. Here's Lawn Dart first mod. Span equals 24".



MM

[BillLee]

Bob, is that the one with 26" span 15" root and 8" tip?

Most all of the Texas Slow Rat and SSR models are 30"+/- span with appropriate chord to get 300 sq.in. Some are 10" constant chord, some are tapered with an average 10" chord.

Bill

[Bob Heywood]

So far no one has mentioned the lines. Line drag is the major drag contributor in C/L. The high aspect ratio F2A models are all about reducing line drag.

I am not a classically trained aero engineer but have listened to those who are and the one thing that keeps popping up in discussions is the Reynolds Number. Our models operate at low Reynolds Numbers which, if I understand correctly, make classical aero calculations a bit fuzzy.

A former B Proto flyer, Carl China ( Viper ) was a proponent of low aspect ration wings in order to help increase the Re No. So was Tom Cassutt of Cassutt Racer fame. But when all is said and done I would go with a wing that covers up more of the lines.

I have been looking at the Count Clipper wing and at the NACA 63-010A airfoil. Both are promising. Symmetrical airfoils work for me. So little AOA is required to generate the lift needed that there is negligible out of trim drag needed to maintain level flight.

The other thing is line rake. Racers, especially profiles, need to be trimmed right to the ragged edge on rake. You do not want to fly around the circle in a skid or a slip, dragging the fuselage sideways. You need just enough margin to transition from launch to race speed and not turn in due to the combination of line mass inertia and drag build up. The pilot needs to be on his tippy toes.

[Motorman]

The Count Clipper is a cool plane. I always had trouble getting the lead outs far enough forward on wings with that much LE taper but they look so good.

My next wing will have adjustable lead outs. After flying electric stunt, that has the same thrust every time, and seeing a significant speed difference with small changes in line rake it's worth the extra trouble.

I think F2a and slow rat are apples and oranges. F2a wing is 1/8 at the tip and there's no wing on the other side of the plane. I wonder how long they would make the wing if they had to make it 3/8ths thick and double it up on the faster side of the plane.

Looks like a 10" chord works out perfect for NACA 63-010A.

MM

[Bob Heywood]

I think F2a and slow rat are apples and oranges. F2a wing is 1/8 at the tip and there's no wing on the other side of the plane. I wonder how long they would make the wing if they had to make it 3/8ths thick and double it up on the faster side of the plane.

MM

I used an F2A only as a reference. Using the wing, even on a slow rat design, to cover some of the lines is advantageous. The thing is, with the 1" thickness rule you can't have a too narrow chord or you end up with a tadpole airfoil. I guess, as in all things and as noted earlier, moderation is good.

[Bob Heywood]

One more thing to consider: The Balance Point. In aero terms that involves the Static Margin along with the Tail Volume Ratio. Most C/L racers are too nose heavy. This then requires up trim to maintain level flight, which becomes out of trim drag. The late Bill Falck had the CG moved aft on Rivets to the extent that the plane had to be flown at all times, sort of like trying to balance a cone on its point. But, it was fast.

The important thing is that the plane is rigged to fly as efficiently as possible.

[phil c]

the main reason for a symmetrical, or nearly symmetrical, on a racer is to reduce climbs and dives due to wind.  A semi-symmetrical airfoil makes the wing's response to up wind and down wind more exaggerated and harder to fly.  For you X factor I'd guess making the upper half50.25% of the thickness and the lower half 49.75% of thickness.  Half a percent camber, you can place the high points anywhere you want.  Even a 300 sq.in. wing doesn't have to work very hard to lift 2lb.

Frontal area of the wing accounts for about 1% of the profile drag of the wing.  A profile fuselage probably accounts for twice as much drag.  Depending on speed, the lines account for ~25% to almost all the drag.  Most speed planes have very long span inner wings because a good wing has less drag than the same length of 2 lines.  So try an assymetric wing.  The outboard tip could be the fuel tank on the side of the fuselage.

A double tapered wing will have less drag than a straight wing, especially when passing.  Figure a 8.5-9 in. tip and a root chord to give 302 sq. in.  I believe the Nashville slow rat used a 9in. tip and an 11in. root, 1.032 thick along the whole span and squared off tips.  they rarely got beaten.

Regarding the yellow plane- Fair in the landing gear to a smooth symmetrical section.  Round wire is the worst.  Put the control inside and run the pushrod out the back end.  Redesign the tank to fit completely behind the engine and extend the tail a couple inches, and/or use plywood for the stabilzer and elevator.  The rules appear to allow anything for "cheek cowels" so fair the engine into the tank.

Practice pit stops endlessly.  Buy, build, or borrow the best engine you can find.

Local rules may vary, but the basics don't change.

[frank williams]

The attached photo is of a slow rat from days past.
The design goal was to take advantage of the high Reynolds number capability of a slow rat, and the low drag  possible with some laminar flow airfoils.  A slow rat with a generous chord could get a Re  in the neighborhood of 10**6.  The NACA 63-209 airfoil has a nice "drag bucket".  The bucket is slightly shifted to the right, Cl at zero alpha is about 0.2.  The "shift to the right" means that as you climb to pass (shift to higher alpha), the airfoil drag doesn't increase.

[Motorman]

Great day racing with the Middlesex modelers. Turned a 3:10 heat with the tapered wing plane and GO25, lead outs were way too far back. Later in practice I got into the 14's with the straight wing plane and OS 25XZ but it goes flat rich mid tank. Also swapped engines and won the new Heavy Metal event with a Nitro Star SS25. Can't get anymore than 19 sec out of that cheap truggy motor. MM

[Jim Thomerson]

I built a Goodyear racer with a bit thicker wing, flat center section with a hair of positive incidence, transition out to tips with symmetrical airfoil at 0 degrees. I had seen the typical thin pointy wing try to stall out when passing.  Mine passed just fine.  I did the same with a slow rat and concluded that it had enough wing area that an all symmetrical wing at 0 degrees was the best.