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Oct 25, 2014, 12:29 PM
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Wright Stuff Planes for 2015 - Science Olympiad

The Wright Stuff task in Science Olympiad flies a more challenging indoor free flight plane than usual. I've posted the construction rules on another thread

but I thought I'd start one that's easier to find, since it isn't mis-named F1D.

These 50 cm (nearly 20 inch) span planes weigh at least 8 grams without rubber and are powered by at most 2 grams of rubber with a 240 mm (9.5 inch) propeller.

One challenge this year is getting enough lift out of the wing and stab at slow speed to take advantage of a big slower turning prop.

Bruce Matthews over on the Chuck gliders thread was discussing neutral point calculations, which are important with a big stab. The calculations can be simple if you assume that the lift coefficients of the stab and wing are the same. For tandem wings of the same size and shape, both at the same incidence, the obvious aerodynamic center for the setup is half way between the centers for each wing.

If you locate the reference point in your moment calculation at the aerodynamic center for the wing, then things are simpler. You have a moment from the stab that is L X Area(Stab) where L is the distance between the stab and wing centers. The distance back of the wing center to the center of the plane is then LX Area(Stab)/(Area (Stab) + Area (Wing)). The formula came up in the Chuck gliders thread, and this is where it comes from, Of course, no lifting fuselages are in the picture. If you use a lot of negative incidence in the stab you throw away a lot of its lift, especially in a shallow climb.

There is a fine article on "Pitch Stability in Indoor Models" by Steve Gardner in INAV #93 which also can be found in The Best of INAV at p. 115. It has lift coefficient charts for various situations and hints for picking the best incidence settings to get efficiency without losing stability.

Look up the setup for Cezar Banks Novice Penny plane in INAV 137 to see a plane with a 42 % stab and target CG at 70% of wing chord.
Last edited by DuPageJoe; Oct 25, 2014 at 01:37 PM. Reason: Add reference
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Oct 25, 2014, 01:39 PM
B for Bruce
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Joe, the thing is that with a strongly rearward CG such as allowed for by using a very large stabilizer there simply isn't any way to use a strongly negative angled rear surface. It would result in a stalling flight trim if you tried it. So the final front to rear surface angle is largely set during the trim flights when we alter the angles to obtain our nice slow cruise flight trim for a given CG location.

There's a bit of history to the method for finding the NP and how to use this information. Back when I was designing, building and flying my own indoor designs in the early to mid 1970's the method was to use the usual calculations and set up for a 0% stability. That's right, they were saying to set the CG right at the NP. What provided the pitch stability was then the high drag couple of the post mounted wing.

These days there is apparently a spreadsheet page out there which factors in the drag of the high mounted wing and the resulting lower position of the CG relative to the wing and gives us a more accurate ability to set the CG location. It showed up in a link in one of the previous threads here. Some searching should turn it up. Once found it should work well for the usual large stabilizers even to the point of becoming a tandem wing style.

More lifting area for a lighter overall wing loading is always a good thing on an indoor model. So if the rules allow for it the key is to build light and build as much lifting surface area as possible while still keeping the weight down to the minimum required. But the surface areas for the Wright Stuff models are totally defined. So all that can be done to load the stabilizer more is to open up the tail moment to as long as possible/practical. So there would be some advantage to making the model lighter than the rule minimum and adding ballast at the nose so that the main wing can be further forward. The resulting longer tail moment would allow the NP to be further back and that means the CG can be further back and the tail is then loaded more positively. Of course it's possible to go too far with such a plan all too easily. The model structure still needs to be stiff enough to avoid inflight deformation that alters the angular setup. For example with the wing further forward on the motor stick one has to take into account the bowing of the motor stick and the effect it will have on the wing to tail angular setup. But if done right the flex could result in a reduced decalage angle for a more controlled climbout instead of a rocket like climb.

Speaking of controlled flex I don't see any minimum sizes for the wood sizes. So there's some serious gains to be had there. A prop design that flexes to flare in a higher pitch angle under maximum torque is another way of both controlling the climb as well as extending the power burst over a longer time.

It also allows for deeper and wider wing spars near the root that taper out to small sizes at the tips to again centralize the rigidity of the surfaces to where it's needed.

I know that this is an event for students. But within those rules I wonder how well a serious long time indoor flyer could make such a model perform?
Nov 03, 2014, 10:23 PM
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Bucket Props for Wright Stuff 2015

The prop diameter of 240 mm maximum is larger than for previous Wright Stuff versions. Plastic props for outdoor free flight planes are nearly as heavy as the whole Wright Stuff plane at a 240 mm size. One solution would be to thin the prop blades but this can be tedious. Making props with formed balsa sheet blades needs a bit of practice and someone who knows how to "cook" them in an oven.

Plastic beverage cups and deli tubs can be a source of thin, already curved plastic to give light prop blades. I have built 14 gram cabin planes called Bostonians with 6 inch props where the blades came from 2 Liter soda bottles, that fly just fine. I found the original pattern in a plan for the "Bostonian Celtic" by Don DeLoach.

I came across a decent source of 0.4 mm (0.016 inch) thick polypropylene at a neighborhood grocery that sells freshly cored pineapple in a 32 ounce tub. The tubs come from E. Hofmann Plastics in Ontario and weigh 19 grams in a 32 0unce size. There is a conical section of the tub which has a slant height of 4.4 inches, a top diameter of 4.0 inches and a bottom diameter of 3.5 inches that should be OK for making prop blades nearly 4 inches long. A blade weighing 1 gram would have an estimated area of 27 sq cm (4.3 sq in) if made from this plastic tub at 0.9 g/cc polypropylene.

There is a design program in Excel form available for figuring out where to put the blade patterns on the cup to get a prop with nearly helical pitch for good efficiency. Fred Rash has posted it on the files section of Yahoo's Group on Indoor Free Flight. (Yes. there are enough indoor free flighters for a Yahoo Group)

I put the bucket dimensions in the spreadsheet and fiddled around until I got an optimum angle of 21 degrees from the vertical to place the pattern. I have attached a picture of the 32 oz tub with 2 blades drawn on it. I'm not sure whether the picture wasn't taken with the blades on the other side of the tub from the camera. The target prop has a 9.5 inch diameter and 16 inch pitch, and each blade is 3.75 inches long. The prop blade angle to the thrust line is 45 degrees at a radius of 2.50 inches. 2.25 inches from the tip.

The other picture shows two blades weighing a total of 1.7 grams made from this tub. I will mount them in slotted wood hubs and put those hubs into a rigid piece of tubing. The pitch is adjustable on the ground, not after launch.
Nov 09, 2014, 12:07 AM
The thin "crispy" foam cups available at many convenience stores can be used for indoor prop blades also. I've used them with 2 strand 1/8" rubber motors.
Nov 10, 2014, 11:33 PM
Indoor Free Flight Modeler
I just drew up my plan for my 2015 SO model. Here is the general layout.

Nov 11, 2014, 10:00 AM
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Your design looks like a good straight forward build to the rules. I see that turning circle is controlled by stab tilt and left pointing thrustline. I was wondering why you didn't put any other auto turn features that are in the Markos' Double Whammy, like offset tail boom and offset wing center line. That long tail boom looks like a way to get the stab to share the load, as Bruce discussed earlier. It wouldn't take much offset to build in some turn.

The picture shows my latest Double Whammy with flat plate wing and Delta Dart prop. It weighs 7 grams. It flew 47 seconds at a Cat 1 site with about 1100 turns on a 14 inch loop of rubber weighing 1.7 grams. it circles left, roughly 30 ft diameter, right off the building board. The wing is bigger than the one for new Wright Stuff plane but has no camber. A decent prop would certainly add to the time.
Nov 11, 2014, 12:43 PM
Indoor Free Flight Modeler
It will have about 1/8" tail boom offset. Just did not show it in the CAD screenshot. The wing and stab have offset in them. I printed my plans out last night so will get started shorty cutting wood. I may re-release my SO building CD. Have not sold it in 10 years but the 2015 rules are similar to the B division back around 2004.
Nov 11, 2014, 10:23 PM
Indoor Free Flight Modeler
Started the wing on my model.
Nov 11, 2014, 11:59 PM
Indoor Free Flight Modeler
Wing and stab are all done, weight is 1.61 grams.
Nov 12, 2014, 01:29 PM
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That 1.6 g build weight for all the surfaces of your SO plane looks pretty good for about 100 sq inches. Are you going to use tissue, at about 0.9 g/100 sq inches, or produce bag plastic, at about 0.5 g/ 100 sq inches, to cover it? I found the offsets in your plans.

My Double Whammy is not quite 90 square inches total surface area, and uses nearly 100 lineal inches of 1/16 inch square stock (10 lb/cu ft roughly) to build the surfaces. They weighed 1.2 grams before covering, and 2.3 grams after adding preshrunk Japanese tissue with water based contact cement. The heaviest part of the Whammy was that Delta Dart prop and bearing at 2.8 grams. Building to 8 grams minimum shouldn't be too tough.
Last edited by DuPageJoe; Nov 12, 2014 at 01:35 PM.
Nov 13, 2014, 12:13 AM
Indoor Free Flight Modeler
I covered with Esaki tissue. All parts covered are 2.6 grams. The motor stick blank I cutout is 1.64 and the boom is .65 grams. So 8 grams will be easy. Here are some photos.
Nov 14, 2014, 11:09 PM
Indoor Free Flight Modeler
Made a thrust bearing and rear hook and glued on the tailboom. So far just under 5grams with no prop. Prop will be about 1.2 grams so will need about 1.8 gr ballast.
Nov 18, 2014, 01:30 PM
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Bucket Prop test

Maybe you don't need ballast, if you just go with a more robust prop. Are you making one with balsa blades?

I tested the first bucket prop I made close to SO Spec. This is the one from the "needs help with F1D" thread. It weighs 3.1 grams with a hanger from a Delta Dart and a 1/32 wire hook. It measures 250 mm diameter which is easy to trim to 240. It was a little slow on .080 wide rubber, so I tried 1/8 inch. At 850 turns on a 10.3 inch loop, weighing 1.8 grams, I got 200 seconds run time. Average rpm was about 250. I'll need to mount it on a Double Whammy to see if it has enough thrust for climb at 8 gram airframe weight.
Last edited by DuPageJoe; Nov 18, 2014 at 02:26 PM.
Nov 18, 2014, 09:49 PM
Indoor Free Flight Modeler
Just finished the model. My weight is 6.02 grams with a wood prop so I need 2 grams of ballast. Here is how it looks.
Nov 21, 2014, 11:04 AM
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Prop tests

That's a nice looking prop. What size rubber does it take to turn it fast enough to fly the plane?

I took the Double Whammy to the gym last night and mounted the bucket prop from post #13 on it. I didn't get a whole lot of climb with a 10 inch loop of 1/8 rubber wound to a bit more than 800 turns. Looks like I'll need to narrow the blades some to be able to use 1/8 rubber, and the higher rpm will shorten the motor run.

I had better luck with the 225 mm diameter Ikara prop in the picture. I got a 69 second flight with climb to the 20 foot ceiling with about 800 turns in the same 10 inch loop of 1/8 inch rubber. However the motor still had 50 seconds' run left when the plane hit the floor. The Whammy was pretty heavy and draggy, so a better plane might use more of the turns. I tried the Ikara prop with a 14 inch loop of 3/32 and there wasn't enough thrust, even at 1500 turns, to get much climb.

I have flown the same Double Whammy with the 5.25 inch Delta dart Prop it was designed for, and it will climb well on 1200 turns in a 14 inch loop of 0.080 inch rubber, for a 50 second flight in the same gym. Motor run is just over a minute, so I'm still looking for something better.

Maybe a 200 mm diameter bucket prop with smaller blades, sized to run 2 minutes on a 10 inch loop of 1/8?

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