Oct 25, 2014, 12:29 PM
Registered User
Discussion

# 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

https://www.rcgroups.com/forums/show...1&postcount=29

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
Nov 03, 2014, 10:23 PM
Registered User

# 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)

https://groups.yahoo.com/neo/groups/...0Props%202013/

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.

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 Nov 09, 2014, 12:07 AM SlingWinger 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.

Don

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Nov 11, 2014, 10:00 AM
Registered User
Don
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.

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 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.

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Nov 11, 2014, 11:59 PM
Indoor Free Flight Modeler
Wing and stab are all done, weight is 1.61 grams.

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 Nov 12, 2014, 01:29 PM Registered User Thread OP Don 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.

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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.

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Nov 18, 2014, 01:30 PM
Registered User

# Bucket Prop test

Don
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.

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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.

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Nov 21, 2014, 11:04 AM
Registered User

# Prop tests

Don
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?