## Wright Stuff B

bjt4888
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### Re: Wright Stuff B

Jgri,

Thanks for the data. I’ll run the numbers in the design spreadsheet shortly. Having done this exercise may times, I already know that I’ll be able to give you some direct input to correct your flight issues.

Brian T.

bjt4888
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### Re: Wright Stuff B

Jgri,

Here’s the result. The current configuration places the Neutral Point at .21” in front of the wing TE. With the CG at 4 cm behind the TE, the airplane has less than -40% static stability margin. A workable SSM would be positive 15% for this year’s rules. This configuration is highly unstable and will some times appear to look ok in the initial climb and will usually lose its nose up character with a sort of wallowing stall and will usually nose down into a shallow straight or rolling dive to the floor.

To correct, remove the tailboom and remove 6” from it and reattach carefully with the same zero degrees stab incidence, 2 degrees of left turn offset and 2 degrees of left turn stab tilt. Move the clay ballast all the way to the nose (or within about 1” of the nose) until the CG is hopefully about 1.8” in front of the wing TE. Total length of this airplane will be about 21” (13.75” motor stick and 6.25” tailboom). This is plenty long enough and is the needed length to get the CG in front of the neutral point.

This is the configuration we are using (along with the other trim settings I noted above) and we are getting a pretty easy 2:30 at about 26 ft. We could do more with a better, flaring propeller.

This setup places the neutral point at about 1.1” in front of the wing TE and the CG in front of the neutral point where it needs to be for stable flight. All of these calculations assume that your clay ballast is about .65 grams. If the clay ballast is much different from this, send me it’s weight.

Probably, you will be fine with 1/4” of wing incidence instead of 5/16”, but test fly first to see if it stalls before decreasing.

Lube the rubber, stretch it seven times its relaxed length and (if 19.5” of .037 g/in rubber) put in 2,800 turns to 0.8 in oz torque, back off 100 turns to a launch torque of 0.4 in oz and fly a nice long flight. Look at all of my historical posts on this forum for a description of sequencing test flights.

Brian T

bjt4888
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### Re: Wright Stuff B

7.25” tailboom after cutting off 6”, sorry.

bjt4888
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### Re: Wright Stuff B

Probably closer to .0355 g/in rubber.

jgrischow1
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### Re: Wright Stuff B

bjt4888 wrote:Probably closer to .0355 g/in rubber.
Thanks for all the info. A few questions:

-I am unfamiliar with dealing with rubber in terms of length and density (or whatever you call mass per length). I had thought you find the thickness that matches your prop and go as close to 1.5 g as possible. Am I mistaken? I am not even sure what thickness corresponds to .0355 g/in but I doubt our plane gets off the ground with something skinny enough that one can get 2800 winds out of it (of course that is with our current configuration).

-I am nervous about telling my kids about your recommendations (conceding of course that you have been doing this since 1972) and suggesting they radically alter their plane with only 3 1/2 weeks to go until States and limited gym time to test it. I am assuming you think it's worth it?

-Our state's height is about twice our own gym's. FFM recommends using a 1/2 weight/length spacer and getting proportional results for a double sized gym. Is that what you would do?

Once again, thanks for all the tips.

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### Re: Wright Stuff B

Jgri:

We almost exclusively test with 1/2 rubber due to gym size. You can learn a LOT in a shorter time, and don't risk hanging in the rafters. Two years ago at Nationals my HS team worked in a 20' gym near Nationals with half rubber. Their event flight altitude was within inches of the rafters, exactly twice the height we tested to, and the time was almost exactly twice.

I agree with Brian's assessment, though I have not run the spreadsheet. With the small stab, the CG has moved well ahead of the TE, 30-50mm depending on decalage. Another approach you can take (since Brian did not know the weight of the ballast) is move all of the ballast to the nose, then shorten the tail until the CG is in the range of 40-50mm in front of the TE.

We have also found that this year's plane is quite sensitive to decalage, especially at high torque launch. 1mm changes are large.

1-2 hours in the gym, if you systematically trim the plane after taking a big swing, should be good enough to start optimizing the rubber for your prop/plane. So, yes, it is worth it even this close to State.

Coach Chuck
Coach, Albuquerque Area Home Schoolers Flying Events
Nationals Results:
2016 C WS 8th place
2018 B WS 2nd place
2018 C Heli Champion
2019 B ELG 3rd place
2019 C WS Champion
AMA Results: 3 AAHS members qualify for US Jr Team in F1D, 4 new youth senior records

jgrischow1
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### Re: Wright Stuff B

coachchuckaahs wrote:Jgri:

We almost exclusively test with 1/2 rubber due to gym size. You can learn a LOT in a shorter time, and don't risk hanging in the rafters. Two years ago at Nationals my HS team worked in a 20' gym near Nationals with half rubber. Their event flight altitude was within inches of the rafters, exactly twice the height we tested to, and the time was almost exactly twice.

I agree with Brian's assessment, though I have not run the spreadsheet. With the small stab, the CG has moved well ahead of the TE, 30-50mm depending on decalage. Another approach you can take (since Brian did not know the weight of the ballast) is move all of the ballast to the nose, then shorten the tail until the CG is in the range of 40-50mm in front of the TE.

We have also found that this year's plane is quite sensitive to decalage, especially at high torque launch. 1mm changes are large.

1-2 hours in the gym, if you systematically trim the plane after taking a big swing, should be good enough to start optimizing the rubber for your prop/plane. So, yes, it is worth it even this close to State.

Coach Chuck
Thank you.

bjt4888
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### Re: Wright Stuff B

Jgri,

Good advice from Coach Chuck, as always.

If you want to send me the weight of the ballast, I can enter it in the spreadsheet and recalculate.

Working with rubber using mass per unit length is what all competition modelers do. It is really not possible to measure thickness of a rubber band and the rubber strip actually varies in width, thickness and density continuously along its length. The variation I am measuring from this year's batch (9/2017) is about 3.8%. If you don't have a rubber slitting tool, and you buy a set "width" from a vendor of, let's say, 0.055", what you will get is rubber that could vary from .0355 g/in to about .038 g/in. So, you can see from this that if you are treating all of this rubber as if it is the same, you will get unexplained variance in testing and will have trouble determining the best rubber density and even knowing if the next piece of rubber you cut is best or not best. Weigh everything you cut and select. Vendors need to sell by "width" as this is about the only way to produce and sell any volume.

You can back into an average "width" by knowing the average density of the stock 1/8" strip that most of us use when slitting rubber. This average is about 1 gram per foot, or .0833 g/in. Knowing this average would allow you to think of .0355 g/in as 0.05325" wide rubber. I didn't have much time to work on Wright Stuff this year as I am coaching four high school teams in the Helicopter event, but we did try rubber between .0355 g/in and .039 g/in (17.5" to 19.5" motors weighing 1.48 grams each with two black rubber O-rings that weigh .04 grams each). For our airplane and the same prop you are using, the best flights were on the .0372 g/in (approx. .056") 18.5" 1.48 gram motors. The thinner 19.5" motors were looking promising but we didn't get much testing time into them. Our best flights were 2,680 turns on the 18.5" motors with 70 backoff turns (2:30 at 26 ft). This motor should have taken 3,137 to break and the students should have been able to get in 2,800 turns (approx. 90% breaking turns), but their skills at winding are still in the developing stage.

Brian T.

jgrischow1
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### Re: Wright Stuff B

bjt4888 wrote:Jgri,

Good advice from Coach Chuck, as always.

If you want to send me the weight of the ballast, I can enter it in the spreadsheet and recalculate.

Working with rubber using mass per unit length is what all competition modelers do. It is really not possible to measure thickness of a rubber band and the rubber strip actually varies in width, thickness and density continuously along its length. The variation I am measuring from this year's batch (9/2017) is about 3.8%. If you don't have a rubber slitting tool, and you buy a set "width" from a vendor of, let's say, 0.055", what you will get is rubber that could vary from .0355 g/in to about .038 g/in. So, you can see from this that if you are treating all of this rubber as if it is the same, you will get unexplained variance in testing and will have trouble determining the best rubber density and even knowing if the next piece of rubber you cut is best or not best. Weigh everything you cut and select. Vendors need to sell by "width" as this is about the only way to produce and sell any volume.

You can back into an average "width" by knowing the average density of the stock 1/8" strip that most of us use when slitting rubber. This average is about 1 gram per foot, or .0833 g/in. Knowing this average would allow you to think of .0355 g/in as 0.05325" wide rubber. I didn't have much time to work on Wright Stuff this year as I am coaching four high school teams in the Helicopter event, but we did try rubber between .0355 g/in and .039 g/in (17.5" to 19.5" motors weighing 1.48 grams each with two black rubber O-rings that weigh .04 grams each). For our airplane and the same prop you are using, the best flights were on the .0372 g/in (approx. .056") 18.5" 1.48 gram motors. The thinner 19.5" motors were looking promising but we didn't get much testing time into them. Our best flights were 2,680 turns on the 18.5" motors with 70 backoff turns (2:30 at 26 ft). This motor should have taken 3,137 to break and the students should have been able to get in 2,800 turns (approx. 90% breaking turns), but their skills at winding are still in the developing stage.

Brian T.
Very interesting. I was unaware of this method of measuring rubber. Thanks for sharing. I have similar issues with my students' winding skills still needing some work.

The current clay weighs 1.10 grams. Of course, that plane weighed 7.10 grams as I said, and we just hacked off 6 inches of tail boom, as you recommended. So, the new clay will weigh a different amount. We have not had time to test with the new configuration yet, but will let you know what we find. Again, many thanks for all your help.

bjt4888
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### Re: Wright Stuff B

Jgri,

Did removing 6" from the tailboom and moving the clay ballast cause the CG to be about 1.75" - 1.875" forward of the wing TE? If so, after setting the wing incidence to about 1/4", and the other trim settings per my earlier post, and winding the rubber fully (and backing off enough turns to keep off the ceiling), hopefully you'll be seeing flights in the 2:00 to 2:30 range.

Good luck. Be sure to post any other questions you might have.

Brian T.

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