Instructions for building plane without kit?

[bjt4888]

Kyle,

Good job supplying the stick and tailboom length and stab dimensions. I'm assuming that your wing dimensions are close to the maximum per the rules of 45 cm x 11 cm.

Another question:

When you state that you are "placing the center of gravity at 1/2 inch above the trailing edge", what do you mean? Is this where you hope to locate the center of gravity, or is this where you are actually measuring the center of gravity to be? Usually, the reference to the center of gravity would refer to a distance in inches or cm forward of the trailing edge or behind the trailing edge as measured along the motor stick (not above or below). Center of gravity on an airplane refers to the balance point fore and aft and is measured by hooking a sample 1.5 gram motor onto the prop hook and the rear hook and letting it just hang there, then flip the airplane over (wing facing down) and use a straight edge, like a ruler balance the motor stick on the ruler and mark the balance point and measure the distance from the trailing edge.

Do this measurement and reply.

Brian T.

[Kyle_Guo]

Kyle,

Good job supplying the stick and tailboom length and stab dimensions. I'm assuming that your wing dimensions are close to the maximum per the rules of 45 cm x 11 cm.

Another question:

When you state that you are "placing the center of gravity at 1/2 inch above the trailing edge", what do you mean? Is this where you hope to locate the center of gravity, or is this where you are actually measuring the center of gravity to be? Usually, the reference to the center of gravity would refer to a distance in inches or cm forward of the trailing edge or behind the trailing edge as measured along the motor stick (not above or below). Center of gravity on an airplane refers to the balance point fore and aft and is measured by hooking a sample 1.5 gram motor onto the prop hook and the rear hook and letting it just hang there, then flip the airplane over (wing facing down) and use a straight edge, like a ruler balance the motor stick on the ruler and mark the balance point and measure the distance from the trailing edge.

Do this measurement and reply.

Brian T.

Sorry, I was holding the plane vertically so I said above instead of forward. Im hoping to place the center of gravity 1/2" forward of the trailing edge. The current center of gravity (2 1/2" nose length and max size wing) is 1/16" forward of the trailing edge

[Kyle_Guo]

Also here is another picture of the plane:

Link (shared to anyone on the internet): https://drive.google.com/file/d/0Bwl5kC ... sp=sharing

[bjt4888]

Kyle,

The Neutral Point (NP), which is the aerodynamic center, of your airplane is at about 0.6" forward of the trailing edge with your current 2.5" nose. In order to have pitch stability (pitch refers to nose up/nose down movement) the CG must be forward of the NP. For this year's design, with the wide chord wing relative to the wing span, the CG should be between 10% - 20% of the wing chord distance forward of the NP. This would place the CG at about 1.0 " forward of the wing TE as measured with the 1.5 gram motor installed on the airplane. You will need to add all of your ballast to the nose of the airplane and probably move the wing back till you have a nose length of about 5.5" in order to have a workable static stability margin of about 8.5%.

This may sound a little strange, but the reason for the long nose length is that your motor stick is a quite a bit heavier than ideal. In the post earlier in this forum, where I suggested part weights and densities and a 2.5" nose length, you will note that I suggested a 14" x 3/16" x 3/8" motor stick that weighs 2.0 grams. All of the part weights and densities would have to exactly match my example for the 2.5" nose to be correct. Also you will note that my part weights build an airplane that is quite a bit underweight, which allows for quite a bit of ballast clay to be added to the nose. The need for this is partly that the propellers for this year's rules are so small and light and partly so that the tail moment arm can be kept nice and long to allow for reduced incidenc, lower drag trim. Your airplane will fly fine with the long nose though. It will probably have to fly with a little bit more incidence that my sample as the tail moment arm is quite a bit shorter. With a 5.5" nose, your airplane's NP will be at about 1.1" forward of the TE and the CG will be at about 1.45" forward of the TE.

I would suggest about 3.3 degrees of incidence as an initial trim setting, which means the leading edge to fuselage distance is 1/4" more than the trailing edge to fuselage distance (leading edge 1/4" higher than TE) and zero degrees incidence for the stabilizer for a total decalage angle of 3.3 degrees. The airplane will probably fly with a slight and repeating stall with this wing incidence angle. If this is so (test with few winds, like 500 -700 initially, and then full winds with backoff to about 0.30 in oz torque), all you will need to do to finish trimming is maybe reduce the wing angle about 1/32" of 1/16"

Brian T.

[Kyle_Guo]

Kyle,

The Neutral Point (NP), which is the aerodynamic center, of your airplane is at about 0.6" forward of the trailing edge with your current 2.5" nose. In order to have pitch stability (pitch refers to nose up/nose down movement) the CG must be forward of the NP. For this year's design, with the wide chord wing relative to the wing span, the CG should be between 10% - 20% of the wing chord distance forward of the NP. This would place the CG at about 1.0 " forward of the wing TE as measured with the 1.5 gram motor installed on the airplane. You will need to add all of your ballast to the nose of the airplane and probably move the wing back till you have a nose length of about 5.5" in order to have a workable static stability margin of about 8.5%.

This may sound a little strange, but the reason for the long nose length is that your motor stick is a quite a bit heavier than ideal. In the post earlier in this forum, where I suggested part weights and densities and a 2.5" nose length, you will note that I suggested a 14" x 3/16" x 3/8" motor stick that weighs 2.0 grams. All of the part weights and densities would have to exactly match my example for the 2.5" nose to be correct. Also you will note that my part weights build an airplane that is quite a bit underweight, which allows for quite a bit of ballast clay to be added to the nose. The need for this is partly that the propellers for this year's rules are so small and light and partly so that the tail moment arm can be kept nice and long to allow for reduced incidence, lower drag trim. Your airplane will fly fine with the long nose though. It will probably have to fly with a little bit more incidence that my sample as the tail moment arm is quite a bit shorter. With a 5.5" nose, your airplane's NP will be at about 1.1" forward of the TE and the CG will be at about 1.45" forward of the TE.

I would suggest about 3.3 degrees of incidence as an initial trim setting, which means the leading edge to fuselage distance is 1/4" more than the trailing edge to fuselage distance (leading edge 1/4" higher than TE) and zero degrees incidence for the stabilizer for a total decalage angle of 3.3 degrees. The airplane will probably fly with a slight and repeating stall with this wing incidence angle. If this is so (test with few winds, like 500 -700 initially, and then full winds with backoff to about 0.30 in oz torque), all you will need to do to finish trimming is maybe reduce the wing angle about 1/32" of 1/16"

Brian T.

Strange, I followed your recommendations with the 5.5" nose and the CG was 2 1/4" forward of the trailing edge ( I haven't added my clay to the tip yet, is that the problem?)
Also, I can extend the MS as it is 2 pieces of balsa that are held together at the ends with cement. I could un-bond those and extend the MS about 3 3/4" and move the wing up 3 3/4 to make the distace to the tail 3 3/4" larger. Will that help with making it fly with less incidence? Does lengthening the MS have the same effect as lengthening the tail boom?

[bjt4888]

Kyle,

When you are balancing, is the rubber motor installed?

Brian T

[Kyle_Guo]

Yes, Maybe the rubber is hanging differently each time. The CG is 2 1/2" front of the trailing edge without the rubber (with Prop) and 1 1/8" in front of the trailing edge without prop and rubber.

[bjt4888]

Kyle,

I should explain the NP and CG spreadsheet calculator a bit more. The spreadsheet has data entry cells for the stabilizer and tailboom as separately weighed items. Since you supplied the weight of the two together (no problem with this) I had to estimate each part's separate weight and enter it into the spreadsheet. Also, the spreadsheet wouldn't be able to adjust for a motor stick that might be slightly heavier on one end or the other.

Also, the calculator spreadsheet is originally designed for very light international class models and I do find that it calculates the CG slightly forward for heavy models like WS. I like to use the calculator even though it has this slight variance as a way to determine a good general range for the weights of the various parts for WS (which tells you what dimensions and wood densities might be best) and i like to build about 1.0 gram underweight and then use the clay ballast position to get the final CG correct.

Try attaching the clay to the nose of the airplane on the top of the motor stick and moving the wing forward about 0.5" and remeasure the CG. If it is not at 1.5" from the TE, repeat the process (move the wing forward again about 0.5" and measure CG again). Repeat this process until you have the CG at 1.5" forward of the TE.

Also, a bit of experimentation may be necessary for both the CG and the wing incidence. Some have found that the particular configuration of this year's WS model have made the NP calculators a little imperfect for determining ideal CG. My guess is that the combination of a very large wing with relatively highly cambered airfoil and a very small stab seems to make the airplane behave as if the NP were further back than the calculator predicts. This means that the ideal CG may also have to be further back than 1.5". Here is how you would determine this. If in your first test flight with full power (it's difficult to determine much during the initial, low power flights; remember full power means winding to 85% or 90% of maximum turns and torque and backing off to a safe launch torque to keep the airplane off of the ceiling) if the airplane does not gently stall all the way through the flight (especially during the descent) then either the CG is too far forward or the wing incidence and decalage angle are too great. Since the 3.3 degrees of wing incidence and decalage angle are almost certainly too much for this airplane, the best thing to test is the CG. So, again, if the airplane is not stalling, move the wing forward about 0.5", measure the change in CG and record in your flight log and fly it again. If still not stalling, move the wing forward 0.5" again and record CG in the log again. I would think that you would start stalling by this flight (if you weren't already stalling on the first flight). Once you have a plane that is gently stalling and have found a good working CG in this way, you can try removing a small amount of wing incidence (maybe about 1/16" lower LE) to see if this removes the stall. If this does it, you have a perfect setup. Mark the wing posts at the point they enter the tubes with something you will remember (like a red pen mark) and mark the final CG with a red pen.

Once the above trimming steps are completed, best possible flights will be achieved by experimenting with increased pitch on the propeller and varying motor thickness.

Let me know how it goes.

Brian T.

[bjt4888]

Kyle,

I should have wrote in paragraph, "...either the CG is too far forward or the the wing incidence and decalage angle are too low."

Brian T.

[bjt4888]

Kyle,

Sorry, I didn't want to give you lots of things to consider all at once, but i will be pretty busy for the rest of the day and wanted to comment on a couple of things that I think I noticed in the picture of your airplane. First of all, congratulations on constructing a very nice airplane! Please don't take these comments as criticism in any way. I just want to see if I can help.

I am guessing that the fuselage, tailboom and stabilizer on your airplane are from the Freedom Flight Kit. If this is so, did you attach the nose bearing hanger exactly per the directions? I tried to zoom your picture to get a good look at this attach point as it appears to be a little delicate. However, I could not zoom you picture without it going out of focus. It looks like your nose bearing is projecting forward of the motor stick a little. This joint needs to be very strong in order to take the torque force of the rubber motor and in order to withstand the airplane possibly bumping into walls and ceiling obstructions. If you want me to inspect this joint, take a close-up picture of it from the side and post in the wiki.

One other thing, it may be just a slight distortion in you picture due to the angle of the camera relative to the wing, but it appears that your wing tips are splayed out to the side slightly. Maybe the wing tip plate vertical components are not exactly 90 degrees to the wing spars. If this is so, and if the wing spar lengths are close to the 45 cm allowed by the rules, the wing span as measured at the top of the tip plates (tip plate to tip plate) may be greater than 45 cm. It may be the case that not all judges will catch this, but the proper way to measure a wing with tip plates (or dihedral,or really any wing) is from the wing tip to the wing tip. Double check that the wing span measured from tip of tip plate to tip of tip plate is not greater than 45 cm.

Also, per your previous question about changing the fuselage motor stick length in order to lengthen the tail moment arm. I wouldn't bother with this. Increasing tail moment arm length in order to allow reduced decalage angle and reduced drag is a very minimal factor compared to the other trimming items I mentioned in my last post.

Brian T.