Design

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Re: Design

Post by torqueburner »

bernard wrote:How close should our team go to the 2 gram limit for them motor? I've heard some recommendations of making a 1.95 gram motor. How likely is it for a balance to measure it as more than 2 grams at the competition due to error?
As Jeff Anderson has stated previously, the more rubber you have, the more energy you can store. My students try to tie their motors as close to 2 grams as possible - 1.98 or 1.99 is the goal. The method for hitting this goal has been described previously. Cut the rubber, with o-rings, down to the target mass, and do the double overhand knot thing, with the magic part where you pull the loop to slide the know out the the very end . . .

However, you raise an interesting question about the balance at the competition. First of all, is it calibrated properly? Don't take this for granted. At a recent invitational we attended, there was a table with a balance for the students to mass their plane and motors before checking in with the judges. Our student adjusted her plane to 8.03 g, something like that, took it to the "official" balance, where it massed 7.94 g! The supervisor didn't want to allow her to adjust this, saying that she should have done it right with the other balance, but he relented after some discussion. We always take a 2 gram calibration mass with us to the competition in case this is an issue.

But another issue is the fact that some electronic balances can be adversely affected by static charge. We have a small digital balance that we carry in our flight box. You can put a rubber loop on it, and see just under 2 grams. Pick it up, rub it against your shirt or sweater, and wow, the mass has increased! I've seen as much as a tenth of a gram increase when we do this. Now you will obviously not be rubbing the motor on your shirt, nor will the ES. But this can be a problem, especially if you are making your motors very close to 2 grams.

One thing that can be done to reduce this problem is to elevate the motor above the balance with something like a length of cardboard tube from a roll of paper towels. Another thing that we do is to wrap our hand around the motor and squeeze down on the motor to ground it just before putting it on the balance. This seems to be pretty effective. Finally, you could make a mechanical balance that is set up to balance exactly at 2 grams. We have one like this, and it is amazingly sensitive. You can put a 2 gram calibration mass on this device, and it balances exactly. Then, add a tiny piece of masking tape to the mass, only 0.01 grams or so. This throws the balance way off. We have a device like this, and take it with us to competitions, just in case. . .

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Re: Design

Post by torqueburner »

jander14indoor wrote:If I had to chose, I'd make the stab adjustable. . . .

Regards,
Jeff Anderson
Livonia, MI
Thanks, Jeff, for your insights. Pretty much the same thing as my students are doing.

On another note, you may remember a previous post, where we were having a problem with the plane not turning properly at the beginning of the flight. We discovered that the thrust bearing was loose, and thought that was the reason. Well, the problem persisted, but we recently discovered the reason. The motorstick on our planes has a balsa truss on the top, which makes it quite stiff in the vertical plane. So we never have issues with changes in decalage due to bending. However, this apparently does not stiffen the stick adequately to resist twisting. Now that we are looking for it, we can see quite clearly that the stab tilt decreases quite a bit when a motor wound to higher torque is mounted.
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Re: Design

Post by bernard »

torqueburner wrote:However, you raise an interesting question about the balance at the competition. First of all, is it calibrated properly? Don't take this for granted. At a recent invitational we attended, there was a table with a balance for the students to mass their plane and motors before checking in with the judges. Our student adjusted her plane to 8.03 g, something like that, took it to the "official" balance, where it massed 7.94 g! The supervisor didn't want to allow her to adjust this, saying that she should have done it right with the other balance, but he relented after some discussion. We always take a 2 gram calibration mass with us to the competition in case this is an issue.
The 2 gram calibration mass sounds like a really good idea! McMaster-Carr sells Class 7 calibration weights for $6.67 which isn't too much. I can imagine some supervisors trusting only their balance; in that case what could we do to show that our rubber is under 2 grams if the balance reads more than 2 grams?

I have a story similar to your 8.03 gram plane story, but for Scrambler. Last year at state every team that went anywhere near 2.000 kgs had to redo their mass at impound because the balance was reading a higher number than the balances of a number of teams. The supervisor ended up allowing anything under 2.015 kgs because it affected so many teams and seemed like an issue with the balance.
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Re: Design

Post by torqueburner »

bernard wrote:. ..I can imagine some supervisors trusting only their balance; in that case what could we do to show that our rubber is under 2 grams if the balance reads more than 2 grams?

If this happens, I advise my students to offer the calibration mass to the supervisor. If it reads more than the motor, I'd think the supervisor would understand. At least, I'd hope so. . .
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Re: Design

Post by jander14indoor »

To TC-SciOly, all suggestions made are important, but the big reason is insufficient winds. New teams often have this problem in that they don't realize how many winds these motors will take. You are flying with a quarter tank of gas or less with those winds, 30 seconds isn't bad with those winds.
When you start winding, those other factors become important to take advantage of of the full tank.

Mass discussion.
Here's where knowing the rules and being able to speak convincingly and respectfully to the Event Supervisor is important. I's say as an ES, I'm much more amenable to such questions BEFORE the competition starts than in the middle. I generally have more time and can do something about your concerns if necessarily. But half way through I may agree there is a problem, but cant change at that point because half the teams have already competed. It is generally better to treat all teams the same, if slightly wrong, then half one way and half another. That gets events thrown out.
Oh, offering the ES a calibration weight. Why do you expect them to accept that? Do they know the source, how do they know it wasn't modified etc...

But that doesn't always work.

Thats why you need to explicitly consider the risks and how to mitigate them.
- Example, plane mass. You can bring two. Make sure they both fly well, then trim weight on one as close as you dare to minimum. Then trim the other more conservatively, say .05 to .1 gm over minimum.
- Motors, there is no limit how many you submit. Make some up RIGHT at the limit, some just a little under, and some a little more under. SOME should pass. Fly on what they say are OK.

As an ES, I use the exact same scales and dimensional gages for student self checks as my official checks. That way if I'm off a little (my error is generally smaller than most teams can evaluate, but...) everybody at least gets the same benefit/penalty.
I also only use mechanical scales. One each for 8 gm & 2 gm set as go/no go. I don't care how much your plane weighs, only that it is more than 8 gm. Although crude looking, I can easily demonstrate my scales balance against reference weights and they are sensitive to less than 10 milligrams.

For the rubber, best method to adjust has already been mentioned.
For the planes, I recommend teams build to under min weight by a couple of tenths to half a gram. Then use clay to ballast up. That makes it easy to adjust at the contest, just add or remove a little clay at the cg.
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Re: Design

Post by TC-SciOly »

Thank you to all who helped! Unfortunately, when we tried flying with more winds today, the plane just plunged at a faster rate... Not sure what is still wrong. We tested with different attack angle, rubber band sizes, and also the number of winds. Seems like the best times for us are the ones with ~40winds.
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Re: Design

Post by InfiniCuber »

TC-SciOly wrote:Thank you to all who helped! Unfortunately, when we tried flying with more winds today, the plane just plunged at a faster rate... Not sure what is still wrong. We tested with different attack angle, rubber band sizes, and also the number of winds. Seems like the best times for us are the ones with ~40winds.
Perhaps your horizontal stabilizer is at a weird angle? or you have too much angle on the thrust bearing? That's all I can seem to think of !
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Re: Design

Post by jander14indoor »

That starts making me suspect something fundamental wrong that isn't coming across in chat like this.
Wrong angle of attack for wing or tail or really strong down angle on prop shaft are possibilities. But if you are flying OK on 40 winds that shouldn't be a problem. I'd expect problems with power stalls if anything. Though if your prop angle is REALLY that down it could cause this.

But I'm a suspicious that you have a perhaps more subtle problem. Is your motor stick bowing significantly as you add winds above 40? If yes, you need to fix that. If you fly OK at 40 winds, describe the flight to us. Then describe what happens when you wind to 50. Look at the motor stick and plane as you increase winds. Do you notice the tail drooping? Then try 60. etc.

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Re: Design

Post by torqueburner »

TC-SciOly wrote:. . .Unfortunately, when we tried flying with more winds today, the plane just plunged at a faster rate...
TC-SciOly and Jander: here is something that may help to sort this out. Yesterday, I helped a student with his FF plane. This was my first hands-on experience with this design. Detailed below are my observations, and some speculation. Jeff Anderson and Brian T. have much more flying experience that I, and can no doubt offer you better advice that might help with your problems, some of which sound familiar based on yesterdays flying session.

We started out with some low torque trim flights to adjust the plane for the best cruise and descent, as recommended to me by Bill Gowen some years in the past. When we do this with our own student planes, as we increase decalage, we see times increase up to a point, then start to decrease, usually as the plane starts to stall.

First, I asked the student to trim his plane as he had been doing in the past. When released from shoulder height, the plane cruised poorly, and descended pretty rapidly. We increased the wing incidence, the main trim adjustment available for this design. This helped at first, but soon the times started to decrease, and we just couldn't cause the plane to stall. At this point, the LE of the wing was about 8 mm higher than the TE, so I wonder if this doesn't result in too much down thrust, since the thrust line was parallel to the motorstick?

Upon closer inspection, we noticed that the stabilizer actually had some positive incidence due to the fact that it was glued to the top of the tailboom, which was tapered on the top. So I'm thinking that this would require even more wing incidence to get the plane to climb?

The student had glued carbon fiber to the motor stick, but it was on the bottom. I've never used carbon, but it seems to me that it should be on the top. We saw classic signs of motorstick flex with launch torque as low as 0.3 in-oz. Just flexing the stick by hand showed it to be pretty springy.

Throughout the day we were frustrated by really quirky behavior. The plane would fly a little better for a while, then worse, with the same trim settings. We then discovered that the cg was way back - about 5 cm behind the TE of the wing. If memory serves me, I recall a past message board discussion that suggested if the cg is too far back, that the plane might be extremely sensitive to tiny changes in trim. Again, perhaps Jeff and Brian can comment on this. We tried moving the cg forward, but this required even more wing incidence, which worsened the problems.
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Re: Design

Post by bjt4888 »

torqueburner wrote:
TC-SciOly and Jander: here is something that may help to sort this out. Yesterday, I helped a student with his FF plane. This was my first hands-on experience with this design. Detailed below are my observations, and some speculation. Jeff Anderson and Brian T. have much more flying experience that I, and can no doubt offer you better advice that might help with your problems, some of which sound familiar based on yesterdays flying session.

We started out with some low torque trim flights to adjust the plane for the best cruise and descent, as recommended to me by Bill Gowen some years in the past. When we do this with our own student planes, as we increase decalage, we see times increase up to a point, then start to decrease, usually as the plane starts to stall.

First, I asked the student to trim his plane as he had been doing in the past. When released from shoulder height, the plane cruised poorly, and descended pretty rapidly. We increased the wing incidence, the main trim adjustment available for this design. This helped at first, but soon the times started to decrease, and we just couldn't cause the plane to stall. At this point, the LE of the wing was about 8 mm higher than the TE, so I wonder if this doesn't result in too much down thrust, since the thrust line was parallel to the motorstick?

Upon closer inspection, we noticed that the stabilizer actually had some positive incidence due to the fact that it was glued to the top of the tailboom, which was tapered on the top. So I'm thinking that this would require even more wing incidence to get the plane to climb?

The student had glued carbon fiber to the motor stick, but it was on the bottom. I've never used carbon, but it seems to me that it should be on the top. We saw classic signs of motorstick flex with launch torque as low as 0.3 in-oz. Just flexing the stick by hand showed it to be pretty springy.

Throughout the day we were frustrated by really quirky behavior. The plane would fly a little better for a while, then worse, with the same trim settings. We then discovered that the cg was way back - about 5 cm behind the TE of the wing. If memory serves me, I recall a past message board discussion that suggested if the cg is too far back, that the plane might be extremely sensitive to tiny changes in trim. Again, perhaps Jeff and Brian can comment on this. We tried moving the cg forward, but this required even more wing incidence, which worsened the problems.
The trim method I use is borrowed/stolen and a combination of methods that I have learned from others. I start with the Bernie Hunt design spreadsheet and use a CG location that gives the desired minimum static stability margin (SSM). For my students, I did not have them construct the fuselage with the positive incidence in the tail boom. Although positive incidence in the stabilizer is not an uncommon trim for long tail moment arm designs like this, I recommended to the students to try a little more conservative trim. We set the initial decalage angle using a little negative incidence in the stabilizer (about 0.65 degrees for the Freedom Flight) and a little more positive in the wing (about 2.3 degrees, or 1/8" longer front wing post than back wing post) for a total initial decalage of 2.95 degrees. With the nose moment arm set at 1.5" (we did not use the adjustable wing mount as it was apparent that the kits would weigh 1.2 grams under minimum and this would allow a good-sized lump of clay that we could move around to adjust the CG if necessary, we selected a SSM of 15% (hopefully to provide a little better recovery from ceiling hits), which is a CG at 2.5" behind the wing TE. The Freedom Flight kit measures a neutral point of 2.99" behind the wing TE and has a recommended CG of 2.75" behind the TE for a calculated SSM of 8.5% (this setting would potentially be very low drag, but maybe not so good recovery from ceiling hits). The recommended incidence settings for the kit are 0.62 degrees positive for the stabilizer and 2.3 degrees positive for the wing for a total decalage angle (difference between the two) of 1.69 degrees. This is a pretty low SSM, which correlates well with the low decalage angle and the very long tail moment arm. We assumed that the 2.95 degrees of total initial decalage we selected (along with the 15% SSM caused by the 2.5" behind TE CG) would result in a stalling flight and we were correct. Every Freedom Flight kit we setup this way stalled slightly during the first low power flight (about 80x15 turns and .30 in oz torque). Almost all of the airplanes my students built needed about 1 degree less wing incidence for a total decalage angle of about 1.9 degrees. The other very significant trim factor that we encountered was the need for a minimum of left wing washin. It is typical of a design with a very rearward CG and neutral point to be very sensitive to decalage angle and left wing washin. Washin settings my students ended up with were 3/32" at the most and usually about 1/16".

All this being said, I am thinking that based upon your note that the airplane you were testing had a CG 5 cm behind the wing TE that it is very nose-heavy. 5 cm behind the wing TE or 1.97" (sorry, I am still better with inches than cm) would calculate to a 32% SSM if the nose moment arm was 1.5" (I'm not sure if it was this, but if not, you can use the Bernie Hunt spreadsheet to recalculate SSM). Typical good flying SSM range would be from 10% to 25% (10% should require about 1.5 to 1.8 degrees of decalage and 25% should require 2.0 to 2.5 degrees). 32% is indicative of very nose heavy and would require the extreme wing incidence that you are trying (8mm would calculate a wing incidence of 5.8 degrees, or almost triple what should be required). If the student tries these settings (as long as the wing and stabilizer are flat except for 1/16" of left wing washin and as long as the tailboom is installed correctly with the bottom edge of the tailboom parallel to the bottom of the motor stick), they should hopefully be successful: nose moment arm = 1.5", CG with rubber motor installed = 2.5" behind the wing TE (move the clay ballast around to get this), wing incidence = 2.3 degrees (LE 1/8" higher than the TE as measured from the top of the motor stick as a reference point).

A long tail moment arm, rearward CG design like the Freedom Flight kit is capable of very high performance but is a little sensitive to decalage angle and CG. However, once my students found a good trim setting (one flight did it), they carefully marked their wing posts at the edge of each wing post socket and they have had no trouble getting repeatable performance since.

The motor stick wood for all of the Freedom Flight kits we built was pretty good and most motor sticks weighed about 2 grams or a little less. I recommended to the students that they bend custom rear motor hooks and custom front hooks and use Harlan bearings. We mounted the rear hooks at about 13.5" instead of at the end of the 16" motor stick in order to create less bending force on the stick and to avoid possible creation of unwanted downthrust. We have had no issues with bending motor sticks flying up to about .48 inch ounces torque. We have not tried flying at higher torque as we do not have a high ceiling site for testing. I would think that the carbon attached to the fuselage would not be necessary unless flying at high ceiling sites and using 1.0 inch ounces of torque or more. It is true that the Freedom Flight kit did recommend the carbon be attached to the bottom of the fuselage if applied to only one side. I have never tried gluing carbon to only one side of a structure, but I would think that if Dave Ziegler recommended this, I'm sure that he tested it and it should not be an issue.

Sorry for the very long message. Your questions and observations are very good.

For TCSciOly, I agree with the other recommendations you have received. All trim settings (CG, stab incidence, wing incidence) need to be checked and matched to the excellent Freedom Flight kit instruction book and all flying surfaces need to be verified to see if they are flat and straight except for the 3/32" of left wing washin. Also, let me know where your wing is mounted relative to the front of the motor stick. Is the wing leading edge about 1.0 or 1.5" back from the end of the motor stick?

Good luck to all,

Brian T.
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