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School Project - Help

Old 09-26-2013, 04:52 AM
  #26  
Petkov
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Originally Posted by j.duncker View Post
I don't see any reason to limit the cells cram as many cells as you can into that 1.5 lb pack. the more volts you have the more watts.
The mission score is divided my the empty weight of the aircraft and that includes batteries. They call it RAC (rated aircraft cost). Sacrificing a bit of speed and power for mission 1 and 3 will be worth it in the end but for right now, im not sure exactly what the ratios are. Will need to test. I will definitely try different power combinations to see what the results are before the actual completion.

Great references to the Gossamer Albatross and wing sailing. Will definitely read up on those. I do like canard designs but due to having to land in one piece and strong winds, ill take your advice on not using it.

The main problem with the plane last year is that we could not get much trust out of the motor. We could only get about 1.5 lbs of force compared to other teams getting 6-8lbs. Even in the air, the pilot had to fly it sideways even with small winds. I will also try to make the design more aerodynamic this year. maybe adding balsa sheets around fuselage to give it more of an airfoil shape. Any advice as to construction methods are appropriated.
....
Jester, great explanation.

Last edited by Petkov; 11-12-2013 at 06:34 AM.
Old 09-27-2013, 11:07 AM
  #27  
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For construction tips, visit http://www.airfieldmodels.com/. Some of it won't apply to your project, but this guy focuses on making things as light and as strong as possible, regardless of how much effort has to be put into it. For your project, a truss type fuselage makes a lot of sense with either a foam or built up wing. Foam is faster and easier to make (especially since you are scratch building), while a built up wing is lighter. You may even consider using foam for your prototypes until you get the performance you want, then design a built up wing to replace it in order to take that extra few ounces off of your unloaded weight.
Old 10-01-2013, 09:59 AM
  #28  
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A note on take-off, they only have 40 feet of take-off permitted so the plane will have to come off the ground pretty fast.

I did some modeling of motors using WebOCalc - using a 230 kV outrunner on 24 cells/28/8V with a 15X10 prop you would pull 15 amps and produce 74 oz of thrust and 50 mph pitch speed on 432 input watts.
http://www.maxxprod.com/pdf/HC6332-230.pdf

If your plane weighs 6 pounds all up that would be about 72 watts/pound. That should get the job done.

Now, you don't have to use THIS motor. It is just for reference to get a starting point.

Similar results on this 200 kV motor with a 16X12 prop at about 69 watts/pound.
http://www.himodel.com/electric/EMAX...GT5335_10.html

Last edited by aeajr; 10-01-2013 at 10:04 AM.
Old 10-16-2013, 11:01 AM
  #29  
Petkov
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This is kind of interesting. I found an old thread about this competition on here.
http://www.rcuniverse.com/forum/brus...y-contest.html
Old 10-16-2013, 11:03 AM
  #30  
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Update:

The team has decided on a high wing design, standard tail, 4 lb max weight plane, and 250-280 watt power output.

We will be testing different dihedral, taper and wing areas soon to see how the plane performs during taxi and in the air to find the best wing design. For the prototype the wings will be made from foam.

We will then test tail designs. Do you guys have and advice on the tail set up and construction methods? Currently we are not sure how large or how far away the tail should be.

Last edited by Petkov; 10-16-2013 at 11:13 AM.
Old 10-16-2013, 12:30 PM
  #31  
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Here's a helpful thread using information that one of the founders of my club put together. Many, many, many models planes have been successfully designed using his principles. The dimensions he gives don't necessarily extract every possible bit of performance from a design, but they do yield a plane that is friendly to the pilot and flyable in a wide range of conditions. Personally, I wouldn't suggest using any dihedral. Dihedral on a high wing plane is for making it self righting in calm conditions, not for making it carry more weight or be more efficient.

http://www.rcuniverse.com/forum/scra...formation.html
Old 10-16-2013, 01:28 PM
  #32  
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Thanks jester, I shared it with the group
Old 10-21-2013, 03:16 PM
  #33  
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An additional question that has come up. If we fly with 1.2V batteries but the motor draws a higher voltage per battery, should we expect any battery damage? If so, what are the ranges?

Batteries we are currently looking at are the elite 1500 which are rated at 10 for the C-rate.

Also, how would we know if a cell has gone bad inside a pack?
Old 10-21-2013, 03:27 PM
  #34  
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Originally Posted by Petkov View Post
An additional question that has come up. If we fly with 1.2V batteries but the motor draws a higher voltage per battery, should we expect any battery damage? If so, what are the ranges?

Batteries we are currently looking at are the elite 1500 which are rated at 10 for the C-rate.

Also, how would we know if a cell has gone bad inside a pack?
First - bad cell - either the pack will not come up to full voltage when charged or the voltage will sag badly under load. Bad cells might even kill the pack.

You can also tell you have a bad pack if you cycle it and it won't take full capacity of the charge. Cycling a battery pack is often a feature of the charger and you have to have a charge with a meter on it to know how much charge you put in. Cycling is a common practice to test a pack. If it cycles and takes less than 80% of its rated capacity it is considered bad.

Motors don't draw voltage they draw amperage. Read the first article (post 2) of the book. Title of the article is AMPS vs. VOLTS vs. C. Once you read that you will be able to answer you own question.

EVERYTHING YOU WANTED TO KNOW ABOUT ELECTRIC POWERED FLIGHT
http://www.rcuniverse.com/forum/m_7100376/tm.htm
Old 10-22-2013, 06:57 AM
  #35  
Petkov
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Thanks Ed, great help as usual. I'll post my comment here from the other thread as well.

"How accurate is the c-rate on the batteries? I know from these specific ones (elite 1500, 10c rate) other teams were capable of getting 20+ amps on several flights. Last year we had issues with the batteries we used (elite 2000) which are also 10c rate but had trouble getting over 10 amps. Could any problems arise from the methods we use to charge the batteries?

My next questions is about airfoils. The team has had instructions on how to interpret airfoil characteristics and how to use the graphs, but we are having trouble figuring out which which airfoils to compare.
What I mean is, I dont want the team to start out comparing the wrong airfoils and find the right one in that batch. Do you guys have have any suggestions of a list of airfoils that we should look at for our type of missions?

Some of the airfoils we started out looking at are the USA 35B (pipercup airfoil) and naca2012 which is a very common airfoil used. Any recommendations as to where else we can expand our airfoil research?"

Last edited by Petkov; 10-24-2013 at 11:21 AM.
Old 10-22-2013, 07:15 AM
  #36  
jester_s1
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On batteries, C rating means jack squat. Internal resistance is the spec that actually determines how many amps you can draw (both the absolute limit and the relationship between amp draw and heat production), but good luck getting a manufacturer to release that information. Your problem last year was likely due to a bad connection or just missing the setup with your motor and prop combination.

On airfoils, I don't know enough to recommend specific ones, but in general flat bottoms and even undercambered ones are going to give you the most lift for the least speed. So you probably don't need to bother with anything that is convex on the bottom. As you probably know from your studies, sharp leading edges and high lift designs will make the best use of your thrust, but also will stall quickly leaving an inexperienced pilot little opportunity to recover. So consider the actual flying conditions and pilot workload when making your choice.
Old 10-25-2013, 11:19 AM
  #37  
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I thought that you are restricted to NiMH packs?

A123 are Lithium batteries violating the restriction.

If you can use A123's you might as well go with LiPo's.

Aren't the Eflite 1500's LiPo's?

Last edited by opjose; 10-25-2013 at 11:23 AM.
Old 11-04-2013, 01:56 PM
  #38  
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The Elite 1500 are a little bit different than the Eflight's. I believe they are still NiMH. For the team's first prototype flight this year, we did cheat a little bit and used LiPo's.

Which brings be to our team's first milestone: We got our first flight on Sunday!

Although our plane doesn't look very pretty yet, I will post pictures soon. Hopefully after putting this one through some brutal testing the second prototype will look more like a plane.
Old 11-06-2013, 06:09 AM
  #39  
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first flight:
https://www.dropbox.com/s/ilq7kbyswy1smt4/MVI_1832.MOV

We had some stability issues. The plane behaved as though it has tail heavy although before flight we picked it up be the 1/4 cord and it seemed fine. The plane kept going up and up until the propeller was pointed straight up and could not hold the 5lb plane, even full down elevator could not get the plane stabilized. With the c.g. being in place we are having trouble locating the problem for the plane behaving as though it was tail heavy.

One of the problems are the control surfaces, which are too small when compared to other rc aircraft so we will need to rerun the calculations on those (this should not be the problem of the plane being tail heavy but should help with the stability afterwards). The fuselage is a little thick and ends right before the tail begins so I am thinking there could be a little turbulence causing low pressure right under the stabilizer/elevator making the tail produce a moment downward.

For the second test flight (hopefully this weekend), we will have a slightly bigger tail with much bigger control surface and a more aggressive taper for the trailing edge of the fuselage. Hopefully the stability can be fixed and we can continue with testing.
Old 11-06-2013, 06:44 AM
  #40  
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Did you use the Chuck Cunningham proportions that I gave you a while back? It does look like your tail is too small in proportion to your wing, and with that draggy fuselage the stability problem is going to be exacerbated. The flight didn't look tail heavy to me. It looked like the plane was just severely out of trim, possible due to too much wing incidence relative to the horizontal stab which was too small to compensate.
What you don't need is bigger control surfaces. Look at planes like the Sig 4 Star or some of the various pattern planes on the market. They don't have particularly big control surfaces yet they have plenty of control. Big control surfaces lead to twitchy planes and overcontrolling, not to mention a greater tendency to flutter. What you do need (most likely) is more stabilizer area. That's not the same thing. And yes, tapering the fuselage to give smooth airflow to the tail will both increase your stabilizer's effectiveness and reduce the plane's drag.
Old 11-06-2013, 09:40 AM
  #41  
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Great input jester. I will definitely look into the sig 4 star plane to see what kind of ratios it has. We did look at the Cunningham articles as well as other information but the wing size and tail distance from cg changed so it might explain those errors. The tail group is rerunning their calculation so everything should be updated withing a few days.

The reason why I said it was tail heavy is because the plane did not want to level off after take off, it just kept going steeper and steeper upwards (this cant really be seen very well from the video). Theoretical without a tail the plane will want to point down due to the moment from the wing. Only explanation I have is the turbulence from the end of the fuselage. I really hope that fixes the issue.

Additional question:
There are mainly 2 types of ailerons, strip and barn door. Do you guys know where I can look to see what the benefits or disadvantages of each are?

Last edited by Petkov; 11-06-2013 at 02:05 PM.
Old 11-06-2013, 01:57 PM
  #42  
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You might also check the relative incidence angles - the interplay between engine thrust line, wing incidence, and stab incidence can really affect the way a model flies

Check out the following ref (not complete by any means, but a good start)

http://www.rcflightschool.com/Inter_...ter_0A4-13.pdf
Old 11-07-2013, 05:35 AM
  #43  
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I have an observation about some newer jets I have seen that have the wingtips flipped up to verticle to reduce the wing turbulence which causes drag. not sure if this would be aproblem with slower flying aircrraft or whether it would help to increase the planes ability to carry more.
Old 11-07-2013, 07:10 AM
  #44  
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There may be some aerodynamic advantages in full scale planes between the two aileron types when you are trying to get the last little bit of performance out of a design, but in models it really doesn't matter. Build it based on what's easy for you to do, which usually means strip ailerons.

The fact that the nose wanted to pitch up doesn't necessarily mean the plane is tail heavy. It just means it's out of trim. If you are balanced at 25% chord (and are sure about that) then the pitching is a trim issue, not a balance issue. It could have been your engine thrustline is off, or it may well have been an incidence problem. In ferreting it out, don't make big changes. Make incremental ones so you can measure the results. A big change may cause other issues that you then have to figure out how to explain and fix, and of course, if you make a change in the wrong direction you'll just crash the plane and won't have any data to work with.
Old 11-07-2013, 07:12 AM
  #45  
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Petkov, first off, Jester is giving you some excellent advice
second.. it looked to me as if the airplane was taken off at full throttle and the throttle was held full...
I'm not sure how much experience your pilot has, but that throttle is a proportion control, and not an on / off switch
the empanage (tail surfaces) certainly look very small to me, and the OP makes a very good point about the interaction between the thrust line of the engine and of the relative incidences of the wing and horizontal stab.
as for strip vs barn door ailerons, the subject is as complicated as which prop / motor / esc / battery combo to use...
typically for a utilitarian type airplane like this one, I would go with barn doors. (larger surface area further out on the wing)
Old 11-07-2013, 12:00 PM
  #46  
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Thanks for the advice everyone. We rechecked the tail and there was actually a negative angle of attack on the tail (about 3 degrees). This seems to be another large part of the problem! Once we fix the fuselage, tail angle and add a little more control surface to the tail - we should be ready to go for round two. Ill keep everyone updated.

I think it would be a good idea to stick to strip ailerons and see if we can get a performance boost in later stages with the barn doors.
Old 11-07-2013, 06:43 PM
  #47  
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I called it! Too much wing incidence relative to the horizontal stab.

You'll make your life a lot easier if you will set the tail incidence as zero to your datum line, then set everything else relative to that. Buying or making an incidence meter will make that process a whole lot easier and less prone to errors.
Old 11-18-2013, 04:07 PM
  #48  
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Update!

We have fixed the issues we saw in the first flight and we got a second flight in. Posted below are pictures of our Frankenstein plane (prototype 1) and the video of the flight.




https://www.dropbox.com/s/k0n6aaj5fs...115_153501.mp4

We resolved all the longitudinal stability problems by resizing the tail control surfaces, removing the negative angle of attack on the tail, and improving the fuselage design. We tried the second wing and reballasted the plane to 5 lbs for the second flight. It was a much smaller wing than the first and in combination with the extra thrust we were getting from the lipo battery, our plane was really quick. If we test the wing again we will have to limit the htrust to replicate the thrust we would be getting with the calculated nimh batteries.

As you guys can see we had some major issues with lateral/roll stability. Every time the plane went for a turn, it would go out of control. We are currently analyzing lateral stability to try to resolve the issue. I do want to get a second flight with the bigger wing that we used the first time to see if the wing area is the issue in order to get a better control system. Either way its back to the flight text books/ articles.

I appreciate any feedback. Tell me what you guys think!

I have updated the original post to reflect teams progress.

Last edited by Petkov; 11-18-2013 at 05:31 PM.
Old 11-18-2013, 04:35 PM
  #49  
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That "going out of control" thing you mentioned is called a spin. Your wing is way too small to carry the plane's weight, and your elevator control is too hot. So every time your pilot pulls back on the stick he tip stalls a wing, which puts the plane into a spin. Calculate your wing loading. A good flying RC model at this size will be between 15-20 ounces per square inch. I'll bet you had 35 ounces per square inch with that tiny wing.

You said you have a bigger wing? You need to size the tail to the wing, so if you are changing the wing size much you'll need to build a new tail too.
Old 11-18-2013, 07:52 PM
  #50  
Petkov
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Jester you are right on the money. Our wing loading is 34.9 oz/ square foot. We didn't really base our wing area on wing loading so I will do my research on that. Hopefully it will shed some light on the lateral stability issues.

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