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Mar 25, 2020, 03:27 PM
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thumbs1's Avatar
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Discussion

STC Mod questions


First of a the STC was the plane I learned to fly RC. It was a while ago and used a brushed motor and gear box. since then I built a couple more, along with other planes. Funny I still have the plane and I never crashed it. Not so much after the original STC LOL. Anyway the question.

I haven't flown the modified STC since last year. As a matter of fact I cant get her to fly at all. Since I have some time at home now I decided to see if I can get er back in the air. This plane flew fine without the mods but the problem came up after the mods.

The mods are mainly the installing of ailerons. I built another wing took the the dihedral out of the and installed the ailerons. After the roll out the plane takes to the are she turns or rolls to the right and I can not control it. She just gains altitude turns right and right into the ground. Any ideas?

What is the offset for the motor? Right, left, up and down? As of now the motor is mount with no offset at all. I can see this may have something to do with it but Is there anything else I should be considering?

Thanks for the help
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Mar 25, 2020, 07:27 PM
flyin' fool
goldguy's Avatar
Warped or twisted wing??
Mar 25, 2020, 07:32 PM
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thumbs1's Avatar
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Thanks for the reply. She looks straight. I know the plane wasn't designed for ailerons but she should fly with them. (I think) LOL I think I'm going to put a couple of washers under the motor mount and pitch the motor a little down and to the right and see what happens. Could be the motor torch is the problem. Idono
Mar 25, 2020, 09:46 PM
treefinder
springer's Avatar
Take some pics of the plane, top view, side view from both sides, front view and attach them to your post. That way we can look for dimensional issues.

In general, did you use the original undercambered airfoil? How big are the ailerons? Single or two servos? Wing is flat, no dihedral at all? Do you have the original incidence? Did you perhaps make the new wing a KFM but use original incidence?

As far as motor thrust angle, I only use a bit of down, no side thrust. I compensate on takeoff with rudder.

Also, try a glide test and see if the plane glides straight and level, or rolls right without power.
Mar 25, 2020, 10:52 PM
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Thanks

I can answer some of the question now.

Yes the original undercamber air foil. Hummm maybe the wing should be flat with no undercamber???hummmm come to think of it my other planes with ailerons are flat wings no undercamber. Never thought of that. Later you will see I have only flown two aileron planes. The other planes are built but not flown. I was afraid of ailerons for quite a while after starting with three channel. Only planes I have flown with ailerons are a Blue P51 which I scratch built and a Eachine P51 I just bought a few weeks ago. Now I'm a believer in ailerons. LOL

The ailerons are one inch wide and run most of the way across the wing. They start about an inch from the fuselage and continue to about an inch from the wing tip at the trailing edge. Because the way the trailing edge or the last inch or so of the wing attaches to the fuselage the ailerons on both sides at the fuselage are about 1/4" higher than the contour of the wing at the fuselage. Hummmm


Single servo mounted to the wind with the horn on top of the wing.

Wing is flat no dihedral.

I used the original incidence. Another possible problem??

Don't know what KFM is. Might sound dumb but there ya go. LOL Maybe I do but not by the name.

Basically I duplicated the original wing without dihedral put a couple of ailerons on it and went to the field. I know I probably did something stupid but tryin to learn a bit. Probably a basic no no but I don't have a clue. I have only flown two planes with ailerons. Most of my flying has been three channel.

It could be it should be a wing with no undercambre and move the incidence to being flatter. I made the wing removable so I could make another wing with little problem if I knew how to correct it. I still have the original wing I made for it. She flew fine with that wing but there is a lot more going on with the new wing with ailerons.

Still looking for your thoughts but funny how your questions gave me a lot to think about.

thanks for the help
Last edited by thumbs1; Mar 25, 2020 at 11:17 PM.
Mar 26, 2020, 05:10 AM
Learning to make
Quote:
Originally Posted by thumbs1
Don't know what KFM is. Might sound dumb but there ya go. LOL Maybe I do but not by the name.
I can solve that for you:
this is a KFm
Post #2 is particularly enlightening.
Basically a stepped airfoil wing, that works great on RC airplanes. Easy to build, plays well with alierons.
Mar 26, 2020, 05:21 AM
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thumbs1's Avatar
Thread OP
Quote:
Originally Posted by fossil1999
I can solve that for you:
this is a KFm
Post #2 is particularly enlightening.
Basically a stepped airfoil wing, that works great on RC airplanes. Easy to build, plays well with alierons.
Thanks
Nope LOL I did not know that one. I built a fixture that allowed me to heat each of the wing panels and using a heat gun to bend them to produce an undercamber shape.
Mar 26, 2020, 08:37 AM
treefinder
springer's Avatar
Thumbs, there are those who say dont do ailerons on undercambered wings, but I'm not one of them I have half a dozen UC (actually KFM5a - follow fossil's link for pix and descriptions) planes with ailerons and all fly and handle fine. The only thing I have found relative to that is if I put too much camber in the airfoil, the ailerons don't work as well. Perhaps the airflow over wing fights airflow over ailerons. I forgot to ask how big your plane is, but 1" wide ailerons are ok for a 30" or so span plane. I wouldn't worry about the aileron inboard tips being "high" I have several planes where that happened - foam control surfaces aren't as stiff as other types, but still work fine.
Another factoid, the flat bottom KFM wings like zero incidence, whereas undercambered and uc KFM like a few degrees positive, about 1/8" up at LE for a typical 30" span plane. Too much incidence will cause plane to balloon and roll off, but it would do that with original wing too if that was cause. I am leaning toward warp in wing, like goldguy said. A good straight on pic should tell us if one side is up or down.

Also it just occurred to me, is the throw on each side the same? Easy to check, and if it is greater one side vs the other, normal control could be wonky.

Lastly, high wing planes with ailerons often show "adverse yaw" where the plane won't turn, but when ailerons are applied it just noses up then rolls off in wrong direction. Aileron differential is the fix, and with your single servo on top, you have built in at least some already, so let's look for warps first.
Mar 26, 2020, 10:37 AM
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Thread OP
Springer

I did forget to mention I did screw up on one of the control horns. Thats a surprise LOL. (I make my own) The throws on the right aileron were more than the left. Normally I drill the holes through both horns at the same time to avoid this. I found that yesterday and corrected it but have not flown the plane yet. I haven't flown her in over a year because of the problem. I guess I didn't look to hard last time out.

I am going to fly the plane the way she is with the control horn corrected. Maybe its as simple as that. The undercamber of the wing is approximately the same as the "normal" wing but without the dihedral.

By the way I watched quite a few YouTubes about the KFM wing along with reading many articles on the subject. It got a bit technical for me but the wing is interesting. I may try one on a future plane. If nothing else it will eliminate the forming of the undercamber which is a little time consuming.

I hope I can get her out today, its a beautiful day, I just have to charge a couple of batteries. I also did some wing repairs on my Corsair that I need to test. After a major crash.

If you still want some picts I will try to post them later today.

thanks
Last edited by thumbs1; Mar 26, 2020 at 10:42 AM.
Mar 26, 2020, 11:58 AM
treefinder
springer's Avatar
definitely post the pics, It's always fun to see another scratchbuilders plane! and, of course, will help us to see if there is anything that we from our experiences might find and suggest an "improvement".

So you had more throw on the right? Could be a contributor. One thing that you need to do, and I expect you already do naturally, is to add opposite rudder to compensate for takeoff torque reaction. IF you put the aileron servo on the right stick, and rudder on the left (typical for 4 channel) after having the rudder on the right stick (typical for 3 channel), then you might be using the right stick to counteract torque, where you now need to use the left stick. (I just re-read that sentence, wow, that's confusing!)

If you get out, and have some tall grass near, try a glide test power off into it and watch if the plane glides straight. If it rolls off power off, then you have some twist. If no rolloff, then try with a little power and see how an extended glide goes. Then do full power takeoff if everything is good to that point.

Charge those packs and good luck!
Mar 26, 2020, 12:52 PM
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Can I ask an off topic question?

I am in the process of charging two 1500ma 3s batteries. Its been a while since I have flown and really cant remember. When I put the batteries on Lipo balance charge the charger will go to over 12v. It got as high as 12.3v and I shut it down to ask if this is correct. I know wet cell 12v can charge at more than 14v but Lipos are tricky little devils and I don't need a fire. I have two different chargers one works off a power supply the other has it built in. They both did the same thing so I think its ok but gotta ask. One charger allows me to put in the total ma of the batteries to be charged the other one I guess figures it out by itself. idono. I always keep an eye on charging but before I do something stupid I thought I would ask.

I'll send picts later. She's not pretty I hope fun to fly once I get her straightened out.

thanks.
Mar 26, 2020, 12:56 PM
Registered User
Ace12GA's Avatar
LiPo cells peak charge at 4.2V (4.35V for HV variants), so a 3s will be ~12.6V at full charge.

EDIT: Be sure to balance charge, and to monitor cell voltages. If you see one cell significantly lower/ higher than the rest, you will have a hard time getting a solid balance charge out of it. It will take longer. Most basic 4 button balance chargers have a view to see the individual cells; advanced ones should have that as well. Otherwise, it's worth making a Y harness so you can attach a cell meter while charging.
Mar 26, 2020, 01:33 PM
treefinder
springer's Avatar
My chargers all charge each cell to 4.17 volts. That is a good target, just under the 4.2volts/cell that shows a fully charged cell. While some chargers show percent charged or percent "full", I like to know the voltage. Voltage is roughly equivalent to percent charged. I have used this chart for years and not had any issues.

Lithium Polymer charge
% charge 1 cell 2 cell 3 cell
100---------4.20---8.40---12.60
76 --------- 4.05---8.10---12.15
52----------3.86---7.72---11.58
42--------- 3.83---7.66---11.49
30--------- 3.80---7.60---11.40
11--------- 3.70---7.40---11.10


Your charger may show higher than the max when charging, as it has to have a higher voltage to push the electrons into the battery. But once it is done, the pack voltages should all be no higher than the max in the chart.
Last edited by springer; Mar 29, 2020 at 07:26 PM. Reason: fixed the 30% line to show correct voltage
Mar 26, 2020, 01:41 PM
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thumbs1's Avatar
Thread OP
Quote:
Originally Posted by Ace12GA
LiPo cells peak charge at 4.2V (4.35V for HV variants), so a 3s will be ~12.6V at full charge.

EDIT: Be sure to balance charge, and to monitor cell voltages. If you see one cell significantly lower/ higher than the rest, you will have a hard time getting a solid balance charge out of it. It will take longer. Most basic 4 button balance chargers have a view to see the individual cells; advanced ones should have that as well. Otherwise, it's worth making a Y harness so you can attach a cell meter while charging.
Thanks thats what I was looking for. I know these things can be touchy so I didn't want to continue until I found out. Both are pretty good chargers. I can monitor each cell and total ma put into the batteries.

thanks.
Mar 26, 2020, 01:44 PM
Registered User
Ace12GA's Avatar
Quote:
Originally Posted by springer
Code:
Lithium Polymer charge
% charge	1 cell	2 cell	3 cell
100		4.20	8.40	12.60
76		4.05	8.10	12.15
52		3.86	7.72	11.58
42		3.83	7.66	11.49
30		8.80	17.60	26.40
11		3.70	7.40	11.10
Your 30% line looks a little loopy.


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