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CG on an old Piper Cub ?

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Old 06-14-2017, 03:34 PM
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schomper42
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Default CG on an old Piper Cub ?

I have recently converted an old Piper Cub , Balsa wood from nitro .46 to an electric . G46 400kv from Value hobby with an 80amp ESC. I have figured out the CG as I do for all my planes. Between 1/4 and 1/3 back from the leading edge as a starting point. But if anyone out there knows the correct CG for this plane I would appreciate any info before my maiden voyage. I have never flown this plane before. It was given to me by someone who had it sitting around for 30 years amnd he doesn't remember anything.
Also , I am going with the biggest prop I can fit and that's a 14-8 prop. powered with 6s power. Before the conversion it was 7 1/2 pounds and its now 7 pounds AUW. The wing is 77 " long 10 3/4" wide
Any tips or help would be greatly appreciated.
Thank you
Bob
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Old 06-14-2017, 05:05 PM
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Oh man that will be a super sonic turtle of a Cub. I would say way more power than needed. But hey, there is always the left stick.

A good guess would be on the spar. Front side or back, but on or very near the spar.

Buzz.
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Old 06-14-2017, 06:20 PM
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Simply enter your numbers for this calculator to do the math:

Aircraft Center of Gravity Calculator
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Old 06-15-2017, 02:55 AM
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Quote:
Originally Posted by blvdbuzzard View Post
Oh man that will be a super sonic turtle of a Cub. I would say way more power than needed. But hey, there is always the left stick.

A good guess would be on the spar. Front side or back, but on or very near the spar.

Buzz.
haha.. yes that thing your left thumb is on is a throttle.. Not an on/off switch. Wouldn't know it watching some guys fly
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Old 06-15-2017, 06:09 AM
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Center of Gravity Calculator
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Old 06-15-2017, 06:24 AM
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All of my cubs, some five or more, all flew best with a CG slightly more forward than most CG calculators suggest. Also make sure you have some toe in on the landing gear to minimize the tendency to ground loop on take off.
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Old 06-15-2017, 07:04 AM
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Quote:
Originally Posted by Rodney View Post
All of my cubs, some five or more, all flew best with a CG slightly more forward than most CG calculators suggest. .
Exactly, sage advice by Rodney if you want a good flying and enjoyable Cub do not balance at one third, instead balance at the quarter chord. Also expressed as 25%mac, on many this quarter chord is at or near the the main spar however if using this methods balance at the forward edge of the main spar Not the rear edge of the spar. With any straight chord wing its easy to arrive at the 25%mac (by the way that stands for mean aerodynamic chord) simply by dividing the distance from the leading edge to the trailing edge by four and that sum is the distance back from the leading edge is your target point.

Also when balancing never balance with the nose slightly down for "mother and country" always balance with the fuselage Level. If you do that you have not balanced at you target point but instead at some unknown point forward.

John
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Old 06-15-2017, 09:49 AM
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Ok so here is my dilemma. General consensus is 25% back from leading edge. And also it should be at or near the spar. If the wing measures 10 3/4" inches , then 25% back from the leading edge should be at 2.687 (2 11/16") " But here is the conflict. The spar is 3 3/4" to the front edge of the spar and 4 1/8" to the back edge of the spar which is even more then 1/3 back . Even if I went 1/3 back , that doesn't even reach the spar. So do I say the heck with the spar and just set my CG 1/4 back ?

any help is appreciated
thank you
bob
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Old 06-15-2017, 11:50 AM
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Bob go with 25% if you want the sweet flying airplane that Cubs are supposed to be famous for, only go for 30% and beyond if three D flight is the goal and the airplane is plane is intended for the vast majority of its flight time to be prop hanging a current catch phrase for that type of flight is sometimes called high alpha.

The spar reference is only because it been traditional to express that way for newbies and of course its only a crude reference and every design can an will vary in that regard.

John
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Old 06-15-2017, 12:58 PM
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I have a Tacon 46, 60 amp ESC, 5s battery swinging a 13x6 prop and it will hover a 7 pound plane. I would guess your plane will fly at about 1/4 throttle or less. Or you could enter the pylon race and see how fast it goes and how far back you can get those wings to flex backwards

If you fly it with floats, you will have plenty of power to fly off the water.


Buzz.
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Old 06-15-2017, 03:17 PM
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I truly appreciate all this good info. I will for sure start out with 25% on the CG. And I guess I have way to much power for this plane. I will start out with the 6s , but it sound like I will be able to go down to probably 4s or 5 s. . I always believe its better to have to much power then not enough. So I guess that what I have.
Thanks again , and I will probably be flying this within a week. So I report in and tell you how I made out.
Bob
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Old 06-16-2017, 09:02 AM
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Quote:
Originally Posted by schomper42 View Post
. Before t
Also , I am going with the biggest prop I can fit and that's a 14-8 prop.
Any tips or help would be greatly appreciated.

Bob
There is a price to be paid these decisions to severely over power as well as using such a long prop and the price may not be so pleasant. First this is not a large Cub and a fourteen inch prop is certain to cause poor ground clearance with many, many ground strikes becoming the norm. Now as direct effect of first overpowering and the long prop P factor as well as gyroscopic precession and a number of other factors will collectively gang up to give your airplane all the poor takeoff and ground handling chacteristics, that cubs are also famous for. This is made worse by the very high deck angle of a Cub when resting on all three wheels along with the narrow tread main gear.

The old saw that states shoot you can always just pull the power back may not be such good advice when you are dealing with an out of control takeoff and fighting the temptation to rip it off the ground far to early as you are
running off the side of the runway.

John
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Old 06-17-2017, 08:00 AM
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To compare your Cub, I flew my very over weight Goldberg Cub with an O.S. 48Fs engine. I would hazard a guess that is about the same power as a Tacon 25 with a 3s battery and a 12x6 prop. Might I suggest, run a 12x6 prop, 5s or even a 4s battery. If you find you do need more power, setup up to a 13x6.

Here is a tool you may well need. It will tell you how much power you really have. A watt meter will help you decide how your setup will preform. Plus it will help you to keep the magic smoke in. So your plane is 7 pounds, so if you get 800-850 or so watts, you should be good. If you test and you have 1500, you know you can step down a bit and still be safe.

https://hobbyking.com/en_us/turnigy-...-analyzer.html

https://www.amazon.com/Power-Analyze...=RC+watt+meter

Let us know how she flies and pictures would be great.

Buzz.
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Old 06-22-2017, 05:41 AM
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Old 06-22-2017, 05:41 AM
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Old 06-22-2017, 05:49 AM
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Well i need to figure out how to attach a picture of the plane
Ok , so the long awaited report on my first flight with this converted Piper Cub. I took your advise and sent the bird up in the air with a 12x8 prop ( only 12" prop I has on hand) still with 6s power. It took off ok , but I needed to give the plane full throttle the entire time. It was severely under powered. I brought along a 13x6 prop and tried that. What a difference ! The plane flew great at half throttle, just the way I like it. Giving me reserve power if I ever needed it. No doubt this type of plane is a slow flyer and it was. But it flew beautiful. It is by far the largest and heaviest plane I have ever flown so I was a little nervous. Take off and landings were a little different , just something I need to get used to. I will now try to come up with something for cowling. Oh by the way, I did set the CG for 25% back off the leading edge and it was perfect. So a plane that's been sitting around unused for about 30 years is flying again, but now with electric power.
Thanks again for all the advise from everyone, it is much appreciated.

Bob
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Old 06-22-2017, 05:49 AM
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Well i need to figure out how to attach a picture of the plane
Ok , so the long awaited report on my first flight with this converted Piper Cub. I took your advise and sent the bird up in the air with a 12x8 prop ( only 12" prop I has on hand) still with 6s power. It took off ok , but I needed to give the plane full throttle the entire time. It was severely under powered. I brought along a 13x6 prop and tried that. What a difference ! The plane flew great at half throttle, just the way I like it. Giving me reserve power if I ever needed it. No doubt this type of plane is a slow flyer and it was. But it flew beautiful. It is by far the largest and heaviest plane I have ever flown so I was a little nervous. Take off and landings were a little different , just something I need to get used to. I will now try to come up with something for cowling. Oh by the way, I did set the CG for 25% back off the leading edge and it was perfect. So a plane that's been sitting around unused for about 30 years is flying again, but now with electric power.
Thanks again for all the advise from everyone, it is much appreciated.

Bob
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Old 06-22-2017, 05:58 AM
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I tried to attach a photo in the post and that didn't work out very well. So I added a picture of the plane in my gallery
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Old 06-22-2017, 06:11 AM
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Post images by going to "Go Advanced" next to "Post Quick Reply". You'll will see a paper clip in the header. Click on that. Dan.
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Old 06-22-2017, 09:00 AM
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Default trying to clip a picture ....

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Old 06-22-2017, 09:01 AM
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Thanks Dan , that worked
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Old 06-24-2017, 02:34 AM
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Glad things worked out for you,, add some pitch and you'll make better use of that motor's power,,, 13-8 or 13-10 and you'll be cruising around at more like 1/4 throttle I bet,,


Good luck.

p.s.
Contact Callie Graphics for some numbers for the wing
https://callie-graphics.com/collections/piper-cub
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Old 06-24-2017, 03:02 AM
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I will give a 13x8 a try , and thanks for the graphics link !
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Old 06-28-2017, 12:05 PM
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Glad you had a successful flight. See you can teach old planes new tricks.

You might be able to find the cowl here.

Goldberg Anniversary Piper Cub 40 Kit Parts Listing

Cowling & Accessories for Kits


Buzz.
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Old 06-28-2017, 05:24 PM
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Default Piper Cub Cowl

I have recently made a cowl for the plane. But thanks for those links. I'll take a picture of what it looks like with the cowl.
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