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Dec 20, 2012, 08:49 PM
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Batteries


Finishing up my first Crash Test plane. Am getting ready to put my radio gear in . My question is this.. as far as getting in the ball park for correct CG what size batteries are you guys using? I was thinking two 3 cell 4000mah. Or maybe two 3000mah?
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Dec 20, 2012, 09:03 PM
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Wing Tips and fins


I pretty much built my deep reaper from the instructions on the CTH website.
However came across instructions on here also the RCgroups. I noticed there was some instuctions on wing tips that were not in the one on site. I have not put my fins on yet . Is there some cuts that i should have made on tips. I have stock wing tips with no modifications . Will i be o.k just slapping fins on with a little toe in??
Dec 20, 2012, 10:37 PM
Lee
Lee
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The instructions at the first of this thread are out dated although the basics are still there. Go with the instructions here http://crashtesthobby.com/products/grim-reaper. They are regularly updated.
Dec 20, 2012, 10:40 PM
Lee
Lee
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Quote:
Originally Posted by Dubjay
Finishing up my first Crash Test plane. Am getting ready to put my radio gear in . My question is this.. as far as getting in the ball park for correct CG what size batteries are you guys using? I was thinking two 3 cell 4000mah. Or maybe two 3000mah?
Lighter flies better but you still need to get CG correct and I am a believer in not adding lead when you can add batteries for longer flights. I think either way it will work depending on placement. Difference in weight between 2X 3000s and 2X 4000s will be 8+ oz.

Lee
Dec 21, 2012, 06:55 AM
Registered User
Quote:
Originally Posted by Dubjay
Finishing up my first Crash Test plane. Am getting ready to put my radio gear in . My question is this.. as far as getting in the ball park for correct CG what size batteries are you guys using? I was thinking two 3 cell 4000mah. Or maybe two 3000mah?
To add to what Lee said... It's really about how fast you fly and how long a flight time you want. The Reaper should balance fine with a single 3 cell 3000mah battery up against the forward spars. I use a 2300 mah battery and the flight time is fine for me. But I'm not usually flying full throttle either, and I keep a spare battery in my pocket.

You don't have to get it perfect the first time. It's easy enough to cut a new battery hole, perhaps stuffing the old one with a chunk of scrap EPP.
Dec 21, 2012, 07:58 AM
UH60 Pilot
E1Allen's Avatar
Quote:
Originally Posted by Lee
Lighter flies better but you still need to get CG correct and I am a believer in not adding lead when you can add batteries for longer flights. I think either way it will work depending on placement. Difference in weight between 2X 3000s and 2X 4000s will be 8+ oz.

Lee
I agree, none of the wings I've built from Lee required lead. Battery always went in last. If I've ever added anything just to make it that much closer it would be tape which just protects. Also I've built a few light and heavy wings. Seems like getting the CG right is the most important thing.
Dec 21, 2012, 08:32 AM
wheeeeee!
elCapitan's Avatar
Indeed, getting CG correct is the real important thing, here, and I think you should be able to accomplish that with either of those battery setups.

The instructions used to include a really good, widely used method to make sure you get the proper CG, but it seems to be gone now. Basically, once everything is laminated, and the elevons attached, you flip over to the bottom of the wing, and measure back from the nose the proper distance for CG. Then use a square to measure out from that, onto either side of the wing, and make symmetrical marks. (A line between the two would then be the CG axis). Place a pushpin in the marks.

Now flip the wing back upright, and lay out all your electronics, including the batteries, where you think you want them. Try to lift the wing from underneath, balancing it on the pushpins with your fingers. If it tips forward, you know you're nose-heavy, and have to move some of the electronics back. Vice versa for tail heavy. Once you get things as close as possible to balancing on those pushpins, mark where all your electronics will go, and then you can actually cut or melt the bays.

The good things about this method are that it's proven (lots of people build this way and accurately attain the proper CG), and versatile. In other words, it doesn't matter what wing you're building, or which electronics you're using. You can always make sure you're on CG by building this way instead of relying on measurements that may not work for your particular setup, or trial and error guessing based off pictures of someone else's build.
Dec 21, 2012, 08:44 AM
Aviation Enthusiast
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Thanks you guys . That helps a lot. Nice to get feedback so quickly!!
Cant wait to get this bird in the air. Is like no other that ive built thats for sure.
Dec 21, 2012, 08:53 AM
UH60 Pilot
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Elcapitan's method is what I usually do as well. I did that to make sure I could keep my batteries within the spars when I added the 2x8000mah batteries at 600g each. Almost 2.7lbs in batteries alone.
Dec 21, 2012, 10:59 AM
Registered User
Quote:
Originally Posted by E1Allen
make sure I could keep my batteries within the spars .
This is important. Once you've covered the plane it becomes hard to see the white spars, and even the extreme tape. So, it's not like you can just put the electronics and batteries anywhere you want. You can't cut the spars, and I assume you want to minimize cuts into the Extreme tape.

elCapitan, I think the instructions still include the "move the battery to adjust the CG" steps. I know it does for the Grim Reaper, because I just looked. Maybe it's not as explicit as before, but it's still there:

Installing the electronics.
...
b) Set the parts on the top of the plane where shown and see if the CG needs to be adjusted.
...
e) Move the battery forward or back to get the CG 10.5" back from the nose of the plane.
Last edited by michaelingp; Dec 21, 2012 at 10:59 AM. Reason: oops, misspelling
Dec 21, 2012, 01:40 PM
Lee
Lee
PERFECT LANDING !!!
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I see conflicting weather reports for today.

http://www.google.com/imgres?hl=en&s...2&tx=179&ty=82

http://www.google.com/imgres?hl=en&s...9,r:1,s:0,i:95

http://www.google.com/imgres?hl=en&s...r:11,s:0,i:130

Happy End of the World!!!!
Dec 21, 2012, 01:54 PM
Fly Now.
This too is how I determined gear placement. Forgive me if this was already mentioned, but be sure to have your winglets installed as this adds a significant weight behind the CG.

As far as batteries, on my DeepReaperXL with a 3536 motor, and lots of camera gear up front, I still needed nose weights with 2x 3300mah3S. I've recently bought some 4400mah, but haven't tried the CG yet.

If you're going with a similar setup as mine but don't plan on a lot of FPV gear, I would think about moving your motor closer to the CG than what is in the instructions. I was really worried about being nose heavy during the build, but the opposite happened.

-Parke

Edit: I just looked at your pict, and your motor looks pretty far forward already. You should be good shape.

Quote:
Originally Posted by elCapitan
Indeed, getting CG correct is the real important thing, here, and I think you should be able to accomplish that with either of those battery setups.

The instructions used to include a really good, widely used method to make sure you get the proper CG, but it seems to be gone now...
Last edited by Parke_ParkeFlyer; Dec 21, 2012 at 02:02 PM. Reason: addendum
Dec 22, 2012, 07:53 PM
Lee
Lee
PERFECT LANDING !!!
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We shot this video this afternoon of and with the 63" Herc Pusher.

Herc Pusher 63" with 808 #16D (5 min 20 sec)
Dec 23, 2012, 10:38 AM
Registered User
I fly my Grim Reaper with an NTM 3536 1800KV with and 8X4 prop. Overkill I know but this is what I like, power on tap. I recently experimented and put a 7x6 prop and a 4s battery in her and when I opened her up to full throttle the plane started to I guess you call high speed flutter. Am I going to fast is there a limit or is something wrong It flew fine as long as I didn't go full throttle. I switched back to 3s on 8x4, it's also less noisy. Any thoughts (I know this plane is not designed to be a speed racer)?
Dec 23, 2012, 10:53 AM
Registered User
Quote:
Originally Posted by cmaynex
I fly my Grim Reaper with an NTM 3536 1800KV with and 8X4 prop. Overkill I know but this is what I like, power on tap. I recently experimented and put a 7x6 prop and a 4s battery in her and when I opened her up to full throttle the plane started to I guess you call high speed flutter. Am I going to fast is there a limit or is something wrong It flew fine as long as I didn't go full throttle. I switched back to 3s on 8x4, it's also less noisy. Any thoughts (I know this plane is not designed to be a speed racer)?
Traditionally there are two causes of flutter. The first is a hinge gap, but most Reapers have continuous hinges. The other cause is play in the control rods or in the servos. You need to beef up the servos and control rods. CTH now recommends standard (40g) servos for the Reaper. If you're not going to fly combat, I'd recommend solid control rods, like dowels. The CTH instructions are optimized for combat survive-ability, and the thin rods and the staple used to minimize play in the rod are probably not adequate for really high speeds.


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