grosbeak

After I put my Pulse 125 together I did a preliminary CG check - it's definitely tail-heavy. I found an old thread about calculating counterbalance weight and I figure I can apply it here. So:



I'll check CG again and note the weight where the top of the vertical stab touches. I'll measure the distance from the CG to the stab contact point and the distance from CG to spinner and I should be able to calculate how much weight I would need. Before I add any weight, however, I'll moved what I can forward and check it again.

ram3500-RCU

My Feedback: (221)

The weight needed to balance will vary depending on how far forward you can install it. The farther forward it is, the less you will need. I see no use for the scale under the tail for this reason.

SeamusG

Someone shared a rule of thumb (guideline) with me - 1/4 oz. at the tail is equivalent to 1 oz. at the firewall. Once you have it close to allow you to safely maiden then 1/4 oz. on the tail will move the CG rear 1/8", 1 oz. at the firewall will move the CG forward 1/8".

ram3500-RCU

My Feedback: (221)

Yes, that is the time-honored rule of thumb. This varies somewhat with the type aircraft. For instance, a WWI bi-plane has a very short nose. More is needed in the nose of that type. The best way is to physically set the plane on it's designed CG and move necessary weight like batteries around, then add as little 'dead' weight as possible. When extra ballast is needed, our Steel Powder works great. Mix it with epoxy to make a very heavy paste and simply pour it in cowls, or around firewalls. It can be used it wing tips as well for lateral balancing. A slightly heavy right wing is a good thing.

Scar

My Feedback: (3)

Grosbeak wasn't asking a question, guys. He shared the solution, he showed the calculation.

Very good, and a good illustration, too. Might be good to point out that the balance weight "y" doesn't have to go at the spinner. You can pick the mounting location (b) for your balance weight wherever you want it, the solution is still correct.

Best wishes,
Dave Olson

SeamusG

Hey ram - I was born, raised and graduated from N. Canton. [8D]

Gotta say that I cannot for the life of me tell if a plane is nose down, tail down while I'm holding the darn thing. My approach is to set up the plane on my build table (hollow core door supported by a couple of saw horses) with a line bubble on the stab. I then adjust the plane until the stab is level (incidence 0) by shimming the wheels. I mark the target CG with a piece of painter's tape. Then I use the fingers at the target CG point on the wing to lift. If the tail dips - tail heavy, if the nose dips - nose heavy. Easier to set up than my Vanessa rig. Gets me close enough to maiden.

Yea, yea - on low wings it's upside down so I'm using bags-of-sand to support the fuse.

Cheers,

SeamusG

How's about moving thread to "Tips and Techniques" instead? That way the reader has the expectation that it is a suggested approach. Us dummies just "ass"ume that it was a question, not a statement.

captinjohn

I 2nd the motion to move it too Tips and Techniques" instead.......................

ram3500-RCU

My Feedback: (221)

What year did you graduate? I'm the class of "71". And I have 6 kids that graduated from Hoover as well.

ram3500-RCU

My Feedback: (221)

Thanks for the tip. I'll continue to do it in the balancing process. I find it very easy to do without actually even knowing how much it is taking. It is what it is.

MinnFlyer

My Feedback: (4)

MOVED TO TIPS & TECHNIQUES

Ok, a few observations...

First, I think you're WAY over-analyzing th situation. Place the plane on a balancer (Or lift it at the CG) and place chunks of weight on the firewall area until it balances - No math involved!

Second, Ram, I think you've got a great product there, but there is no way in hell that I would add weight to a cowl! Most cowls are held in place with 4 screws going through the fiberglass wall. Adding all that weight to the front of a cowl - then factor in vibration - and I can see those cowl mounting holes opening up like crazy. Weight should be added to the firewall, engine mount, or something similarly SECURELY attached to the airframe.

Third, in the diagram, it shows adding weight to the spinner. Again a really bad idea! Throw a prop in flight, and you now have to land a tail-heavy airplane!

Square Nozzle

My Feedback: (1)

Gary, where will I find steel powder? I like the idea of being able to make ballast to conform to the irregular surfaces. I've always used lead bar which I am able to shape to the rough configuration of the area. I think your suggestion to use steel powder rather than lead is the powder can be made more dense and conforming than lead shot or bar. PM sent. Thanks.

grosbeak

That's true, Dave, and thanks for pointing that out. I was pleased to find the calculation and I wanted to share it - and before I moved things forward or added weight I wanted to know what I was shooting for - especially since the inverted airplane wound have to come off of the CG stand before anything could be done.

The measurements:
[ul][*]a = 36-5/16"[*]x = 2.18 oz[*]b = 17.5"[*]y = ax/b = 4.52 oz
[/ul]

The idea to add weight inside the spinner came from an online RC guide. After learning more here and elsewhere I can see why this is a bad idea. As it happens I was able to move the ignition and its C-sized battery pack forward a couple of inches. I'll re-balance the plane when I can and re-weigh the tail. If I still need more weight I'll see if it can be added to the back of the firewall. I'll take the new measurement for b and I can proceed from there.

I'll let you know how it goes.

kenh3497

Full scale (experimental in particular) use the three scale method to calculate CG. One scale under each wheel.

It would be hard to suspend a home-built upside down to check weight and balance Just another way to accomplish the end result. Kind of like walking east around the world to to wind up west one block of where you started.

Ken

raptureboy

I was thinking the same thing about the weight in the cowl. I have added some weight to a cowl before, maybe an oz, but 1/2 a pound or so?? I would agree it's going to weaken the cowl mounts unless there is some kind of heavy duty mounts. Still I like the idea of steel powder that can be moulded into shapes without having to heat it or deal with toxic fumes.

pkoury

My Feedback: (7)

[quote]ORIGINAL: ram3500-RCU


Explain to me why a slightly heavy right wing is a good thing? Along with the basic CG check I always check the lateral balance, a heavy wing will make flight trimming harder.

gsoav8r

My Feedback: (6)

Thats the method I use for large or heavy models that I do not want to put on any type of balance machine. (have never understood the need for a vanessa either)
Only difference might be is I dont use three scales. I use one scale and two blocks/chocks that let me note the weight at the wheels with the horizontal stab level. An added bonus of the method is that once you note the weight on the right and left wheels, you can also calculate the difference in the lateral balance.

kenh3497

Three scales is the "high end" way of doing it. I've seen your method used many times with a bathroom scale.

grosbeak

With the battery moved forward the weight to x is 1.87 oz. From the CG to inside of the firewall is 10-3/4". Long story short, 6.3 oz needed. I thought I might try killing two birds with one stone, the other bird being a combination of large prop, flexy landing gear and a reported tendency to nose over while taxiing... so I turned the gear 180ΒΊ.



I'm in the process of reversing the wheel pants now.

bkdavy

My Feedback: (1)

An inexpensive digital kitchen scale is a great tool to keep around. They're usually accurate to tenths of an ounce, and don't require suspending a plane upside down. I use my old encyclopedias for blocks under the wheels (about the only thing they're good for anymore). You can also pick your reference point to make the calculations easier. Normally I just use the main landing gear as the reference point. Then the hardest part is measuring the distance to the tail wheel from the mains, and knowing the desired CG relative to the mains.

I've tried the string rigs, balance stands, etc. The scale method is the easiest for me.

Brad

Lnewqban

Be aware that by advancing the front wheels you are reducing the yaw stability during take-off and also increasing the pitch-bouncing effect during touch down.

Wheels in line with the leading edge of the wing has proven to be a good compromise.

http://www.rcuniverse.com/magazine/a...article_id=866

jester_s1

Yes, the takeoffs and landings with the LG that far forward are going to be a huge pain.

gsoav8r

My Feedback: (6)

IMO if the plane is slowed well in ground effect and the nose is up, tail down... close to a three point landing, the plane will settle in fine. Have flown some friends planes with gear flipped around like that many times. Not my preference thou. Also Low rate rudder or an increase in expo can take care of the increased sensitivity of the rudder [while on the ground].

For what its worth, I fly from grass and in the summer when the grass is thick ground handling can be challenging sometimes. But most of the time taxiing, taking off and landing with high rate elevator cures the tipping issues. Once the planes in the air I can go to low rate elevator if desired.

Cheers.

Bundubasher

Man, a PULSE is one of the easiest and nicest planes to fly - they have no bad manners. If it is a H9 kit, just follow the cg instruction.
Don't get too technical, enjoy flying it...

Cheers


Bundu