c of g suspended from ceiling thingy!

[Steve Dunne]

10 hours ago, Bruce Collinson said:

oo err ... 3rd para might be capable of misinterpretation ...

???

[Ron Gray]

Just seen this from Dane RC

 

C of G balance

[Peter Jenkins]

Looks nice and substantial.

[Ron Gray]

Plus it has its own storage box!

[Peter Jenkins]

You could probably make one yourself.  Thing is, by the time you've bought all the bits it might cost as much!

[Ron Gray]

If my experience with their Leprechaun is anything to go by it will be extremely well made and will assemble really easily.

 

[John Minchell]

Ron, that Dane RC device looks like a rip off of the MPX design which I have been using for the last 20 years+ 

Apologies to Dane if it is not, but the working principals are exactly the same.

MPX is cheaper too - from Nexus (and other MPX vendors).

Edited September 26, 2021 by John Minchell

[Ron Gray]

I think that the Dane one can handle much larger ‘planes than the MPX one can. Looking at the 2 I wouldn’t say that it’s a rip-off the MPX one!

[Andy J]

Can I open this old thread as I have an issue establishing the location of the CoG on a heavy 83 inch model.

 

Would have liked to build a Vanessa rig but lack a suitable hanging point from which I can suspend the rig from. Therefore wondering if I can use a measurement of weight as a method, however the lack of a clear forward reference point due to wheel spats and forward rake on the undercarriage leg prevents using the main wheels as one of the measurement points.

 

What I propose to do is given the underside of the fuselage just in front of the wing is flat is simply to block this up with a piece of timber such that the model weight is taken up by the support. Place the scale under the wood and measure W1 (of course subtracting from the measurement the weight of the support wood). Measure then the weight at the tail wheel ensuring the model remains level in pitch between the two measurements.

Position of the CoG would then be determined by the equation W1xL1/(W1+W2) as declared in an earlier post to this thread where L1 is the distance between the two measurement points.

 

[Peter Jenkins]

An alternative solution is to sit the model level flying attitude and stick some masking tape to the fuselage so you can mark the position of the mains and nose/tail wheel.  Make 2 blocks the same thickness of your weighing machine (I'm assuming you are using a digital scale) then move the scale and thd blocks round to get 3 weight readings from main wheels and tail wheel (or nose wheel) that will give you total weight and mains and tail/nose weight.  Use a plumb bob or any piece of weight on a piece of string and use that to mark the wheel positions on the masking tape.  Measure the distance between the marks and then plug those into the equation and that will give you the CG position.

[Andy J]

Was allowed to play indoors today given it was wet and there was no way the model would fit in the workshop.  Disappointed to find the CoG was way way back from the desired point using the method of measuring under each wheel. So yet more church roof is required.

 

Do have my doubts as to my measurement technique of using a simple kitchen scale, so will now build a pivoted support as pictured at an earlier point in this thread just to give me some confidence in my measurements.

[Zflyer]

I take it the Ecalc available on the 'net hasnt been of use. I have used it quite successfully

[Andy J]

Just took a look at that site and it does seem a little over the top as I simply used an XL spread sheet with the equation W2xL/(W1+W2) once that distance was calculated I then added an extra measurement between the wing LE and main gear wheel ground contact point (given the mains wheels are inboard of the LE) to get a traditional reference point for the position of the CoG at the wing root chord.

One observation was that I did not bother adding shims under each main wheel which replicates the depth of the kitchen scale to ascertain the measurement W1. 

[Peter Jenkins]

A simple kitchen scale is what I use Andy.  You don't need to get the CG position absolutely right just in the safe zone.  The only real way of checking CG position is to fly the aircraft and see how it responds.  You can just roll inverted and see if it heads down hill quickly!  Just roll back to upright to recover.  Then move the CG aft by adding say a 10 g lead weight (wheel balance weights are good) until when you roll inverted the aircraft doesn't need a vast amount of down elevator to fly level.  If you barely need any down or else you need a tad of up - you have gone too far aft!  Remove the weight that caused this situation.  Go fly.  Always do the check with the tank near empty though.  If you are electric then you don't have that problem as electrons don't weigh anything!

[Andy J]

Agree with your second statement Peter that the CoG does not need to be perfect but unfortunately it seems that I am a long way adrift as my first measurement came out as being some 60mm aft of the desired point. By adding more lead to the nose managed to get it down by 30mm so will need to think how to proceed to gain the other 30mm

Could remove weight at the tail but this would be a major undertaking, moving the three flight batteries further forward are possibly a simpler option.

[Andy J]

Having moved two of the flight batteries to just rear of the engine bulkhead,  spent another morning checking the CoG using the kitchen scales. Calculation is indicating the CoG is still a good inch rear of the desired point but after adding yet more lead to the nose I hit the 5Kg limit on the scales which prevented any further checks.

Therefore spent the rest of this afternoon knocking up a simple balance support system similar to the post by eflightray in September but yet to try sitting the plane on top so don't know if its any better than the weight calculation method.

Given the model is now over 20lb also beginning to doubt the DLE 35RA engine as fitted will be up to the job. Should have purchased a DLE 55RA but thought at the time the suggested power of 62cc was excessive.

 

[Andy J]

Well the balance rig works a treat and with adjustment of the CoG to 6inches back at the root, I managed to remove just short of 2lbs of lead from the nose.

 

Took the picture with the model supported on the forward stop of the rig but obviously the measurement was taken with the model in a neutral attitude.

[Peter Jenkins]

Well done!

[Paul De Tourtoulon]

I prefer balancing low winged planes upside down, good luck with the first flight.

[Peter Jenkins]

Andy  In your post of 3 Nov, you said you had a way aft CG.  However, in your post of today, you said you'd removed 2 lb of lead from the nose.  I presume your 3 Nov post you meant nose heavy and not tail heavy?

[Andy J]

Well using the kitchen scales Peter the value I was aiming at was 5.4 inches but the calculation was giving me values a good inch further back ie 6.4 inches. Was therefore surprised that using the balance method the CoG indicated close to my original aiming point so it was a good inch difference between the calculated weight method and the balance method.

 

Having more confidence in the balance method I was then able to lower the weight in the nose accepting a revised CoG of 6 inches which others back in 2011 had recommended as a good starting point.

[Martin Harris - Moderator]

With a tail wheel, there may be a significant difference depending on the attitude it’s measured at. Normal full size practice is to raise the tail to a predetermined point - nominally flying attitude - while taking the measurements. 
 

Calculation depends on accurate measurement in correct relation to the datum. 

[Andy J]

Did raise the tail Martin to assume a flying attitude. The error is possibly the scales as each measurement on the main wheels was close to the scale maximum of 5kg.

 

Laser was also used to determine the lever arm and main wheel datum so I dont think the length measurement was out by much.

.

[Peter Jenkins]

You can use a 4 kg weight to zero the scale and then you would be within the scale weight.

[Paul De Tourtoulon]

What's all the fuss about weight ?, if it is pretty straight and powerful enough and the balance point is right it will fly, ok a bit of weight will make it fly a bit quicker but if fly it and see for yourself.

 

 

 Peter, you are right about the weight planes fly better in kilos,,,my old LMC Hawk worked out at 136 grams D2, it was fast with a nice slow nose up landing ( like a delta ).