How to Set the Correct Centre of Gravity (RC Plane)

Another video in our "Sharp Focus Featurette" Series;
In this video we investigate why our Faltus (HF Model) Duo Discus was such a handful to fly at the recent Sherlowe aerotow meeting.
We also explain the "Dive Test" to check if the CG is correct
Suspicions (0:58)
Checking Tailplane Incidence (1:27)
Dive Check (2:07)
Visit our site; / scalemedown
C/G Calculator; rcplanes.onlin...
Music Link;
Lost Frontier by Kevin MacLeod is licensed under a Creative Commons Attribution 4.0 license. creativecommon...
Source: incompetech.com...
Artist: incompetech.com/

[@ianwalker8978]

Hi again, infomation and explanation you just keep giving us novices is superb, many thanks Ian.

[@ScaleMeDown]

Thanks

[@johnkinnane547]

G’day and greetings from Tasmania Australia, that was a very good explanation on how important the centre of gravity is and how it affects the flight characteristics of the aircraft. Thank you for filming that I have subscribed although I haven’t flowed for years since I was sixteen I am now 67. That’s one beautiful glider kind regards John

[@ScaleMeDown]

Thanks. Nice to hear from you

[@tomralph4301]

Really useful and helpful video 👍

[@ScaleMeDown]

Thanks

[@sideshowbob5237]

"If it pitches up (in the dive test) the centre of gravity is too far forward". Are you sure? The "Aircraft Centre of Gravity Calculator you refer to (correctly) states "For an aircraft to be stable in pitch, its CG must be forward of the Neutral Point (NP)". Since the NP is a characteristic of the model shape, how can moving the cg back increase the pitch stability. In my experience it certainly doesn't. Although you say you moved the cg bit by bit, you don't actually say whether you moved it forwards or backwards. The model as originally flown seems to have a centre of gravity too far back and the corrected model to have a more forwards cg. Increasing the tailplane lift by increasing the tail incidence - as you appeared to do - moves the net neutral point rearwards, making the model more stable. However I'm surprised and informed if making the tail produce more lift overcame moving the cg in the "wrong" direction to produce a stable model. Please tell us what you actually did to the cg position and tail incidence.

[@ScaleMeDown]

Hi. Yes, I am sure. The Neutral Point (NP) or more accurately the centre of lift is a function of the airfoil used. The centre of gravity must be ahead of this position for an aircraft to have static pitch stability. The further forwards the CG is ahead of the centre of lift, the more stable an aircraft will be at the expense of aerodynamic load on the tailplane. Also, the greater the distance between the centre of lift and the centre of gravity, the bigger the trim change will be with airspeed.
If the model is trimmed for cruise conditions, the dive test highlights if the elevator is deflected up to achieve straight and level flight (a shallow glide angle in this case). Hence, if the model pitches up strongly after releasing the stick in the dive, then there is too much 'up' trim, and the amount of nose weight should be reduced to reduce the overall trim drag.
This model required a change of tailplane incidence (LE of tailplane UP by around 1.5 degrees), and was almost unflyable without this change. This angle of incidence on the tailplane then required a reduction in nose weight to achieve stable flight (the centre of gravity was moved aft by 7mm).