C of G sensitivity

[Chris Walby]

General question for the model gurus amongst the forum as I suspect there is no specific answer, but its opinions I am after.

I assume some aircraft are more tolerant of C of G movement and some less so, my question is what affects this?

As an example I have a foam EDF that is 1500mm long, AUW 2.5Kg and its C of G moves backwards when the gear goes down by close to 25mm. From gear up to gear down this causes a noticeable pitch change (which with a bit of mixing I can sort out if I can keep it in the air long enough!) and sensitivity. It’s the sensitivity change which is the surprise/hard work aspect. If I move the C of G forwards I reduce the sensitivity, but with wheels then up run out of up elevator

Examples of aircraft C of G tolerant and non-tolerant please?

[Klippy]

Hi Chris, I have found the Multiplex FunCub and Blizzard to be very tolerant, but at the cost of increased elevator sensitivity the further back you go. I have a feeling that this is normally the case.

[Denis Watkins]

We always compromise don't we Chris, the decision made for C of G set with wheels up, due too the greater amount of flight time made with wheels up.

Flying IC, we also consider tank empty for set up, so take off is slightly forward C of G.

My preference is for good elevator for landing, so set up in between, depending on the model.

Contraptions, or should I say devices, have been used to slide a small counterweight forward and aft with the retracts

[John Lee]

Aircraft with a large tail volume (tailplane area x tail moment) tend to have the less sensitivity to the CG position, but still in every case become more sensitive/responsive as the CG moves rearwards.

All the classic sports models & trainers are more tolerant to CG shifts: e.g. The Sticks, Pulses, Maricardo, SportAirs, Discovery, Arising Star etc etc

[Bob Cotsford]

In my experience the CofG sensitivity is dependant largely on the tail moment (distance between tail and wing) and the tailplane area. Wing section can be a factor too, and a larger LE radius seems to help. Higher airspeed will magnify trim variations put in to balance out CofG changes so that could be counted as a secondary factor.

So a slow flying model with lots of tail area and a long tail moment with a thick section wing will be more tolerant than a flying wing with a thin section and a big motor, though sweepback seems to help on a flying wing.

[Former Member]

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[Former Member]

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[Bob Cotsford]

And mine crossed with John's, so we're all in agreement! One of the worst models I had for CofG intolerance was a constant chord 'Little Bit' flying wing with no sweep, one of the best was a '362' delta.

[Former Member]

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[Biggles' Elder Brother - Moderator]

I agee with the above, but one slight different way of looking at this is via the whole stability issue.

If you take a typical trainer, long tail moment, big tailplane and probably generous dihedral. This will have a high pitch and lateral stability margin. The precise fore/aft location of the CoG will not be critical. Any change brought about by moving the CoG will be slow and gradual. There is a "best" position but its actually quite hard to find because if you imagine a graph of "how well it flies" vs CoG position it would be a rather broad hump. Yes it has a peak, but not a well defined one.

Now contrast that with something like, say, a Pitts Special. Very short tail moment, small tailplane, no dihedral.This aircraft has a very small stability margin. In fact it might even be "marginally stable" ie it's boarding on being actually unstable. The graph for this would have a very localised sharp peak, there is a very small "sweet spot" - outside that narrow band lies much unhappiness! So the position of the CoG is critical, small changes of a few millimetres can have quite dramatic effects.

But of course these two aircraft are designed for very different purposes. The first needs to be forgiving, both in set up and in flight. Hence the high stability. But it pays a price for that, it's very slow to react to control inputs and needs quite a heavy-handed, very positive and definite, control input to produce a change.

On the other hand the Pitts is an aerobatic specialist, agility is everything. Its low stability means it reacts very swiftly to even small control inputs. It also means its has no "prefered orientation". You can't make an aircraft that is equally highly stable in any orientation. Trainers have a very definite "this way up" sticker on them! Try flying a trianer inverted for any length of time - you'll find its very laterally unstable like that, difficult to fly and constantly wants to right itself! The Pitts however is equally happy in any orientation - it accurate to say its equally unstable in all orientations!! Of course you need this in an aerobatic aircraft. If you put it in knife-edge you don't want to have to fight it to keep it there.

So, in short, as a generalisation CoG positional tolaerance tends to go with naturally high levels of stability. CoG positional criticality tends to go with agility and marginal levels of stability.

BEB

[Former Member]

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[Piers Bowlan]

Chris, you say that there is a pitch change with the gear going down in flight but you don't say whether it is a negative or positive change. I assume it is a negative change (the model pitches down when the gear is lowered) causing you to dial in some up-trim. So perhaps it is not the C of G that is changing but the extra drag from the 'dunlops' dangling in the breeze, moving the centre of drag downwards and consequently a pitch down couple.

I remember flying my Little Bit wing with a Cox babe bee and I concur it was the most critical model for C of G, like many flying wings. It was also the noisest!

[Bob Cotsford]

My Bit started with a Cox 049 but ended it's life with an electric motor, on 2S it was nimble, on 3S it was just plain daft. A bundle of fun - but only as long as the balance point wasn't allowed to move back! Very pitch sensitive, a few mm was all the movement needed on the elevons.

In contrast my King Altair aerobat with it's extemely long fuselage, large tailplane and thick wing really couldn't care less where you balance it between 25% and 50% chord. All that happened was it needed more or less movement on the elevators.

[Ken Lighten]

I built an enlarged ‘Bit’ of about 55” span many moons ago (before computers made that sort of thing easy!) and it flew brilliantly (till I forgot up is down when inverted!) and didn’t seem too twitchy at all, balance point would have been as per the original plan and any reflex was purely on the elevons. The wing section was quite thick if I recall correctly

[Bob Cotsford]

Ken, I bet you never experimented with moving the balance point back

[Former Member]

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[Ken Lighten]

Bob, nope! 😁

[Ken Lighten]

As an aside, my avatar is/was another slightly enlarged plan for a forward swept wing slope soarer (Predator?) that I modified to take a .25, all I did was to make the wing section a symmetrical one using the top surface of the original as a pattern, the original had fixed canards (I think) which I dispensed with due to the tank etc, again, the balance point would have been as per the plan and it flew superbly, I also added a rudder and could knife edge relatively well, can’t remember what happened to it but I know I didn’t scrap it. I reckon I must have got lucky!, might do another now thinking about it.

[Chris Walby]

Thanks for all who have contributed...I'll read through in detail and digest, then see if it helps with what I have.

[Former Member]

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