Centre of Gravity

[FiddleSticks]

Always been puzzled by people talking about moving the centre of gravity as if it's some sort of fixed point. On plans I often see it marked on the wings between 1/4 to 1/3 back from the leading edge as though it's fixed.

 

I've trained as a forklift operator on pivot steer forklifts instead of normal counter balance. The question of centre of gravity confused my colleagues but I've always understood it to be a theoretical point which can move - which was confirmed by our instructor.

 

So, what's with it on RC aircraft? From my POV it's just a case of adjusting weight to make it balance how you want it but in this hobby people refer to our like it's some fixed point on the aircraft (when in reality it changes with every change of direction, angle, even the fuel tank emptying etc).

 

Thanks for your input..

[Robin Colbourne]

There is the designer's intended centre of gravity position, or range, and the actual centre of gravity position.  If you want the aircraft to fly efficiently and controllably, you ensure the actual centre of gravity position is within the designer's recommended centre of gravity range; usually by adding or removing weight from the extremities.

[Geoff S]

The fixation with CoG is that if it's wildly wrong (particularly too far back) a model can be almost impossible to fly.  There's a broadly true maxim that a model with CoG too far forward will fly badly but one with it too far back will fly once.  It's not absolutely a fixed point but within a range and the variation can be personal preference.  If it's too far forward it can be difficult to get a model to spin for example.  It's certainly a very important parameter that needs to be carefully checked before a maiden.

[Cuban8]

Many years ago, a friend was training a beginner on buddy box when at altitude, the engine and firewall fell out of the model.

The model made a very hairy landing by flying as slow as possible and with full down elevator that was just enough to keep control.

We checked its balance point which was at the wing's trailing edge.

The engine remained unfound in the adjacent field of crops.

[Simon Chaddock]

The CoG applies to full size too.

Many years back I flew a commercial flight in an Islander to the Channel Islands. The pilot walked out with passengers and directed each to sit in a particular seat, no argument! The biggest went to seats under the wing. I was only slight so was directed to the rearmost one.

Only once all were in did a ground staff remove the prop from under the rear fuselage.

Getting the CoG within the range matters.   

[PatMc]

I nearly always adjust it over a few trial flights to suit my preference for the particular type of model.  

BTW, shouldn't we be referring to the lengthwise balance position not the CoG ?

[Chris Walby]

3 hours ago, FiddleSticks said:

1/ Always been puzzled by people talking about moving the centre of gravity as if it's some sort of fixed point. On plans I often see it marked on the wings between 1/4 to 1/3 back from the leading edge as though it's fixed.

 

2/ I've trained as a forklift operator on pivot steer forklifts instead of normal counter balance. The question of centre of gravity confused my colleagues but I've always understood it to be a theoretical point which can move - which was confirmed by our instructor.

 

3/ So, what's with it on RC aircraft? From my POV it's just a case of adjusting weight to make it balance how you want it but in this hobby people refer to our like it's some fixed point on the aircraft (when in reality it changes with every change of direction, angle, even the fuel tank emptying etc).

 

Thanks for your input..

Might be completely wrong, but point 1 Yes the C of G is a fixed point when its measured, if you add weights at either end then the C of G changes

 

So being a FLT operator you forktruck has a C of G, if you pick a pallet up then the C of G moves. Pick an excessively heavy pallet and the C of G will move outside of the FLT wheelbase and it will tip up

 

The C of G does not change for the model unless something changes or moves. Tank emptying, UC that retracts/lowers fore sand aft etc. For IC (as tanks are normally infront of C of G I check C of G with them empty. I have a very tricky foam model with canards which is very C of G sensitive, bad news is the C of G moves over 25 mm reward when the UC goes down (which makes it very lively just when you don't need it!). 

[Geoff S]

25 minutes ago, Simon Chaddock said:

The CoG applies to full size too.

Many years back I flew a commercial flight in an Islander to the Channel Islands. The pilot walked out with passengers and directed each to sit in a particular seat, no argument! The biggest went to seats under the wing. I was only slight so was directed to the rearmost one.

Only once all were in did a ground staff remove the prop from under the rear fuselage.

Getting the CoG within the range matters.   

 

When I was learning to fly gliders, the instructor always asked my weight so the CoG was right.  One of the club's regular pilot/instructors was a small woman who had to cart around a cushion filled with lead so she (and her cushion) kept the CoG within limits.  She also needed it when she flew a single seat glider.

[Peter Miller]

Once the battery fell out of my Moon Dancer. (The hatch was underneath)  Amazingly the model flew on and made a perfect landing out of sight several hundred yards away.

[Peter Miller]

A little story related to CGs.  

At one stage In the RAF I was on second line servicing of Vickers Varsities  (Also  known as "the Pig") and we ha one in the hangar up on jacks. About three of us were standing at the nose when the engine men removed a propeller.

All of a sudden the nose started rising up. It is had gone too far it would have come off the main jacks which would have caused a lot of damage.

We reacted fast and threw ourselves onto the nose wheel. How we all managed to cling on I don't know but it was enough to stop that aircraft coming off the  jacks.

Someone had forgotten to put the tail trestle into place!!!  

You never forget incidents like that!!!

[Christopher Wolfe]

As I understand it, the CoG is a point (usually) within the airframe such that if the aircraft is suspended (any way that you are game to hang it from) and a plumb-bob is dropped from the suspension point then the line from the plumb-bob shall always intersect at certain place.

 

This special place is usually referred to as the Center Of Gravity, CoG or CG.

 

Unless one adds or subtracts weight from a model then this cannot change.

 

Just be aware that the CG symbol printed on many plans does not take into account the vertical or lateral position of this symbol.

 

I actually have a model where the CoG is actually outside of the aircraft. It is called a 'Pushy Cat' and it is below and behind the pusher propeller of a mighty Mills .75 - I just hope that it won't get blown away by the propeller wash.

 

* Chris *

[Rich Griff]

I have no doubt that in a few moments lots of pictures will be posted showing ww2 aircraft flying with half a wing missing.....

[Martin Harris - Moderator]

I’m missing the point there Rich. 
 

Mention of WW2 planes though reminds me that people sometimes have problems establishing the “CofG” (longitudinal balance point) on typical fighters with low wing positions - and of course this applies to any low wing model…

 

The reason is that the true CofG is above the wing so balancing the model on fingertips or a jig is almost impossible - the slightest movement shifts the CofG in the same direction so the model tips rapidly as it’s inherently unstable.  The answer is to invert the model so the CofG is below the pivot point making it stable and allowing the slight nose down stance often recommended by the designer to be checked accurately. 

[Rich Griff]

See the c of g thread, the last one I think it was, during the summer.....picture of a "tank", thunderbolt, with half the wing missing....don't think my thunderbolt would do that when made....I will try and find the picture.

[Martin Harris - Moderator]

I’m still none the wiser!

[Robin Colbourne]

The centre of gravity changes as the fuel tank empties.  Particularly when you drain the fuel and forget that there is a tank in the fin:

[Shaun Walsh]

16 hours ago, Simon Chaddock said:

The CoG applies to full size too.

Many years back I flew a commercial flight in an Islander to the Channel Islands. The pilot walked out with passengers and directed each to sit in a particular seat, no argument! The biggest went to seats under the wing. I was only slight so was directed to the rearmost one.

Only once all were in did a ground staff remove the prop from under the rear fuselage.

Getting the CoG within the range matters.   

Many years ago I was on a flight from Liege to Heathrow via Calais.

At Calais three people built like Cyril Smith got on and the only seats were at the tail of a 15 seater twin turboprop aircraft. 

Needless to say when the last one boarded the plane sat on its tail.

The pilot and co-pilot laughed and the co-pilot shuffled down the narrow aisle and moved everyone around resulting in the aircraft resuming the correct attitude. 

As I was sitting directly behind the co-pilot I had a clear view of the instrument panel and watched as they waited for the engines to reach maximum rpm before releasing the brakes. 

I did that trip many times and had never before seen the end of the runway appear with all three wheels still on the tarmac.  We cleared the perimeter treeline at a much lower altitude than normal and both the pilot and copilot looked very relieved as the wheels went up.

 

Edited December 2, 2023 by Shaun Walsh

[Robin Colbourne]

1 hour ago, Rich Griff said:

See the c of g thread, the last one I think it was, during the summer.....picture of a "tank", thunderbolt, with half the wing missing....don't think my thunderbolt would do that when made....I will try and find the picture.

Rich, I'm not sure whether you are referring to the P-47 Thunderbolt or the A-10 Thunderbolt II, however the latter was designed to be able to lose an outer wing panel and half a tailplane and fin and keep flying.
There is also the Israeli F-15 Eagle which lost most of one wing, yet landed successfully (albeit a little faster than usual):

Edited December 2, 2023 by Robin Colbourne

[Robin Colbourne]

I've always wondered what happens to the centre of gravity on long aircraft such as the C-141 Starlifter when doing air drops.  That last pallet moves all the way from the front of the cargo bay to the back, then goes off the ramp.  Life must get interesting in the cockpit when its a twenty ton armoured vehicle going off the ramp.

[Rich Griff]

Hi robin,

 

Did not know about A10 or f15, both having relatively powerful "jet engines".

 

Yes, a paradrop must be "interesting" in the cockpit. The cargo roll time prior to leaving the ramp could be quite long, 15 seconds or so ?

 

But that aircraft would have all sorts of aerodynamic aides we can only dream off I think.

 

Tank, thunderbolt, p47 picture of one  flying with half of the starboard wing missing, kind of throws doubt on importance of c of g, for and aft and side to side....but I still thing if I made mine like that ( guillows r/p FF/ or control line, but will be made RC electric, and a sloper copy )t would only be airborne for a few seconds, 3 seconds probably.

 

 

[leccyflyer]

This is the picture that you are referring to,

 

 

Not sure what relevance you feel it has to this thread. The picture was posted to illustrate the sort of control compensation available for lateral imbalance, not particularly relevant to the discussion of this topic.