CG position 25% MAC versus supplied instructions

[Graham Bowers]

Hello All

 

Last time I flew my Hobbyking Sport Jet 70mm was off tarmac (Fradley) and I didn't do much more than set the CG to instructions and a couple of beeps of trim to get straight and level, and it only got a couple of flights. For me at the time, it was small, fast and scary!
 

Fast forward about 10 years or so and it's now dusted off and back in the air - just. I'm flying off a small grass strip that's mown to around 25 metres square, and it's into the rough before unsticking. It needs a LOT of "down" to maintain level flight when inverted. I reckon that can be improved.

 

CG is set to the instructions, so at the back of the given range of 65 to 75mm rear of the LE at the junction of the wing and fuselage.
 

Having recently read a certain book on precision aerobatics, I set about establishing where the CG position would be if set to 25% of Mean Aerodynamic Chord. I was surprised to find it came out at 110mm rear of the same datum – so 35mm rear of the instructions. I’m happy with my sums as for a sanity check I calculated the wing area inboard and outboard of the MAC line and they are equal. I generally like my propeller models set with a small static margin, so a touch of down to maintain level flight when inverted.
 

I realise there are other factors such as main undercarriage location, wheel size etc – however they’re fairly basic stuff.
 

However the CG discrepancy seems huge, or am I missing a trick here?

 

[Frank Skilbeck]

Probably not, they probably just flew it from tarmac at that setting and said yep good to go., a good program to calculate the c of g is the one in Ecalc, it even allows you to take into account the fuselage.

 

But rather than jump to 110mm you could move the c of g back in steps (say 10mm at a time), until you reach a point that suits you.

 

 

 

 

Edited September 2, 2022 by Frank Skilbeck
Removed video which wouldn't play

[Shaun Walsh]

37 minutes ago, Frank Skilbeck said:

Probably not, they probably just flew it from tarmac at that setting and said yep good to go., a good program to calculate the c of g is the one in Ecalc, it even allows you to take into account the fuselage.

 

But rather than jump to 110mm you could move the c of g back in steps (say 10mm at a time), until you reach a point that suits you.

 

 

 

 

I agree, cgCalc is very good. A fellow club member had a Hunter PSS that just wouldn't fly and suspected that the CG was wrong. Ran the dimensions through cgCalc and it recommended a CG some way back from the original position. After the removal of some nose lead it flew perfectly.

[Graham Bowers]

48 minutes ago, Frank Skilbeck said:

Probably not, they probably just flew it from tarmac at that setting and said yep good to go., a good program to calculate the c of g is the one in Ecalc, it even allows you to take into account the fuselage.

 

But rather than jump to 110mm you could move the c of g back in steps (say 10mm at a time), until you reach a point that suits you.

 

48 minutes ago, Frank Skilbeck said:

😉Yes, a step-wise approach would be the way to go . Edited to add, I see I messed up the quoting ................

 

 

 

Edited September 2, 2022 by Graham Bowers
Messed up the quoting. Didn't fix it, either!!

[Graham Bowers]

2 minutes ago, Shaun Walsh said:

I agree, cgCalc is very good. A fellow club member had a Hunter PSS that just wouldn't fly and suspected that the CG was wrong. Ran the dimensions through cgCalc and it recommended a CG some way back from the original position. After the removal of some nose lead it flew perfectly.

Thanks. Two recommendations for cgCalc makes it appealing. I had a quick look and see that it's based on the same methods I used manually. I'll give it a proper read though.

[Martin Harris - Moderator]

The “magic” 25% only works with a conventional tail layout. If the tail is unusually large or small with shorter or longer coupling, the “correct” C of G could be anywhere from a fraction of an inch behind the leading edge (e.g. a tailless model) to over 50% on a vintage style lifting tail model. 
 

A proper C of G calculation takes the tail moment into consideration. 

[Jesus Cardin]

Graham, your considerations are totally correct and I am a real believer that a 25% MAC Centre of Gravity is just a WINNER!

 

In fact I always calculate my own CG based on the 25% rule against the proposed one in the instruction manuals and it always proved safe and successful but...... ONLY FOR PROPELLER DRIVEN PLANES.

 

In jet models -being EDF, glow Ducted Fan or turbine powered- other factors add to the calculation, mainly the fuselage lift contribution but also the normally long tail moments as Martin already explained. In model jets it is normal, referred to MAC, to get the proper CG between 15% and 20%, so most probably in your Sport Jet HobbyKing did his job correctly!

[Jesus Cardin]

Extracts from Jet expert Mr. David James´  classic book "Ducted Fans for Model Jets" regarding C.G. location on this type of model....

 

 

[Graham Bowers]

Many thanks for all inputs, this has been a real education.

 

The as-is situation, with the CG at 75mm rear of datum, is 9% MAC. 15% MAC would be 88mm rear of datum.

Think I'll pull it back 10mm and see how we go.

 

The main undercarriage would then be 95 mm rear of the CG, that seems like a long way. Admittedly, all I'm going on is viewing some plans in Outerzone - so hardly a comprehensive study. Anybody care to offer an opinion on main undercarriage location please? I know I can do the experiments, however wantonly going off the strip into the rough grass on takeoff makes me cringe, apart from risking damage.

 

[Denis Watkins]

Nosewheel where room and best fit under front canopy instrument bulge, and

Main wheels, just behind the C of G

[Graham Bowers]

13 hours ago, Jesus Cardin said:

Extracts from Jet expert Mr. David James´  classic book "Ducted Fans for Model Jets" regarding C.G. location on this type of model....

 

Thanks for drawing my attention to this.  A copy now winging it's way to a bookcase near me 😉

[Jesus Cardin]

9 hours ago, Graham Bowers said:

Thanks for drawing my attention to this.  A copy now winging it's way to a bookcase near me 😉

 

A real honour and a pleasure being of help and inspiratiuon, Graham!

[Simon Chaddock]

Graham

As suggested don't ignore the aerodynamic effect of the fuselage, particularly if it has significant area ahead of the CofG.

Any area ahead of the CofG is more 'effective' aerodynamically than area behind, as canards tend to show.

A configuration may be dynamically stable at a low angle angle of attack but fuselage area alone can seriously erode the stability margin as the AOA increases. A fuselage may not really stall so can continue to provide a lifting force well after the wing itself has stalled.

 

My most extreme example of this effect was my Bachem Natter where the fuselage area is a considerable compared to the wing and tail areas. On the other hand the tail moment looks almost generous.

  

Given that models tend to go to the more extreme areas of the flight envelope it did not exhibit 'safe' flight characteristics until the CofG was at just 16.6%!    

At a normal 'safe' 25% above a certain AOA it would tend to pitch up by itself. Recovery from the resulting stall was by no means certain.

[Graham Bowers]

3 hours ago, Simon Chaddock said:

Graham

As suggested don't ignore the aerodynamic effect of the fuselage, particularly if it has significant area ahead of the CofG.

Any area ahead of the CofG is more 'effective' aerodynamically than area behind, as canards tend to show.

A configuration may be dynamically stable at a low angle angle of attack but fuselage area alone can seriously erode the stability margin as the AOA increases. A fuselage may not really stall so can continue to provide a lifting force well after the wing itself has stalled.

 

My most extreme example of this effect was my Bachem Natter where the fuselage area is a considerable compared to the wing and tail areas. On the other hand the tail moment looks almost generous.

  

Given that models tend to go to the more extreme areas of the flight envelope it did not exhibit 'safe' flight characteristics until the CofG was at just 16.6%!    

At a normal 'safe' 25% above a certain AOA it would tend to pitch up by itself. Recovery from the resulting stall was by no means certain.

Many thanks for that insight Simon.

I hadn't come across the Natter before, so just did some interesting reading.

Thought I replied to this earlier, so if a duplicate occurs, it's gremlins.