Cof G location in delta wing models?

[John Dennier]

I am building a second Avro arrow from scratch with help from a three view drawing plus a few cross sectional views. My first effort was overly nose heavy.

I had used a geometric plot prescribed on a website to estimate a CG location which was obviously in error. I am appealing for some wisdom on the subject of acquiring a CG location on delta wings and would be obliged to hear from any modeler who has used a process successfully.

Cheers JD

[Denis Watkins]

You have to use the online calculator John

And yes, the C of G looks odd, as it is way way back, further than normal wing,

And can tolerate a slight rearward c of g

[cymaz]

This is thread may help you.....I didn't quite get the gist of it

[graeme jones]

John - on a true delta i.e. a triangle, balance at 50% of the root chord, since that is 25% of the Mean Aerodynamic Chord. I've just checked my Pete Russell 363, with a root chord of 39" it balances at 19" from the tail. As you are building a scale aircraft, it should balance just forward of the main undercarriage.

Graeme

[Simon Chaddock]

The other aspect to consider is the degree of trailing edge reflex (if any) built into the delta. More reflex improves longitudinal stability but requires the CofG to be moved forward.

Remember also that a delta continues to create lift in a controlled way up to extreme angles of attack and the shape of the delta can effect the way the centre of pressure moves. It is possible to have a delta that flies quite normally but at high alpha it enters a situation where the elevator is no longer effective. It does recover but it requires considerable altitude to do so.

For models that have 'extreme' shape aerodynamics I like to build a 1/2 size simple Depron profile 'chuck' glider to get an idea of the likely CofG position and also to see how sensitive it is to small changes. A technique that worked very well for my EDF Concorde with its true scale wing.

[John Dennier]

Many thanks for your constructive comments gents. In checking the RC groups thread offered by Cymaz I can confirm that my CG was at the spot indicated by the red box. This location is at odds with the general drift suggested by the other responses. I wonder if the online calculator suggested by Denis will correspond with the rearward preference? Could you send me the online calculator web site please Denis?

The model I am building is made from blocks of pink insulating foam sculpted with various foam cutting wires around ply templates. The wing section was basically flat bottom curved upper surface and quite thin with zero degrees angle of incidence. Power is from two 70 mm fans powered by two 4s batteries. Another issue the plane exhibited was very twitchy aileron response. I had full span elevons and plan to reduce the span of these on the next effort.

Thanks again to you all, any further discussion on the above would be welcome.

Cheers JD

[Denis Watkins]

https://www.ecalc.ch/cgcalc.php

[Chuck Plains]

Here's the flying wing CG calculator I use.

**LINK**

And here's my previous result for my 55 degree delta.

It's great, because it changes shape to match your wing after you click the update button!

Oh yes, when you open that page click on "Extended version" at the bottom, it may be of more use

Edited By Chuck Plains on 14/05/2017 19:06:28