Need help building a large Mk3 plane

[TheNoobHunter24]

I've started doing replica planes and I decided to do a Ilyushin Il-78, however it's my first big plane like that. It gets going to around 150 m/s on the runway so speed isn't a problem, but it won't get off the ground even a tiny bit. Picture of it:  

 

[qzgy]

Your CoL is wayyy too far back relative to your CoM. Either move the wings up or mass back. Also helps to tilt the wings back 2 or 3 degrees to create angle if incidence, which helps with lift while also reducing drag.

[TheNoobHunter24]

I think it might be just far too heavy. I got the CoL a lot closer to the CoM and got the weight down to 60 tons but it still doesn't get off the ground on it's own, and only glides a little when it goes up a slope. Also after pressing F12, the lift lines are incredibly high.

Edited August 20, 2017 by TheNoobHunter24

[Foxster]

Where are the rear wheels in relation to the CoL? 

My starting rule-of-thumb is to have the CoM a ball's width in front of the CoL and the stanchions of the rear wheels in-line with the CoL...

 

[Wanderfound]

5 hours ago, TheNoobHunter24 said:

I've started doing replica planes and I decided to do a Ilyushin Il-78, however it's my first big plane like that. It gets going to around 150 m/s on the runway so speed isn't a problem, but it won't get off the ground even a tiny bit. Picture of it: 

Does it fail to rotate (front wheel stays on the ground) or does it rotate (lift the front wheel) but fail to lift off despite that?

[Spricigo]

I'm blind or the horizontal wing/elevon in the tail is missing?

[Curveball Anders]

26 minutes ago, Spricigo said:

I'm blind or the horizontal wing/elevon in the tail is missing?

Hard to spot but T-tail.

[TheNoobHunter24]

It doesn't get off the ground at all. Front wheel stays firmly put on the runway.

[Wanderfound]

52 minutes ago, TheNoobHunter24 said:

It doesn't get off the ground at all. Front wheel stays firmly put on the runway.

In that case, the problem is some combination of poorly placed landing gear and insufficient pitch authority.

The rearmost landing gear should be just behind CoM. Judging by the picture, that seems to be already okay or close to it.

The elevators require sufficient power to control pitch. Are you certain that pitch is activated on your horizontal tail surfaces? And is it deactivated on the ailerons and flaps on the wings?

If you try to lift the nose while the plane is stationary, what do the control surfaces do?

[Gaarst]

I think @Wanderfound's got you.

I build a similar plane to yours, trying to match the overall layout and characteristics; and I could get it off less than halfway across the runway, while under 100m/s and with half the engines.

I've made the plane similar to that of your screenshot, but I have made changes to bring the CoL and CoM closer together. Other than that, I have a similar lifting surfaces, 2 Goliaths instead of 4 and a very similar mass.

As I couldn't see the tail very well on the screenshot, I've made something out of type C wings, with pitch controlled by four Elevons 1. Even with the improvements, the pitch authority is very small and the CoM/CoL mismatch means that the pitch is almost at maximum just to keep the plane levelled (see the Pitch gauge on the screenshot, I wasn't touching controls then).

Seeing your original craft, It's no wonder you couldn't get it off the runway: the pitch authority is way too insufficient to overcome the CoM/CoL induced pitch-down you seem to have on your screenshot. You have enough lift, more than enough power and your weight is very reasonable.

 

As been said before, to increase the pitch authority you need to move your CoL/CoM closer together by moving your wings or fuel around and to use larger control surfaces on the tail. Moving your rear gears closer to the CoM will help you take off, but your plane will still dart to the ground if you can't keep it levelled.

[TheNoobHunter24]

Thanks! I forgot to make an update, but I made it a bit longer and took fuel out to lower the weight but..it got off the ground and flipped over (Like nose kept going up and plane goes upside down and over and crashes) . Landing gear placement and pitch control IS generally the usual issues my planes tend to have, so it's kinda expected. This is again also first mk3 plane I tried to make, everything before that was mk1 fighter jets.

 

Update: It gets off the ground on it's own now and has great pitch control. Problem now is roll control.  I'm not sure how to describe it but its awful. It moves the pitch a little as it does it and takes forever to respond. Currently its two Aeroplane control surfaces under the wings but gonna experiment with it as I feel thats why it messes up the pitch. Before it was on the tail wing but it wasn't strong enough either there.

 

Edited August 21, 2017 by TheNoobHunter24
Just adding a statement.

[Wanderfound]

Isolate your flight controls: set elevators to pitch only, rudder to yaw only, ailerons to roll only.

If you still have instability in the roll axis, you can correct it with dihedral. Angle some of the lifting surfaces (wings or horizontal stabiliser or both) up at the wingtips.

https://en.wikipedia.org/wiki/Dihedral_(aeronautics)