In airplanes design, if CoT is ABOVE CoM, how to put CoL?

[SalehRam]

I started to mess more around in my sandbox and SPH, I am looking to create MK3 airplane designs that have 2 rear engines, and other designs with 3 rear engines, something similar to these:

The first image is a DC10, with 3 engines, 2 under the wings, 1 at the tail... the second image is a DC9, with 2 engines at the rear...

From visual observations of the DC9, the rudder has 2 large elevators on it, but that because of the 2 engines location which taken the place for the elevators (if I am right), also it could be to balance the CoL with the CoM and CoT, because if I understood that correctly, having CoT ABOVE CoM, will cause the plane nose to go down, thus unable to fly...

For the DC10, it has 2 wing engines, and 1 larger tail engine, but I guess it is either the 2 wing engines got more power than the larger tail engine, or the angling of the 2 elevators (and their size), will compensate for the extra thrust that will cause the plane nose to go down...

Though I am interested in the physics of airplanes, I am not much into details (yet), and I would love to know more about these 2 designs so I can get to do something similar in SPH...

Appreciate your feedback!

Edited July 29, 2015 by SalehRam

[T-Bouw]

Though I haven't experimented a lot with off-center thrust myself, from the picture I can deduce something:

The upper engine is 2 engine heights above the wing, the other two engines are directly under the wing.

Thus the upper engine has a twice as long arm, thus it generates approximately the same pitch-down force as the other 2 engines do pitch-up.

Also, almost any plane has at different speeds a different pitch-angle and trim needed in level flight.

Almost never is everything perfectly canceled out with each other all the time. Instead, manufacturers try to get the most optimum setup for a plane at cruise-speed/height.

[Red Iron Crown]

I don't think either of those designs have the CoT significantly misaligned from the CoM. The DC-9 has two engines roughly aligned with the fuselage centerline vertically, the nacelles are angled for airflow reasons but the nozzles are not. The DC-10 has three engines, but the wing mounted ones are only half the vertical distance from the fuselage center as the top one, so the net thrust should be aligned with the CoM.

Moving this over to Gameplay Questions.

[Sharpy]

Right where it was, and move CoT in line with them.

CoT above CoM will pitch your plane down the stronger the thrust.

CoM in front of CoM will pitch your plane up the higher the speed.

Since the two are independent from each other, there's no way to cancel it out. If you start from the runway, your plane will be pushed nose down because your speed=lift is low. If you fly very fast and throttle down to slow down, your plane will do a backflip.

One way to do it is to angle the engine (upwards) so that CoT vector goes through CoM, that way it won't introduce any torque (although it will be a little less efficient and need some more thrust as the engine pushes it a bit downwards.)

Edited July 29, 2015 by Sharpy

[Brofessional]

If you want CoT above CoM, check out the He162.

[SalehRam]

I have managed to build something similar to DC10, DC9 in SPH, the thing is due to the nature of KSP, I could not manipulate the design of engines too much... I used engine pre-cooler as a main engine body, and I got a very close to stable plane with that...

Because I am on the work PC, I could not do much, but once I get home I'm sure I'll get something better.

Also for the DC9, I just realized what R.I.C. was talking about... if you look at its engines from the side, you will notice they are not completely horizontal with the main plane body... and the nacelles are a bit higher than the nozzles...

HE162 design is very interesting, and I'll try to get it to work once I go home as well... I have a feeling it is better on fuel and power than standard inline designs.

[plotz]

Btw: The DC-10 in your picture does not have a "larger" engine on top of the fuselage, but three CF6-6D / -50 powerplants rated at exactly the same net-thrust. The bigger cylindrical shape is for air intake reasons.

For another interesting spin on Tri-Engine aircraft, take a look at the TriStar design which incorporated an S-Duct to feed air to engine #3.

[SalehRam]

Will attaching 2 engine nacelles together work out? I mean will the front intake feed air to the first nacelle, then it will feed air to the back one, then to the engine?

I am asking because of what plotz said about the tail engine of the DC-10, when the tail engine got a larger intake area, it means it works more efficient than the other 2 right?

So looking at the main wings, and the other 2 engines under them, and elevators, I see some sort of a balance that way...

Edited July 29, 2015 by SalehRam

[Red Iron Crown]

The intakes don't need to be directly connected to the engines at all. As long as there are sufficient intakes on the craft they will feed air to engines that require it.

[luizopiloto]

I made this plane a month ago...

You're right, elevons are at the top of the tail because of the engines... what balance this type of plane are the lift generated by the wings. I placed wings with a slightly angle of attack, to compensate engine torque...

http://kerbalx.com/luizopiloto/Vikkers-VK-12

Edited July 29, 2015 by luizopiloto

[Hodo]

I started to mess more around in my sandbox and SPH, I am looking to create MK3 airplane designs that have 2 rear engines, and other designs with 3 rear engines, something similar to these:

http://cdn-www.airliners.net/aviation-photos/photos/0/8/3/1101380.jpg

http://www.aviationexplorer.com/Assorted_Aircraft_Pictures/images/finnair_dc9_jpg.jpg

The first image is a DC10, with 3 engines, 2 under the wings, 1 at the tail... the second image is a DC9, with 2 engines at the rear...

From visual observations of the DC9, the rudder has 2 large elevators on it, but that because of the 2 engines location which taken the place for the elevators (if I am right), also it could be to balance the CoL with the CoM and CoT, because if I understood that correctly, having CoT ABOVE CoM, will cause the plane nose to go down, thus unable to fly...

For the DC10, it has 2 wing engines, and 1 larger tail engine, but I guess it is either the 2 wing engines got more power than the larger tail engine, or the angling of the 2 elevators (and their size), will compensate for the extra thrust that will cause the plane nose to go down...

Though I am interested in the physics of airplanes, I am not much into details (yet), and I would love to know more about these 2 designs so I can get to do something similar in SPH...

Appreciate your feedback!

If you look a them the CoT actually drives through the CoM of the aircraft and that is what matters.

The DC-10 is a great example, the CoT is split between the two wing engines and the tail engine. This gives it a good deal of power for the size but also lets it cruise faster and farther with less fuel use. At least at the time it did.

The DC-9 the engines actually are driving just above the CoM. But this is countered by the a the engine is shifted the exhaust port.

A large amount of the time these aircraft have to be trimmed to maintain level. In stock it is difficult to deal with this, but in FAR it is quite easy to deal with.

[Red Iron Crown]

A large amount of the time these aircraft have to be trimmed to maintain level. In stock it is difficult to deal with this, but in FAR it is quite easy to deal with.

What makes it easier in FAR? Alt+WASD works pretty well in stock, does FAR add something else?

[SalehRam]

A large amount of the time these aircraft have to be trimmed to maintain level. In stock it is difficult to deal with this, but in FAR it is quite easy to deal with.

What makes it easier in FAR? Alt+WASD works pretty well in stock, does FAR add something else?

Since this got mentioned... I still never understood the function of ALT+WASD and its relationship to CapsLock...

Can I get some explanation about this...

[Red Iron Crown]

Capslock is fine control, pressing a WASD key moves control surfaces/gimbals by a smaller amount. The control markers at lower left turn light blue to indicate this mode is active.

Alt+WASD is trim, it sets the control surfaces/gimbals to stay off center when no input is received. You can see the little control markers move as you adjust. Alt-X cancels any trim settings.

[Hodo]

What makes it easier in FAR? Alt+WASD works pretty well in stock, does FAR add something else?

FAR's atmospheric modelling makes it easier to handle offset CoT vs CoM aircraft. Stock it has a tendency to over do the drag at some altitudes and ignore it completely above others.

As in this old aircraft picture.

That one its CoT is below the CoM of the aircraft, but yet it flies fine in atmo, as soon as it gets to space it gets a bit more of a handful.

Edited July 29, 2015 by Hodo

[SalehRam]

I have managed to build a good size MK3 airplane, with 2 engines mounted on the tail, the CoT was way up compared to CoM, and after placing elevators on the tail rudder, and lowering the position of the main wings, I was able to fly it perfectly and easy... using Alt+WASD, I was able to maintain stability all the time...

Your posts were so much informative and helpful guys.

Here is the craft file and a screenshot in the hangar for the plane with its CoM, CoL, CoT.

The craft file has some issues with the rear landing gear, as this was just a test, I did not give much attention to details other than CoM, CoL, and CoT.

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[cantab]

In an aircraft it doesn't matter much if the thrust isn't through the centre of mass, because the wings and tail will keep you flying straight. You do have to be mindful of it, changes in throttle will require changes in trim, but it need be no barrier to flying. The centre of lift should as usual be a little behind the centre of mass.

[ajburges]

I strongly recommend RCS build aid if you want to dabble with asymmetric engine load outs. Stock tools are horrid for aligning CoT and CoM.

A little torque to pitch up can be handy for jets. It tends to be a nightmare in vacuum.