Why Does My Plane Go Backwards On The Runway

[MajorGosnell]

https://youtu.be/49TUJX8Vr5c Anyone able to explain why on earth my plane was reversing on the runway? Note i had'nt powered up or used any input.is the runway slightly sloped?,or was it a bug?

[Friend Bear]

Sloped. It does that to me.

You can either engage brakes if they are not already or set them to start engaged in the building by right clicking them.

[cantab]

The runway is a flat plane on a curved planet, which means it effectively slopes up at either end.

[paul23]

The runway is a flat plane on a curved planet, which means it effectively slopes up at either end.

That would move the plane towards the center of the runway, not off the runway.

[Chaos_Klaus]

The runway is a flat plane on a curved planet, which means it effectively slopes up at either end.

Only that this would make you go fowards ...

In this case it is the damn unupgraded bumpy runway.

By the way: you can hit C to activate angle snap. That let's you place things at specific angles, like the wheels.

Also: Basically everything about your design is not working.

Your wings are too far fowards. Even if you get it in the air, it will want to fly backwards. Turn on the markers for center of mass (CoM) and center of lift (CoL). They are located at the bottom of the parts menu. The blue lift marker needs to be behind the yellow mass marker. That way the craft has a chance of not flipping.

Look at real planes and the kind of wings they have. Your plane lacks a vertical stabilizer. That would be a vertical fin at the tail. You also need to control pitch, so you need elevators either at the front or at the tail. Roll control comes from controlsurfaces on the main wings. If you have swept wings, you can combine roll and pitch control with one set of control surfaces.

With the gear: make sure the rear gear is behind the center of mass so your tail does not immediately fal to the ground. But don't place it too far back. For lift off, you need to lift the nose and the gear is your pivot point. The farther back the gear the worse the lever is going to be.

[paul23]

Look at real planes and the kind of wings they have.

Please don't. KSP doesn't come close to real world.

In reality the total lift vector (tail + main wing) is typically 5-10 meters ahead of the center of gravity. This is to provide static stability due to the lifting moment. (The lift vector also has a moment, as the distribution of lift is not constant of the wing).

It's not for nothing the name for a forward control surface is called a "canard"! Canard is french for "duck", but is also used to specify something is a hoax in the media. When the wright brothers made their first powered flight, in france they couldn't believe it. - Physics can prove that a tail in front of the main wing will never be stable and will always require active control, hence they named the news a "Canard". Back in the US the media picked this up again and thought the french were talking about the way the aircraft flew, with a vertical control surface in front of the main wing.

[cantab]

Please don't. KSP doesn't come close to real world.

In reality the total lift vector (tail + main wing) is typically 5-10 meters ahead of the center of gravity. This is to provide static stability due to the lifting moment. (The lift vector also has a moment, as the distribution of lift is not constant of the wing).

This contradicts everything I've read about aerodynamics, which says that for longitudinal static stability the centre of gravity must be ahead of the aerodynamic centre.

Even stock KSP is now close enough to reality that you can expect a design that works well in real life to work well in KSP. The converse does not hold though.

[paul23]

This contradicts everything I've read about aerodynamics, which says that for longitudinal static stability the centre of gravity must be ahead of the aerodynamic centre.

Even stock KSP is now close enough to reality that you can expect a design that works well in real life to work well in KSP. The converse does not hold though.

That's not entirely correct:

read this analysis.

The main wing is always so many more times larger than the tail (quite often the tail provides negative lift for equilibrium, but that needs to be calculated) that the total center of lift is almost as the main wing's. A simple line may already give an indication:

It is convenient to treat total lift as acting at a distance h ahead of the centre of gravity, so that the moment equation may be written...

This already implies the total lift is ahead of the center of gravity. Now a center of mass in front of the main lifting surface can also be done quite easily - however other constraints make this unfeasible (such as that a tail is always force to be behind the main wing).

Edited October 29, 2015 by paul23

[FlyingPete]

This contradicts everything I've read about aerodynamics, which says that for longitudinal static stability the centre of gravity must be ahead of the aerodynamic centre.

This was my understanding as well, both from pilot training and engineering at university. It means that a conventional tailplane is more often than not producing a downwards force to correct the pitch-down moment of the main wing.

It's the reason canards are always of interest as their pitch-up moment produces positive lift (but with other stability and control issues to contend with.) It has to be balanced by differing lift coefficient slopes of the main wing and canard- an increase in AoA should result in a greater change of lift coefficient on the main wing compared to the canard. Remember that the first powered flight was in an aircraft with a canard configuration.

[mabarry3]

It's not for nothing the name for a forward control surface is called a "canard"! Canard is french for "duck", but is also used to specify something is a hoax in the media. When the wright brothers made their first powered flight, in france they couldn't believe it. - Physics can prove that a tail in front of the main wing will never be stable and will always require active control, hence they named the news a "Canard". Back in the US the media picked this up again and thought the french were talking about the way the aircraft flew, with a vertical control surface in front of the main wing.

Are you sure? If I remember my ground school correctly, when we got into canard wings a bit, the story behind the name was a Santos-Dumont (Santas-Dumont? I don't remember) experimental plane that was said to look "like a flying duck", which was coined a canard plane by a French person since, as you pointed out, canard is French for "duck". The name stuck. I don't recall hearing anything about it being called a news hoax.

[cantab]

This already implies the total lift is ahead of the center of gravity.

Later on the same analysis states "h is known as the static margin. For stability it must be negative"

[CrashTestDanny]

In reality the total lift vector (tail + main wing) is typically 5-10 meters ahead of the center of gravity. This is to provide static stability due to the lifting moment. (The lift vector also has a moment, as the distribution of lift is not constant of the wing).

You will find very few aircraft in the real world with CL in front of CG. At least few that fly more than once. This is true of conventional configurations, canards, and flying wings. The reason is that the moment of a wing increases with AoA. For symmetrical airfoils, including flat plate, it is 0 at 0 AoA (along with total lift) and so with a symmetrical airfoil, you can place CG at CL and have a neutrally stable (neither stable nor unstable) aircraft. This is popular in high-alpha aerobatics where you really don't want the plane to right itself. For any stable configuration, since the moment of the wing would tend to lift the nose relative to the CG, moving the CG forward slightly is a good way to offset that moment. If your CG moves aft of the CL, then when your craft pitches up, the increasing AoA will mean that the pitching moment is increasing with AoA and is causing AoA to increase - kind of a viscious cycle, often resulting in death for pilots, passengers, and aircraft. Usually, the ideal CG position is between 1/4 and 1/3 of the mean aerodynamic chord ahead of the CL. This configuration produces an aircraft with positive longitudinal stability in normal flight attitudes. A few aircraft have intentionally flown with negative longitudinal stability, but these require complex fly-by-wire systems to interpret the pilot's input and produce the correct control surface movements to keep the aircraft from tumbling.

Now, I'm sure there is someone who doesn't believe this, so I'm going to tell you how to prove that what I have said is true. Go pick up a cheap balsa or foam glider at the hobby shop or toy store - something that flies well before you modify it. Measure its CG and figure out it CL. Now slowly move the CG back and record how far, how long, and how true it flies with each change. Observe how it tries to fly backwards when the CG moves behind the CL... There ya go. Same thing, BTW, happens here in KSP. Check the successful designs being put out there - All the ones that I've seen that fly well have the CG just in front of the CL and when they burn fuel off it does not move behind CL. If it does, I recommend not trying it.

Danny

[Vanamonde]

If the front or back landing gear is even slightly shorter, when the craft is generated on the runway and gravity is applied, it falls slightly until all the gear are in contact with the ground. This also applies a tiny bit of lateral momentum, causing the craft to roll.