Placing rudders in front of the CoM

[roblamb98]

Hey guys! I'm sure you read the title already so I'll just get on with it, while building SSTO's I noticed that I can't always place the rudder behind the center of mass because the extra engines move it so far back, so usually I just place it in front and it seems to work fine, it made me wonder why airplanes in real life all have a rudder on the tail, and what drawbacks having one closer to the nose of the plane made them decide not to utilize it.

Thanks for the help!

[EdFred]

Because it usually helps to, um, see where you are going in real life. And even if it were a twin boom set up, they would still be in your field of vision which makes for seeing other traffic more difficult. And no (at least here in the US) air traffic control doesn't keep most traffic separated. That's all on the pilots to do so.

Edited June 30, 2014 by EdFred

[roblamb98]

Because it usually helps to, um, see where you are going in real life.

I don't understand, am I supposed to take that seriously?

[Hodo]

Hey guys! I'm sure you read the title already so I'll just get on with it, while building SSTO's I noticed that I can't always place the rudder behind the center of mass because the extra engines move it so far back, so usually I just place it in front and it seems to work fine, it made me wonder why airplanes in real life all have a rudder on the tail, and what drawbacks having one closer to the nose of the plane made them decide not to utilize it.

Thanks for the help!

They did, the Wright Flyer had its rudders in front of the pilot. There was a couple of designs that were like that prior to WWI. They were as Roblamb pointed out, poor for seeing.

[EdFred]

I don't understand, am I supposed to take that seriously?

Yes. Imagine trying line up yourself on a runway while you have a rudder blocking your view.

But what would I know? I'm only a commercially rated pilot and flight instructor, so I probably don't know what I'm talking about.

[Red Iron Crown]

Putting rudders at the rear helps keep the center of drag behind the center of mass. This is important in real life to help keep the plane stable, the "lawn dart" effect; it takes some careful design to make a forward mounted rudder plane balanced.

[EdFred]

Putting rudders at the rear helps keep the center of drag behind the center of mass. This is important in real life to help keep the plane stable, the "lawn dart" effect; it takes some careful design to make a forward mounted rudder plane balanced.

But even your jet fighters which have the COM very near the COL for maneuverability, have rudders in the rear. Most drag on planes is from frontal area.

[Red Iron Crown]

But even your jet fighters which have the COM very near the COL for maneuverability, have rudders in the rear. Most drag on planes is from frontal area.

That's true, but I don't think CoL is an important factor here (or did you mean CoD?).

The problem with front mounted control surfaces is that their drag increases quite a bit when they are deflected, moving the CoD forward and potentially upsetting the balance. Some designs have used forward mounted elevators though, these require careful balance and have the benefit of stalling before the wings stall when increasing AoA. There isn't really such a benefit with a forward mounted rudder, which is why I suspect it's so rare.

[EdFred]

No I meant CoL, since the relation of that to CoM has more to do with stability than drag. Most of the drag is already ahead of the CoL/CoM except in probably some very odd designs.

[Red Iron Crown]

Wouldn't that make the plane flip around when not under power? Much of an aircraft's drag is lift-induced vortex drag, as I understand it, which would be centered further aft. I didn't think the CoD was at the front of most aircraft, but I could be wrong.

[Jimbimbibble]

Typically, a forward rudder causes unstable equilibrium and a rearward rudder causes stable equilibrium. Think of it like a weather vane. Do weather vanes ever point backwards?

[EdFred]

Wouldn't that make the plane flip around when not under power? Much of an aircraft's drag is lift-induced vortex drag, as I understand it, which would be centered further aft. I didn't think the CoD was at the front of most aircraft, but I could be wrong.

Most drag is frontal area. Windscreen, nacelles, etc, and my plane doesn't flip around when I pull power to idle. Either way, there's a lot more reasons to put the rudder in back, than in front.

[costermonger]

It's all about directional stability. A vertical stabilizer (note: only the moving bit is called a rudder, and it's less important than the stabilizer) is tasked with keeping the aircraft pointed in the same general direction it is traveling. It does so by creating copious amounts of drag if it is no longer aligned with the direction of flight:

This aircraft is in what's called a sideslip, in that it's traveling through the air in a different direction than it is pointed. In this case, the aircraft is traveling to the left of where it's nose is pointed. Since the vertical stabilizer now has angle of attack, it generates a force pushing the tail to the right, and aligning the aircraft more or less back into the relative airflow, when the vertical stabilizer will go back to 0° angle of attack and revert back to it's fairly low-drag condition of doing nothing in particular. The rudder is needed to fine tune this, but the majority of the work is done by the vertical stab.

If the vertical stablizer was in front of the CoM, it wouldn't be a stabilizer - as soon as you enter that slipping condition the aircraft would actually deviate by a greater amount. That's negative stability.

You don't really *need* a vertical stabilizer, but it sure does help (swept wings can do the job too). Or at least having something to keep the pointy end pointed forward.

[RocketPilot573]

Yep, plane would flip around...

I ran into this problem before while trying to make a space shuttle with walls made of wings. It took me sooo long to realize what was wrong.

[cantab]

So ultimately what matters is that the vertical stabiliser, or tailfin, is aft of the CoM. The back of that fin is then an obvious place to put a rudder. A forward-mounted rudder would need a separate structure to hold it.

[roblamb98]

Yes. Imagine trying line up yourself on a runway while you have a rudder blocking your view.

But what would I know? I'm only a commercially rated pilot and flight instructor, so I probably don't know what I'm talking about.

okay okay no reason to get snippy, im just curious as to how a rudder ON TOP of your plane gets in the way IN FRONT of it, thanks again!

[roblamb98]

It's all about directional stability. A vertical stabilizer (note: only the moving bit is called a rudder, and it's less important than the stabilizer) is tasked with keeping the aircraft pointed in the same general direction it is traveling. It does so by creating copious amounts of drag if it is no longer aligned with the direction of flight:

http://2bfly.com/assets/verticalstabworking2.png

This aircraft is in what's called a sideslip, in that it's traveling through the air in a different direction than it is pointed. In this case, the aircraft is traveling to the left of where it's nose is pointed. Since the vertical stabilizer now has angle of attack, it generates a force pushing the tail to the right, and aligning the aircraft more or less back into the relative airflow, when the vertical stabilizer will go back to 0° angle of attack and revert back to it's fairly low-drag condition of doing nothing in particular. The rudder is needed to fine tune this, but the majority of the work is done by the vertical stab.

If the vertical stablizer was in front of the CoM, it wouldn't be a stabilizer - as soon as you enter that slipping condition the aircraft would actually deviate by a greater amount. That's negative stability.

You don't really *need* a vertical stabilizer, but it sure does help (swept wings can do the job too). Or at least having something to keep the pointy end pointed forward.

Thank you for the illustration that really helps me understand why.

[kujuman]

I'd also add that it makes sense to have control surfaces as far from the CoM as possible if you're trying to induce rotation because the increased lever arm will lower the force required. Roughly speaking, a vertical stabilizer/rudder 2x as far from the CoM only needs to generate half of the force to get the same stability. Smaller control surfaces => less drag, less structure, and can induce the same torque with weaker materials.

Consider that most early aircraft have a big ol' engine in front, leaving the front of the aircraft very close to the CoM. You'd need an enormous rudder if you placed it in the front, and a much smaller one if placed in the rear. So for structural reasons, in addition to the other aerodynamic reasons, an aft rudder makes sense.

You don't really *need* a vertical stabilizer, but it sure does help (swept wings can do the job too). Or at least having something to keep the pointy end pointed forward.

I was half expecting you to link this video of a B-52 which lost most of its vertical stab.

Edited June 30, 2014 by kujuman

[costermonger]

So ultimately what matters is that the vertical stabiliser, or tailfin, is aft of the CoM. The back of that fin is then an obvious place to put a rudder. A forward-mounted rudder would need a separate structure to hold it.

Pretty much. And that structure/forward mounted rudder would actually work against the vertical stabilizer, necessitating a larger stabilizer, resulting in more drag (real world, can't speak to KSP) and heavier structure.

[EdFred]

okay okay no reason to get snippy, im just curious as to how a rudder ON TOP of your plane gets in the way IN FRONT of it, thanks again!

You said you placed the rudder in the front of your plane, and asked about real life. In real life, you generally look forward, and sit somewhere near the center of mass. So in the real world it would be in your way vision wise.

[O-Doc]

I see this has been pretty much answered. I already wrote out my reply on page one so I'll post it anyway.

Putting a rudder at the rear of the plane vs putting it at the front is like difference between dihedral and anhedral wings. A rear rudder will always return to the neutral position. Whereas on the front it wants to turn more. That way, if you lose rudder control and it's mounted on the rear you wont lose the aeroplane. There's a good chance of crashing an uncontrolled rudder mounted at the front.

A: Safety.

[costermonger]

I was half expecting you to link this video of a B-52 which lost most of its vertical stab.

Hah! It was definitely one of the examples I was thinking of. Their version of "something else" was to extend the rear landing gear & outboard spoilers.

There are also neat solutions like this aircraft:

They managed to retain directional stability by washing out the wingtips enough that they basically don't produce lift (just drag) and since they're behind the CoM, job done.

[roblamb98]

You said you placed the rudder in the front of your plane, and asked about real life. In real life, you generally look forward, and sit somewhere near the center of mass. So in the real world it would be in your way vision wise.

what i meant by in front was on top above the nose and any observation point in the aircraft, resembling a shark's dorsal fin