Horizontal Stabilizer/Center of Gravity/Center of Pressure Question

Alchemy

Well-Known Member
I understand why we need horizontal stabilizers. The center of pressure of most wings is 2/3 of the way to the trailing edge, while the center of gravity is further forward. To keep the nose from dropping, we need the horizontal stabilizer to apply a tail down force and allow us to sustain a level flight attitude.

My question is: Why can't all planes be designed so that most of the mass of the fuselage is mostly aft of the wings, compensating for the forward center of gravity of the wing, and eliminating the need for the horizontal stabilizer. Isn't this the way most delta wing aircraft, like the concorde or mirage fighter are designed? What are the advantages and disadvantages of that type of design?

Thanks.
 

E_Dawg

Moderator
Airplanes are stable because of the location of the CG vs CP. i.e. you pitch up and tail down force decreases as speed bleeds off, and you pitch back down again.

I'd guess the long EZ type planes with the cannard design are more difficult to design and more expensive to build... so even though they're more efficient they're less popular.
 

Alchemy

Well-Known Member
The concorde didn't have a canard or a horizontal stabilizer, did it?

I think the reason for this design being less popular, especially with reciprocating engine driven aircraft is because to get the CG far enough aft to avoid needing an H-Stab you'd need to mount the engine(s) somewhere other than on the front of the fuselage/wings. This would result in cooling problems. The point you bring up about stability is also an important consderation, aft CG's do hinder stability.
 

E_Dawg

Moderator
Oh, I see what you mean! Only thing I can think of is gliders: they are supposed to be the most efficient planes out there and even THEY have horizontal stabalizers - and they don't have problems mounting engines.

Another thing is the planes you are talking about have delta wings which are crappy at low speeds - no idea how they're stable either.
 

Minuteman

“Dongola”
(The) Concorde and most other supersonic aircraft have to deal with a varying-location center of pressure. Once its going supersonic, the center of pressure moves aft (shock waves advance on the wing and the pressure distribution changes), somtimes to much as 50% MAC. More stability comes with a bigger separation between the CG and CP, which isn't necessarily a good thing unless you didn't really want your pitch inputs to do much anyway. The Concorde solves this problem with a trim tank in the tail that moves the CG aft and reduces the spread when its tooling along at supersonic speeds.

You're right, though, that in an ideal situation there would not need to be any lift produced by the stabilizer because the airplane would always be balanced properly to counter the moment from the primary lifting surfaces. Less lift from the stab means less drag and faster airspeeds.

Unfortunately, aircraft aren't constant-weight vehicles. After ten percent of the gross weight has disappeared as fuel burn, the plane can't still be in balance without either maintaining the CG location of the remaining mass or having an additional moment about the CG (lift from a stabilizer).
 

Mike Lewis

Shadow Administrator
Staff member
[ QUOTE ]
The concorde didn't have a canard or a horizontal stabilizer, did it?

[/ QUOTE ]

It had two itty bitty winglets underneath on either side of the windscreen that I have heard classified as canards, although I don't think they are canards in the traditional sense. I think they might be there for stability?

The B-1 has similar small winglets on the front of its fuselage. Anyone know what purpose these things serve?
 

JHines

New Member
[ QUOTE ]
The B-1 has similar small winglets on the front of its fuselage. Anyone know what purpose these things serve?

[/ QUOTE ]

They are part of a structural mode control system (SMCS). Operated by an automatic control to reduce flexing of the long fuselage. Supposed to improve the ride and fatigue life.
 

Alchemy

Well-Known Member
So I guess the main answer to my question is:

The change in center of gravity due to fuel burn is significant enough to prevent any design that eliminates canards or h-stabs from being stable unless the distribution of payload is somehow shifted continuously in flight.

For some reason I thought the shift in center of gravity but not be significant enough to prevent such a design, but now that I think about it, this explanation makes sense.
 

PhotoPilot

New Member
[ QUOTE ]
My question is: Why can't all planes be designed so that most of the mass of the fuselage is mostly aft of the wings, compensating for the forward center of gravity of the wing, and eliminating the need for the horizontal stabilizer.


[/ QUOTE ]

If the CG of the aircraft was located to the rear of the Center of Pressure (CP), a horizontal stabilizer would still be needed to counteract the associated nose up tendency and stall recovery would be virtually impossible. Because the nose is normally held up by the tail down force, when the plane stalls and the horizontal stabilizer stops 'flying downward,' the nose drops, the angle of attack is reduced, and the stall can be easily broken. In a rear CG aircraft, when the stall occured, the nose would remain high and the stalled stabilizers might not have the authority to recover. Flaring for the landing would also be risky and I'd think a lot of tails would be hitting runways.

If the plane was neutral, with no nose up or nose down tendency to fight, handling would be touchy and unstable. The slightest disturbance would upset the tenuous and light balance.

I'll stick with my horizontal stabilizers and tail down forces . . .
 

seagull

Well-Known Member
Actually, the wing itself has a forward pitching moment as part of the nature of the lift curve itself. As the AoA increases, the CP moves forward, which is obviously not ideal for stall recovery.

The other interesting aspect is that this is only a discussion of static stability and not dynamic stability, the latter of which is where you get discussions of short term pitch occillation and phugoid, etc., major issues in handling qualities.

Canards have many disadvantages, such interfering with the airflow over the wings.

The issue of delta wings is similar to a large swept wing. The wing has washout, so the tips, which are aft, provide the downforce. The delta wing just has the space between filled in.
 

ready2fly

Well-Known Member
That's the way I understand it: as your angle of attack increases, your center of pressure (center of lift) moves forward - the reverse being true for a decreasing AoA = As AoA decreases, the CP moves aft....

That's what I understand from my studies... however... the visual of it in my mind makes no sense.

I would think (wrongly) that as more of the wing( \ ) is exposed to the relative wind (----->), the more pressure would strike the AFT part of the wing:


------> \


and as the AoA is decreased, the RW would be relegated to the forwad part of the wing ( ----- ).

------> ----

Does that make sense (that I would see it that way)?
 

Rugby51

New Member
[ QUOTE ]

Does that make sense (that I would see it that way)?

[/ QUOTE ]

I definately understand what you're saying.


If my foggy recollection of school serves me correctly, the CP movement goes something like this:

As the air hits the leading edge of the wing, there is a point where the air will be split to either go over the wing, or under. In level flight this happens right in the 'middle' of the leading edge.

As AOA is increased, the air is coming up 'under' the wing. The point where the airflow seperates (the stagnation point) will then move towards the bottom surface of the wing. Now, with the increased AOA, air traveling over the top must travel an increased distance, and will travel at a faster speed, especially as it accelarates up and over the leading edge. As described by Bernoulli, there will be a decrease in pressure, near the leading edge, as a result.

So while the wing is exposed to a bigger shot of air underneath, the CP will move forward as a result of more 'lift' being generated towards the front of the upper surface.

Does that make sense to anyone?!?
 

seagull

Well-Known Member
I don't have this time, but it is clear from the last two posts here that there are some real myths being perpetuated about what creates lift in the first place. Perhaps time for a new thread on this topic. I'll try to get back to this later tonight, gotta get some tax stuff organized first!
 

Minuteman

“Dongola”
Oh Boy!!! Here's comes the "Newton vs. Bernoulli and Circulation vs. Matched Streamlines" debate! (I love these!
)
 

B767Driver

New Member
[ QUOTE ]
I understand why we need horizontal stabilizers. The center of pressure of most wings is 2/3 of the way to the trailing edge, while the center of gravity is further forward. To keep the nose from dropping, we need the horizontal stabilizer to apply a tail down force and allow us to sustain a level flight attitude.

My question is: Why can't all planes be designed so that most of the mass of the fuselage is mostly aft of the wings, compensating for the forward center of gravity of the wing, and eliminating the need for the horizontal stabilizer. Isn't this the way most delta wing aircraft, like the concorde or mirage fighter are designed? What are the advantages and disadvantages of that type of design?

Thanks.

[/ QUOTE ]

We all know that the two major forces acting upon the wing are the CG and CP (center of pressure). These two forces constantly "challenge" each other and require a "referee"...a stabilizing force that we call tail down force provided by the horizontal stab/elevator. Simply increasing mass to the rear of the fuselage will not provide the balance necessary to stabilize the CG/CP relationship.

Let's look at how this affects a longitudinally static airplane. Pitch the nose up and let go of the controls. Since the CG is always designed to be forward the CP...the nose will lower. When the nose lowers, the airspeed increases over the wings and the downwash impacts the horizonatal stab at a greater velocity. This provides the increased tail down force to pull the airplane out of a dive and start climbing again. As airspeed bleeds off in the climb, tail downforce is reduced and the forward CG again causes the nose to lower. This causes the aircraft to begin a series of oscillations.

If the airplane is dymamically stable, each oscillation becomes smaller until the aircraft returns to level flight.

I think this answers your question about "mass concentration" to the rear of the fuse. I'm not sure about Concorde's aero design...I'm not exactly sure what provides the balancing force...but I'm not sure it is mass related.
 

B767Driver

New Member
[ QUOTE ]
Oh Boy!!! Here's comes the "Newton vs. Bernoulli and Circulation vs. Matched Streamlines" debate! (I love these!
)

[/ QUOTE ]

I like to think there's a bit of Bernoulli and Newton in the total scheme of things....but Bernoulli gets most of the credit (he provides the majority of the lift!)
 

Mr_Creepy

Well-Known Member
[ QUOTE ]
It had two itty bitty winglets underneath on either side of the windscreen that I have heard classified as canards, although I don't think they are canards in the traditional sense. I think they might be there for stability?


[/ QUOTE ]

I thought they were there for a place for the FE to put his coffee during the walkaround.
 

Rugby51

New Member
[ QUOTE ]
...it is clear from the last two posts here that there are some real myths being perpetuated...

[/ QUOTE ]

OOF! I think that there is at least a bit of truth in my explanation!
It's probably lost in translation...

If you do have a minute to expound, I'm all ears.
(Good luck on the tax stuff, hope your return works out favorably!)


As far as Bernoulli v. Newton, it's a win-win situation, cause we get to have them both!
 

ready2fly

Well-Known Member
Seagull. Please do whenever you get the chance. It'd be GREATLY appreciated.

That said....

I spoke with my instructor about this last night and he explained that I was confusing the CP, which is basically a spot on the wing where lift is concentrated, with where the realitve wind is striking the wing. The two are not synonymous.

Any thoughts?
 
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