Tail Heaviness in WW-1 aeroplane ?

[womenfly2]

Question for you: Many people refer to a WW-1 plane like the Camel or Dr.1 as being tail heavy because it climbs under full power and has to be flown with much forward stick.

Is this (tail heavy term) being confused with the amount of high lift from the wings and not so much as the plane being extremely tail heavy as flying with the CG full aft.

Your comments welcome.

[hank jarrett]

Wow, you could discuss this for weeks. It is a complex topic, but a couple of basic concepts will give you a start towards understanding what is going on.
First, it is NOT a matter of high lift. It is the point on the aircraft where all of the lift can be shown to be acting. An airfoil produces lift from front to back and tip to tip. It does NOT produce the same amount of lift over the whole surface. If the wing planform is symmetric, and the aircraft is in unaccelerated flight (not turning, climbing or changing speed) the lift distributed across teh surface will behave like it is concentrated in a single spot. That spot is usually between 1/4 and 1/3 back from the leading edge at the center line of the wing span.
"Tail heaviness" is just what it says. The CG is too far aft. When the GC is aft of the center of lift (actually the Center of Pressure, but we are trying to simplify this) the tail has to lift UP to keep the plane level. With the CG forward, the tail would lift down to keep the plane level.
When the CG is aft of the CP, there is negative static margin, and the plane is unstable. When the CG is forward of the CP, that is positive static margin and the plane is stable.
Many modern fighters have negative static margin and use computers to make the plane feel stable. In WW-I, the Pilot was the computer. You moved the CG aft to make the plane more maneuverable. It also made it less "fun" to fly. The pilots and planes that could survive the aft CG could turn better and climb faster than those who couldn't.
Eliminating the aft CG of a Fokker Triplane or Camel will go a LOOOONG way towards making it a nicer to fly plane (since no one is shooting at you). Eliminating that huge gyroscope from the front will go even further.
So, getting back to your original statement. The original Camel and Dr.1 WERE tail heavy, to make them more efficient killing machines. There is NO reason to reduce static margin that much in a modern reproduction.
The lift is the same in magnitude from the wing wherever the CG is, but with the CG aft, the tail isn't lifting DOWN as much, so the plane will climb faster, at the expense of stability.
One of the first things you would notice, all other things being the same, is that an aft CG plane would require more forward stick. This can be removed with trim, but then all things aren't equal any more and you are hiding the lack of stability from the pilot. For the kind of flying we do with replicas, having the CG a little forward from the original as a GOOD thing. We will get to fly for more years without entering a spin that can't be recovered.

Hank

3.9 years more and you get a degree in Aero Engineering, but if this is clear, you understand one of the key elements of the subject.

[womenfly2]

Thanks HJ for the great explication. Being an ME myself and building a few planes I fully understand what you are saying.

I was just wondering if the planes were intensionally built with a very after CG. As you stated adding incidence to the horizontal stab only masks it. I see on the Dr.1 the designers did put a good amount of incidence to the horizontal stab built right into the fuselage frame itself to help some want. With this all said, I can only imagine that as fuel and ammo was used up, would the aft CG, planes negative stability, even get worse?

So, thanks again!

[US95Damiani]

Originally Posted by womenfly2

Thanks HJ for the great explication. Being an ME myself and building a few planes I fully understand what you are saying.

I was just wondering if the planes were intensionally built with a very after CG. As you stated adding incidence to the horizontal stab only masks it. I see on the Dr.1 the designers did put a good amount of incidence to the horizontal stab built right into the fuselage frame itself to help some want. With this all said, I can only imagine that as fuel and ammo was used up, would the aft CG, planes negative stability, even get worse?

So, thanks again!

One thing I've noticed in airplanes with an aft Cg even with the down trim unless the airplane has a flying Tail (IE a Jenny which has a very aft CG), the airplane will tend to "hunt" in pitch during flight,

[hank jarrett]

The "hunting" is an indicator of low static margin.
The topic is actually much more complex than I presented. It is difficult to tell what the static margin is on a plane when it is test flown and requires a number of control inputs and measurements of the responses. In the "old days" they would start out CG forward and slowly move the CG aft, doing the control response tests over till the test pilot soiled his drawers, said that's good enough or spun in and died. We used up a lot of test pilots back then.
Now the analysis is done using Computational Fluid Dynamics, wind tunnel tests and flight tests. We rarely loose test pilots any more.
If you don't have a super computer or a wind tunnel, be real careful and stick with what the designer has already tested. A good indicator is if the designer can give you stall spin characteristics. If they haven't done stall tests, be very careful. Low static margin designs are known for NOT recovering from stalls.
Hank

[US95Damiani]

Absolutely Agreed Hank,
Aft Cg's are not to be trifled with. Thanks for the great information!

[Lufbery]

Hank,

Thanks for the great explanation. There's a lot to think about here.

One thing you wrote caught my eye:

Originally Posted by hank jarrett

The original Camel and Dr.1 WERE tail heavy, to make them more efficient killing machines.

I've often wondered which egg came before which chicken when talking about World War I aircraft design. I do not think that the designs were "primitive," but early aircraft designers did not have the same level of understanding of aerodynamics that their sons (and daughters?) did twenty to twenty-five years later. (Of course, transonic flight changed everything again. . .).

In short, did the Sopwith folks say, "We need to move the center of gravity aft so the Camel can be more maneuverable," or did they say, "let's put the pilot, guns, ammo, engine, and fuel tank, all in the first six feet of the fuselage and HOLY COW, this thing is maneuverable!"

I suspect the design came first and then the understanding of why it worked or didn't work. World War I aviation history is litter with the wrecks of designed that were pretty good, but lacking in some key aspect: the v-strut Albatross fighters losing their lower wings; Spad fighters with thin wings and wicked stalls (especially on landing); Nieuport 28s losing their upper wing fabric because their construction wasn't suitable for fast dives.

I strongly suspect that WWI aircraft design was very much a matter of trial and error. Which, of course, in no way takes away from your very cogent explanations of why those planes behaved the way they did.

Regards,

[hank jarrett]

In early aviation the designer was usually also the test pilot. We eliminated a lot of bad designers very quickly.
The Wright brothers didn't believe in natural stability. According to their papers they believed instability was the foundation of control. If you ever get a chance to fly a "Flyer" sim give it a shot, and good luck, I couldn't do it. Those two were real cyclists!
The designers of the Tauben were just the opposite. A Taube was generally so stable pilots were known to climb out of the cockpit and lie on the wing root. The weight of the pilot off center was just enough to make a Taube make gentle turns in the direction of the heavy wing.
A lot of the effects of aft CG were known by early designers. Not all knew why, but most knew. Taking advantage of what we know, even when we don't know why, is a standard of cutting edge technology. You write the rules in blood, and figure out why they are rules later.
I used to get into lots of arguments with Graham Lee on the WHY of his designs. His theories on why airplanes behaved the way they did were just a bit "out there". They worked, but NOT for the reasons he thought. I really miss those long discussions. He was born in the wrong era. I don't just miss the discussions, I miss him. If you are right for the wrong reasons, you are still right.
Hank

The library at the NASA Langley Research Center has ROOMS full of old papers from WW-I with the early theories of aerodynamics. Fascinating reading! Mostly in random piles.

[snj5]

Hank - many thanks for the wonderful explanation.
Much as you suggest, in our Camel replica, we are trying to move the CG a bit more forward (e.g., moving the fuel tanks from behind the pilot to where the ammo was. Hopefully, in conjunction with less gyroscopic effect from the small radial the Camel will be a sweet plane.

As someone once said:"Nose heavy planes don't fly well, and tail heavy planes don't fly long"

Again - well written explanation - thanks!

[Jeff Brooks]

I am not an expert, but ... one of the things that I have learned plotting the CG of the DVII, is that the tail surface was a lifting surface, not a downward lifting tail of the modern design. This changes the definition of tail heavy a little.

On modern designed planes, the tail-plane pulls downward. When airspeed is reduced to where the tail stalls while the main wings still have lift, the nose pivots downward and plane will automatically dive and recover.

On most WW1 types, this isn't the case. The tail helps lift the empinage. When the tail stalls, the nose of the plane pitches upward and stalls the main wings too. When all the lifting surfaces stall, the weight of the engine eventually pulls the nose down so you can recover.

The CG has to be moved back from the normal 25% (or 23% for biplanes) to around 33%, like on the DVII.