Lifting Tails?

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Largeprime

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I have read several places that conventional aircraft can be loaded such that the CG will force the tail to create lift.

I assume this means the CG will be aft of the 25% cord line, forcing the tail to lift, thus behaving like a tandam wing (canard)

Are there any certified or common kit build craft that allow a CG in this range? Or is this an obvervation that there are no examples of?

Thanks
 

orion

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The short answer here is that most conventionally configured airplanes can be flown with a CG location far enough aft that it actually results in a positive lift vector on the horizontal surface. However this needs a bit more discussion.

First, the allowable CG range on an airplane is a function of quite a number of variables so I won't get into that but, the results are the same for all - that being a forward CG limit and an aft CG limit. The forward CG limit is set so the aircraft can be brought to full stall, with flaps fully deployed, and in full ground effect. The latter is especially important since it removes the downwash field off the horizontal tail so the tail has to be of sufficient size to maintain full authority without the downwash flow.

If the horizotal does not have the power to do this then it either has to be larger or, the CG forward limit has to be set further aft so that it does not create as large a nose down moment.

The aft CG limit is a function of neutral stability. This means that for any flight speed, if the airplane is disturbed from a straight and level cruise, it must have the ability to return to that flight condition on its own, without pilot input. It also must be able to do so with the stick fixed and the stick free (this actually results in two aft CG limits).

For most conventional airplanes, the allowable CG loading envelope may range from just aft of the leading edge, say ten to fifteen percent MAC, to almost about 40% of the MAC (usually the aft limit is set about 5% MAC forward of the aft stick free CG limit).

In short, airplanes do not have to be balanced at the quarterchord, the loading range is much wider.

However, whether the horizontal tail is lifting or not is a function of the airpalne's design, the incidence angles of the wing and the tail, the pitching moments of the airfoil(s) used, the airplane's overall layout, etc. But yes, most airplanes can do this.
 

Largeprime

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Just so it is clear, your typical spam has a CG range that allows a tail lift senario in typical operations?
 

orion

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Yes, if I recall my basic aero education, the conventional spam-can can be loaded so that approaching the aft CG limit, the horizontal is trimmed so it generates a bit of positive lift.

For instance, I used to own a Grumman Lynx (two place Grumman). That airplane, when loaded fully, including baggage, often flew over gross and at or very near the aft CG limit (actually, depending on the baggage, it calculated a bit beyond the aft limit, but still flyable and controllable - a hair ahead of the neutral point). At that condition, the elevator was trimmed significantly TE down. Even if you consider the downwash over the tail, that particular trimmed configuration (still considered stable) was generating positive tail lift.

Do keep in mind though that all planes are rigged a bit differently and I am not 100% sure that you can make the general statement that all general aviation airplanes can achieve this condition. I am however sure that it can be done. As a matter of fact, I think I recall my flight instructor telling me about this through my flight training (before I went to college) in regards to the Cessna 150 and 172. But again, please don't quote me since that's been quite a while ago.
 
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Largeprime

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Ho no you dont!
I'm quoting you and theres nothing you can do about it!!!!!!

When ever I am asked if spam cans can be loaded so the tails are generating positive lift I am gonna say...
Brace yourself now
Orion on that forum told me EVERY SINGLE ONE can do it without a doubt. Yes siree bob.

No doubt about it.

Ok enough fooling.
So it seems this occures at the edge of the envelope more than a typical setup, right? It is not a typical or probable case?
 

wsimpso1

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While it is possible to load aircraft to give ppsitive lift on the horizontal tail, the biggest reason that it is not done is stability. Once your CG is aft of the neutral point the aircraft will not be stable.

Stability is defined as tending to return to its trimmed point following a disturbence... Stick fixed stability is demonstrated by holding the stick fixed in one place and power setting, and the airplane will head back to the same airspeed and attitude after a disturbance. Likewise, stick free stability is demonstrated by setting power and trim tab position, with the stick released, and again it will head back to the same airspeed and attitude after a disturbance.

If the CG is aft of the neutral point, pitch disturbances will lead to the further deflection in the direction of the disturbance. To manage instability, once you have the nose moving upwards to where you want it, you would then have to apply nose down pitch control to stop things, and then actively fly it to keep it there.

The F-16 is a well known example of an airplane that is unstable, but is stabilized by the autopilot. The pilot applies control pressures like normal, the autopilot interprets the pressures, and maintains attitudes by actively flying the control surfaces... I don't want that in my airplane, but with sufficient redundancey and development, it can be a reliable and terrific package, as demonstrated in the thousands of reliably operated F-16's in many air forces around the world.

Let's lay out the math for stability (simplied to pitch here):

The sum of all lift and all weight must equal ZERO;

The sum of all moments about any point must be ZERO;

The sum of the first derivative of pitch with respect to angle of attack must be NEGATIVE;

Stall must occur with the forward plane first;

The first two make possible a particular deck angle, and the third one means that it can hold that deck angle without having to actively fly the airplane in pitch, and the fourth one keeps the airplane from entering an unrecoverable deep stall.

The result of this knowledge is that simple aircraft, where we move surfaces directly, can have an aft horizontal tail with negative lift (conventional), or a forward horizontal tail with positive lift (canard), or a tandem wing with similar limits to the canard.

As Orion has pointed out, you may get mildly into the regime of positive lift on the tail, with the craft still flyable. I would not try to live there though... Real aircraft need a range of CG available, and this would be at the extreme aft end of the range. Further, and the craft is unflyable.

Small aircraft respond to this sort of instability so quickly that they can be uncontrollable by a human pilot... For instance, the Wrights' were initially flying gliders and airplanes that were unstable in pitch and roll, and they were an incredible handful to fly. After all, they were inventing this stuff ahead of the airfoil understanding that this stuff is based on. Later on, they moved the CG forward, which loaded up their canard, and the airplanes instantly became flyable for longer durations, they taught others to fly their airplanes, and so on. Roll was still unstable, but is a much slower mode and we continue to have airplanes that have marginal roll stability today. And we seem to like them that way...

Thurston's book Design for Flying actually has a fairly good discussion of this...

Billski
 

orion

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If I recall my math right, the conditions for stability Billski lists above can still be met with a positively lifting aft horizontal but as he points out, and as I tried to indicate above, the condition happens only over a small range at the rear end of the allowable CG envelope.

Based on flight tests conducted by NASA in the early seventies (when NASA still paid a bit of attention to general avitaion), it is possible for a human pilot to controllably fly an airplane that is as much a three to five percent unstable (CG located up to 5% of the MAC aft of the stick-fixed neutral point). Not that I would want to be there for too long as it would be an extremely uncomfortable flight, but I guess it can be done.
 

wsimpso1

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Orion and I must've learned the same stuff.

In general, a conventional layout would be negative lift tail. A little corner of the envelope can be in positive lift, but not much...

Once you get your CG aft of the neutral point, if the airplane wanders off slow enough, you can keep up with it. If it is fast, you can't.

Three anecdotes about aft CG:

In the 90's, a twin engined jump plane (Twin Beech if memory serves) crashed in a flat spin, with only a couple of the jumpers getting out. It seems that rather than queueing up from the door forward, and then wrapping aft when the queue hit the cockpit, they had queued up from the door to the aft bulkhead, then wrapped around to the front. This drove the CG aft. The jumpers that made it out of the bird reported that the airplane was riding a roller coaster with it pitching up gently, then down gently, then up on a period of a several seconds. Apparently the pilot was able to detect the pitch deviation, put in some elevator to counter it, then catch the next one, and so on. Then, as the first jumpers exited, the queue bunched up and shifted aft... The lift required of the tail apparently became more than it could carry, the tail stalled and sagged away, and the twin entered a flat spin all the way to the surface. A couple of divers were outside when it started, and a couple more got out, but the crash killed everyone else...

In 1995 a Sonerai was built by a guy (a division chief engineer at a major automaker) who decided to change from VW to O-200 power, change from taildragger to tri-gear, both of which shifted the CG way forward. He then calculated his cg shift and placed a weight in the tail to correct his CG. There were no records of his ever having weighed the ship nor establishing CG. His high speed taxi tests were reported to have included the airplane alternately banging the tail skid and the nose gear. It apparently was not even stable with the mains on. He flew it anyway, trying to keep up with it tried to pitch up, then down, then up, until it got away from him. Lawn dart. Evidence of where the CG was during the flight was now anyone's guess, but it sure looked like it was aft of the neutral point.

1990, a experienced Bonanza pilot and some of his buddies (engineers at a major automaker) went on a golf weekend, and aft loaded a Bonanza (back seaters, bags behind the back seat). Now this loading worked for the trip out, but now they had less fuel, and the Bonanza is noted for having CG shift aft with fuel burn. On climbout the airplane got away from him in pitch and killed all of them.

CG aft of the neutral point is a bad place. In close coupled birds it can be too fast to keep up with. In bigger airplanes, you might keep up with it for awhile, but a moment of inattention can kill you. In bigger airplanes still, it can probabaly be kept up with, but from there, your margins to tail stall may not be much.

Billski
 

StRaNgEdAyS

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It has been said that flying with CG aft of the neutral point is where even test pilots fear to tread, and apparently rightly so.
Most of the aircraft we work with as home builders are closely coupled, and as such there is not a great deal of distance between the flying surfaces, so the moment arm is fairly short, this is why smaller aircraft should have comparatively larger tail surfaces than larger aircraft. The shorter arm requires a greater moment or force to maintain trim.
This is not always how it happens, where designers try to compensate to acheive a more aesthetic tailplane (I have been guilty of this one :whistle: ) and sacrifice stability for looks, resulting in an aircraft that can easily become dangerous. :eek:
The plane I am designing is to be fairly maneuverable, so I want to try to keep the it close, and since it is not to be carrying cargo or passengers, I can do so. However I still want it to be safe, and as such, my horizontal is still fairly large, for as the (considerable :suprised: ) fuel load burns, the CG will still make a reasonable migration aft, so I have had to design to give me good moment fully loaded (much negative lift on the horizontal) yet retain the same positive stability when coming in to land, with minimal fuel, a time when you most DEFINATELY do not want to be fighting for stability. Working out this balancing act has been quite a challenge, one I may yet stilll have to overcome. Time will tell.
I couldn't imagine anything much worse than coming in to land in a plane that you have to hold a lot of forward stick into to maintain a good glide path, then trying to flare.
This is just my limited understanding of the subject. I, as always, will welcome any corrections of my interpretation, but I hope I am close to the mark (after all this studying something just HAS to have sunk in by now! :gig: ).
 

pylon500

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Long story short, Stay away from aft CofG!
While all the above explanations and stories give all the variations of problems with aft CofG, the real clincher is...

Stall must occur with the forward plane first
Once your tail starts carrying weight, it becomes a wing, usually a wing with a not very good section, and with the possibility of having it's angle of attack changed rapidly (pilot input).
Picture this; you manage to get your tail heavy bird off the ground and find you're holding a lot of forward stick to stop it nosing up all the time, you suspect you have a stuck trim or something so you come around the circuit to try to land, as soon as you let off some of the power, things will start to feel weird, it will be harder to keep level (no down thrust and less elevator authority).
Once on final you might put on a bit of flap, depending on how you particular aircraft reacts to flaps, you could die right about here! :eek: or it may help a little.
However, as soon as you try to reduce your speed to land (even at 10ft!) your plane will start to pitch up, and you will be pushing all the last remaining amount of forward stick you can get.
Again, depending on aircraft, you may get it to fly onto the ground, or you might find that last amount of down elevator is just enough to stall the tail whereupon you could end up about 30ft in the air pointing Vertically straight up with about ZERO airspeed! :wail: (you figure what happens next!) :(
Most likely though before you got to the vertical attitude, your craft would probably have rolled onto it's back and speared into the threshold ‡b is your insurance paid up?
Arthur.
 

orion

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Arthur is correct in his description of a potential problem however, this is an extreme case where the airplane is most likely flying at or beyond the static stability neutral point anyway. If your airplane is running out of control authority and you're still within your allowable envelope, then it is a poor design.

As I said above, flying with the aft tail lifting should be OK as long as the airplane is still stable.

We should also point out that the magnitude of positive lift on the tail we're talking about is relatively small so the danger of the aft surface stalling is relatively minimal.

If we look at the practical case, the danger is actually when the aircraft has insufficient horizontal and the CG is well forward. At that point you're really trying to load the horizontal to counter the heavy nose and at times, you could run out of elevator authority (hit the control stop) or if you go through an inadvertant bit of wind shear, even stall the surface. At that point the airplane will nose over and of course if you're on appraoch, you may not have sufficient altitude to recover control.

This is especially critical as you transition from free air into ground effect. If you're running out of elevator authority and you lose the downwash, you then need to pull further back on the stick in order to come to flare. If however you're already near the control stop, you may end up making expensive noises all over the runway.

I know the extremes of this in my Cherokee. With full fuel and two front occupants, the airplane is extremely nose heavy (my Cherokee is the one with a O-540 on the nose). Coming in trimmed for a short strip, I have seen a couple of times that if I let my speed drop a bit too far I make a bit of a hard landing since I have no control left for flare.

On the other hand, about a week ago I had to haul some gear in the airplane. It was just me in the front seat, low fuel, and about 350 pounds distributed in the baggage compartment and where the aft seats normally are. The airplane was actually beautiful to fly. It trimmed well, cruise was actually a bit faster than when I trim for the nose heavy condition, and the landing was almost automatic.

Flying at or near the aft end of the CG envelope is actually OK and in some airplanes, beneficial. The bottom line however is simply that the airplane has to be designed so it can operate safely even at the extremes of the specified allowable CG envelope. This includes flying loaded so the airplane is at the aft end of the envelope and the horizontal is developing positive lift.
 

StRaNgEdAyS

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I know it's not exactly on topic, but here is an article I got in my latest AVflash c/- NewsWire that describes a crash that occured during an aft CG test. It just goes to show that even when it has been planned for, aft CG can still thorw up a few surprises.
The NTSB says over-control by the pilot and inadequate preflight briefing contributed to the crash of a Challenger 604 test aircraft at Wichita in October of 2000. The plane was rigged to be aft-heavy to test the pressure-feel simulator. During the takeoff run, the NTSB report says, fuel rushed to the rear of the tanks, adding to the unbalanced state. The plane stalled twice after takeoff and crashed in flames after 10 seconds in the air. The pilot and an engineer died the crash and the co-pilot died 36 days later. The NTSB said the crew talked about the aft center of gravity before the flight but, according to the Associated Press, Bombardier didn't tell the crew how that might affect the plane's performance. The NTSB also said the pilot, Bryan Irelan, had never flown the plane in that configuration and was too aggressive on the controls, resulting in a high pitch rate on takeoff. The report also cited lack of direct oversight by the FAA and Transport Canada over the testing. Bombardier has since redesigned the fuel system to reduce fuel migration.
 

wsimpso1

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I just went into my stability program to explore the stability vs CG question a little, and then I remembered something.

Tractor configuration airplanes are destabilized by the engine driving the prop and are stabilized with the prop driving the engine. Pushers are reversed, with power stabilizing them. How much? RV6's have been cited (an article in Sport Aviation by David Lednicer) as having the neutral point move about 10% MAC forward with power on, so they have to place aft CG limit forward of the neutral point by more than 10% MAC.

Some folks will point out that power adds a lot of airflow over the tail, but only a little to the wing, so they would expect that power would be stabilizing. True it is, but in general, the prop pulling on the air contributes more instability at the nose than stability at the tail.

So, the horizontal tail pays its way by contributing stability, but the cost is the induced drag of the negative lift and of the extra lift that wing also has to make.

If you find a configuration that satisfies the criteria I stated in an earlier note with all lifting surfaces, you will have invented a more efficient airplane.
 

NEMuzzy

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Originally posted by Largeprime
would not a pusher canard do just that?
The canard airfoil is relatively inefficient and draggy. It is also quite heavily loaded. Thus, what you gain in having two lifting surfaces you give away in efficiency of the canard.

The disturbed airflow into the prop also hurts on the efficiency side.

The concept of the canard being the ultimate design because it has two lifting surfaces may make good salesman banter, or bragging stories, but it doesn't hold up to serious analytical review.

Build a canard because you want to, you can afford it, you like the history of the plans, you like the way to look, you want to fly a plane that is different, etc. All good reasons.

The typical canard is very fast because they are light, have small frontal areas, and are aerodynamically very clean (even with the canard). They do not somehow violate the laws of physics, and they do not prove every other type of plane in the sky to be inferior.

-Norm (flying a canard beacuase I can!)
 

StRaNgEdAyS

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The disturbed airflow into the prop also hurts on the efficiency side.
What if you made it a JET....
:gig:
:roll:
:D
:whistle:
Seriously though, has anyone thought about putting a tubine into a cozy? not a turbo prob, but a real live turbo fan or jet?
Intakes would not pose a problem, but the structural mods to deal with the extra power and speed might put it out of the park.
Guess that's one for the resident cozy nuts to debate.
 

pylon500

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I know, off topic but,

but the structural mods to deal with the extra power
Not many people would want to replace a 200hp engine with a 1000 lb thrust jet! :eek:
The idea behind gas turbines is that they maintain efficiency up through altitude, and can maintain thrust through speed, as such, if your 200hp prop puts out 500lbs thrust static (I'm guessing here :rolleyes: ) then a 500lb thrust jet will give you the same take off performance, but with the correct intake design will give you continued thrust to a greater speed or altitude (or both) than the prop.
The down side is that the 500lb thrust jet will use nearly twice the fuel of the 500lb thrust prop! :mad:
Arthur.
 

wsimpso1

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Pusher canard ships have a lifting tail (forward) that has to run at a higher surface loading and a higher Cl than the main wing to satisfy stability and prevent main wing stall. This looks efficient as can be initially.

Then you start looking at the rest of the issues. Geez, in a simple airplane, the minimum landing speed is predicated upon the airplane weight and airspeed where canard stall occurs, so you end up setting canard size and then wing size based upon what landing speed you will tolerate. Now, the wing area is larger and draggier for an airplane of this landing speed and weight than would be a conventional airplane with flaps. Ultimately, the drag (and speed and climb) end up about the same, and more dependant upon just how clean the craft is, how well cooling is managed, etc.

Well, we are not comparing apples to apples - we have flapped vs non-flapped wings. Yeah, and how are you going to achieve higher Cl for landing from a canard ship? Hmmm, well we can flap the wings, but the flaps are already applied in the canard at landing speed (elevators are deflected down already), so you have to do something like the Beechcraft Starship, and swing the canard forward while you are deploying the flaps. OK, big bearings, linkages, some sort of fairing glove at the fuselage-canard joint. Hmmm, this is getting ugly, because not only has the ship gotten heavier and more complicated, the interference drag at the wing fuselage joint has gotten big and you have to deal with stall behavior of varying sweep angles - you can not just install vortillons on variable sweep canard to make it behave...

The next step is a three surface ship, like the Rutan Catbird or the Piaggio Avanti. Now we have both a lifting canard that is fixed, flaps on the main wing and canard, and a horizontal tail. And you now have three planes with fuselage-plane inteference drags, wake interference from the canard on the main wing and tail, wake interference from the wing to the tail, and (in the pusher twin Avanti) all of those wake inteferences messing with the efficiency of the pusher props (the prop blades see highly varying angle of attack around the prop disc), which makes both the Avanti and the Starship slower than expected. Burt Rutan placed his props on the front of the Catbird and the Boomerang. The Catbird is really a pretty marvelous piece of airplane design and execution. It came out of action only when Burt needed another engine and prop for Boomerang.

Now you folks flying your Long Eze derived ships don't have much of a prop inteference problem because the wing is big and the cruise Cl is low to keep the landing speed reasonable, so the wash angle variation through the prop is small, and you consider 200+ mph to be adequately fast. In our size ships, the compromise is a pretty good one. And different is a good thing to have out there. I LOVE explaining that the little ship with the prop on the back and the nose on the ground was built by some intrepid homebuilder out of foam and fiberglass and travels at around 200 mph.

But please don't make it out to be inherently better than some other scheme. The right hand gives and left hand takes away in all of these cases. By the time it is actually executed as a practical airplane, it may actually be slower, or land faster, or use more fuel than some other scheme that appears in a cursory examination to be inferior....

It all really boils down to what shape and construction method appeals to you and how good a job you do of executing it. And we know that everyone paying this much attention to the build will do a great job and draw a crowd on the ramp during the pancake breakfasts ...

Billski
 
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