Tail Questions

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Heliano

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Billski is right: tail volume affects both stability and control. As for one compensating shorter arm with a larger vertical tail surface, yes, larger surface usually ends up being a heavier solution. But I would like to call attention to another aspect: dynamic stability. Yaw damping is proportional to arm^2, which means that, with a shorter arm and same volume, one has less dynamic stability. In other terms: the aircraft is more subject to dutch rolls and yaw oscillations, which makes flying more difficult, especially with turbulence and landing/takeoff with gusty winds.
 

Pilot-34

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How big a difference are we really talking?
Yes I am aware that in many planes were striving for ounces but if I add two or three feet to the tail of a tri-pacer while converting it to a Taildragger what does it really cost me ?
A few pounds in structural weight, A few feet per minute in climb, A few yards per hour in cruise and a few ounces per hour in fuel use?
 

TFF

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Plenty of Hot Rod Cubs have longer tails. They also have bigger engines on the other end.

For just numbers sake the original tail is at ten foot back from the CG and you make it twelve feet back. This is all simple weight and balance like you did for your pilots license. If your engine stuck out 4 ft and weighed 75 lbs. your tail sticks out 10 ft and weights 30 lbs. You move the 30 lb back 2 ft, the weight at 4 ft needs to be 90 now. Now you have to decide how to get more weight forward. Make the engine mount longer, bigger engine that just weighs more, more accessories like AC, or bolt on some lead. A mix probably. If you don’t, you will crash right away. No difference than bolting a cinder block to the tail and trying to go fly.
 

Pilot-34

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But arnt you forgetting a couple of things? For instance that 30 pound tail that sticks out 10 feet will have its center of gravity only 3 or 4 feet back due to the structure being heaviest closest to the cabin And the weight that you add will be the closest to the cabin.
But more importantly wont that extra length of tail give you more authority? Will That be enough to counteract the inbalance?
 

Victor Bravo

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When you move the parts of the airplane further away from the CG, the handling of the airplane usually gets WORSE.

Just like the best handling sportscars have the weight of the engine close to the CG, you can worsen the handling qualities of an airplane (or at best slow down the control response time) by moving the tail structure (or baggage weight) further back and then moving the engine forward to compensate.

The best example (in my own personal experience only) is the Taylorcraft. When their chief engineer Jack Gilberti created the Model 19, they increased the size of the baggage compartment (from a small laundry sack to a medium size dog playpen), which required the engine to be moved forward.

But the (old-school, heavy) starter and generator on the C series Continental required the battery to be moved back to that baggage compartment. Everything else about the airplane was identical to the previous model B Taylorcraft. The airplane immediately lost the light "sportscar" feeling and delightful two-finger handling in the process.

This rant is not to prevent anyone from increasing tail volume for safety, but only to remind ourselves that every single design choice is a compromise that comes with some cost, or loss of some other advantage. The people here who have been privileged to fly one or more airplanes with truly magnificent handling, and then flown 'average handling' airplanes or worse, will back me up on this.
 

Topaz

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How big a difference are we really talking?
Yes I am aware that in many planes were striving for ounces but if I add two or three feet to the tail of a tri-pacer while converting it to a Taildragger what does it really cost me ?
A few pounds in structural weight, ...
A few pounds at the site of your change. Now figure in the additional structure needed to carry that extra few feet of tail. You've just lengthened the arm of the tail, which means the moments it can apply to the part of the fuselage just aft of the cockpit are much larger. That structure should be beefed up as well to accommodate the extra load. A longer tail can also mean more-effective controls - meaning they can command a higher pitch or yaw rate. Does your engine mount still have enough factor-of-safety at those higher rates? Maybe it needs to be beefed up, too. Of course, both the engine mount and the aft fuselage attach to the cockpit area, and are imparting higher loads to that structure. Is it still strong enough, or does it need some reinforcement?

How many pounds are we looking at now? Enough to change the performance with the existing wing? Maybe. Does it need to be enlarged to capture back that lost performance? Maybe. How much weight does that add, to both attach the larger wing and to carry things like landing impact loads with that larger wing?

Nothing on an airplane "stands alone." Change one part, and the consequences of that change ripple through the entire airframe. None of this is "simple" if you're going to do a better job than a shade-tree mechanic. Airplane designers are striving for ounces, so there's likely less margin than you might think in these structures to "play around with." Redesign one part, and you need to examine the consequences of that change throughout the airframe.
 

wsimpso1

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How big a difference are we really talking?
Yes I am aware that in many planes were striving for ounces but if I add two or three feet to the tail of a tri-pacer while converting it to a Taildragger what does it really cost me ?
A few pounds in structural weight, A few feet per minute in climb, A few yards per hour in cruise and a few ounces per hour in fuel use?
Let's run an example.

First Tail Power Coefficient:
  • Lets say the existing tail arm is 8 feet with a 15 ft^2 tail area, wing is 100 ft^2 and chord is 4 feet. Tail volume is 8*15/(100*4) = 0.30 - Paz says that is just adequate for an airplane with split flaps and small Clmax, and a small MAC. Pitch control will be kind of weak, the horizontal tail will run at fairly large Cl, etc;
  • Now let's slide that tail aft to 12 feet. Tail volume becomes a somewhat better 0.45, which Paz says is adequate for plain flaps, moderate Clmax, and moderate MAC. You can control a much more useable airplane;
  • Let's get there a different way, and increase the tail from 15ft^2 to 22.5ft^2.
Then how much weight to do this?
  • Well, let's say your longerons are 1-1/4 x 035 steel tubing and they are pretty well optimized. To extend the tail by four feet, you will need to bump that new section of fuselage to 1-3/8x035, you will need 16 feet of it. I get that at 8 pounds for the longerons. Say 15 pounds to include uprights and diagonals.
  • To increase the tail area, you add 7.5 feet square times 0.8 pounds per square foot, or about 6 pounds. Now if the fuselage was fairly well optimized, you will probably have to bump all the tubing in the fuselage and I get that around 8 pounds, for a total of about 14 pounds increase.
Then what does it do to stability?
  • Base airplane has the neutral point somewhere around 0.8 feet aft (9.6") of the 1/4 MAC point. If your aft CG was on 6" aft of the 1/4 MAC point, the original airplane only had 3.6" or 7.5% MAC of static margin. Tractor props use up close to that amount. This airplane is likely to wander about in pitch, and be a real handful on a gusty or turbulent day;
  • Shove the tail aft by four feet and the neutral point is now about 1.2 feet (14.4"), and if the CG moved back to 8" (tailplanes are further aft), your static margin is now 6.4" or 13.3% MAC. The prop uses up less than that, and you now have a bird that will stay nose on without constant attention;
  • What about making the tailplane bigger? Neutral point is 1.15 feet (13.8 "), and if the CG moved back to the same 8" as lengthening the fuselage did, your static margin is 5.8" or 12% MAC.
Drag is comprised of Induced drag and skin drag?
  • The existing tail is our base;
  • The extended tail requires less downforce to balance the airplane reducing induced drag while it adds some skin friction from increased wetted area of the fuselage - usually it is a net decrease in drag;
  • The enlarged tailplane makes the same downforce so induced drag gets no worse, but the wetted area is increased by about as much as the fuselage wetted area is increased for the longer tail - usually a net increase in drag.
Then damping. Well, it goes with area and arm squared. Let's normalize on the base area*l^2"
  • Base is 1;
  • Longer tail is 2.25;
  • Larger tailplane is 1.5.
  • If the base was good, then the others will be fine too. If the base wobbled around and would barely stop oscillating after a gust, then the larger tail might be OK, and the lengthened fuselage will be pretty close to dead-beat in response, which is a good thing...
So, shifting this example airplane's existing tail aft gives more stability and more damping than just making the tailplane bigger. For only a tiny weight increase over the enlarged tail, you get a bunch of benefits.

If any of us is designing an airplane, we really ought to do a downtown job of this sort of analysis. This means estimating the shortest tail we would consider, the longest tail we would consider, and something in the middle. Then we can put enough tail area on each one to give a decent tail volume coefficient for our airplane, then figure out the weight and drag and maybe the damping of each. Once we have done that, we have a basis for picking the actual tail position and tail area for our airplane. Have fun.

BIllski
 
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Lendo

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John Roncz also has information on Tail volumes in his papers, he also mentions HT span AR to Wing AR.
The more information you can get from different sources the better. I'm happy with one 'golden nugget' from anything.
George
 

Heliano

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As for "negligible" weight increments, the oldest aero engineering problem is still the most difficult to solve: weight. As Burt Rutan once said: "if you want to evaluate if the addition of something to an aircraft is worthwhile, throw it upwards. If it falls back to earth, it is NOT worthwhile."
Seriously: adding weight has a domino effect. Always. In my 45 professional years, I haven't seen a single new aircraft whose weight was equal to or lower than the design estimates.
 
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Since I have a Taildragger that was made from a Pacer I can vouch that the tail is much heavier when flying solo. I have moved everything forward that I possibly could, adding a longer/larger tail would just add weight on an already tail heavy plane.

It is very responsive in the stock position, why would you want to ruin that?
IMG950575.jpg
 

Riggerrob

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The Pilatus PC-2 military trainer plane is a classic example of how tail length affects flying qualities. During the original design, structures engineers argued with aerodynamic engineers about the length of the aft fuselage. Eventually structural engineers won because their short aft fuselage was lighter and less expensive to manufacture. After a few months in service, instructor pilots complained about poor spin recovery characteristics. Pilatus then did an exhaustive series of spin tests, then they added a dorsal fin, a huge ventral fin and horizontal strakes in front of the horizontal stabilizer. All subsequent Pilatus trainers have similar fins, strakes, etc.
We wonder what would have happened if the PC-2 started life with a longer aft fuselage??????

Most other military trainers (Epsilon, Tucano, etc.) also a full set of fins and strakes. A few military trainers even have extra strakes on their noses (Cessna T-37 jet trainer, RCAF Musketeer and Tutor) to help with spin recovery.
In summary, cruising airplanes have long tail moment arms for stability, while aerobats have short tail moment arms for rapid maneuverability.
 

Pilot-34

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Since I have a Taildragger that was made from a Pacer I can vouch that the tail is much heavier when flying solo. I have moved everything forward that I possibly could, adding a longer/larger tail would just add weight on an already tail heavy plane.

It is very responsive in the stock position, why would you want to ruin that?
View attachment 115663
Is that airplane the length of a stock pacer or was it lengthened in the conversion process?
 

Victor Bravo

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Is that blue biplane built on a Pacer fuselage? There's a four-seat open cockpit biplane out there flying around that's smaller than the couple of huge ones from 1930 that give rides at fairs?
 

llemon

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An old rule of thumb is that for adequate dynamic stability the horizontal should be ≥2.5x MAC and the vertical should be ≥0.4x span. Obviously, do the actual analysis but as a starting point this is a pretty good place to begin.

As for "negligible" weight increments, the oldest aero engineering problem is still the most difficult to solve: weight. As Burt Rutan once said: "if you want to evaluate if the addition of something to an aircraft is worthwhile, throw it upwards. If it falls back to earth, it is NOT worthwhile."
Seriously: adding weight has a domino effect. Always. In my 45 professional years, I haven't seen a single new aircraft whose weight was equal to or lower than the design estimates.
The only aircraft I've heard of coming in under weight was the Canberra. It beat the early weight estimates by almost half a percent, which says a lot about the quality of the team put together for that one. And they were pushing into new territory to boot!
 

wsimpso1

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An old rule of thumb is that for adequate dynamic stability the horizontal should be ≥2.5x MAC and the vertical should be ≥0.4x span.
What dimension of the horizontal should be more than 2.5 MAC? Arm? Area? Arm looks most likely, but this leaves tail area open…

What dimension of the vertical should be more than 0.4 x span? Arm? Area? If arm, most sailplanes ever built violates this rule. And if this rule is arm, it also leaves the tail area open…

There has to be more to it than this…

Billski
 

Heliano

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LLemon: Is the Canberra you're talking about the English Electric (british version), or the US-made Martin B-57? This team must have been really top notch, cetainly very experienced, especially because in those years there was no software for weight estimation one has today.
 
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