larrysogla

What's up with some full scale and some RC aircraft with lifting airfoil tail horizontal stabilizer................what is the reason for using such a setup?
Thanks for your valuable reply.
larry

da Rock

The Hercules for example? With it's lifting tail?

The heavy lifters like the Hercy often have highly cambered horizontal tails. Why? Well, since they're cambered DOWN, it might be a puzzle to some.

Yeah, the tail lifts down. Not what you see on some models, right.

Real life birds often have design goals we don't have in our models. The C-130s carry lots of weight. And need to carry it as efficiently as possible. It was discovered long ago that one of the most efficient ways to deal with heavy loads was to design the lift into the wing and then deal with pitch stability mostly with the tail. They chose to use an airfoil for the wing that produces considerable pitching moment when max loaded, so they developed the horizontal tail with sufficient negative lift capability to deal with the wing's pitching moment. They made it's area adequate and applied an airfoil to suit. And the airfoil needed to be "upside down" to counter the wing's pitching moment.

Doing it that way, the CG range the loadmaster had to deal with matched the cargo space better. And the tail was small enough that it's drag didn't become a problem. The L/D of a tail matters to the range of the airplane. And full scale planes have range requirements just as they have load capability requirements. Our models don't.

da Rock

The horizontal tail on the Hercy (and many other full scale cargo birds) needs a very large negative lift vector when loaded. It doesn't when unloaded. Using a negative camber and adjustable incidence gives a larger downward lift. When the bird's empty, the AOA can be trimmed to give lots less lift.

Basically, the tail on that airplane can give whatever negative lift needed when it's needed and then trimmed to give as little lift as needed when empty. That range gives better efficiency both ways. And military planes need to be efficient when they can be.

larrysogla

Gentlemen,
Thanks for that good info.
larry

Jim Thomerson

A lot of duration freeflight models have lifting tails because they are balanced well behind the center of lift of the wing. There are some with the camber down because they are blanced ahead of the center of lift of the wing. At least I think that is why.

Champ-RCU

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[img][/img]Here's an example of the inverted horizontal stab airfoil. This is on a F-4G Wild Weasel at the Air Force Museum in Dayton Ohio.

larrysogla

Champ-RCU,
The image is missing in your post.
Thanks
larry

Champ-RCU

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I wish some people would learn how to resize their pics properly. Could someone help a poor boy out?

BMatthews

We can't resize something small to large. Only large to small.

Larry, as noted the free flight models of old and new use an upward lifting tail because the CG is set to far back. This is because the aircraft neutral point is that far back and climb to glide stability on such a model is aided when the positive stability factor is close to neutral. I know it sounds counter intuitive but it works. The idea is to minimize the pitch stability action from the rather high speed powered climb and the slow speed glide so the model doesn't want to try to loop in small circles under power.

On the other hand large cargo haulers such as air liners and military cargo haulers are mandated to operate well in the very conservative zone. Hence the CG ranges up in the 15 to 20% range on these big birds. Such a forward CG point means that the tail will always be operating in the negative lift region so a negative camber airfoil makes sense from a minimizing drag standpoint.

Note that in both cases the airfoil of the stabilizer does NOT set the CG point. Rather the CG point sets the sort of airfoil that will result in the least drag. This last bit is something that eludes a lot of folks. As a result some seem to think that if they put a lifting airfoil on a stabilizer that suddenly they can move the CG back. But it ain't so. First the model must have a tail volume coefficient that supports a radically rearward aircraft neutral point. Only then can the CG be placed well back on the wing chord but still ahead of the neutral point if only by a small margin. Try to put the CG behind the neutral point and it won't matter what sort of stabilizer airfoil you have bad things will happen.


Note that there are a lot of the flat foamie RC 3D models that fly with the CG located at or slightly behind the NP. But when I say slightly I mean JUST A BIT! ! ! ! Such models are still flyable but require regular small corrections. Not something you want normally. But since such models are flying with symetrical airfoils (a flat plate is still a symetrical airfoil) they would not benifit from using a lifting airfoil on the stabilizer.

larrysogla

Champ-RCU & BMatthews,
Thanks
larry

Top_Gunn

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One use for a lifting stabilizer is to make the plane less prone to nosing up when the airspeed increases. A flat-bottom wing will generate more lift when throttle is added, and this will cause the plane to climb. If you've got a lifting stabilizer, it will also generate more lift when the speed increases, and this will tend to lift the tail, reducing the plane's tendency to climb. To be sure, the stabilizer must still "lift down" overall; this is achieved with a negative incidence for the stab. If I remember right, the Telemasters are an example of this: lifting airfoil on the stabilizer and a whole lot of negative incidence, too.

(I think this explanation may just be a simple-minded version of the BMatthews explanation above. I'm not an engineer, so I like simple-minded.)

BMatthews

Up to a point you've got the right idea TG. Yes even on a short tailed design with a forward CG a lifting airfoil on the tail will do what you're saying but it won't have the lowest drag for that case.

One error you made is that the up or down lift provided by the tail is dependent on where the CG is located. That whole neutral point and CG in relation to that spot thing. On any design with the CG well forward, typically at the 25% or more forward point, the tail will always lift downward. On in between CG locations between 25 to 30% the lift will be slightly negative to slightly positive depending on the airfoils and the angle of attack forcing the lift coefficient to specific values. On models where the CG is more rearwards than 30% the tail will always be lifting upwards the same as the wing. So where does the stability come from in that case you ask? It is because the wing ALWAYS LIFTS MORE THAN THE TAIL.

Now there's some other issues that come into this. On a rearward CG on a plane with a smaller tail there is not enough tail boom to tail area factor, called the tail volume coefficient, to provide the leverage to control the wing and such a design can prove to be unstable in very bad ways. This is because the pitching forces of the wing overpower the stabilizing forces of the tail. Giving the tail more to work with by increasing the area helps. As does giving a smaller tail more leverage by using a longer moment arm. Both of these factors increase the value of the TVC.

The whole reason the old tyme free flight models used lifting tails to such good effect was because the CG was so far back that they gained some lift by doing so. Sort of like a tandem wing. However I've been bitten on at least one model by trying to use an airfoil that lifted almost as well as the wing. It was a little P-30 rubber model that had higher cambered airfoils for both the wing and the tail. For most flying it was fine but about every 3 or 4th flight in more turbulent conditions it would get kicked into a dive and then the stablizer would lift the tail to a steeper dive that the wing's lift could not counteract and then it was lawn dart time.... Nothing I did fixed it. I know that if I had pushed the CG well forward it would but then I'd loose the extra lift of the big tail. So what I did was build a new tail of the same size but with a lower camber or "flatter" airfoil. From that time on it was fine.

Does that help show how these things all fit together?

tweedy

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Richard

BMatthews

Sort of.  It has a lot to do with other stability factors as well but you're 100% right in that it's related to avoiding a variable pitch response in any passenger carrying full sized plane.  Some of the jet fighters are set up to be closer to neutral stability or even negative stability in a small way.  But with those the on board computer stability augmentation system makes the pilot FEEL like it has a more normal and consistent pitch stability.  For this one case they sacrifice a true positive pitch stability for a more agile aerobatic performance.  Heck, the old X-29, which was the test bed for this idea, was said to be like an arrow flying backwards but stabilized with computers.  If the 'puters failed apparently the plane on it's own would try to swap ends.  Apparently this did happen once and they did lose the airplane to the resulting crash.  The pilot bailed when he realized that nothing he did helped.