Why trainers balloon and pattern ships don't. Also a consideration of lifting tails and down lift tails. These figures are from Model Aviation which had a article about canards. I forget the exact date and author (may I be forgiven?).

This first one is a typical trainer.
The second figure is a pattern ship (well a flat bottom wing pattern ship with wing at some angle, the force diagram is the same with a symmetrical airfoil and angles at 0-0).

The second figure.

Ben,
Neither of your figures showed up on my computer. It kind of takes away from the story when there are no pictures. Is it my computer or do others have this problem?

Ummmmmm..... working on it. It shows up on my Mac which is my main computer and not on the Compaq (used for Realflight playing).

I input them as jpg files in the browse box. Is there something else I have missed or need to do to make the show on a windows system? Any help would be appreciated.

Ben,
Both of these pictures are labeled "conventional aircraft". How did you determine the first was a trainer and the second was a pattern ship?

The diagram seems to be showing the effect of CG location.

Tom

You are right in that it does show CG position.

The typical trainer uses a forward CG that ends up with a down load on the tail. It tends to give an airplane that has an exaggerated nose-up-with-speed-increase effect (our old contest rudder only airplanes were set up that way). For a beginner it means that if he gets out of sorts with the nose headed down and the speed increases the airplane will have some pitch recovery tendency built in. A more experienced flyer typically will not like it.

The second figure (for those of you that can see it) is the more aft CG location. The up load on the tail gives an airplane that doesn't have the exaggerated nose-up-with-speed-increase effect. It makes for a smoother more aerobatic machine.

The forward Cg plane is also less efficient because the down load on the tail must be balanced by increased lift from the wing.

On the other hand, with an aft Cg, the tail is producing lift which means the wing doesn't have to work so hard.

Tom

But the wing produces lift much more efficiently than than the tail so it is about a wash on overall efficiency. Sailplanes, which are very sensitive to how efficient the lift process is , have about the same minimum sink rate over their range of useful cg positions. Aft cg produces increased response to EL inputs but at the loss of pitch stability.
Nose heavy sailplanes have a strong tendency to pitch up when sped up by making a shallow dive - same idea as increasing speed by advancing the throttle.

Ben,
Your figures are coming thru on my computer now. It could be my end that wasn't working right. I didn't change anything , consciously, tho.

If the CG is behind the center of lift on the wing then isn't plane laterally unstable? Don't we always fly our planes with the CG forward of, or at, the center of lift, never behind?

The exception being reflexed airfoils used on tailless aircraft.

Effect of CG location is aptly demonstrated by this web page:
http://142.26.194.131/aerodynamics1/...ity/Page7.html

Another question:

The center of pressure of a flat bottom airfiol moves forward when the angle of attach increase. It seems that that will leave the plane out of trim when a gust (with an upward component) hits... potentially exacerbating any tendency to balloon in response to a gust.

My question is answered on this page.
http://142.26.194.131/aerodynamics1/...ity/Page9.html

So Ben... what you are saying is that ballooning is a side-effect of slow pitch recovery rate combined with moderate damping (the forward CG position)? Am I interpreting that right? That alone wouldn't account for ballooning I would think... maybe in combination with ground effect.

I would expect a lifting tail to stay in the air like the main wing due to ground effect. The downward-lifting tail would not have that ground-effect bouyancy though... lifting the main wing and leaving the tail where it is... pitching up that aircraft. Maybe that's it.

I believe trainers balloon because of their low wing loading combined with the fact that many beginner pilots fail to reduce speed. When you get into the flare with excess speed, it is very easy to balloon with any plane. More skilled pilots maintain proper speed control and make better landings.

Tom

In the context discussed here, I understood "ballooning" to mean large responses to gusts.

Jim, I was discussing this in the context of this thread:

http://www.rcuniverse.com/showthread...032&forumid=19

where I think all this ballooning talk started..

I was thinking mostly out of ground effect although I should have thought that most folks think of ballooning in the landing phase, I need a better title.

The balloon, nose up with speed increase, is there at any altitude. But coming into landing if the speed gets higher than a nicely trimmed landing speed (which indeed is a beginner's problem as Tom mentioned) the trainer airplane will trade the speed for a nose up attitude and potential problems.

I guess in that respect the trainer isn't a good trainer in the landing process.

At altitude it can save a potentially bad death dive caused by disorientation by zooming upward as the speed increases. I know the effect has saved my airplane many times in the good old days (I learned to fly on my own, not a really good approach).

Ken - I think our pitch damping is fairly high in most of the airplanes we fly. Our horizontals are large and we do have a reasonably high static margin. The ballooning is mostly a dCm/dVelocity effect.

Certainly when in the ground effect there can be some effects. One of the interesting tests we did on our airplanes at McDonnell Douglas (before Boeing) was the slow speed ground-effect testing. They would insert a "board" and them take data. There is a variation in downwash and lift due to wing-board interaction and lift on the tail due to downwash and tail-board interaction.

If you come in at a good (+ or -) landing attitude the overall effect with a trainer configuration would seem to be that the tail, regardless of whether lifting up or down, would get an incremental up load due to ground cushion effect. It would stop the ballooning very close to ground (maybe).

Basically what you have illustrated is that an airplane of conventional configuration is more stable with a more forward cg and less stable with a more rearward one. Less stability is desired for an aerobatic plane and a little more stability is desired for a trainer. (The more stable in pitch, the more tendency to “balloon”) You are absolutely right.

As regards lifting tails, a tail with a cambered airfoil shape (referred to as a “lifting” tail) was all the rage in free flight models in the 50’s and 60’s, but the reason was different. The model was trimmed to glide at the minimum sink rate angle of attack but had to climb under power at a lessor angle to gain the maximum altitude during the limited engine run. Since there was no control once the plane was launched, many schemes were tried to make the transition work. One of these was the “lifting” tail. The flat bottom airfoil section of the stabilizer was in the slipstream of the propeller and it was thought that the increase airflow across the tail would “lift” the tail when the engine was running, reducing the angle of attack of the wing, and allowing a more efficient climb. Many contest winners of that era had this configuration. The incidence was such that in the glide the force on the stabilizer was nearly neutral or even down.

If the cg is moved far enough aft (on an airplane with typical sized tail surfaces) to require lift at the horizontal stabilizer, it may be in balance but will likely be unstable.

Lou,

Do a look up of the stuff I have written and you will get a ton of stuff about lifting tails (it tends to be a fanitical thing with me, sorry). Actually if the CG is between 25%-anything aft of that,with everything else at 0-0-0 degrees, the tail is lifting. It doesn't have to be unstable. It is just the result of the force and moment balance around the CG. Certainly it will go unstable if the CG is aft of the Neutral Point of the airplane regardless of other stuff.

The old (and new for that matter) free flight models with the long tail arms could have the CG as far aft as the wing trailing edge of the wing (and farther but always not aft of the Neutral Point) which meant that the tail was always lifting regardless of speed or usuable incidence.

The wings couldn't input a nose down moment due to camber large enough to need a down load at the tail. With a cambered wing with its nose down moment it does add some moment that might cause a incremental nose down load but the net load is usually up.

In free flight glide you want a dCm/dvelocity curve at the lowest sink rate. This is still a lifting tail. Note that even with the high thrust line models they still used the same tail sections. The high trust line gave a power moment to counter the higher climb speed nose up pitch characteristic from the dCm/dvelocity curve. It did move the tail out of the slipstream somewhat. The variable incidence tail that is used nowadays is basically moving from one dCm/dvelocity curve to another.

The free flighters thought that a lifting airfoil would certainly be more appropriate if the tail was lifting anyway. For a given tail load the choice of a tail section was a trade off of the drag of a flat plate, symmetrical tail, flat bottom or cambered airfoil.

Obviously the tail configuration varies on a continuum, the extreme being the canard where the “tail” provides nearly all the lift. There are also the “Flying Flea” and the Delane Duo-Monoplane of the 50’s where the wing and tail share the lift somewhat equally. All can be brought into balance with an acceptable degree of stability. Free flight models like you mentioned not only have long tail moments but rather large tail surfaces, which allow a more rearward cg (even to the trailing edge) with positive stability. The free flight designer has the luxury of designing to only two flight conditions, climb under full power and glide at minimum sink rate.

For airplanes of more conventional configuration, and proportion, the normal force on the tail whether positive or negative should be held to a minimum consistent with the stability desired. This is not only more efficient but makes an aerobatic ship respond more nearly the same upright and inverted.

Again, what you have said is merely that trainers balloon because they are more stable in pitch and pattern ships do not because they are much less so.