Although the ilustration is mostly correct its not accurate. Two airplanes of same size and wing area, one with symetrical airfoil, zero engine, horizontal stab and wing incidence would not behave like a trainer if the CG is moved forward. On the contrary, given the CG is exactly the same on both wings you still have two different flying machines. But by no means would the symmetrical airfoil aircraft behave as a trainer. A trainer with flat bottom wing tends to right itself as destabilizing conditions affect it (wind gusts, etc). Now a symmetrical airfoil aircraft, lets say an Extra 300, will not right itself even if the CG is forward. It relies on operator input to continue level flight.
The balance of forces are completely different between the types of aircraft thats the reason why you dont see , for example, engine downthrust on symmetrical airfoil aerobatic aircrafts.

Don't be too quick to dismiss it until you have tried it. There is nothing unique about a flat bottomed wing that makes it fly more stable nor is there anything particularly unstable about a wing with a symetrical airfoil. It all depends on balance.

As I said, there are other things that can be designed in that make an airplane more suitable for a particular purpose, but moving the cg can make a dramatic difference in the way a particular machine flies. For instance the reason you don't see downthrust on aerobatic airplanes with a symetrical wing is primarily because you want the plane to fly just as well upside down as right side up. This is the same reason you don't use washout at the tips, because it messes up the inverted performance. Fortunately such aircraft are balanced much nearer the neutral point so the tendency to climb under power is not very pronounced to begin with. An airplane with a symetrical airfoil, a reasonably light wing loading, and a forward cg will indeed fly much like a trainer, as far as pitch stability goes.

You are right when you say if the cg is exactly the same on both wings the one with the symetrical wing wouldn't behave like a trainer. As I said earlier in this thread, due to the absence of the negative moment produced by the cambered wing, the neutral point is further forward on the aircraft with the symetrical wing so if they are balanced the same, it is much closer to it. But if you take that airplane and move the cg forward from whatever position it is, it will behave more like a trainer.

I don't know what you're flying but if it's an Extra 300, move the cg forward a significant amount and go fly. It will climb when you add power, and will recover from a dive without your input. Aerobatics won't be much fun because it will be sort of "loopy". If you have a trainer, move the cg back to the rear limit (or just a tad more) and go fly. You will be suprised.

Boys,boys...
Me thinks we are getting far from the simple question originally asked. I know I helped start it but the highly technical argument is probably misaimed at our thread starter.
I think we all agree that if he wants a plane that doesn't climb excessively when power is added he has chosen the wrong design. We are not sure of the airworthyness of this scratchbuilt. If he wanted a plane that won't climb under power he could:
*change the CG
*use a symetrical airfoil
or
*add incidence to the rear stab
all good ideas and most likely helpful to the original thread starter

move the cg forward and it will climb when power is added? Do a free body diagram using figures for a stable aircraft then move the cg forward.....now tell me what kind of moment would you have. Doesnt seem its counterclockwise so how would it climb?

Capio,
If you move the CG forward, then to maintain level flight at a cruise speed, more "up trim" is needed. Increase power (and speed)... and presto: the model climbs. Hence using throttle to control altitude and elevator to control speed.

-David

*or take a minute or two at the field and raise the aileron's neutral position. I like to tell guys to raise them one clevis turn and fly. Repeat until the plane flies the way you want it to. It usually only takes a couple flights. Until I started solving pitch trim problems this way, I never would have believed such a simple minor change could have such a drastic influence on the way a plane flew.

lets keep beating up this dead horse........basically any aircraft that is trimmed for level flight will climb with the addition of power regardless of the cg location. This is because there is no vertical component of a balancing force to keep the nose from rising.

unless we add downthrust!...... tadaaaa!


[sm=stupid.gif]

I always thought it was kind of goofy to add downthrust... it's basically like adding weight to your aircraft. Props are for pulling/pushing, wings are for lifting.

Why not just accept the fact that an airplane is typically designed for a cruise speed... if you go faster than this, the plane will climb unless you compensate by adjusting your elevator. If you go slower than this, your aircraft will sink, unless you compensate with your elevator.

I can only guess...

How about: The plane operates at, or closer to, minimum drag conditions if it doesn't have to fly with loads of elevator trim. That must be worse than the cost of power wasted by pointing the engine down.

ummm... a plane that is climbing, but not accelerating, has no "excess" lift. So we can't really say the wing's lift is in excess of that required to support the plane's weight, hence it climbs. At least not when it is a steady state.

When power is applied, the plane will speed up, react transiently, and the balance of forces will reestablish the plane in a climbing condition. The engine is delivering power in excess of the rate at which it is being sapped by drag, hence the plane is climbs. I feel much better now.

cap, this is for fun, and personal edification, not animal cruelty. If we stopped beating dead horses this site would shut down. Not exactly the cutting edge here... these horses were dead from the get go.

I am trying to understand that concept.......an airplane that climbs has no "excess" lift? Shouldnt it be the other way around? In order to maintain level flight shouldnt all the forces balance and weight = lift, therefore no "excess"? And in order to maintain dynamic equilibrium while in a climb shouldnt lift > weight?


The waste of power by pointing the engine down a few degrees is relatively small, just the sine of the angle multiplied times the amount of thrust.


......maggots are eating the horse's flesh.

Cappio777,

I would agree that the amount of power wasted is relitively small but I would not agree that the effect on top speed is not something to be concerned about... Lets say we have a prop at 14.3 degrees at 75 percent diam roughly a 10x6 prop now if you add two degrees of down thrust this effectively becomes a 10x5 prop. Lets say that the prop is running at 12000rpm (200rps). The 10x6 prop would reach the zero thrust point at about 129 fps...the 10x5 prop would reach this point at about 114 fps. Assuming a linear dynamic thrust curve and an static thrust of around 4 pounds then at 100 fps the 10x6 prop would still be generating .9 pounds of thrust or 90 fp/s of power....the affective 10x5 prop would be generating about .5 pounds of thrust or about 50 fp/s. At 90fps the 10x6 would be generating 153 fp/s....the 10x5 126 fp/s At 80 fp/s 10x6...156 fp/s....10x5 133 fps. This does not take into account the thrust being used as a dynamic weight or the interferance drag because of down thrust. I'm estimating that the overall lost to top speed would be between 3 to 5 percent...or about 1 to 2 times the amount of down thrust used. This is conservative estimate it might be as high as 10 percent. I not quit sure what the tail loading on both planes would be.

Beating a dead horse is a realitively amusing.....maybe this will help anyone that is confused

[link]http://142.26.194.131/aerodynamics1/Stability/Page12.html[/link]

Steve

I agree 100% that the effective power loss is within that range. What we have failed to look here is that we are talking about a trainer with a considerable amount of drag. If we were to calculate the efficiency of a PT-40, LT-40, ...etc, compared to the amount of thrust put out by the propeller its easy to see that the loss of power for using downthrust is practically neglible. The idea here is to provide dynamic stability to a trainer thats being accelerated while level flight. The argument was never made for any other type of model.


[link=http://www.dwtoons.com/Horse/Horse1.htm]Beating a dead horse[/link]

I guess it depends on how you want your airplane configured. The elevator is there to make adjustments to pitch. That's it's whole point for being. Worse than pointing the engine down? I guess it depends on personal preference... I still think it's a goofy idea.

There is almost no way to measure this in a model... mostly because a model is tough to get into a steady state. You can be climbing without L > W... true. But the climb will diminish due to drag. If you want to keep climbing L -must- be greater than W. But this is almost never going to be the case in a model because most of what happens is transient (finger twitches, gusts, etc).

Speed will go up until drag=thrust.

If you apply more power and are now going faster.... L is greater than W... you're going to be accelerating upwards unless you compensate with your elevator, or reflex both ailerons, or, as some have said, use downthrust, which is kind of self-compensating.

If you want to keep climbing, lift doesn't have to exceed weight. Perhaps we should say "net vertical force" rather than lift. If net vertical forces exceeds weight you not only climb, you would accelerate up. Which is not the case when a plane is in a steady climb - it's not accelerating. The engine must, of course, provide more power because now in addition to performing work to over come drag it is performing work to overcome gravity.

I have horse steak and potatoes for dinner, everyone is invited. Dont forget to wash your hands with propwash.

Cappio777,

I thought I saw that horse twitching so we better make sure its dead before we cook it....should I wash my hands with 10% or 15%?

Here is a link to reflex rons:
[link]http://142.26.194.131/aerodynamics1/Stability/Page10.html[/link]

Looks like this has some merrit.

Steve

Any airplane with enough stability to be flyable will tend to climb when power is increased and dive when it’s decreased. Specifically how it reacts and what sort of handling characteristics it displays can be dramatically different between airplanes. If this were the “beginners” forum a general simple statement might be sufficient, however I suspect that many people come to the “aerodynamics” forum seeking a better understanding of why their airplane behaves like it does, and what can be changed to make it behave differently. With that in mind, I think this discussion has been both informative and appropriate.

For those who already know it all or don’t want to contribute, they are free, as always, to go to another thread.

Steve, no, I got tired of beating the horse and brought my shotgun and finished it a while back.....I got a big piece for you on the grill so come on over. And yes, that link has a lot of merit...........as long as it applies to tailless aircraft.


Anyone wants barbecue sauce on theirs?

Louw,

I agree....learning is the reason I read the aerodynamic forum....

The main reason I got in this discussion is that I was attacked on another forum by moderaters(it will remain nameless) for the following statements:

All planes have a tendency to climb under power..... the throttle on a plane is not like the accelerator on a car....any power above the power needed for level flight results in climb.

I am no aerodynamic expert but from what I know about the subject these are true statements.... I really have not seen anything stated on this forum or anywhere else that would lead me to believe that an attack was appropriate for making these statement....as a matter of fact I don't believe, no matter how wrong a statement might be, that it ever needs to be attacked....persuasion and education are much better tools.

Hopefully this has been educational....I know I've learned some things and have a lot more to learn....but at least this forum give me some direction.

Steve

I had this problem with my LT-40. At half throttle (.40 engine and 10-6 prop) the plane would climb like a skyrocket. It wouldn't pitch up, just gain altitude.

I reasoned that the wings were creating too much lift (imagine that?) and that this might be caused by having the ailerons drooped down a little, effectively creating more lift on the wing.

I took about 3 turns out of the aileron clevis's and this seemed to tame the problem. I still need to do more, but didn't want to get back into the wing again. I'll take 2-3 more turns out of the aileron before I fly it again.....

I was just thinking that this might be another reason for the rapid climbing that you're experiencing...

Bob

Basic flight aerodynamics 101

Very basic flight dynamic info for Full sized planes from the smallest trainers to the biggest jets and applies well to R/C flying. Of course fully aerobatic planes are capable of changing the rules to some degree

More airspeed = more vertical lift

Less airspeed = less vertical lift

Turn changes lift vector sideways in direct relation to angle of the bank which will cause loss of vertical lift.

Trim is always used once the desired airspeed is reached.

You also need to remember that more power does not always give you more lift as angle of attack (AOA) of the wing is everything.