Will an aircraft stall and a higher or slower airspeed with a forward CG vs. and aft CG?

Airspeed has nothing to do with it. It will stall when the critical angle of attack is exceeded.

As for flying the plane, a more rearward CG will let the nose be lifter higher by the elevator during slow flight. A nose heavy plane will stop flying at a higher airspeed simply because it can't keep the nose up any longer, but this in not a stall.

Schmleff

Great answer! I can't add anything else.

Schmleff

I put the question out to see what some other thought on the subject. I disagree with you on airspeed. You are correct that a wing will stall at the critical AOA. However the actual airspeed of the aircraft will be different depending the the location of the C.G. You are correct that and extreme nose heavy aircraft will not stall because it simply runs out of elevator however lets just say the forward most C.G. that will still allow a stall v.s. the rear most C.G. the aircraft is still stable.

What makes and aircraft stable... it is having the C.G. in front of the center of lift (C.L.) The farther the C.G. is in front of the C.L. the more down pressure is needed on the horizontal stab to keep the aircraft level thus creating more wing loading. The higher the wing loading the higher stall speed. Thats why LearJets land at 130 kts and Pipers land at 65kts. When you increase the wing loading i.e. moving C.G. forward, increasing aircraft weight, making wing smaller etc. you increase the speed at which the wing will reach the critical AOA. Unless they changed the books on aerodynamics lately or models do not fly the same and full scall...I believe you are incorrect.

Regards
Mike
ATP/MEII/LR-JET

You might want to take a look at this web link... http://www.safetydata.com/manual/poeg11.htm
Here is an excerpt from it.


4. What effect does a forward center of gravity have on an aircraft's flight characteristics?

Higher stall speed - stalling angle of attack is reached at a higher speed due to increased wing loading.

Slower cruise speed - increased drag; greater angle of attack is required to maintain altitude.

More stable - the center of gravity is farther forward from the center of pressure which increases longitudinal stability.

Greater back elevator pressure required - longer takeoff roll; higher approach speeds and problems with landing flare.

5. What effect does a rearward center of gravity have on an aircraft's flight characteristics?

Lower stall speed - less wing loading.

Higher cruise speed - reduced drag; smaller angle of attack is required to maintain altitude.

Less stable - stall and spin recovery more difficult; the center of gravity is closer to the center of pressure, causing longitudinal instability.

Um,

From my exposure, wing loading is a loose term that describes the weight of the craft verses the area of the wing, on the ground.

You seem to be interchanging the term "wing loading" for load factor (which by judging by your ratings, you are familiar with).

CL has nothing to do with critical AOA, but like you said, everything to do with stability.

Either I wrong and I am about to learn something, or you and your resourse are wrong. I have no horse in this race.

Schmleff

PS:I have flown with enough CFII's and MEII's to know that the rating does not make them good pilots.

PSS: what you are doing is called trolling.

So this question was a trap for your own entertainment? [sm=rolleyes.gif]

Jogng

"So this question was a trap for your own entertainment?"

No JohnG. This question was to get other opinion's on the subject. By reading some of the other threads in this area I can see that we have many very knowledgeable people and I wanted their thoughts. I know what my opinion is I just wanted others. It has nothing to do with entertainment. Just getting the facts straight. I don't claim to know everything and would like to have a better understanding of these subjects.


Schmleff and I did not agree on another thread. I did not expect him to be the first to answer this one. Hopefully others will help answer this question.

Respectfully
Mike

Aeronautical fact has nothing to do with opinion. You have answered your own question quite accurately. Yay.

Of course, you did not qualify your question with any description of the stall e.g. "level flight" or "during landing" - so schmleff would be right in reverting to the basic description of a stall being at a critical AOA above all else. The AOA argument holds true in a vertical climb, balistic zero-g flight, and any other situation you can get a plane into. The texts you have quoted do not. Being that our models often see much more unusual attitudes than GA airplanes, taking the wider definition would seem prudent.

If you had pasted in the info you already had first, it wouldn't have looked like you are trying to be a know-it-all.

Johng,

Had I already found the text before my post I would have included it. However I found it after the fact. In fact I did not look it up until after my second post. This is not a witch hunt. Just looking for some second thoughts. You are right everything in arerodynamics is a fact, but not all of us know all the facts so we have opinions untill we know the facts.

I disagree with you as well. The question has all it needs for qualification. Regardless of the situation of flight when you apply "Load Factor" to the aircraft be it in vertical, inverted or other situations the C.G. position will still affect the airspeed at which the the wing reaches the critical AOA.

A vertical ballistic zero g path would not count because technically the wing in no longer flying or producing lift. It is at zero AOA provided it is symmetrical. The thrust source would be providing the lift. However when you pull the stick back now you are increasing the angle of attach. Centrifugal force is pulling at the point of C.G. and lift is pulling at the center of lift. If you have a forward C.G. the airplane is going to stall sooner than with an aft C.G. If the stall come sooner than it would make sense that the airspeed was higher.

I am not saying that speed is always the same. You pull hard enough you can stall at any speed.

I have some experience in full scale aerobatics. When I am by myself in an extra 300 and try to pull 6 g's at 130 kts it is no problem. When I throw a 250 lb pound guy up front and do the same thing I get an accelerated stall, regardless of the attitude of flight. This is why I need to add some speed to some manuvers to prevent accelertated stall problems.

Your thoughts....

I think it goes like this.

First case -
Looking at just the wing the stall is angle of attack related for a given Rn and Velocity, wing airfoil, roughness, etc. Assume we have an airplane that stalls at 1 g load factor. With the CG at the 25% chord point of the wing the tail load is zero and the aircraft stall is at a given angle of attack.

Second case -
When incorporated into an airplane with a forward CG the stall of the wing is still angle of attack related. Since the tail is down loaded on the airplane the wing will need to increase the angle of attack to try to keep the same 1 g load factor as in the first case. But it will stall if increased. So at the same given speed, etc. as the first case the airplane will stall at a lower load factor as a result.

Thrid case -
Since the tail load with an aft CG is up on the airplane the wing will need to decrease the angle of attack to try to keep the same 1 g load factor as in the first case. At the same given speed, etc. as the first case the airplane won't stall. It can go to the case one angle of attack and a higher load factor will result before stall as a result.

If you attempt to keep the load factors all the same before stall, say 1 g, then the comment

"4. What effect does a forward center of gravity have on an aircraft's flight characteristics?
Higher stall speed - stalling angle of attack is reached at a higher speed due to increased wing loading. "

seems to occur. But the airplane is stalling at a higher speed only because the speed to reach a 1g load factor requires an increase in velocity. It is just the reverse with the aft CG results.

But the comment - "stalling angle of attack is reached at a higher speed " isn't true. The quoted statement should read,

"Higher stall speed - Airplane stall occurs at a higher speed when attempting to maintain a constant load factor."

and similiarly stated for the aft CG cases each of which is not the same as the "stalling angle of attack". The stalling angle of attack of the wing is the same in each case. Johng is correct in his statements.

Ben,

Lets say we are dealing with a load factor that is not constant. Would you agree that it still holds true. Lets say its variable between 1g and 3 gs. Would you agree that an increased speed would be needed to keep the wing from reaching critical AOA as the load factor increases. Also their would be a margin of speed differences depending on the location of C.G. Also I am not saying that margin is constant but still exists.

Example

Lets say for a constant aircraft weight: G = Load Factor

1 G aircraft stalls at 50 kts at 6degree AOA forward C.G. condition.
1 G aircraft stalls at 40 kts at 6 degree AOA aft C.G. condition.

2 G aircraft stalls at 60 kts at 6 degree AOA forward C.G. condition.
2 G aircraft stall at 50 kts at 6 defree AOA aft C.G. condition.

3 G aircraft stalls at 70 kts at 6 degree AOA forward C.G. condition.
3 G aircraft stall at 60 kts at 6 degree AOA aft C.G. condition.

I agree with you and johng that the AOA does not change. however their is still a 10 kts margin even thought the speeds are changing as the load factor changes. (10 kts margin is just for the example and may not always stay constant) This margin is due solely to the position of the C.G. Would you agree with this?

MT

OK. Here is my penny worth of thoughts.

First of all, according to NASA ( http://www.grc.nasa.gov/WWW/K-12/airplane/cg.html )
”The center of gravity is a geometric property of any object. The center of gravity is the average location of the weight of an object.” Thus all the CG is a center of the mass of the object. This is mechanics, at best kinematics, not aerodynamics.
Stall, AOA, etc. IS an aerodynamics. The relationship – zero to none! The relationship is only in the amount of energy (I am using mechanical term on purpose) the engine produces TO COMPENSATE ALL OTHER forces, including gravitational force, the weight, so the plane can fly.

Technically, even “perfectly balanced” plane will eventually stall when the speed is low so wings will not be able to “hold” the plane in the air anymore. With CG moved forward or aft, left or right you just adding another variable to the equation. All you do is shifting weight, thus creating an additional force which causes the plane to go towards CG, and all it does is TRYING TO FIND THE POSITION at which it will be perfectly balanced again (kinematics)!!! Unfortunately, with all these MECHANICAL changes you are inevitably affecting planes AERODYNAMICS by shifting AOE, thus you causing stalls at higher speeds, but again, because you are changing aerodynamics of the “wing against wind” condition, and not because you have shifted CG.

That example has more to do with the fact that wing loading is up by what, 15-20%?, than it does with CG position.

Now, pull up into a 45 degree climb and neutralize the controls, idle throttle. At some point you will hit the apex of the climb, and it can easily be at a speed that is below the 1g stall speed, yet the wing hasn't stalled - lets say 30 kts. Now apply full up elevator. Wing stall at 30 kts. Same for any CG position. Essentially it's the same for the vertical dive where speed is above 1g stall speed. As long as you keep to certain assumptions about continous flight conditions at constant or near constant load factors, your speed comparisons hold. But it is not a universal fact. Speed is not a good predictor of stall.

You are saying that the extra downforce on the tail is causing more total downward force on the craft. This increase of total force must be countered by the wing, therefore requiring more velocity into the relitive wind.

Or put more simply yet, the wing has to make up for the extra downforce created by the tail so it requires more lift genereated by the airfoil.

I agreee with that idea.

Point the aircraft straight up and it is all out the window. Your data is only applicable to flight at a constant angle of climb or decent.

So lets go back to your original question:

"Will an aircraft stall and a higher or slower airspeed with a forward CG vs. and aft CG? "

The answer is "it depends" it is either yes or no, or neither yes or no.

This entire topic is the foundation of canard theory. A canard adds no downfoce so it requires no additional lift to counter the downforce, thereby reducing total drag (in theory).

"Now, pull up into a 45 degree climb and neutralize the controls, idle throttle. At some point you will hit the apex of the climb, and it can easily be at a speed that is below the 1g stall speed, yet the wing hasn't stalled - lets say 30 kts. Now apply full up elevator. Wing stall at 30 kts. Same for any CG position. Essentially it's the same for the vertical dive where speed is above 1g stall speed. As long as you keep to certain assumptions about continous flight conditions at constant or near constant load factors, your speed comparisons hold. But it is not a universal fact. Speed is not a good predictor of stall."

I'll buy that John. I think we have all learned a little bit on the subject.


"You are saying that the extra downforce on the tail is causing more total downward force on the craft. This increase of total force must be countered by the wing, therefore requiring more velocity into the relitive wind."

This was my point exatcly. However I have learned that this does not always hold up. But I think it can help some to understand the dynamics of a stall an factors related to it.

Thanks you for all your comments.
Regards,
Mike