C Earnest

Let me explain to you why the CG position has an effect on the G load vs. lift requirements of the wing. For a conventional aircraft (tail aft), static stability occurs when the center of gravity is ahead of the center of lift. To maintain level (1G) flight, the tail must generate a downward force to offset both pitching moment of the airfoil and the forward CG.

Pylon racers generate very high G loading in the turns. This is evidenced visually by the nearly 90 degree angle of the wings when pulling at the pylon. The actual load depends upon both the speed of the aircraft as well as the radius of the turn. Use 170 mph (about 250 feet/second) and a radius of 71 feet for the radius of the loose turn at pylons 2&3, or a radius of 50 feet for a tight turn at pylon 1 for your own calculations.

Now as the CG is moved forward, the down force from the tail must increase. As you increase the G loading, this force multiplied. This means that the wing must also increase the amount of lift generated. Greater lift generated corresponds to greater induced drag. Therefore, for minimum drag, it is common knowledge that you fly with the CG as far back as possible. Typically, the trailing edge of the elevator will only move about 1/8” from neutral to full up elevator. I have flown some with even less throw.

BTW, Dr. Dolittle wrote a rather nice article on the concept of balance and speed of aircraft back around 1928. Who was Dr. Dolittle? He was kind of well known as the first pilot to hit 300 mph, and flew the Tokyo raid in 1942. His first name was Jimmy, he raced the GeeBee and he was also a General in the Army Air Corp.

Oh -a calulated G load --
We did our speed/balance stuff in the fifties- on controlline-then flying quickie in the 70's - the forward cg was to increase drag at the rear of the plane.
Better stability in really hard pulls - yes I know about the CG setups currently used .
Thanks for the "nuts" remark - I have been called worse by professionals ---

Hi Dick,

Well it DOES say that you are certified on the left.[8D]

High Plains,

Oh, a positively stable airplane. Again......more variables. I suppose we should define the rules before we start the game. O.K., you prevail.

"......as evidenced by the 90 angle of the wings as the a/c is ""pulling at the pylons""........" May I assume by that statement that when my a/c is is in knife-edge flight that I should be concerned as to my main spar failing....? (Variables, variables, variables. Define your parameters. It is just not that easy! It seems that at times you are trying to play baseball on a squash court.)

Might we assume that as this increased g-load is occuring, the lift-induced drag is increasing, that the a/c is slowing and thereby decreasing the g-load, or are we increasing the thrust or the AOA, or both, as we "round the pylon"? Might we assume that the increasing g-load (seperate from wing-loading, which is a constant resulting from a/c mass, wing area, and fuselage wetted area) is a resultant of increased AOA? Those nasty variables again! Nothing is static. Everything is dynamic. It seems to me that you are mixing your parameters whenever you think it's advantageous. Logic is dangerous when abused.

There are too many of these nasty "gotcha's" for me to comment furthur in this string. Thanks very much for the aviation history lesson.

BTW, I am very familiar with Gen. Jimmy Doolittle and his stellar aviation career. let me explain to you how his name is spelled: it is Doolittle......two O's. I believe the Dr. Dolittle of whom you speak is the one that "Talks to the Animals". You would do(little) yourself well to check your facts before you pretend to educate others. There are lots of folks out here reading what everyone says, and one's veracity is being constantly evaluated through one's writings.

Anyhow, I've had my fun with this one. I am mortally wounded and I'll ask Dick to carry on for me. He has my proxy vote.

Regards, and have much success and fun with your pylon efforts. Add me to the "nuts" list, please. :-)

CE

Now, after we've learnt that Dr. Dolittle in fact is Dr. Doolittle, let's learn about the difference between the centrifugal force and inertia... and how... and when... and why or why not they affect wingloading.
- any ideas?

Adam_one,

Knowledge is found where one looks if he is judicious in where he looks and to whom he listens. As Albert Einstein once said to a 1st year math. (or physics?) class: "Two plus two equals four. If you don't understand this, all else is irrelevant." Perhaps there is a lesson there.

Oh, no, not again!! It goes a little deeper that that, Adam_one. Actually, we have learned that the person to which High Planes (and yourself) has attempted to refer is, in fact, General James "Jimmy" Doolittle. He is the one of the two which Does Not "talk to the animals", and not, to my knowledge, ever had either a medical nor a doctorate in any profession. He was far beyond any attempt to classify his level of expertise. I am acquainted with the navigator of B-25 #6 and he has related many details of Lt. Gen. Doolittle as to how his men revered him and some revealing insights into his personal character. He deserves the respect of having his name and rank referred to correctly, as does any national hero. Pardon me (Not!) for getting picky. Learn history or learn not to attempt to use it to bolster your case.

If some cannot even get people's names and titles correct, they should not be mentioning them by (incorrect) names nor titles. It's sorta' a sign of respect..... annnnnnnnndddddd:

Under those conditions, how in the heck can anyone pretend to discuss a complex subject such as physics/aerodynamics, and how much creedence should their words carry?

"God is in the details............."

No thank you, I do not have any ideas that I desire to proffer.

Offered in all best wishes, Regards,

CE

Wing loading is most definitely NOT independent of G-loading. Given a constant-mass aircraft (and don't go preaching details like "the fuel is being burned, so mass is not constant" ) with a fixed CG, G-loading and wing loading are almost directly proportional. (they are not directly proportional, since the fuselage and thrust vector may start to contribute to the lift at high angles of attack)

An aircraft flying with the CG located near the neutral point will fly either faster or more efficiently than a nose-heavy plane. I can speak from personal experience in a Cessna 172. During cruise, a piece of heavy cargo was moved from the co-pilot seat to the baggage compartment. Following this move, the plane accelerated from 110 knots to 125 knots, and required a re-trim.

An aircraft flying as above will also be more responsive to small elevator inputs. Since the wing is essentially acting as a fulcrum against which the tail operates to keep the CG balanced, moving the CG effectively increases the mechanical advantage that the tail has.

I realize that the comment bashing the "near 90 degrees" observation was intended as an agressive nitpick, and that you probably understand full well the difference between knife-edge flight where the weight is supported by thrust and fuselage lift, and a high-G coordinated turn, where the wings are actually perpendicular to the load vector formed by combining gravity and centripetal acceleration. Do we really need to insist on absolute precision in communication, when the intent should be clear to anyone with even a rudimentary understanding of flight physics?

C Earnest,

I really didn't mean to hurt your national feelings (or heroes), but I just can't keep track on all national heroes from all over the world - you know internet is global - and I'm not a US citizen.
By the way, I don't think that a wrong spelling or missing a title is much relevant for the main issue here.


Crewguy25,

Thanks for addressing the point.
I don't understand why people starts bashing each other instead of focusing on what really matters here - "aerodynamics".

Being nuts , I have to ask a nutty question.
"why would you shift heavy cargo whilst in flight?"
Also Tho it is common practice to trim in elevator for max speed -I never have been asked to "move to the rear to speed this thing up" --
The lift required for flying straight and level is dependent on the speed and weight --
Now then-- how much speed is to be gained --percentage wise --changing ONLY a moving CG --and retrimming (no weight change.)
Lets understand that CG was "close enough" to get the thing in the air ---
Has A-- the lift actually changed ?" B-- trim drag is reduced?
C ---something else - gremlins maybe?

Hey Dick,

Riddle me this: I was always taught that you trim for zero pressure on the control column regardless of flight attitude (AoA). If not, constant pressure is required on the flight surfaces to maintain level-climbing-descending-turning flight. This translates to some combination of deflected surface/s (induced drag) in 99% of all situations notwithstanding perfection in all relevant circumstances. Now.....if one trims for "max speed" is that not the same as "balanced" control column pressure with all parameters considered and will that not also result in induced drag? I fail to fathom the relevance of "trimming for speed", unless the pilot trims max "down elevator".

Hep me; hep; hep me........

Regards,

CE

Good info for the kids.

dear 8178 - thanks for the anon tip-----
However, why not simply answer the question if you have the answer?
Is the matter of speed as related to trim drag that obscure/simple ?

True, I can't spell and I should have check on Dr. Doolittle's name.

But I guess that you don't really know as much as you pretend.

Have you ever heard of MIT?

Born at Alameda, California, on December 14, 1896, Doolittle was a junior at the University of California when the United States entered World War I. He enlisted as a flying cadet in the Army Signal Corps, which gave him a commission. he spent the war as a flying instructor in the United States.

Remaining in the Army after the war, he earned a B.A. degree in 1922 and then studied aeronautical engineering at the Massachusetts Institute of Technology, from which he received both a Masters and Doctors degree in science. He took a leave of absence from the Army in the period before World War II,but returned to
active duty when the war began....

well, you get the basic drift.

I'm sorry, that post was really for earnest

So -the Dr. who lived at Puddleby On The Marsh was only famous for discovering the Cessna Push Me Pull You?
Maybe I better check with my library again---
Maybe take the Giant Moth--

High Plains,

I stand corrected, and thank you.........and yes, I have heard of MIT.

Perhaps our foreign friend Adam_one will also learn a little about Gen. Doolittle from this. The internet is truly global, and one only has to enter "Gen. Doolittle" into any search engine to learn about that distinguished gentleman. 'Tis better than using one's nationality as an excuse for ignorance. MAN, that's nasty........., and I apologize......... ;-(

Regards,

CE

“Lucy, you still have some �xplaining to do’

Well, now that we have established the limits to your omnipotence, perhaps we should examine your other snotty comments. Plainly, you don’t understand my point about load, balance, and drag. Observing an aircraft’s wing angle in reference to the horizon during a level turn is a simple method of estimating the g load. Since we were discussing a pylon racing turn, a level turn is implicit. Otherwise, contact with the ground is the usual result. I can also calculate G loading from the parameters given. Of course it is dynamic, even wind shear can adversely effect the instantaneous value when in a constant turn. We could debate nit’s till the cows come home, but this forum is somewhat limited in scope. Find a larger sword to fall on.

Dick,
The total lift required for straight and level flight is definitely dependent on the weight (it is the weight), but I don't think depends on speed.

Almost all RC airplanes have irreversible control systems (the position of the flight control surfaces with neutral control sticks should depend only on the trim setting). If you neglect the effects of Reynolds and Mach number, the pitching moment coefficient about the Aerodynamic Center of an airplane with an irreversible control system and neutral controls depends only on the trim setting (the Aerodynamic Center or "AC" is the point along the fuselage where the pitching moment doesn't change with AOA). For an airplane with positive static longitudinal stability, the center of gravity is forward of the AC. In order for the airplane to be in pitch equilibrium, the "aerodynamic moment" about the AC must balance the "gravitational moment". For a given trim setting, fixed CG, and neutral controls, there is only one airspeed where the moments balance, and the airplane will seek this speed (assuming that the airplane also has positive dynamic longitudinal stability). You can change the trim airspeed either by moving the CG fore/aft or by clicking the longitudinal trim to change the neutral position of the control surfaces.

Gliders sometimes "retrim" by moving water to shift their CG. Is this a more efficient way to trim than by changing the neutral position of the control surfaces? Kind of. By shifting the CG, you can get the airplane to fly at the same airspeed with different control deflections. Different control deflections will result in a different drag coefficient and different performance. I would guess that "trimming for speed" would mean moving the CG to the point where:
1) The airplane flies level "hands off" at full power
2) The control deflections result in the best level speed at full power
This is not necessarily at max down elevator. Would the airplane trim at a faster airspeed if you put in down elevator trim? Yes, but if you met conditions 1 and 2 above, the plane would not have enough power to fly level at the faster speed.

One drawback to shifting the CG is that you also change the airplane's "static margin" and longitudinal flying qualities. If you are concerned about performance and are willing to put up with more workload, shifting CG may be the way to go.

Dick:

A GA plane will definitely fly faster at a given power setting by moving the CG aft.

For safety's sake they are built with the CG far enough forward to require stab/elevator down force for level flight. By moving the CG aft, less down force is necessary, and with the resultant lessened induced drag more power is available to pull the airplane forward. Therefore, a higher speed at the same power.

Bill.

That was a concise and straightforward explanation, just as simple as "two plus two equals four".
And we really don't need to know about the fella Doolittle in order to understand that.

Thanx

C Earnest,
Yes, that was a nasty one.
So, why should I bother about your Doolittle?
But, if I were interested in details of your history I'd seek another forum.

Now after you've boasted about your knowledge on your hero, why don't you tell us about what you know on aerodynamics, or better said, tell us about what you don't know as your ignorance in this area seems much more evident... [8D]

Adam_One,

Why should you care about "my" Doolittle? Because, my friend, lacking Gen. Doolittle and hundreds of thousands like him who, by the way, are buried all over the continent of your homeland, you would be speaking German (Nazi dialect) with a smattering of Italian and Japanese. BTW, there are a few heros from your area of which you should be aware, as you probably owe your life to them. Does "Heroes of Telemark" mean anything to you? Without them your country would still be glowing.

As to what I don't know ablut aerodynamics, it would fill volumes. If I were able to tell you what I don't know about the subject, then I would know it, wouldn't I? Really.................! I freely admit my ignorance when and where appropriate.

My most profound apologies to all the participants of this string. I did not want to let this part of the discussion get to this point, but like most, I just can't put my keyboard down.

And, High Plains and all, I sincerely apologize for all my "snotty" remarks. I, however, do not hold the patent on those, but if I did, a lot of people here would owe me thousands in royalties :-).

My regrets also go out to the fine folks at RCU. Their efforts do not deserve these irrelevant posts. I will, however, stand behind my entries re: General Jimmy Doolittle. (except for the Doctorate thing.)

Again, sorry for staining this string.

I will fight no more forever..........................

Regards,

CE

Shoe,

As was stated, 2+2=4; Einstein would be proud of you. Very clear and concise. Adam_One and I finally found a common point. GA aircraft might benefit from this sort of scheme were it not for the obvious cost and increased "induced liability". My comments about trim presupposed a non-movable CG, and perhaps I should have stated that. Precision...........

Thanks for the illustrative remarks.

Regards,

CE

Thanks for the responses on "trimdrag"
If anyone knows, please toss in any SWAGS about percentages of speed improvement -thru reducing trim drag --on models! (not on F16's)
When I noted way back -that I put CG way forward on my racers -I was not kidding .
I kept the stab volume low -so I could really crank in trim change -and elevator throw --with no real chance of throwing the plane into a high speed stall.
Setting it up for marginal stability really didn't appeal to me .
Current racing trends may go for marginal stability - their choice.
I found that the guy who could hold a perfect racecourse track had the best chance of winning .
I am positive the current models are faster - the winning flyers can fly on the edge better etc..
Nothing new there.
I would like to know tho what measured -NOT calculated G's are on current stuff also if anyone knows just how they measured the improvement of speed by shifting the CG.
We used to SWAG the G loads- found out we were waay off the mark.
Sure would like to see measured results

C Earnest,
Ok, I’m aware of all those who sacrificed their young lives to liberate Europe and/or other parts of the world from tyranny.
I really do admire and respect them, but I would never require you to know the names of my country’s heroes nor spelling them correctly here in this forum… but never mind, I didn't mean to be rude, I just tried to keep this thread on the topic.

And of course you can tell us about what you don’t know… that’s what most of us are doing here, posting questions about things we don’t know.

Just keep on fighting for whatever you think is worthwhile.

Regards,