RE: Suggestion for moderators
 

Dick,
Can you post a scale drawing of one of your foam 3D models, with the CG labeled... just so the pencil pushers can do the math and see how things look?

thanks,
-David

RE: Suggestion for moderators
 

Hello guys,

I see this discussion is really vivid.

Just was thinking about- if CG does not matter, does then existence of stabilisator (tail surfaces) matter? Obviously the fin is good to have for control.
But just mix the ailerons to be flaperons and you will be fine without the tail.


I am really newcomer to aerodynamics, but my gut feeling say, that very light plane with lot of thrust (twice the weight) is flying more in a way the Extra is flying the knife edge. And wingloading is not a major player there- the thrust is. I dont know if there is a defined static margin for an aeroplane to be stable in the knife edge regime. I doubt.

Given the low weight and low wingloading it actually might be of benefit to have unstable aircraft- pilot can deal with gusts and turbulence of the air with better control response. And to have stability in this size- it will only help in calm weather or indoors.

But to really answer if CG matter- why not to compare time in the air for airplanes of the same wing area and same weight, same engine and battery, one with oldfashioned CG and other with random CG. Or maximal distance flown. Perhaps also in different weather conditions (indoors vs outdoors).


I think even small size aircraft will fly more efficiently when stable.

Zero weight does not apply to aeroplanes- definition of airoplane is that it is heavier than air. And it fly only because air has mass. Go to orbit in outher space- no gravity and no air, and no thrust from propeller.
By saying zero weight- what you really mean is to have infinite thrust to weight ratio, low weight only helps to keep things slow, otherwise there is no difference with the bigger plane (except Re number and its consequences).
This beast with rocket engine will propably be stable and controllable as long center of drag is behind the center of thrust (if such thing can be defined- it is CG normally).


Once upon a time in between sessions in the school i made a paper plane (actually during the session). It happened to be precisely balanced- i could leave it in the air (without throwing) in what ever position- nose down, nose up, side up or upside down- it always returned to perfect straight gliding flight from height of less than 3 meters. It was simply beatiful- if I only could repeat this with something bigger. The plane was propably around 1-1,5 grams heavy and had approx one square decimeter of wing area, and it was delta- aspect ratio much less than one.

Hopefully we will find the thruth about the gravity and its center.

Happy flyings.

RE: Suggestion for moderators
 

Here it is - very conventional layout -
I typically fly it with the cg at about 40% of the root - but once, when the battery pack came off -and I flew it to a normal landing - it took lots of down elevator and some aileron fussing -but it still flew.
The drawing is exactly to scale -so anyone can pick the cg which makes them comfortable - anywhere from th LE back to 40% is flyable for most people -but - you can go back further
By the way -I flew a heavy pig of a scale P51 yesterday -and the cg on that turkey was critical -
Read the prior post on eliminating the stab - and flying with flapperons only -
This is not only possible but done many times and kits were on the market .
Glad to see this guy is thinking!

RE: Suggestion for moderators
 

Dick,
Thanks for posting that. Now that I see it, I think you posted it before... sorry for my short memory. It does look very conventional and, since the spanwise datum line for the wing is ahead of the 50% point at the root, it's not at all surprising that you can fly it well at 40% of the root chord. Eyeballing it, that will be in the 30-35% MAC range. Very normal for an aerobatic plane. And no, I'm not saying this to dispute your observations... just making an observation myself.

-David

RE: Suggestion for moderators
 

Dick,
Could you tell us about the wingspan and the flying weight?
By the way, locating the CG at 40% from LE seems to me quite close to the NP, which would make it more neutrally stable than positive stable.
The low wingloading along with rather large control surface areas would make it flyable, but I'd not recommend more than 40% to a beginner.

RE: Suggestion for moderators
 

34" nothing special about it - all up weight is 10-13 ounces depending on battery packs selected - wing loading is under 5 ozs/ ft
The typical popular foamie is quite a bit longer in the fuselage -to make it more of a hover forever model
This was of no interest to me - I was after a model which had instant abilities to snap and roll and turn in extremely tight ,flat circles etc.
This it does.
placed in a knife edge- one can simply do a flip -in knife edge thru 360 degrees.
OR do a pirouette.
It flies on edge very easily with virtually no couple induced - very stable
This is because the AOA of the deep/short fuselage is extremely low for level knife flight.
These types may not throw away YOUR rule book but the results are quite surprising.
And one hell of a lot of fun.

RE: Suggestion for moderators
 

people fly capiche's at my club in the UK with the CG way back - like 55% back.

Hands off and inverted they apparently climb - wing loading I guestimate at about 15oz.

I think a lot of people thought you had your CG behind your NP.

Even paper planes set up like that tend to tumble!

One thing I would like to mention though is that if the plane's CG is behind the NP and the thrust force is greater than the weight of the airframe, then the wing isn't doing much, so perhaps CG doesn't matter then - it's effectively a helicopter ;)

Is that what you are saying Dick? or are you making the point that perhaps birds or insects operate in a weird laminar flow low Re regime where the conventional rules do not apply?

RE: Suggestion for moderators
 

hi again,

if you can loop those birds in knife edge- most propably you dont need wings for lift- at least not as big as they are if the flat fuselage is enough for flying. What is "wingloading" in knife edge? Instead the wings are only for drag and control. Thrust keeps it in the air, big wing creates enough drag to counteract thrust at reasonably slow speed. Make the plane bigger and heavier, put a strong engine with thrust 2x to 3x the weight and make the wing big enough to counteract thrust at 20mph. I believe it will fly the same way as the small one. And it will have low wingloading- just for drag. The problem is purely technology- such bigger plane will fail structurally. Thats why you have it small- for better structural strengh to weight ratio.


The weight does not have to be small- wing loading yes, but rather indirectly- not for lift at small speed but for large drag to counteract the thrust at slow speed. In addition big wings have big control surfaces- which is dood at low speed.

How would glider fly with weight approaching zero?


all the best
A.

RE: Suggestion for moderators
 

sorta - --once you are able to simply vector (with thrust) - then the CG is of little importance
for some reason -this thought makes some guys crazy-
Why?
If all I was doing was flying models like the p51 I flew this weekend - which flew pretty close to upper limits of it;s lift), I would be careful to keep cg forward --and add up trim as needed
as it was the p51 was not too bad (not too good either) -but how in the world the BARF mfgr expects the novice to handle it- especially when it was suppose to have a smaller, lighter engine --is beyond me -- they say " be sure to balance". yeh, right -
This is the kind of model I used to see in this forum where the owner would ask " will flaps make my P51 fly better ?"
The answers were never the correct one .
which is:
Nothing will help it --it is too damn heavy and you haven't enough power.

RE: Suggestion for moderators
 

thrust to weight ratio much more than one together with large drag and you should only be concerned that center of drag is behind from the center of mass.

Anyway- this kind of planes are extremes and the wings lose their meaning- wing is supposed to create lift, with vectored thrust it becomes just a control surface and may be it should not be called wing anymore.

Parashute and dart arrow might be closer to your planes in the manner they move through the air than a regular glider is.

RE: Suggestion for moderators
 

yes - you are correct - but some of the guys did not see that - actually the drag only needs to be behind the point of thrust .
I don't think a pusher prop setup would be controllable.
as a glider this setup is worthless.
The wings still have value tho and just because it operates more like a surfboard - it still works .

RE: basic aerodynamics
 

DickHanson.

I got to say, I think you're nuts. You state that you balanced a pylon racer on the leading edge or very nose heavy? That's NOT how things work in racing. Most Quickie and Q40 airplanes are balanced so close to the center of lift, that full up elevator is usually about 1/8" of total throw at the trailing edge. Since the wing has to offset the downforce of the stab, having an airplane nose heavy just increases the amount of lift the wing has to generate. At one G, not much, but we pull 30-40 G's in the turns and so that extra lift the wing has to make really slows a nose heavy airplane down in the turns. Perhaps, you should stick with your flat plate knowledge and quit wasting peoples time.

RE: basic aerodynamics
 

High Planes,

As to whether or not you are the nutty one:-) or not will have to remain moot. You are not, however very well informed if you state that CG position has any effect on G load vs. lift requirements viv-a-vis the wing. The only thing to which you can be referring is wing loading vs. a/c mass. You are playing around with variables that you cannot play with in that fashion. Nose-heavy aircraft place a greater aerodynamic load on the elevator to obtain the same amount of elevator authority at a given airspeed. (tail-heavy lands "hot" or snaps.....nose-heavy = more stable in pitch mode; more upsetting force required on elevator to cause displacement about the lateral axis of aircraft.) There are several other confusing statements in your post, the meanings of which I will just have to guess and therefore can have no comment.

Regards

RE: Suggestion for moderators
 

30 to 40 G's
interesting --

RE: Suggestion for moderators
 

also moot..............

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

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 ---

RE: Suggestion for moderators
 

Hi Dick,

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

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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?:D

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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?

RE: Suggestion for moderators
 

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".

RE: Suggestion for moderators
 

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?

RE: Suggestion for moderators
 

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

RE: basic aerodynamics
 

Good info for the kids.

RE: Suggestion for moderators
 

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 ?

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

I'm sorry, that post was really for earnest

RE: Suggestion for moderators
 

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--

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

“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.

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

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. :eek:

Thanx

RE: Suggestion for moderators
 

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]

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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,