sillyness

My Feedback: (25)

In a harrier the wing is controllable for different reasons.

The tail... simple thrust vectoring.

The ailerons... the wing is a plank that's pushing through the air. As such, it experiences parasitic drag (or profile drag..., whatever you want to call it). Formula for Drag: D = 1/2*Cd*Roe*A*V^2 if I remember right. Cd = drag coeff, Roe=airdensity, V = speed... all these stay constant. So... apparently all that can change drag wise is surface area... and it does. Surface are is not wing area, it is the frontal profile area as seen by the relative wind.

Picture a plane in a harrier coming toward you... when the plane is mushing forward and you make an aileron input, the wing with the down going aileron appears to grow larger... therefore A increases. The wing with the up going aileron appears to get smaller... therefore A on the wing decreases. You actually have a drag imbalance between the wings. With the angle of the plane, this causes rotation about the roll axis... the wing with less apparent area is allowed to fall forward.

Once in knife edge, the wing is effectively flying again... the AOA relative to the airflow is zero, so the ailerons can work like normal. When you get to inverted, same as above... and so on and so forth.

3D planes have large ailerons.... you don't really need 'em when the plane is flying, but thaose huge panels can effect large frontal surface area changes when in a high AOA situation, resulting in effective roll control.

Just 'cause the plane rolls the same in a harrier as is does when it's flying doesn't mean that it is doing so using the same physics! Nature is the great deceptor!!!

rmh

The Aerodynamics Forum has been a favorite haunt of mine --
I enjoyed throwing out ideas - just to see how others look at a question
some were offended - some understood- some just did not accept other than what they learned years back.
Understanding what can NOW be done -full scale and models -is a bit of a surprise - apparantly.

On our aerobatic stuff - all of this conjecture (and that is what it is ) falls flat once put to the task of huge speed envelopes . Turns out there is no "perfect aerobatic airfoil".
Or shape
for the most part -- a simple rather flat plate - lots of control authority and huge thrust potential- does it all rather nicely--
On the CAP snap problem- 99999.9% of the problem simply too heavy and or underpowered.
If one can kep speed constant -irrespective of attitude - most of the problem ---just goes away.


thank you .

sillyness

My Feedback: (25)

Aero is fun! It's what keeps me interested. If I was driving RC cars around a track I think I'd fall asleep in a week!

Shogun

My Feedback: (2)

As much fun as it is to discuss theory I tend to agree with Dick. In my experience ALL of the snappy planes I have flown have had the same characteristics, HIGH wing loading. With respect to the balance issue, nose heavy WILL produce a very quick and sharp departure IF the AOA is exceeded via to much elevator throw. Moving the balance aft will soften and delay this behavior every time but will make the plane twitchy to fly. In every case simply reducing the elevator throw to the point where applying full throw will not induce a snap has worked for me every time without fail.

Cap's in particular have very effective elevators and as such tend to require no more than 8-10 degrees of throw for general flying and often can fly any and all maneuvers(non-3d) with these low throws.

sillyness

My Feedback: (25)

Never been... I should look.

Yup... heavy is bad... exceeds critical AOA at higher speed and with less pull (i.e.... in a 5 G loop, if the plane is 10% overweight, the wing has to have the capability to generate 50% more lift before exceeding critical AOA than it would if the plane was 10% lighter.)

Forward CG makes it harder to the elevator to cause the wing to exceed critical AOA... it has to work harder.

As a note, elevator authority is a big probelm with flying wing designs (full scale)... since the elevator moment arm is so short, the plane winds up with a very narrow CG envelope... not good for freighters and airliners. The B-2... the bombs always go in the same place.

rmh

It is often claimed ---
1- A little bit heavier makes no difference - why worry -it will fly light
2-So what if it looses 300 hundred rpm - that is a small percentage of 6000

The weight to lift relationship is badly misunderstood as is the power to speed relationship.
It takes at least 4 times the power to double the speed
a 300 rpm drop from 6000 rpm can be easily a loss of over 25% and more -in power. (depends on prop/engine ).
Essentially -on an aerobatic setup---you simply can not make it too light (and stiff / twist free)
You can't have too much available thrust
Why?
well - you just can't get there .

RTK

My Feedback: (1)

Agreed Dick
Lighter always flies better period and I have never owned a plane with to much power. That is one of the functions of the left stick.
(foamies included)

Wings-RCU

My Feedback: (2)

Dick, Here is some more good information supporting the position that a more forward CG is no different than adding weight to a plane. It increases wing loading and like a heaver plane increases the stall speed. Increased power to maintaining airspeed does not change stall speed. Again this is in full scale so the argument could be made that it doesnt work the same way.
http://avstop.com/AC/FlightTraingHan...tribution.html

sillyness

My Feedback: (25)

Wow... can't argue with that!!!

rmh

Yes - that was your basic argument - I understand it
But I do not agree with it
IF - the argument stated that forward cg reduces the "shared load of providing lift " ( lift from the tailplane )- I might go for that
Here is the problem:
if the plane weighs "X", then, ""X" amount of lift is required.
At some given combination of AOA (wing lift) and speed - we will have a steady level flight condition.
Unless the weight changes - that condition remains the same
Shifting the CG- changes the DRAG PROFILE.
Now we need more thrust
So
Lift overcomes weight
but
speed overcomes drag.
another viewpoint
Consider that the tailplane also adds to total lift area. (as does fuselage but that's another story)
IF we change the tailplane lift to a negative lift (I hate that term) we effectively reduce total lifting area of the plane.
Then, in that case, increased speed is needed to produce the "X" lift required. speed times wing at given AOA

Back to the wing -if AOA is increased to compensate for the added drag --this adds more drag -and more power (thrust) is needed

If you buy the argument that increased drag is the same thing as weight- then that argument as stated works
The actual problem is not "an effective additional weight" --it is simply a change in total drag -in this case at the tailplane



What is wrong with this argument?

Wings-RCU

My Feedback: (2)

Your argument adds the components of thrust and drag to the equation, but fails to address the increased wing loading for CG changes or centrifugal forces (a banked turn). You see wing loading as a static function of wing area/plane mass. In reality, wing loading (and stall speed) changes dynamically in different flight conditions. You are correct that lift overcomes weight. Speed however only overcomes certain types of drag (parasitic) while introducing another type of drag (induced). Every plane has a sweet spot where total drag is low and generally referred to as glide speed. It the best combination of thrust (thrust could come from gravity in an engine out condition) and total drag. This is the speed that will give you the max flight range in a power out situation. Your theory would work if the plane is flying under that “sweet spot� where increased speed would decrease total drag.

rmh

OK ---
sticking with original situation--
the wing loading (weight ) has not changed
Only the drag has changed.
The wing does not have to lift more weight
but
the thrust must change to hold the original " balanced " speed.
look at the reverse situation
If you keep shoving the cg (cargo) rearward -- the trim drag must be decreased.
IF we want to hold the exact same altitude , what must we do?
reduce the thrust .
if we fail to do this -- the plane will climb.
N/Y?
( I ain't giving up )
soon--- we are flying a modern fighter -and letting George hold the trim setting
now we have the same speed /same weight / same lift. (a combo of wing and thrust)
and I think, the same AOA on the wing
but not the stab
Gravity as a "thrust " in my story -can't be used because I am trying to hold the plane at a steady speed and altitude irrespective of cg shift.

sillyness

My Feedback: (25)

You need to. You're wrong

There were great pictures in the link to the published work on basic aerodynamics (dumbed down for private pilots). The text was great. Read it and re-read it. You'll figure it out. I'm confident.

Wrong too. Depends on the AOA and airfoil of the stab. Could actually increase trim drag if down elevator is required. So... seeing as how moving CG rearward can actually increase the drag on the tail, please explain to me why less thrust will be required. Read the nice article again... it's in there.

Speed creates drag. It does not overcome it. Only thrust overcomes drag. Drag increases with AOA.

It's call "L over D Max". Also called max excess thrust. It also corresponds with best glide angle (lowest drag... going faster or slower increases drag) and max endurance (min thrust required = min fuel consumption). It is NOT best range... L/DMax speeds are generally very low and have a low specific range (Naut Air Miles per pound of fuel). It also corresponds with best climb angle (not rate... that's max excess power which is totally different).

I don't know what this means. If you are referring to the glider, a cosntant decent rate is the same as level flight... the sum of all forces = 0. It's callen non-accelerated flight... it is not pitch over or pulling up, it's at 1 G,... it is in a steady state. So... constant rate descent at constant speed = level flight... same friggen thing. Steady state, zero-sum force game. It is VERY important to understand that force imbalances create acceleration (friggen Newton again... F=ma), not speed, not lift, not drag, nothing. Force imbalances change the speed, they change the pitch, they change the descent/climb rate. Once established in a climb/descent/level flight, all forces add to equal zero again. Can I say that again?

You need a paradigm shift. It's very obvious that you have developed all of your theories yourself, so they all make sense to you. It's tough to get someone who thinks they have it figured out to accept other ideas no matter how simple they might be.

F2G-1

My Feedback: (13)

Wow, what a juicy thread.

Dick, you had me convinced, untill i drew the problem out on paper.

No one will argue that increased wing loading (a heavy plane) is more likely to have a 'snappy' tendency.

Dick, regarding posts #39 and #43, a forward CG (noseheavy) ABSOLUTLY increases wing loading. (i'll explain a sure fire demonstration in a sec)

Silly, I agree with you, but only to a point. Yeah, extream noseheavy = very bad, increases wing loading, increases stall speed. BUT (ah yes, the big but) BUT I like just a tad of noseheavyness to take the edge off the elevator, and to insure that i'm not too far aft on my cg. To me at least, a desensitized elevator helps me control pitch, regardless of the penalty in net increase of stall speed. It just feels like the lesser of two evils. This penalty may be more pronounced in full scale.

Now, as promissed, heres a little experiment that will put things into perspective on the CG/wingloading dilema.

Lets take our favorite 10 lbs airplane, set it on the CG stand, perfectly balanced, hangin straight and level. On a great big scale. Scale is reading 10 lbs.
Now, lets take that big old battery and slide it all the way forward in the fuse, moving the CG forward, makin the plane noseheavy. What happens? the tail comes up. Scale still says 10 lbs, but plane wont fly this way. Now we add some weights (say 6 oz) to the tail, and naturally the tail goes down, plane is nice and level again. Scale now reads 10 lbs 6 oz, which means wing loading increased. That wing has to create 10 lbs 6 oz lift to fly level with the cg forward. That 6 oz on the tail can be from a lead weight or deflected air on the elevator, its the same 6 oz.

EVERYBODYS RIGHT! OH JOY!

nice thread.

Ted

rmh

The idea of "increased weight" -is a simple explanation.
except no weight is changed

the amount of downforce on the tailplane ( or lessened downforce ) is what changed with the cg change
This changed the WORK load when the downforce changed. (hence the dumbed down explanation about increased weight.

This is an analogy which is fine for the light plane pilot - easy to see that the plane is not operating in it's best condition.
Ted - you added 6 ozs to level the and that rebalanced the plane - different ballgame BUT you also noted the 6OZ of downforce -- THAT, is my point in the entire thing -- this "6ozs downforce" is not additional weight - it is a change in loading on the tailplane
That is a load change of course-and when you add it - what happens? the plane will slow from the added drag
If you add thrust to get back to original speed - the total lift picture should be back to square one . N/Y?

All I am trying to explain is this situation:

1- the weight of the plane in my scenario is fixed
2-The altitude and speed are to remain constant

The entire exercize is regarding what must happen when CG shifts
If you change the actual weights and or -change the altitude - (the glider thing)-the situation changes.

- additional trim drag -IF it increased after shifting CG aft - would be from adding an up force at the tailplane. I understand that.
pretty tail heavy now --

What new laws of physics am I making up?

BaldEagel

Ok Chaps

For a start Silly (post 76) Form drag and Parastic drag are different, there are three types of drag on an airframe, Form drag, Parastitic Drag and Induced drag untill you understand which is which your theries go out the window.

On the wing loading and tail moments try all you have suggested with a flying wing, lets face it the tail is only there to control the Centre of Pressure relationship to the Centre of Gravity.

Disscuss.

Mike

rmh

a flying wing -

a reflex at the TE applies a force to hold AOA
and irrespective of the amount of force, the wing weighs the same
I think where all this got skewed is the semantics
EXAMPLE: a downforce of 6ozs on the tailplane = an additional 6 ounce loading on the wing.
That downforce is not a constant - it is a load which changes with speed
I called it trim drag
you may call it download
or a watermellon
Any addition requires more thrust to hold the original speed/altitude
N/Y?
It does not require a higher AOA
N/Y?

BaldEagel

Dick

Ok now apply all that theory to a swept back wing that does not need reflex.

"Any addition requires more thrust to hold the original speed/altitude
N/Y?
The wing will still produce the same amount of lift at a given airflow irespective of its weight, its just a question of if it can support that weight, glide angle will increase i.e. decay from the original GL with an increase in weight, so to maintain alititude an increase in AOA will be needed, to increase lift and to maintain speed extra thrust. In short Yes

It does not require a higher AOA
N/Y?

Yes see above.

Mike


Mike

rmh

let's see if I have this:
An increased AOA required --is to compensate (along with increased thrust) for the increased download.
even tho the weight of the airframe is still the same
The simplified pics provided showed this as additional weight -which it is NOT- It may act as weight but it is not weight

F2G-1

My Feedback: (13)

Ok, how does this sound:

The forwarg cg example is prone to stalling at higher airspeed because of the 'higher wing loading' induced by the air pressure on the tail required to maintain level pitch. If you increase airspeed via thrust, you will indeed need a little less elevator up input, conversly, you will need more elevator with less airspeed. The force needed is absolutly a contstant for that particular cg setting.

I know what your thinking with the trim drag idea, but its a side effect. that 6 oz wont change with speed, its always there; that is the force required to hold the position of the plane about the pitch axis. the only thing that will change with airspeed is the actual amount of elevator throw required to keep the 6oz force there, in turn keeping the plane level. Although stalls are induced by exceeding max AOA and agravated by excessive wing loading, in this case i think we're talking about flying on the wing, not on the prop.
Dont get me wrong, I agree with you. The tail heavy plane will always snap first.

Ted

RTK

My Feedback: (1)

I am having a hard time with that, Kinda contradictory to your above statements, No?

F2G-1

My Feedback: (13)

Boy, it sure does appear that way!

My posts explain how wingloading is increased with a forward CG. A higher wing loading will make a plane stall at a higher speed.

Planes with the cg to far aft suffer from over sensitized elevator, making it easy to over pitch, creating too great an AOA, causing a stall.

Both these condition cause a stall.

I just think the tailheavy condition is the worse of the two evils

Shogun

My Feedback: (2)

I personally think that as to wether tail or nose heavy is worse depends entirely on the weight and wing loading of the model. As I said earlier if your nose heavy and over weight the model will snap very abruptly and violently, usually with little warning that it's fixing to happen. A tail heavy model on the other hand will usually drop a wing but much more softly when too much elevator is used. Of course the tail heavy model is going to be a different beast to fly as it's pitch unstable and will be doing a few other things that your attention will be focused on probably more than the tendancy to snap.

The real problem occurs with an over weight plane that has the balance forward as these planes tend to fool us because the feel so stable. The first time you pull hard in a turn though they will try to bite you and that is where the "snappy" Cap issue came from in the first place. Light models simply don't try to bite you when you push them hard because the wings stall is so soft to begin with.

sillyness

My Feedback: (25)

'

Yup... to far forward, bad. Too far rearward, bad. There is a sweet spot. The noseheaviness you are talking about is miniscule in the big picturre, and if you ahve a light plane or efficient airfoil, you will not notice it.

You got it!!! The plane does not weight any more, but the wing "feels" the the tail downforce as weight. It is has to increase lift to compensate for increased tail downforce!!! The extra lift requires increases AOA, which increases induced drag (lift drag), which requires the extra thrust.

You are correct... I was trying to simplify it by talking about drag produced by lift and drag not produced by lift. Form drag is from the shape of the airframe. Parasitic drag is from skin friction, etc... You are very correct.

BaldEagel

Silly

Sorry Silly in this case you are wrong, Parasitic drag is not from skin friction, parasitic drag is from parasitic articals on the airframe i.e. Pitot tubes, canopy hinges, tow hooks and undercarridges. Your theriories are still out the window.

Going further on the cofg disscussion, If we think of the moment arm from the cofg too the tailplane of the aircraft on one side and the LE of the wing on the other, the longer the moment arm (forward cofg) the effect of the elevator for a given deflection will be?

Move the cofg aft and the moment arm decreases on one side and increases on the other, and the effect of the elevator is increased or deacresed?

All taking the above as being the cofg is the pivot point of the moment arm. for a given deflection you will move the other side of the moment arm more or less according to how far you are from the pivot point for a given deflection.

50:50 moment arm same deflection both sides.
25:75 moment arm 75% movement of the tailplane only 25% movement of the LE
75:25 moment arm 25% movement of the tailplane 75% movement of the LE
therefore with a forward cofg more movement required by the tail for a given AOA increase. and visa versa, so increasing weight will have less effect with the cofg forward or backward?

Disscuss.

Mike