So you are saying this is all because of reducing air density as the altitude increases. Does air density really varies this much in the 500~600 meter altitude range?

at 600 mtrs it is 8.3%, (see image). It is not the altitude, it is the thrust drop that counts, since the model weight is so low that any reduction affects the climb performance drastically.

Got it
One more question on this Considering the motor can handle both, which prop should i use to get more altitude, a 9x6 or a 11x4 ?

Quote from: augustinev on February 14, 2011, 03:27:16 PM
Therefore reduction of Prop aircraft thrust as compared to a jet aircraft is near exponential. that is why you see not may I/C engine prop aircraft flying very high

Wow.. I didn't consider flying your RC plane for deliveries to the ISS  

It was only around the 400m to 600m heights, that I am able to take my aircrafts vertically to ! (depending on the size of the plane, and I am guessing the heights!).   

Quote from: SunLikeStar on February 14, 2011, 05:58:26 PM
One more question on this Considering the motor can handle both, which prop should i use to get more altitude, a 9x6 or a 11x4 ?

11x4 would generate more thrust than a 9x6. Larger dia props generate more thrust as they are simply large... and the peripheral velocity of the blade tips are high.

Thanks Izmile, i was thinking more pitch speed will take me higher but now i understand its the thrust that matters in this case.

Think of pitch as gears in a vehicle. Fine pitch equates to lower gear and vice versa. So fine pitch will give you better acceleration but less speed, and so on.

makes sense, awesome analogy 

Flat Plate Aerodynamics

1. this is in co-relation to Rep#2 of this thread,
A few things i want to flag off, as regards Flat plate foamies, This does not include Kline Fogleman (Kfm) Aerofoils

(a) Any aerofoil at +ve angle to the forward movement of the aeroplane will produce an upward force and a rearward force (Exactly like the way the hand moving back when you put it out of a running bus or a train in an aerofoil shape) This upward force is called the Total Reaction, This total reaction can be resolved into two components one upward which is called the Lift and One rearward which is called Drag

(b) in climb the lift is less than weight , in true vertical climb and vertical dive lift =0, , Don't believe me ? see image

(c) In a flat plate upto the point of stall there is no movement of C of P, which means in the working ranges of angle of attack , its behavior is quite predictable.

(d) last but not the least, a flat plate has something called a leading edge separation , a small bubble of air, at and near stall this bubble continues to increase and at stall the bubble bursts and the stall is spectacular because there is a sudden loss of lift.

Also Flat plate has the least Supersonic drag, the reason why it is the choice of section in missiles and high speed aerospace applications.

Rep 35 for 1 'G' stall
http://www.rcindia.org/gas-glow-nitro-planes/my-first-plane-with-sunboard-sheet/msg48491/#msg48491

I saw a thread reg kFm buts its a year old so posting it here

I want to construct a kFm3 airfoil of wingspan 48" & 7" chord length using coro.
It is said that kFm3 has (9% to 12% thickness) step at 50% and 75% chord

what needs to be the thickness of the wing at 50% and at 75 %

Is it 12% of chord length(for me its 0.84") at 50% chord(3.5")?
then what is the thickness at 75%
Is the thickness from 50% to 75% of the chord is constant?

A pic with the dimensions marked would be very helpful(7" chord, 48" wingspan).

T/C (Thickness to chord ratio) decides a lot of things including stall alpha, t/c and MAC to has a lot of effect, actually they are decided by your flight performance requirements, short chord also is preferred for ease of carriage,

I want the wing to be used in a pusher model using coro(2mm)
If the wing is 48" long wing having 7" as chord length
what needs to be the values of X, Y , Z & P in the pic attached.
Also does the second step needs elevation(I am sure if Z > P then the second step needs elevation, but just for clarification)

Well, pardon my lack of knowledge, but is there a difference between symmetrical aerofoil and flat plate? If the aerofoil is uniform on both the upper and lower surface, the air pressure will be the same and hence no difference to cause the lift - right?

Flat Plate will have a bubble on the leading edge that makes flat plate behave in an unpredictable snappy manner, at high alpha the bubble elongates , does not elongate equally on both wings, this further addes to the woes, during stall the bubble bursts and stall is jerky, airflow transition from laminar to turbulent takes place not on the wing (at slightly high alpha) but on the bubble, this makes the air flow separate there, sometimes, all in all for low speed applications flat plate isn't good, if you are planning a foamie, Kline Fogleman is better

As you said earlier, the air bubble effect on the flat plate is at the stall speed. but above that? What I meant to clarify was that most of the 3D planes ARF/RTF seem to have a symmetrical wing. So if this is not producing lift, then does it matter for the model to have such a well crafted shape and not just a flat plate with LE and TE shaped properly?

See theoretically, a flat plate without any LE radius will be in a stalled condition of flight at any positive angle of attack, however practically, Wind tunnel Experiments have shown the LE bubble in Flat Plate forms at 2-3 deg angel of attack and it continues to grow, and at stall it engulfs entire wing and bursts. Flat plate is suited for supersonic configurations because after the transonic regime the wave drag of a flat plate is the least, for aeromodelling this info is of no use (Unless you want to do supersonic FPV Flight)

As far as foamies are concerned, it flies because of the Thrust wt ratio being high, when TWR is high it pushes even the stalled airflow over the wing and generates enough lift to keep it aloft, another thing about stall is with the increase in angle of attack when there is no more increase in lift, then we call it stalled condition, however the wing will produce lift well after the stall state (See image ) I hope i have answered the question

I have been doing foamie scratch builds for some time. The best results for a slow flyer / trainers seems to be a UC aerofoil without ailerons, seems to work better that curved upper and flat bottom foil. Any reason for this or just a coincidence ? Does having an aileron have any effect on the lift as far as the model plane goes ( not talking about the actual ones)

What is the advantage of having stabilator instead of elevator and a horizontal stabilizer, in a glider

SLS

That is a very big question. i will restrict my answer to gliders alone. Apart from controlling the aircraft in pitch what is the purpose of a tailplane? ans. it is to balance the aircraft pitching moments caused by lift-weight and thrust-drag couple (Read in conjunction with earlier posts). A tailplane and Stab arrangement is because a stabilizer when deflected, in subsonic speeds, will modify airflow ahead of it and give you more than what you bargained out of the stabilizer. a stabilator or a a all flying tail as it is called moves as one piece and gives desired effect in less of deflection. advantage is less deflection disadvantages are plenty (a) to move a all flying tail is not easy in terms of mechanism involved (b) bigger servo will be required (d) hinge point will be the wing therefore hinge point got to be strong and lubricated as well


Why is it used i a glider
(a) you are supposed to glide a glider not maneuver too much, therefore you reach a trim point and thereafter the movement of the tailplane is negligible, does not affect the glide and the tailplane contributed lift characteristics greatly (tailplane lift and Trim Drag is a great contributor to glide distance)

Why is it there in a jet ?
some other time

PS
There is a lot to think about, think and ask me questions, open to answer all of them , no question is a stupid question remember that

Why in a pusher, If the motor is placed above the wing the motor is angled upwards (looking from back). It will be causing a down thrust   which inturn pushes the plane down  .

For a Conventional Aircraft thrust drag couple is nose up and Lift Weight Couple is Nose Down (See Image)

However for aircraft like easy star or axn floater (Like a sea plane) the thrust drag couple and lift weight both are nose down couple , (Because the thrust line goes above the drag line) that is why thrust line is inclined, inspite of this you will see the trim point of the tail plane is for nose up (tail down).

With reference to 70

there is also split tail, works like a conventional stabilizer at subsonic speeds and the locks to become a all flying tail at transonic and supersonic speeds,

In some aircraft like the Kiran HJT16 and Jet Provost the stabilizer is connected to the pilots joystick and the tailplane to the trimmer, so when you trim the stabilator moves and when during piloting the stabilizer moves , neat yeah !!! can you figure out why shouldn't it be used in RC plane (If it is not already being used)

(In my scratch build I installed the motor alone without the control surfaces cutted out, for testing. the plane is not gliding well as it did without the motor)

So the motor is angled a little up so that the thrust is also angled little down so that the thrust drag couple doesn't push the plane complete down. but the thrust is also pulling the plane down right  . So normally without elevator trim the plane will be going down where other planes will remain in the same level  .