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« Reply #25 on: January 27, 2011, 02:50:37 PM »
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Dutch and Spiral Instability. This has been overcome in all modern aircraft with the introduction of Autopilot and Auto Stabilisers, however this problem is very live in RC flying, One of my colleague crashed his scratch built High winger, i suspect it was due to Dutch Roll. therefore this information.

Spiral Instability. Highly Directionally Stable aircraft (Large Fin, Long Fuse with medium Fin) suffers from this, this is not as dangerous condition as a Dutch Roll, provided you have enough rudder area and rudder throw to pick up a spiraling aircraft. when a yawing moment commences, and if the lateral stability is low, and yaw damping is small, the directional stability keeps turning the aircraft with bank angle increasing continuously, it gets steeper and tighter, ROD increases and the RC aircraft crash is spectacular. on the lighter side people blame it on 2.4 Ghz lock out, etc. Don't confuse spiral dive to a spin, because in spin the aircraft is in the stalled condition of flight and the speed is at or near stall speed, in spiral dive it is fast and the speed continues to increase.

Dutch Roll.   The second one is an oscillatory motion, normally not in sync with each other is a roll and yaw combo motion called Dutch roll, named by Dutch skaters. High wing, Large Dihedral, sweep back etc increases lateral stability and if directional stability is not as high, then you have created a recipe for a Dutch Roll. High lateral stability with vicious bank reversal can lead you to Dutch Roll and with control inputs out of phase, development to crash can be in seconds. You will wondering who switched to your frequency, and or how batt quit or was it sudden wind. not knowing it was Damn Dutch. (See the image in terms of roll and direction of yaw , carefully)

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« Reply #26 on: January 27, 2011, 02:51:43 PM »
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is it making sense ? Huh?
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« Reply #27 on: January 27, 2011, 06:56:44 PM »
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so many people saw and no comments/queries at all. Huh?
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« Reply #28 on: January 27, 2011, 09:51:15 PM »
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lot's of information from your posts,but it's not  digesting for a person like me,that's why i am keeping quiet
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« Reply #29 on: January 27, 2011, 10:01:22 PM »
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These are applicable only in deadstick situations, or when you are really really close to the ground, right ? And that too on planes without ailerons ?  Otherwise aileron (or even enough rudder) corrections should take care of it ? Head Scratching
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« Reply #30 on: January 27, 2011, 10:49:54 PM »
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thumb rule.
1. too much of fin...directional instability
2. too much of Dihedral/sweepback...Dutch roll

Aileron no aileron, rudder no rudder..

of course controls make you recover from the situation. Grin

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« Reply #31 on: January 28, 2011, 08:46:04 AM »
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Propellors
 
Thrust is the force that move the aircraft through the air.Thrust is generated by the propeller of the aircraft through application of Newton's Third Law.  The propeller consist of two or more blades connected together by a hub. The hub serves to attach the blades to the engine shaft. .

The blades are made in the shape of an airfoil like wing of an aircraft. When the engine rotates the propeller blades, the blades produce lift. This lift is called thrust and moves the aircraft forward. most aircraft have propellers that pull the aircraft through the air. These are called tractor propellers. Some aircraft have propellers that push the aircraft. These are called pusher propellers.

Leading Edge of the airfoil is the cutting edge that slices into the air. As the leading edge cuts the air, air flows over the blade face like it does on any wing.
Blade Face is the lower surface of an airfoil (Over a period people think it is that side that you see when you face the aircraft, it is not).
Blade Back or the Thrust Face is the curved surface of the airfoil.
Blade Shank is the Root section of the blade (nearest to the hub).
Blade Tip is the outer end of the blade farthest from the hub.
Plane of Rotation is an imaginary plane perpendicular to the shaft in which the blades rotate.
Blade Angle is formed between the face of an element and the plane of rotation. The blade angle throughout the length of the blade is not the same. The reason for placing the blade element sections at different angles is because the various sections of the blade travel at different speed, because of their distance from the hub, that is why the twist in the blade. Each element must be designed as part of the blade to operate at its own best angle of attack to create thrust when revolving at its best design speed
Blade Element is the airfoil sections joined side by side to form the blade airfoil. These elements are placed at different angles in rotation of the plane of rotation. (Something like a slice of a cake at any point on the prop, you cut and look at it side ways (See image)

Why Twist, say again! Huh? The blade has a small twist (due to different angle in each section) in it for a very important reason. When the propeller is spinning round, each section of the blade travel at different speed, The twist in the propeller blade means that each section advance forward at the same rate so stopping the propeller from bending.

Thrust is produced by the propeller attached to the engine driveshaft. While the propeller is rotating in flight, each section of the blade has a motion that combines the forward motion of the aircraft with circular movement of the propeller. (see image), so if your model is going to travel at lesser speed the second number of your prop should be small, if it is going to screech through the second number should be more.(Not a rule though)

Pitch is the distance a spiral threaded object moves forward in one revolution. As a wood screw moves forward when turned in wood, same with the propeller move forward when turn in the air. More on Pitch later
  

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« Reply #32 on: January 28, 2011, 09:07:23 AM »
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hi,

this is fantastic information for the modellers who design and build models. infact i have problems with my model which will spiral down and crash. hopefully with this information i should be able to rectify my problem.

thanks for the info.

regds
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« Reply #33 on: January 28, 2011, 09:07:53 AM »
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The blade angle throughout the length of the blade is not the same.
So if I have a 10x6 prop and I am cutting it down to 7 inch, will I get a 7x6 prop or not Huh?
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« Reply #34 on: January 28, 2011, 11:13:47 AM »
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augustinev you are a saviour, now i know my scratch built suffers from spiral instability, it banks to one side and spirals before crashing just a few seconds into the flight, please please tell me how do i overcome this. Help appreciated
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« Reply #35 on: January 28, 2011, 03:47:55 PM »
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husein, sandeep, what i follow for my scratch build i can tell you, (i) i keep more than half my fin as the rudder, remember, in pattern flying we rarely use rudder, however it is a gr8 tool for  (a) picking up a dropped wing by inducing something called 'Yaw induced Roll' especially when the aircraft is at low speeds (b) rudder is the easiest way to get out of a spiral.
As i settle down in the model, I reduce the throw of the rudder. i keep it a high winger, di/polyhedral or sweepback a little. thats about it , you are sorted.

'Scratch built' as a rule should not have a flap, it is the cause for most failures, you may want to introduce it later.

Lot of tools are available before sitting down to cut the foam, coro or balsa, run the math. a lot of issues will get sorted on the drawing board believe me, i have heard people say 'iam a hands on old fashion builder, i care a damn about math and drawing' ain't true, in todays age you have gr8 sims available to run and fly your model virtually before you cut it.
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« Reply #36 on: January 28, 2011, 03:48:29 PM »
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also read

#15 and # 17 of
http://www.rcindia.org/beginners-zone/airfoil-for-trainer/new/#new

For Aerofoils
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« Reply #37 on: January 28, 2011, 05:55:04 PM »
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Phrozen Crew member!  Bow not exactly, the tip section of the aerofoil needs to reduce in camber and chord so as to reduce its optimum angle of attack to very less value Huh? (See blade twist image, above), you will ask why ? complicated question, more complicated answer is, 1. to reduce 'Tip Spillage' 2. to avoid 'Blade Interference' 3. to improve 'Blade Harmonics', in a nutshell yes it will become a 7x6 prop but since not designed to be one,  will not perform like one. Bang Head

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Did fiddle around with Softice for a while, till all AV hell broke loose and categorized it a dangerous HI virus  Grin Grin
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« Reply #38 on: January 28, 2011, 05:59:27 PM »
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Santhosh.
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tnx augustine, sure in may
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« Reply #39 on: January 28, 2011, 06:59:23 PM »
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So if I have a 10x6 prop and I am cutting it down to 7 inch, will I get a 7x6 prop or not Huh?
Hi Amit,
After almost more than a year into this racket few people have have been able to give me answers to my queries... Grin... I am still going through the trial and error method Bang Head....I am sure nobody wants to be buried by rocket science Bang Head...But sure Anwar Bhai has been a great help...but of late he also seems to be hard pressed for time Shocked...IMHO simple answers to queries will help all fellow aeromodellers Huh? I rest my case Salute...
Regards,
Mohan
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« Reply #40 on: January 28, 2011, 07:32:00 PM »
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mohan, with you 100%, keeping it CCC (Clear , Concise, Crisp ) and short is not simple, atleast for me, i spend quite a bit of time to simplify and put it in a manner so that it is not intimidating (Unlike other RC forums, which is , like you said, full of rocket science). Have I been able to succeed Huh?, you all should tell me, i am sure some thumb rules will help. problem with the thumb rule is if you forget the rule, it is over, and it doesn't explain the logic to a action, in RC flying and in aviation, it is so very important. Will give an honest try to simplify further so that you don't  Bang Head
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« Reply #41 on: January 28, 2011, 11:08:57 PM »
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Phrozen Crew member!  Bow
Thanks, but those days are gone. Opensource is the new revolution Thumbs Up

I am still going through the trial and error method.
Thats what I do and I'm getting good results. Slow and steady..   Smiley

in a nutshell yes it will become a 7x6 prop but since not designed to be one,  will not perform like one.
I tried this a few days ago and slow speed performance was awesome, maybe because a cut down 10x6 to a 7x6 has much wider blades than a regular 7x6. But open up the throttle and that thing was noisy as hell and not much thrust gain above half throttle.
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« Reply #42 on: January 30, 2011, 12:49:47 PM »
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On ram temperature rise rep #2 of

http://www.rcindia.org/chatter-zone/experimental-flight-at-mach-6/msg47947/#new
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« Reply #43 on: January 30, 2011, 01:10:02 PM »
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From what I understand, augustinev you are an aerospace guy, if yes can you please help with two projects I am currently working on, one is Design of a Ramjet and other one Environmentally responsible aviation, a competition at NASA, please PM me. Thanks
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« Reply #44 on: February 12, 2011, 06:14:24 PM »
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Firstly, Sorry Husein, i saw this now. PM me anytime

Secondly a lot of people are getting confused with these Basics, i guess they know the concept but are caught in semantics. Some of them are defined rather inaccurately on the internet and in other RC forums, so here we are, in the series of more accurate concepts.

Total Reaction (TR).
 The resultant of all the aerodynamic forces acting on the wing or aerofoil section. Imagine this to the direction your hand gets pushed when you put it  flat like a aerofoil (I guess every child does it  Huh?) out of the window in a train or a vehicle

Lift.
 That component of the TR which is perpendicular to the flight path or Relative Air Flow.

Chord Line.  A straight line joining the centres of curvature (It is not the centre, it is the centre of curvature Grin) of the leading and trailing edges of an aerofoil. (See Aerofoil Funda Image).

Chord (c).
 The distance between the leading and trailing edge measured along the chord line.  The mean chord is often used as a datum linear dimension in the same way that the wing area (S) is used as a datum area.

Wing Area (S).
 Area of the wing projected on a plane perpendicular to the normal axis.

Mean line or Camber Line.
 A line joining the leading and trailing edges of an aerofoil equidistant from the upper and lower surfaces.  Maximum camber is usually expressed as a ratio of the maximum distance between the camber line and the chord line to chord length.  Where the camber line lies above the chord line, the aerofoil is said to have positive camber.

Angle of Attack (α).
 The angle between the chord line and the flight path or RAF.  In many textbooks this is referred to as Incidence.

(Riggerís) Angle of Incidence.  The angle at which an aerofoil is attached to the fuselage.  The angle between the mean chord line and the longitudinal fuselage datum.  The term is often used erroneously instead of Angle of Attack.

Thickness/Chord Ratio (t/c).  The maximum thickness or depth of an aerofoil section expressed as a percentage of chord length.

Centre of Pressure (CP).  The point, usually on the chord line, through which the Total Reaction may be considered to act.

Streamline.
 The path traced by a particle in a steady fluid flow.

Aspect Ratio
= span/chord or span2/wingarea.

Wing Loading.
 The weight per unit area of the wing = weight/wingarea

More to Follow Huh? Huh?

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« Reply #45 on: February 14, 2011, 01:02:04 PM »
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Why does a plane going vertical at full throttle stalls after reaching a particular altitude  Head Scratching Shouldn't it keep climbing at speed approximately equal to pitch speed of the prop Bang Head
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« Reply #46 on: February 14, 2011, 02:32:06 PM »
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Some part of the power system must be overstressed and giving up, otherwise you can get unlimited verticals (at least until they are get too small to control visually). 

We have some foamies and helis that can keep going up as long as I want, the same can be done on glow planes if you put the right power system.
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« Reply #47 on: February 14, 2011, 02:57:24 PM »
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Why does a plane going vertical at full throttle stalls after reaching a particular altitude  Head Scratching Shouldn't it keep climbing at speed approximately equal to pitch speed of the prop Bang Head

Hey If I am not wrong you mean to say that it should reach moon till you keep full throttle and straight towards sky  Giggle..... well they will stall after a certain height as gravity is pulling them desperately. TO breach it you need a booster rocket with huge amount of propellant and need a launching station (like sriharikota in india  Giggle) then your rocket will not stall( if everything goes right ..) and may reach moon..... Clap.... ( anyways we are trying hard to reach there through Chandrayana.....)  Smiley
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« Reply #48 on: February 14, 2011, 03:08:08 PM »
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Common man, you know what i mean. Just want to reach a little higher to get to the sweet thermals Smiley

@anwar: I never had any of my planes give unlimited vertical, may be I've never had enough power. I've got a habit of under powering my planes and under propping my motors.
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« Reply #49 on: February 14, 2011, 03:27:16 PM »
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 Giggle Giggle

Anwar Bhai,

It is a gr8 question. Answer is in two fold, they are :-

Reduction in Thrust due reduction in mass flow across the Propeller

Though this is not very accurate, for simplicity reasons I have chosen to show the thrust produced by a propeller using Newtonís laws of motion which give: (See Image)

Force=Mass x Acceleration
Thrust = Mass flow rate of air through Propeller ◊ Increase in velocity of the air
=M x (Vj x Va)
Where M=Mass flow rate of the air
Vj =Velocity of slipstream
Va =Velocity of the aircraft(TAS) (What is TAS ? See below)

In the case of the propeller as compared to jet engine, the air mass flow will be large, and the increase in velocity given to the air will be fairly small.

So as the altitude increases the density reduces and the TAS increase, this makes the Mass Flow Reduce because the difference in the velocity in front of the prop and on the rear of the prop reduces (If at that altitude if you happen to stand behind the prop you will feel less of air as compared to when you launched it at the field)

True Air Speed (TAS)(V). The TAS is obtained by dividing the total Dynamic Pressure (The number of air molecules in motion hitting the prop face for a given time) divided by the square root of the air density.

I have kept this part simple, if you read it couple of times you will get it.

Reduction in Density also reduces engine thrust because of reduction in MAP (Manifold Air Pressure) inside the engine, this further complicates matters. 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

Prop Thrust.jpg
Re: Basic Aerodynamics for RC Flying
* Prop Thrust.jpg (25.3 KB, 800x391 - viewed 548 times.)
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A Great pilot will practice until he canít get it wrong.
 

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