Jan 15, 2003, 01:47 AM
DLG Fan

# Rules of Thumb for Aircraft Design - Angles of Incidence, etc.

On a traditional monoplane with standard rudder/elevator tail, it appears there are a number of 'Rules of Thumb' relating to airplane design. I have a few questions perhaps someone can answer.

Viewing the fuselage from the side, a CG mark can normally be found on model plans with a horizontal line running through it longitudinally along the length of the fuselage. What is this line called and how is it determined? Is it a line that marks the intended direction of flight? I understand that the line passes through the CG but what second point, if any, determines the line?

For the sake of this question, let's assume the main wing has a flat bottom. The main wing is normally mounted on the frame with an 'angle of attack' of some positive angle of the flat bottom of the wing relative to the fuselage line mentioned above. What is the normal angle of attack for planes? I suspect that sport aerobatic planes and jets, for example, may have an angle of attack of 0 degrees, while other planes such as trainers may have a positive angle of attack of a few degrees.

The horizontal stabilizer is also mounted with an 'angle of attack' relative to the aforementioned longitudinal fuselage line. What is the rule of thumb for its 'angle of attack?' I thought I read somewhere that it points up (i.e., has a positive angle of attack) in order to counterbalance some up-pitch effect due to lift from the main wing.

The motor is supposed to be mounted (I assume) with the shaft of the motor along the horizontal longitudinal fuselage line, but at an angle. This is the thrust angle. I have heard that 1 to 3 degrees downward (this should be a negative angle, right?) and 1 to 2 degrees rightward are the suggested 'rules of thumb' for vertical thrust and right thrust. What is the purpose of the downward thrust? To counterbalance the torque of the motor or to compensate for up-pitch due to lift effects or ...? The right thrust is, I understand, there to compensate for the torque effect of the motor.

Any info that you folks may have would be much appreciated. I have been designing planes with Bluecor and have reached the point where this stuff is starting to mean something!

Thanks. My apologies if this sounds a bit muddled or confusing.
Last edited by AZ_Astro; Jan 15, 2003 at 02:07 AM.
 Jan 15, 2003, 07:43 AM Registered User This is really an advanced topic that belongs in modeling science. That said, there's a good amount of info buried in the 'flaperons or spoilerons?' thread in Electric Sailplanes going on right now.. ..a
 Jan 15, 2003, 08:45 AM DLG Fan Thread OP I will take a look at the electric sailplanes threads. Thanks. I have tried a fair number of searches but finding, in particular, the recommended angle of attack for the elevator has proved difficult.
Jan 15, 2003, 11:28 AM
Registered User

# Re: Rules of Thumb for Aircraft Design - Angles of Incidence, etc.

[QUOTE]Originally posted by AZ_Astro
Viewing the fuselage from the side, a CG mark can normally be found on model plans with a horizontal line running through it longitudinally along the length of the fuselage. What is this line called and how is it determined? ?Is it a line that marks the intended direction of flight? I understand that the line passes through the CG but what second point, if any, determines the line? [QUOTE]

This is called a reference line. It is totally arbitrary as to where its located. It is used as the baseline from which dimensions and angles are measured. It is not the intended line of flight.

[QUOTE][/B]For the sake of this question, let's assume the main wing has a flat bottom. The main wing is normally mounted on the frame with an 'angle of attack' of some positive angle of the flat bottom of the wing relative to the fuselage line mentioned above. [/B][QUOTE]

Not "angle of attack" but rather "angle of incidence". Angle of attack is the angle that the wing moves through the air while its flying and it varies depending on speed and weight of the plane.

[QUOTE][/B]What is the normal angle of attack for planes? I suspect that sport aerobatic planes and jets, for example, may have an angle of attack of 0 degrees, while other planes such as trainers may have a positive angle of attack of a few degrees. [/B][QUOTE]

Angle of incidence can be anywhere from several degrees positive to several degrees negative depending on how the plane is designed. A "rough" rule of thumb for sport/scale planes is 2 degrees positive.

[QUOTE][/B]The horizontal stabilizer is also mounted with an 'angle of attack' relative to the aforementioned longitudinal fuselage line. What is the rule of thumb for its 'angle of attack?' I thought I read somewhere that it points up (i.e., has a positive angle of attack) in order to counterbalance some up-pitch effect due to lift from the main wing.[/B][QUOTE]

The angle of the horizontal tail is normally measured reference to the wing and is called the "angle of decalage". For most models the tail will have a negative angle (leading edge lower). The reason for this is that the plane should be balanced so that the nose wants to drop and the tail has to push down to hold the nose up. This can vary greatly depending on CG, wing location, and fuselage shape.

[QUOTE][/B]The motor is supposed to be mounted (I assume) with the shaft of the motor along the horizontal longitudinal fuselage line, but at an angle. This is the thrust angle. I have heard that 1 to 3 degrees downward (this should be a negative angle, right?) and 1 to 2 degrees rightward are the suggested 'rules of thumb' for vertical thrust and right thrust. What is the purpose of the downward thrust? To counterbalance the torque of the motor or to compensate for up-pitch due to lift effects or ...? The right thrust is, I understand, there to compensate for the torque effect of the motor. [/B][QUOTE]

The downthrust helps to keep the plane from climbing too steeply when power is increased. Right thrust helps to control the torque of the motor.
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 Jan 15, 2003, 12:45 PM DLG Fan Thread OP radfordc: Ah, yes. Much clearer! Thank you very much for clearing away those cobwebs of confusion for me. I am indebted to you for your assistance. Reference Line: so it is arbritray then. Ok. Angle of Incidence: of course, and not Angle of Attack is what I meant to say. Thanks for that important clarification. Angle of decalage: Ok, that's new to me. And I see the importance of countering the nose-dropping effect. Down-thrust: ok Right-thrust: ok THANKS!
Feb 03, 2016, 10:50 AM
Registered User

# Editied so it is easier to read.

Question 1: Viewing the fuselage from the side, a CG mark can normally be found on model plans with a horizontal line running through it longitudinally along the length of the fuselage. What is this line called and how is it determined? Is it a line that marks the intended direction of flight? I understand that the line passes through the CG but what second point, if any, determines the line?

Answer 1: This is called a reference line. It is totally arbitrary as to where its located. It is used as the baseline from which dimensions and angles are measured. It is not the intended line of flight.

Question 2: For the sake of this question, let's assume the main wing has a flat bottom. The main wing is normally mounted on the frame with a positive angle of the flat bottom of the wing relative to the fuselage line mentioned above. Is this the angle of attack ?

Answer 2: Not "angle of attack" but rather "angle of incidence". Angle of attack is the angle that the wing moves through the air while its flying and it varies depending on speed and weight of the plane.

Question 3: What is the normal angle of incidence for planes? I suspect that sport aerobatic planes and jets, for example, may have an angle of attack of 0 degrees, while other planes such as trainers may have a positive angle of attack of a few degrees.

Answer 3: Angle of incidence can be anywhere from several degrees positive to several degrees negative depending on how the plane is designed. A "rough" rule of thumb for sport/scale planes is 2 degrees positive.

Question 4: The horizontal stabilizer is also mounted with an 'angle of attack' relative to the aforementioned longitudinal fuselage line. What is the rule of thumb for its 'angle of attack?' I thought I read somewhere that it points up (i.e., has a positive angle of attack) in order to counterbalance some up-pitch effect due to lift from the main wing.

Answer 4: The angle of the horizontal tail is normally measured reference to the wing and is called the "angle of decalage". For most models the tail will have a negative angle (leading edge lower). The reason for this is that the plane should be balanced so that the nose wants to drop and the tail has to push down to hold the nose up. This can vary greatly depending on CG, wing location, and fuselage shape.

Question 5: The motor is supposed to be mounted (I assume) with the shaft of the motor along the horizontal longitudinal fuselage line, but at an angle. This is the thrust angle. I have heard that 1 to 3 degrees downward (this should be a negative angle, right?) and 1 to 2 degrees rightward are the suggested 'rules of thumb' for vertical thrust and right thrust. What is the purpose of the downward thrust? To counterbalance the torque of the motor or to compensate for up-pitch due to lift effects or ...? The right thrust is, I understand, there to compensate for the torque effect of the motor.

Answer 5: The downthrust helps to keep the plane from climbing too steeply when power is increased. Right thrust helps to control the torque of the motor.
Last edited by Creative RC; Feb 03, 2016 at 10:56 AM.
 Feb 04, 2016, 09:31 AM Kamikaze Ace Okay, but why on a 13 year old thread?
Feb 11, 2016, 11:54 PM
Registered User
Are there some good diagrams or other images that explain AOA and AOI posted, I need help visualizing?

Here's a simple plane. I see the horizontal stab has a positive(up)angle to it.

### Images

Last edited by Tommedia101; Feb 12, 2016 at 12:28 AM.
Feb 13, 2016, 04:09 PM
Learn to build with wood.
Quote:
 Originally Posted by Glacier Girl Okay, but why on a 13 year old thread?
I was thinking the same thing. Attack of the zombie thread. Speaking of that...

https://www.rcgroups.com/forums/show...ight=big+floyd
Feb 13, 2016, 04:39 PM
Registered User
Quote:
 Originally Posted by Tommedia101 Are there some good diagrams or other images that explain AOA and AOI posted, I need help visualizing? Here's a simple plane. I see the horizontal stab has a positive(up)angle to it.
Posting a picture of a plane that likely never flew isn't much help in answering the question.

This video may help some:
 Aircraft Design RC Angle of Incidence (5 min 31 sec)

Glen
 Feb 14, 2016, 11:06 AM Registered User Thanks Glen! ��
Feb 14, 2016, 12:02 PM
Closed Account
Nasa has al kinds of free info and charts talking about aircraft design online. They have on line study and reading.