How to calculate the maximum deflection angles of the elevator for a statically stable aircraft. As I understand Cm should be 0 at max. and min. speeds and angles of attack. I've been trying for a while now and it seems to give very large Cm with small deflection angles. I tried to change the angles of the stabilizer, the center of mass. Maybe the aircraft is not stable or I missed something. Any help would be greatly appreciated.
Last edit: Danielius Snitkus 2024-06-04
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Hi,
your polars seem fairly reasonable to me.
Note that the condition Cm = 0 at max. and min. speeds applies to steady level flight. It therefore lets you determine the trim speeds of the aeroplane.
However, your maximum elevator deflection does not depend on this. While you obviously need to be able to trim the aeroplane, the required deflection for this is usually quite small. (Try hooking up a servo to your receiver and transmitter and watch the amount deflection when moving the stick vs moving the trim to the max. You will see that the trim function of the transmitter deflect the servo a small amount compared to the full stick deflection.)
The maximum elevtor deflection depends on what moment you need to achieve adequat controllability. There are more or less three ways to determine this;
1. guessing a reasonable deflection angle and adjusting it while flight testing. While this seems crude it is done a lot for both rc and homebuilt aeroplanes and is generally a good method. Your 15 deg deflection is also within the region of max. elevator deflection I'd call reasonable. With time and experience you can also look at the polars for a fine tuned guess. This is generally the method I'd recommend and also use for my rc aeroplanes most of the time - because it just works!
2. use empiric formulas from conceptual aircraft design. This is pretty much the mathematical equivalent to the first method, with the exception that those formulas can lead to completely unreasonable values if you don't pay close attention to what aeroplane sizes (size in general, aspect ratio, tail volume and and and) they are valid for. Just like method 1 you will adjust the maximum deflection to your liking while flight testing.
3. full on flight dynamic simulations. This requires the program(s) and flight mechanics knowledge to set up a model of the aircraft and look at time responses to control input changes and so on. I won't go into detail on this as it is out of scope for most people.
Cheers
Stefan
👍
1
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
How to calculate the maximum deflection angles of the elevator for a statically stable aircraft. As I understand Cm should be 0 at max. and min. speeds and angles of attack. I've been trying for a while now and it seems to give very large Cm with small deflection angles. I tried to change the angles of the stabilizer, the center of mass. Maybe the aircraft is not stable or I missed something. Any help would be greatly appreciated.
Last edit: Danielius Snitkus 2024-06-04
Hi,
your polars seem fairly reasonable to me.
Note that the condition Cm = 0 at max. and min. speeds applies to steady level flight. It therefore lets you determine the trim speeds of the aeroplane.
However, your maximum elevator deflection does not depend on this. While you obviously need to be able to trim the aeroplane, the required deflection for this is usually quite small. (Try hooking up a servo to your receiver and transmitter and watch the amount deflection when moving the stick vs moving the trim to the max. You will see that the trim function of the transmitter deflect the servo a small amount compared to the full stick deflection.)
The maximum elevtor deflection depends on what moment you need to achieve adequat controllability. There are more or less three ways to determine this;
1. guessing a reasonable deflection angle and adjusting it while flight testing. While this seems crude it is done a lot for both rc and homebuilt aeroplanes and is generally a good method. Your 15 deg deflection is also within the region of max. elevator deflection I'd call reasonable. With time and experience you can also look at the polars for a fine tuned guess. This is generally the method I'd recommend and also use for my rc aeroplanes most of the time - because it just works!
2. use empiric formulas from conceptual aircraft design. This is pretty much the mathematical equivalent to the first method, with the exception that those formulas can lead to completely unreasonable values if you don't pay close attention to what aeroplane sizes (size in general, aspect ratio, tail volume and and and) they are valid for. Just like method 1 you will adjust the maximum deflection to your liking while flight testing.
3. full on flight dynamic simulations. This requires the program(s) and flight mechanics knowledge to set up a model of the aircraft and look at time responses to control input changes and so on. I won't go into detail on this as it is out of scope for most people.
Cheers
Stefan