# How much pressure needed to deflect flaps, ailerons, rudder, elevator?

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#### rtfm

##### Well-Known Member
Well, I guess the title says it all. Any ideas? Are there specs I can read somewhere for our type of airplane. Not Part 103, but close...

Duncan

#### plncraze

##### Well-Known Member
HBA Supporter
Start with Neal Willford's "Handling Qualities" in Sport Aviation.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
I like having a title that reflects the content...

TOWS is always the first stop.

Method 1 - Appendix 1 and find the V/v and/or (V/v}^2 distribution over foils there, focus on the control surface portion only. Once you have v established, you can calculate V and then net lift along each control surface, and then moment about the hinge. Being as (V/v)^2 is essentially linear in the aft half of most foils, this pretty easy.

Method 2 - Chapter 8, figs 97 & 98 and decode those curves to calculate hinge moments and change in moments with pitch and control surface angles. All of these moments are about the leading edge of the control surfaces. If your hinges will move aft of leading edge of the surfaces, you will need to go through the static exercise for that.

I would do both as a check on my math. Actually, I would do both on an Excel page and have learned colleague check it privately (me or someone you trust). Then you go through the mechanical advantage of your control system to get fingertip pressures. Oh, and while rudder and elevator forces are good by themselves, remember that both ailerons are lifting against each other through the linkage, so aileron forces that you push against are the difference in moments between the two ailerons.

All that being said, in a single seat 100 knot airplane, the control forces with leading edge hinges will be pretty light.

Billski

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
Duncan, if you have access to EAA's archives, there is a bit on the subject on page 12 of April 1955's Experimenter.

You might also find some useful things in this book: Practical Aeronautical Engineering.

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#### Rockiedog2

##### Well-Known Member
I like having a title that reflects the content...

TOWS is always the first stop.

Method 1 - Appendix 1 and find the V/v and/or (V/v}^2 distribution over foils there, focus on the control surface portion only. Once you have v established, you can calculate V and then net lift along each control surface, and then moment about the hinge. Being as (V/v)^2 is essentially linear in the aft half of most foils, this pretty easy.

Method 2 - Chapter 8, figs 97 & 98 and decode those curves to calculate hinge moments and change in moments with pitch and control surface angles. All of these moments are about the leading edge of the control surfaces. If your hinges will move aft of leading edge of the surfaces, you will need to go through the static exercise for that.

I would do both as a check on my math. Actually, I would do both on an Excel page and have learned colleague check it privately (me or someone you trust). Then you go through the mechanical advantage of your control system to get fingertip pressures. Oh, and while rudder and elevator forces are good by themselves, remember that both ailerons are lifting against each other through the linkage, so aileron forces that you push against are the difference in moments between the two ailerons.

All that being said, in a single seat 100 knot airplane, the control forces with leading edge hinges will be pretty light.

Billski
LOL
Well OP, you asked.

##### Well-Known Member
Not what you asked, but also important to good flying qualities is the relative force needed to move each of the primary controls. You don't want the elevator, ailerons, and rudder to all move equally. One designer told me the ratios should be 1:2:3 with the rudder being heaviest and the elevator being lightest.

#### rtfm

##### Well-Known Member
Hi guys,
Wow - thank you for your help. This has got to be the most helpful/useful flying forum on the planet.

Someone should really trawl through the thousands of questions/answers and compile this into an online database. Someone with no life, of course...

Regards,
Duncan

#### rtfm

##### Well-Known Member
Hi Aerowerx,
The site is blocked. But thanks anyway.

Cheers,
Duncan

#### pictsidhe

##### Well-Known Member
The earlier NACA flight qualities specified stick forces. I think 8785 was the spec that left the to the manufacturer? A good paper about desirable stick forces is this one: CRITERIA FOR ACCEPTABLE STICK FORCE GRADIENTS OF A LIGHT AEROPLANE
I think Perkins and Hage may have some stuff on actual forces and desirable ones. For my project, I'm going huge on the aileron chord for two reasons:

1. So it will roll slightly faster than a glacier at 103 speeds.
2. So the stick will need more than a tickle to make it roll.

I did read an article about an investigation into the puzzling crash of an LSA. It was eventually determined that the pilot most likely ejected himself through his straps and canopy by knocking the exceedingly light elevator while reaching for something. That persuaded me to try to get Baby Bear's porridge for the stick forces.

#### Lendo

##### Well-Known Member
Hello Duncan,
I recently tried the RV stick forces and they were a good balance light enough but certainly not too light, where as the Jabiru were extremely light. Feed back to me most Light Sport Aircraft have very light stick forces, which suggest to me that if too light some resistance can be made with springs or counter weights.

pictsidhe,
What do you class as huge, recommendations I seen suggest 40% Span (of usable wing) with 20 - 25% chord, Flaps 60% Span with 25- 30% chord, where as Elevator and rudder 40% chord. I would guess 25% chord would provide a very light response.
George

#### plncraze

##### Well-Known Member
HBA Supporter
Funny story Chuck Berthe ( I think) told in an older issue of Kitplanes; he was giving his mentor Bob Harper a ride in his RV and asked him for his "number" in regards to handling. Highest is a ten. Mr. Harper said 9.5. Mr. Harper is the co-author of 8785C which is sometimes called "Cooper Harper."
Mr. Harper's reason for not giving the the plane a 10 was that the control stock was too long.

#### Aerowerx

##### Well-Known Member
Hi Aerowerx,
The site is blocked. But thanks anyway.

Cheers,
Duncan
Just Google MIL-F-8785C yourself. That is how I found it.

#### don january

##### Well-Known Member
Log Member
Each and every bird control force pressure will vary depending on throw and adjustments. You may need a rudder with a trim tab holding a bit right rudder or a wing that seems to want to drop in level flight due to aileron. Elevator wants to follow the incidence your Horz. stab is set at under thrust to maintain level flight. I can't see any book that can aid unless you have square inches and directional force of airflow against said aircraft surfaces. Only then can you get an idea. And flaps are like a on off switch and they boil down to airflow over the wing. FWIW

#### BJC

##### Well-Known Member
HBA Supporter
Funny story Chuck Berthe ( I think) told in an older issue of Kitplanes; he was giving his mentor Bob Harper a ride in his RV and asked him for his "number" in regards to handling. Highest is a ten. Mr. Harper said 9.5. Mr. Harper is the co-author of 8785C which is sometimes called "Cooper Harper."
Mr. Harper's reason for not giving the the plane a 10 was that the control stock was too long.
The only RV that I have flown is the -7. I agree that the stick is too long. That is easy to fix. 9.5 is too high on my scale.

BJC

#### rtfm

##### Well-Known Member
Just Google MIL-F-8785C yourself. That is how I found it.
Thanks. Will do.

#### Aerowerx

##### Well-Known Member
Each and every bird control force pressure will vary depending on throw and adjustments. You may need a rudder with a trim tab holding a bit right rudder or a wing that seems to want to drop in level flight due to aileron. Elevator wants to follow the incidence your Horz. stab is set at under thrust to maintain level flight. I can't see any book that can aid unless you have square inches and directional force of airflow against said aircraft surfaces. Only then can you get an idea. And flaps are like a on off switch and they boil down to airflow over the wing. FWIW
There are two ways to view the OP's question.

One is "what forces will I see on the controls during flight"? The other is "what is an acceptable amount of force"?

The MIL-F-8785C that I cited answers the second question. It is a sometimes quite obtuse description of what the military considers as acceptable handling characteristics, including control forces.

The second question will require some effort to determine. A good place to start is the VNE of the aircraft. Then determine the dynamic air pressure per square foot (or metric equivalent). Apply that to the deflected control surface, adjusting for the deflection angle. Work through the various moments of the cables, pushrods, pulleys, etc. and you have the force seen at the stick or yoke.

#### rtfm

##### Well-Known Member
Hi guys,
There is a third way. How much force needs to be applied at the control surfaces to get them to deflect? By the time the linkages reach the pedals or the control stick, various "gearings" can have taken place to get to, possibly, Radfordc's suggested 1-2-3 ratio.

From what I'm reading above, these forces aren't that great on our type of planes.

Duncan

#### Aerowerx

##### Well-Known Member
Hi guys,
There is a third way. How much force needs to be applied at the control surfaces to get them to deflect? ....
You mean sitting on the ground? Very very little.

Also, there should be very little friction (loss if you prefer) in the mechanism between the control surface and the stick/yoke.

Besides, you don't want them to deflect without input from the pilot.

The "gearings" you mentioned, that is what I meant by "work through the various moments".

Moment = Force x Distance This should remain almost the same through the entire linkage system from the surface to the stick. It is relatively simple to work this out, compared to other aspects of aircraft design.

#### Lendo

##### Well-Known Member
That's right Duncan.
George