Reflex and Incidence

So you got a hot plane, eh? Tell us why you think you're gonna be Top Gun this year!

Moderator: hbartel

Post Reply
wtsidney
Posts: 30
Joined: Wed May 09, 2012 6:46 pm

Reflex and Incidence

Post by wtsidney »

Trying to understand Reflex. Setup a Flying Stab on a Lethal Weapon. The semi-symmetrical wing required considerable more reflex to set the flying stab to neutral and not have the nose drop when cutting power.

Would like to minimize the required Refliex. Was thinking changing the Wing Incidence would be a better solution than the added drag from Reflex.

Tried adding a popsicle stick under the leading edge of the wing and didn't notice much change. Is there relationship between Wing Incidence and Reflex? Is there a better way to minimize Reflex?
Bill Sidney
Round Rock, Texas
RCCA 1036
User avatar
Dane McGee
Posts: 885
Joined: Fri Feb 21, 2003 1:57 pm
Location: Greenville, Texas

Re: Reflex and Incidence

Post by Dane McGee »

By increasing positive angle of attack of the wing, you possibly create much more drag than control surface reflex.

Different airfoils and airframe configurations require different settings of control surfaces....I don't use any rule of thumb except experimentation till I get it to perform as I want it.

Sorry, I can be of more help.
Dane "Mad Max" McGee
Greenville, Texas
RCCA Vice President 684
User avatar
Blue Note
Posts: 221
Joined: Tue Aug 12, 2008 1:02 pm
Contact:

Re: Reflex and Incidence

Post by Blue Note »

In a nutshell, it sounds like a CG issue more than anything. The decreased weight of the flying stab has probably left you nose heavy if you made no other changes.

With a flying stab, changing the wing incidence is essentially the same as adding a couple clicks of elevator trim, especially if you disregard the thrust angle. A semi-symmetrical wing will usually require a little positive incidence to ride in the sweet spot of least drag, but if it flew good before the conversion, you were probably pretty close. From you description of the nose dropping when you cut power you are either nose heavy or have too much up thrust with your motor mounting method. If you are nose heavy, you are required to compensate by adding up elevator or excessive amounts of reflex that emulates the same effect of the up elevator, with much more drag. When you cut the power back, your wing is providing less lift due to the decrease in speed and Reynolds numbers, the wing will probably begin to stall, and the stabulator will have decreased influence on the pitch of the plane. It will usually present as a sluggish flying airplane. Too much up thrust would end up needing down elevator trim and/or camber, which is not the condition you are describing.

The process of dialing the plane in is not too hard, but usually requires a few steps for best results. This is what works for me to obtain the most responsive, minimum drag setup for my Tomahawks. All of this is best done in windless conditions, if possible.

Note : The flying stab should remain and/or be returned to neutral with the resolution of each of these steps.

1. Start with the flying stab set in neutral and the ailerons with NO reflex. The wing incidence requirements are airfoil specific, so stick with the original setting, assuming they are correct. Launch the plane anticipating the need for a little up elevator attention just in case. Go high, idle the engine, and push the plane into a perfectly vertical dive then hands off the sticks. Chances are the plane will tuck under to the belly and indicate the wing needs a little reflex. Land and make some adjustments to the reflex settings. The goal is to set the reflex so that the plane falls straight as an arrow, but the dive needs to be absolutely vertical to negate the CG component. It will probably take multiple adjustments to achieve the perfect fall. If the flying stab is still in neutral, you should have the theoretical level flight trim, not taking into account gravity and thrust angle.

2. Now that the reflex is set to neutral, it is time to evaluate the CG. Take the plane up and adjust the elevator for level flight at full throttle. ( Yes, it is no longer in neutral ) Go high, idle the throttle, glide for a few seconds, and push the nose down so you are in a long 45 degree dive. Release the sticks and observe. If the plane pulls out of the dive, you are nose heavy. The quicker it pulls out, the worse it is. We know this because you have to compensate for the excessive nose weight by adding up elevator trim. If the plane tucks under toward the belly, you are tail heavy and compensated with down trim. Land, adjust the CG and repeat until you achieve a straight dive path at 45 degrees while at idle ( or dead stick ). Your flying stab should be returned to neutral when it is correct.

3. Next, check the thrust angle to make sure it is not causing you problems. From a very good height, repeat the idle dive test at 45 degrees after a few seconds of gliding. About a third of the way through the dive, smoothly but quickly apply full throttle and observe. If there is no pitch change, you are golden. If it pulls out, you have some up thrust. If it tucks under, you have some down thrust. Make sure to repeat it a few times and make the powered portion of the dive long enough to allow the torque changes to settle out to give a more accurate indication of influence. Adjust the trust angle as needed based on the direction and severity of the influence.

4. Now for the final reassessment of the required reflex. At straight, level, full throttle flight...chop the throttle. The plane should settle into a gentle drop in altitude while staying essentially level in pitch. If the nose drops, this is the time for evaluating the final need for additional reflex. Usually, only about 1 full turn of each aileron clevis will be required to achieve the proper hands-off dead stick glide. Again, this setting is airfoil and planform specific while damage to your wing changes it's characteristics mildly. Once adjusted though all 4 steps, your plane should be stable, responsive and about as quick as it can be relative to control surface drag.


Hope this helps. Good luck.
Last edited by Blue Note on Tue Sep 04, 2012 5:28 pm, edited 1 time in total.
Chris Gunter
East TN
RCCA #954
Quickdraw
Posts: 258
Joined: Thu Mar 13, 2003 2:37 pm
Location: USA

Re: Reflex and Incidence

Post by Quickdraw »

Good information, thanks.
David E. Ebers Sr.
RCCA South Central Rep
Quickdraw #338
wtsidney
Posts: 30
Joined: Wed May 09, 2012 6:46 pm

Re: Reflex and Incidence

Post by wtsidney »

Thanks Chris - incredible detail.

I have been performing a couple of these tests but not with the insight you just provided. Will re-trim with better understanding.

Do you typically adjust the thrust angle on your Tomahawks?
Bill Sidney
Round Rock, Texas
RCCA 1036
User avatar
Blue Note
Posts: 221
Joined: Tue Aug 12, 2008 1:02 pm
Contact:

Re: Reflex and Incidence

Post by Blue Note »

wtsidney wrote:Thanks Chris - incredible detail.

I have been performing a couple of these tests but not with the insight you just provided. Will re-trim with better understanding.

Do you typically adjust the thrust angle on your Tomahawks?

My Tomahawks essentially employ a 0-0-0 arrangement ( similar to pattern planes ) and are designed around a fully symmetrical, low drag wing. The stabulator is set perfectly level to the fuselage rails, the symmetrical wing sits perfectly level on the rails, and the engine mounts perfectly level on the rails without any thrust angle deviation for pitch or yaw. I set the reflex position by laying a long straight edge on the top of the wing running front to back and bring the ailerons up to be perfectly flush with the top of the airfoil. This effectively sets my reflex to zero. With the ailerons top hinged with tape and the bevel on the other side, it induces the perfect amount of mechanical differential for axial rolls. This is what works for my airfoil. Semi-symmetrical airfoils will usually have different requirements in regards to final reflex settings. My goal is to keep everything perfectly in line with each other so it produces a low drag, true flying airplane with equal ability to perform equally aggressive positive and negative maneuvers relative to stabulator input. I like my planes to fly straight as an arrow with no tendency to self correct. They should do exactly what I ask of them. Nothing more, nothing less.
Chris Gunter
East TN
RCCA #954
Post Reply

Return to “My plane is better than yours because...”