Help! Thrust angle question on new FT Zero build

[Andy.T.]

Hello! I just got back from a successful maiden flight of my FT Mighty Mini Zero! Successful in that I still have a fully intact plane to fly later in the week when it stops sleeting and snowing This was a speed build kit, not like a blown up version - which I already want!
But, it didn't quite fly as perfect as it could have; better than I expected, given a couple...angles...that came out wrong in the build. With neutral control inputs and nice and leveled out control surfaces, it wanted to go extremely nose up. I was already flying w/ the CG at about 1.75" from leading edge at the wing root (this is 3/4" forward of the recommended). I have space to fly with the battery (3S, 800mah, or 850) maybe another fingers-width forward. But I'm wondering if the thrust angle came out not down enough.
Thanks in advance if you even got thru my long-winded question, let alone any suggestions!

 

[Aslansmonkey]

Looks about right to me, as compared to mine. The mini Zero likes to be nose heavy though. Make sure the battery is tucked well up in the nose. To me, the FT mini zero is the best flying of all the mini warbirds, seconded by the corsair. I've also flown the hellcat, P-40 and trojan and none of those flew as well for me. Keep in mind, it's entirely possible it's the pilot, not the planes in those cases, but I think the Zero has the nicest flight characteristics.

 

[Merv]

....it wanted to go extremely nose up. I was already flying w/ the CG at about 1.75" from leading edge at the wing root (this is 3/4" forward of the recommended)...

The first step, trim the plane to fly straight and level at the speed you prefer to fly, I prefer full throttle. Then fly several hundred feet high, cut the throttle and put the plane into a straight down dive. If the plane pulls out of the dive right side up, you need more down thrust. If it pulls out belly up, you need up thrust.

Why it works, flying straight & level, you are trimming out any errors. When the plane is in a dive, you have removed the thrust angle error and yet are maintaining speed. The way the plane pulls out of a dive it reveals th e error.

How quickly the plane pulls out will give you an idea of how much thrust is needed. If it pulls out in 25 feet or less, 4 degrees of thrust: 50 feet, 2 to 3 degrees: 75 feet, 1 to 2 degrees. The goal, it will dive 150 feet, more or less straight down.

CG is more how stable your plane flies, if you want a stable flying plane, choose a CG slightly forward. If you enjoy a bit of instability, as I do, move the CG aft.

 

[Andy.T.]

Thanks both for the thoughts. Getting to talk to other RC pilots is kind of a brand new thing for me; I've just sorta done it by myself for years, other than sometimes getting my cousin or dad to watch, and sometimes film.
The idea of using a dive like that is super clever. Never would have come up w/ that on my own. If it needs down-trim to compensate for not enough down thrust, when it is in a freefall dive, thrust angle thus not being a factor, wouldn't the plane want to pull out negative not positive? The plane would still have the down trim, but not the excessive up-thrust pull.
I guess the next questions are...
When the kit for the Zero says CG at 2.5" from leading edge, that's supposedly with the battery, measured pretty close to the wing root, right?
If so, how come the plane would want to pitch up so sharply? I guess before anything else I need to eliminate the possibility of needing to adjust thrust angle. Otherwise that would seem like an awful far back CG to suggest. I think I've noticed feeling like I always had CG a significant margin more forward than suggested. Or maybe I shouldn't expect that if the elevator is in a very flat line with the stabilizer that the plane should fly level?

 

[Piotrsko]

Other factors: total incidence is excessively positive. Same symptoms. Having a flat elevator in respect to the horiz stabilizer might be an indication that it is off since typically there is a bit of up elevator. The @Merv dive method works, although I'm not a fan of it, tends to break planes.

 

[Merv]

...When the kit for the Zero says CG at 2.5" from leading edge, that's supposedly with the battery, measured pretty close to the wing root, right?...

Yes, the CG is measured with everything you are going to fly with. The only exception is with a liquid fuel plane. Measure the CG with an empty fuel tank.
With the plane ready to fly, where the plane balances is the CG point. Most all planes will fly well when the CG is between 25% to 30% of the average wing cord. If you want a stable plane, choose 25%. If you want more twists and flips, instability, move the CG aft until the plane flies the way you want it to. Just be careful not to go too far aft. A nose heavy plane will fly poorly, but a tail heavy plane will only fly once.

...If so, how come the plane would want to pitch up so sharply?...

It's out of trim.
The angle of the wing, the elevator and motor all play a roll in the amount of trim a plane needs to fly straight & level.

...Or maybe I shouldn't expect that if the elevator is in a very flat line with the stabilizer that the plane should fly level?

This is only a starting point. Every plane I have ever made has needed some trim to fly straight & level. It all comes down to how accurate you were when building the plane.

 

[Andy.T.]

Thanks again, guys! I love actually getting hear all this wisdom and experience.
I didn't even consider the incidence of the stabilizer. I just assumed it would be pretty close from the build. But, I know I didn't build it as flawlessly as Josh did in the build video 😂. My dad gave me an incidence gauge he got, probably in the early 90s but never even took out of the package. I should try harder to see if I can make it work. My initial inspection, it didn't seem to have aged well. But I admittedly can only guess vaguely how it works.
Good, thank you again, the confirmation of cg check with battery, while basic, I realized I was operating just under my own assumptions. I love comments I've read about tail heavy planes will fly, once! Makes me chuckle.
And, understood about level control surfaces just being a starting point. I will keep that in mind and adjust my mental perspective accordingly. I'm no Bixler with the build skills 😉
I also like the reference to the chord. I thought that would somehow come in to play. I don't think I knew the 25-30% of the chord, but looking at the shape of the wing, I was surprised where the suggested cg was. Where I was thinking I would want the CG was right about at the thickest part of the airfoil. Looking at a diagram on Wikipedia article about chord, that looks like it's just shy of 25% of that chord length. I'm totally going to go measure my Zero now

This has been so informative, I appreciate your collective time and advice!

 

[danskis]

Since it’s a kit the wing and stab are PROBABLY at the correct angle but it’s worth checking. Your grasp of the dive test is good but you may take a look at this.

https://gympiemodelflyers.wordpress.com/2014/01/28/cg-the-dive-test/

 

[danskis]

BTW great looking plane. I’ve put a lot of down thrust in some of the FT planes

 

[Bricks]

You can do the same thing Merv suggests for the dive me personally like going straight up under full power same thing just a different way to do it

 

[Andy.T.]

BTW great looking plane. I’ve put a lot of down thrust in some of the FT planes

Thanks x 2 ! I checked out the link, that was a really great write up, and the drawings help make it nice and clear. Can't wait to for better weather and to get to actually trim out my Zero.

 

[Piotrsko]

Um checking cg by using the chord only works on square non tapered wings with no sweep. Sweep moves the correct cg point as does taper. If they mark the cg on the fuselage then you're good otherwise there should be locations specified on the wing plans. Fyi: correctly cg on my KFM swept wing is somewhere aft of 80% back of the nose when holding the fuselage and I still had to add nose weight....

 

[danskis]

Re-reading your post I would start with more down thrust. I’ve found that maidening a plane with the correct CG and not enough down thrust means you get to fly again…not a bad thing.

 

[danskis]

@Bricks I'm unsure how going up under power simulates the dive test. The dive test uses increasing speed to indicate how elevator trim is compensating for optimum (more or less) CG. All of my FT planes decelerate when going straight up.

 

[Bricks]

@Bricks I'm unsure how going up under power simulates the dive test. The dive test uses increasing speed to indicate how elevator trim is compensating for optimum (more or less) CG. All of my FT planes decelerate when going straight up.

Your under powered at least in my eyes then do as Merv mentions check thrust angle in a dive. I just prefer going straight up less stress when I have a gasser or more expensive plane.

 

[MrClean]

So, they mentioned that when you have the center of mass in front of the planned CG of the airplane you have to trim to hold the nose level. That trim at higher speeds is too much balance and will make the plane climb under full power and dive at less power, well more then a symetrical wing will do. The Dive or Climb test gets rid of the holding the nose part, gets rid of the power part and just leaves you with the aerodynamic forces part (technically if you're adding power in the climb you're adding engine thrust angles into the equation) Normal trim of an airplane will have the dive test pulling to the canopy slightly. IF you fly a zero zero ship where you have to hold up or push down to fly upright or inverted, then the dive test will continue straight down. Generally, we fly sports ships a tad nose heavy so the plane should pull towards the canopy, if it tucks it's nose, the CG is too far back. We call it Errordynamics cuzz few of us went to college on Aerodynamics degree path. Hope that's clear. In design of an aircraft the designer needs to search for the CG range and you put the CG to where you like to fly. When you design, well bub, YOU get to figure it out. It's quite fun but a little nerve racking if you get the cg to the place where you pull up, the tail of the plane goes down but THEN the flight path of the plane starts changing accordingly. (REALLY NOSE HEAVY) or when you think about pulling up and the plane just about stops in the air (really tail heavy). fun fun fun DUCK!

 

[Piotrsko]

Having built and flown zero zero ships, it ain't flying at zero zero, its falling like an arrow. unless you're sloping in a pretty strong spot where the airspeed up is greater than the plane airspeed down.

 

[JDSnavely]

Looks about right to me, as compared to mine. The mini Zero likes to be nose heavy though. Make sure the battery is tucked well up in the nose. To me, the FT mini zero is the best flying of all the mini warbirds, seconded by the corsair. I've also flown the hellcat, P-40 and trojan and none of those flew as well for me. Keep in mind, it's entirely possible it's the pilot, not the planes in those cases, but I think the Zero has the nicest flight characteristics.

My son has made two. Very light, super fast. Great fun. He opened up the square right behind the prop to give airflow to the battery and ESC. It likes to go fast and he can't resist full throttle most of the time. The Trojan is a good flyer too and has room for a 4S 850 battery! But I agree, the Zero flies outstanding!