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Old Apr 17, 2012, 10:46 PM
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United States, MA, Sturbridge
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Having trouble getting low

This seems to be a piloting problem for me.

Im using a Hawk Sky with a Grayson Microjet V3. Here is my issue if I want to fly low at any sort of speed I have to give her lots of down elevator. The plane just wants to pull up all the time and I get a little scared getting under 30 ft at high throttle a quarter mile out. The engine pod is perfectly level with the fuse and Im pretty certain my cg is correct. Any suggestions for getting down low?
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Old Apr 17, 2012, 10:51 PM
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Lakewood, Colorado
Joined Aug 2002
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Sounds like the behavior of a nose heavy plane to me. Nose heavy requires more static up
elevator trim to balance, which means when you speed up, it climbs (weight remains the same
but aerodynamic force of the up trim increases).

The pitch will become more neutral as the CG moves back but you *must* also reduce
your elevator rates, or it'll become pitchy (too sensitive). Many people refuse to lower
their elevator rates, so they simply fly with ill handling nose heavy planes all the time.

ian
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Old Apr 17, 2012, 11:03 PM
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Quote:
Originally Posted by Daemon View Post
Sounds like the behavior of a nose heavy plane to me. Nose heavy requires more static up
elevator trim to balance, which means when you speed up, it climbs (weight remains the same
but aerodynamic force of the up trim increases).

The pitch will become more neutral as the CG moves back but you *must* also reduce
your elevator rates, or it'll become pitchy (too sensitive). Many people refuse to lower
their elevator rates, so they simply fly with ill handling nose heavy planes all the time.

ian
I'll give it a try, but I've got the battery shoved as far back as I can get it right now. So I might have to add some weight to the tail. Which I do not want to do Right now the CG is about 1/4 in front of the spar. Maybe I'll add dual rates to my tx. I;m also noticing a tendancy to stall with little elevator input.

On a similar point Im finding that I am flying almost exclusively in pilot assist mode (which on my system does not control elevator) and Im having some trouble keeping the plane level without it. Granted ive been flying in very windy and gusty conditions lately (15-25mph winds). I added aileron extensions and now the plane seems a bit twitchy so I maybe I'll dial those in.

My rates are all at 120% (max) right now. What should I reduce them by for dl mode?
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Old Apr 17, 2012, 11:40 PM
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120% is way too much for elevator, especially if you get the CG anywhere
close to correct. I often use elevator rates under 40%. On my best handling slope sailplanes
it can be less than 25%. I can't say what is best for that specific plane, but I know 120% is too
much, and the fact that it's flyable at all, tells me that it's nose heavy.

ian
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Old Apr 18, 2012, 12:49 AM
Team Flying Pirates
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Quote:
Originally Posted by Daemon View Post
Sounds like the behavior of a nose heavy plane to me. Nose heavy requires more static up
elevator trim to balance, which means when you speed up, it climbs (weight remains the same
but aerodynamic force of the up trim increases).

The pitch will become more neutral as the CG moves back but you *must* also reduce
your elevator rates, or it'll become pitchy (too sensitive). Many people refuse to lower
their elevator rates, so they simply fly with ill handling nose heavy planes all the time.

ian
right
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Old Apr 18, 2012, 01:29 AM
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i've got about 350 flights on a hawk sky via fpv and you need to look at the way your pushrod is set up. notice that it PULLS to pitch up, and PUSHES to pitch down. well the pushrod length is like 18 inches or longer. and most of the time the white tube pops out. make sure the tube isn't popping out and that you get full travel.

next check the CG, it should be right over the airfoil(the highest part of the curve in the wing)

then check to see if your elevator is trimmed correctly. The plane is so easy to fly that you can half stall it and you won't notice anything. so let's say you set it to 75% throttle. then gradually pull back on the stick until it slows enough that it is at a balanced gliding speed except slightly pitched up. you're in high alpha but you wouldn't know it if you threw it like that and your elevator was trimmed to that position.

FPV TROLLING (3 min 37 sec)


here's one of my hawk sky flights, so you can see it is quite easy to fly low if you have it properly set up. and i did some very fast passes(60mph, 20 feet off beach)
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Old Apr 18, 2012, 08:46 AM
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Interesting. I have replaced my push rod tubes with cf tubes and they are pretty stiff. My elivator works and travels well.

I guess I dont understand what you mean by high alpha. Id be interested in knowing more about your hawk sky setup. What motor you were using and such. Ive got agopro on mine and given the limited ability to move the battery back it might be too much?
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Old Apr 18, 2012, 09:14 AM
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i tore mine apart lool, before i had a 1700kv 480 watt rctimer motor with 7x6 prop

my microjet burned out, once you really start buying a lot of motors and stuff you will realize the grayson motors suck for what they are. they are way overrated.

http://www.youtube.com/watch?v=dzsN-qdyGI4
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Old Apr 18, 2012, 10:14 PM
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Ok some improvement today. I did added dual rates to elevator and added 4 pennies to the very end of the tail. This gave me a pretty good COG. However the dual rates were unnecessary I think.

Now the plane flys well even inverted. Im still having trouble keeping it down, but its better. I might try another penny or two on the tail. But its an airfoil right? and lift increases with speed right? So if im hauling ass 5 ft off the ground shouldnt it naturally be hard to keep it there without some trim?

Its no longer stalling with normal when I accelerate with no elevator input.

Avoided Pilot Assist mode today and boy is it a crutch. But today was NOT windy and I had no problem keeping her level without PA. In fact I realize how much fun I was missing. Now I wonder what will happen if I flip on PA while inverted? will it level my plane inverted? Hmm I'll try it at 400 ft first.
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Old Apr 18, 2012, 10:20 PM
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Long answer.
It's always a balancing act.
The main wing generates lift, but also a forward pitching moment (trying to push the nose down).
The horizontal stab on a conventional plane always pushes down (negative lift) to counter
both the forward pitching moment of the main wing and the weight in the nose.
You can't change the magnitude of pitching moment (without variable camber) but you can move
the CG back while simultaneously removing elevator up trim (sometimes even giving it some
downtrim to counteract the built in negative incidence angle of the horizontal stab). When the
negative lift of the tail is only countering the forward pitching moment of the main wing, then the
plane will fly pitch neutral at all airspeeds. Most people move the CG a little forward of that so
that the plane will naturally pull up a little.

Short answer:
You can still move the CG back more, reduce elevator throws and get it to fly more neutral, even
at high speeds.

ian
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Old Apr 18, 2012, 10:42 PM
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understand your volts change as you fly so you have to compemsate for throttle and your incidences for that plane are most likely for very slow flight

fly slow in very low wind

use the throttle to control the plane rather than the elevator

make the st up so that if you get ito trouble and need to climb for whatever reason like poor video that you can just use more throttle

set up your plane so you can fly straight and level at any level of flight

ezstar as an example has this problem, it just does not fly that great

idea being set up well under full throttle or with no throttle the plane should fly straight, but unfortunately some planes do not do this

elevator compensation with throttle helps but is messted up with mixing due to as I said to volts changes

try a funjet for low flying as an example, it seems to fly pretty flat if set up well



Quote:
Originally Posted by slowjett View Post
This seems to be a piloting problem for me.

Im using a Hawk Sky with a Grayson Microjet V3. Here is my issue if I want to fly low at any sort of speed I have to give her lots of down elevator. The plane just wants to pull up all the time and I get a little scared getting under 30 ft at high throttle a quarter mile out. The engine pod is perfectly level with the fuse and Im pretty certain my cg is correct. Any suggestions for getting down low?
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Old Apr 18, 2012, 11:15 PM
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United States, CO, Parker
Joined Mar 2011
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I also fly FPV with a Hawk Sky and have a GoPro on the nose. You will need to add weight to the tail. Proper CG I've found is right on the wing spar. The MicroJet V3 might also be underpowered for FPV. When I started FPV I ditched that in favor of a 1500 KV motor that can support larger props, though I had to cut out a big chunk of the tail boom to have enough clearance. Not sure if having the prop go inside the tail a little messes up the airflow or not, but it works. Oh, and make sure your tail boom isn't sagging. That was throwing my balance way off until I fixed it the other day.

Here's a picture of my setup (a few minor details have changed since then, but it's mostly current):

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Old Apr 19, 2012, 09:13 AM
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United States, MA, Sturbridge
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yeah my tailboom is reenforced with 8mm carbon tube. its straight as an arrow, well cuz I used an arrow

The mjv3 seems to be doing ok. Its not overly powerful but I can cruise at 40mph, hit speeds of about 55-60 in level flight, do loops and rolls without much issue. I cant climb steeply however.

Dynam Hawk Sky mods - carbon fiber / aileron servo wire mod / motor upgrade (8 min 39 sec)
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Old Apr 20, 2012, 04:08 AM
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you copied me slowjett lol, i have an arrow on mine too!!! i had to cut a wedge out of the tail to level the elevator again since the arrow took the curve out of the foam
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