Inverted flight, how quick should it fall?  
View related threads: (in this forum | in all forums)

Tower Hobbies
Enter up to 4 keywords or Tower stock numbers
Logged in as Guest



Users viewing this topic: none
  Printable Version
       

All Forums >> RC Airplanes >> Pattern Flying >> Inverted flight, how quick should it fall?
Page: [1] 2   next >   >>  

Login
Message << Older Topic   Newer Topic >>
Inverted flight, how quick should it fall? - 7/1/2008 3:20:33 AM   
tschmidt


 

Posts: 244
Joined: 12/4/2002
From: Fort Scott, KS,
Status: offline 		How quick should a plane fall when inverted, hands off the elevator?

I'm carrying .75 positive in the wing and about .25 in the stab to offset down elevator trim. When inverted, hands off the elevator, its diving at a 45 in 3 seconds.

To much positive in the wing?

Todd Schmidt



       Post #: 1

RE: Inverted flight, how quick should it fall? - 7/1/2008 5:07:22 AM   
vbortone


 

Posts: 240
Joined: 4/6/2004
From: Olathe, KS, USA
Status: offline 		Todd,

What is the engine thrust line? With 0.75 incidence, I suggest that the T/L should be -0.25 to + 0.5. If you are too much negative could cause that probem. Also the plane could be too much nose heavy. I am sure that you already took care of the CG.

Good luck,

Vicente "Vince" Bortone

(in reply to tschmidt)
       Post #: 2

RE: Inverted flight, how quick should it fall? - 7/1/2008 9:33:22 AM   
anuthabubba


 

Posts: 176
Joined: 11/9/2007
From: Transylvania, LA, USA
Status: offline
It's normal to have to hold in some down stick to maintain level inverted flight. Does it fly level hands off right side up?

Terry in LP

(in reply to tschmidt)
       Post #: 3

RE: Inverted flight, how quick should it fall? - 7/1/2008 2:24:51 PM   
vbortone


 

Posts: 240
Joined: 4/6/2004
From: Olathe, KS, USA
Status: offline 		Todd,

Are you mixing low Thr. with down Elev.? I usually do very small amount ~1-2-%. If you have a lot of mixing could cause this problem.

Vicente "Vince" Bortone

(in reply to tschmidt)
       Post #: 4

RE: Inverted flight, how quick should it fall? - 7/1/2008 2:25:08 PM   
tschmidt


 

Posts: 244
Joined: 12/4/2002
From: Fort Scott, KS,
Status: offline 		Vicente,

The engine is at 0 degree down thrust and I have no throttle to elevator mix.

Terry,

It flys hands off upright and I do realize we have to push to maintain level flight inverted. Just wondering if I'm having to push to much for the norm. To me, it drops faster than it should.

Todd

< Message edited by tschmidt -- 7/1/2008 2:27:08 PM >

(in reply to anuthabubba)
       Post #: 5

RE: Inverted flight, how quick should it fall? - 7/1/2008 3:30:27 PM   
flyncajun


 

Posts: 120
Joined: 12/25/2006
From: DENHAM SPRINGS , LA, USA
Status: offline 		Hi Todd
your flying wayyyyyy tail heavy,
move you c/g forward probably an inch and your dive will go away ,,, Trust me
remember tail heavy elevator will be sensitive and c/g forward elevator is positive and effective around nuetral
it will require very little elevator to mantain inverted flight ,,,,however you might require more over all throw in the end
when you get way back on the c/g the airplane behaves opposite of what you think it should, very confusing.
make sure your wing inc is 1/2 pos to the thrustline of your model and your motor should be 1/2 neg to the thrustline
your c/g should be around 25% of the MAC not in the 30`S where most models are set up
try this and report back please
P.S.you will find your snaps, spins, knife, up and downlines all improve with this setup
Bryan

(in reply to tschmidt)
       Post #: 6

RE: Inverted flight, how quick should it fall? - 7/1/2008 10:51:19 PM   
dhal22


 

Posts: 1233
Joined: 8/28/2006
From: Roswell, GA, USA
Status: offline 		certainly all of you guys are more accomplished pilots than i but i fly a ultra sport 60 that flies hands off right side up or inverted.

david

(in reply to flyncajun)
       Post #: 7

RE: Inverted flight, how quick should it fall? - 7/2/2008 4:06:56 AM   
billtulsa


 

Posts: 54
Joined: 1/8/2005
From: tulsa, OK, USA
Status: offline 		Hi Todd,
Take .25 degrees of positive out of the wing and reset the stab so the elevator flys just a little down trim. When you roll inverted the same thing will happen so you will end up needing to move the CG aft to get the feel you want. I only recommend resetting the wing because it sounds slightly exceesive but will probably not change the feel when inverted, you will end up moving the CG back. I assume also that the .25 of stab was .25 degrees and not 1/4 inch, correct?

The reason I set the stab to run some down elevator trim is I want my plane to be in perfect trim elevator trim from 1/3 throttle all the way to full throttle and at the changing airspeeds. This also makes the plane go straight up, no pitching. Basically I am flying with a cambered stab when setting it this way.

Bill

(in reply to dhal22)
       Post #: 8

RE: Inverted flight, how quick should it fall? - 7/2/2008 4:44:42 AM   
jim woodward


 

Posts: 117
Joined: 9/29/2006
From: Miramar, FL, USA
Status: offline 		hi Todd,
Personally, I think you should run the least amount of positive incidence you can - somewhere between 0.25 and 0.5 deg +. Too much positive incidence can double-cross your trim efforts and especially for evaluating inverted flight. I'd go with Bill's suggestion and reduce the incidence for starters, and reset the stab to zero. Down thrust is not a crime and you may end up with a little bit when you are done.

Here is a link to the best trim article I've read, written by Peter Goldsmith. http://www.mini-iac.com/Portals/57ad7180-c5e7-49f5-b282-c6475cdb7ee7/PGoldsmith%20on%20trimming.pdf

Regarding CG, give his method a thought...

This is off of the IMAC website and I hope he doesn't mind the plug. His methodology flat kicks the pants off the "trim-chart", in my opinion, and offers great insight and reasoning into how to get you model flying "right" with an eye towards the complete sequence, versus singled out elements.

Its possible to get combinations of exaggerated incidences or CG settings and still end up with a mildly mixed out model. However, the closer you get to a "stable" model that still carries a tad of the great flying qualities of a near neutral plane, the better you are going to fly. IMHO. You really don't want a plane that flies great upright, then dives like hell towards mother earth when inverted, because no matter how much you play with expos or throws to make it "manageable", the model will still have a setup flaw that will bite you some time. Simply said, the further forward you run the CG, and the more positive incidence you run, the worse the "dive" tendancy is going to be when inverted.

This is a great internet topic because opinions and experience vary so much.
Thanks,
Jim W.

(in reply to billtulsa)
       Post #: 9

RE: Inverted flight, how quick should it fall? - 7/2/2008 4:53:55 AM   
mjfrederick


 

Posts: 593
Joined: 8/18/2006
From: Baton Rouge, LA, USA
Status: offline
quote:

Simply said, the further forward you run the CG, and the more positive incidence you run, the worse the "dive" tendancy is going to be when inverted.


The statement about CG made here is only correct for extreme variances from the recommended CG. The same can be said for the incidence... Half degree positive is an excellent starting point for incidence on modern pattern aircraft. I've seen both in practice and know that what flyincajun recommends works. Of course, it might be easier to help if we understood what kind of airframe we're working with here, as well.

_____________________________

mjfrederick AMA 275874 NSRCA 4134
Maybe the Dingo ate your baby

(in reply to jim woodward)
       Post #: 10

RE: Inverted flight, how quick should it fall? - 7/2/2008 5:11:49 AM   
jim woodward


 

Posts: 117
Joined: 9/29/2006
From: Miramar, FL, USA
Status: offline
quote:

ORIGINAL: mjfrederick

quote:

Simply said, the further forward you run the CG, and the more positive incidence you run, the worse the "dive" tendancy is going to be when inverted.


The statement about CG made here is only correct for extreme variances from the recommended CG. The same can be said for the incidence... Half degree positive is an excellent starting point for incidence on modern pattern aircraft. I've seen both in practice and know that what flyincajun recommends works. Of course, it might be easier to help if we understood what kind of airframe we're working with here, as well.



I'd say moving the CG 1.0" forward, as suggested, qualifies an extreme variance. I'm not debating Bryan's methods, mjfrederick, I'm pointing out a fact of airplane life. The further forward you run the CG and the higher the wing incidence goes, the worse its going to dive when inverted. More than one person in the RCUniverse has worked out trim issues before.

Thanks,
Jim W.

(in reply to mjfrederick)
       Post #: 11

RE: Inverted flight, how quick should it fall? - 7/2/2008 3:13:50 PM   
flyncajun


 

Posts: 120
Joined: 12/25/2006
From: DENHAM SPRINGS , LA, USA
Status: offline 		WOW three responses
three different fixes ,,
how confusing can that be ?
Let us know what works Todd,
Bryan

(in reply to jim woodward)
       Post #: 12

RE: Inverted flight, how quick should it fall? - 7/2/2008 4:11:40 PM   
MHester



Posts: 2415
Joined: 10/27/2002
From: Woodstock, GA, USA
Status: offline
quote:

ORIGINAL: flyncajun

WOW three responses
three different fixes ,,
how confusing can that be ?
Let us know what works Todd,
Bryan


The microwave just beeped, popcorn at the ready....LOL

I'm interested to see what ends up being the fix as well. My advice would be the same as has already been offered here, so I'm in curious mode....I know that the actual design of the plane sometimes plays a LARGE part in this, but I have a hunch you already got the right advice.

-M


_____________________________

Owner Custom Airframes of America, Team YS, Mercury Adhesives

(in reply to flyncajun)
       Post #: 13

RE: Inverted flight, how quick should it fall? - 7/2/2008 5:20:23 PM   
flyncajun


 

Posts: 120
Joined: 12/25/2006
From: DENHAM SPRINGS , LA, USA
Status: offline 		Hey Mike,
the design has very little to do with Todd`s problem.
tail heavy is tail heavy
once you get too far back it will act nose heavy inverted and dive . thats why I want him to move the c/g forward one inch
inc. has nothing to do with this , the only thing that plays in to this problem could be airfoil thickness,
the thicker the airfoil the worse the effect on diving inverted at an aft c/g location.
I have felt what what Todd is discribing recently with an experiment
so I know his problem well.

As you know there aint much we have not run across helping every one else fix trim problems and working on our own.
The root problem is c/g more nose weight, elevators are more effective around nuetral,,
and tail heavy airplanes ,,, have no controlable effect around nuetral
they are just sensitive,sowe expo it out to make it "feel" good
I just fixed this problem recently Soooo,

Todd you have three ways of doing this ,,,it`s your move
let us know what happens
Bryan



(in reply to MHester)
       Post #: 14

RE: Inverted flight, how quick should it fall? - 7/2/2008 5:34:52 PM   
tschmidt