SMALL - Telemetry SMALL - Radio
Thread Tools
Old Apr 15, 2015, 12:42 PM
vespa is offline
Find More Posts by vespa
El picudo del agave
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,712 Posts
Trim and c.g. are fundamentally different and completely unrelated settings. Setting the trim by moving the c.g. will only result in increased drag and reduced handling qualities.
vespa is offline Find More Posts by vespa
Reply With Quote
Sign up now
to remove ads between posts
Old Apr 15, 2015, 12:49 PM
savneo is online now
Find More Posts by savneo
Registered User
savneo's Avatar
Cyprus
Joined Mar 2012
201 Posts
Quote:
Originally Posted by vespa View Post
Trim and c.g. are fundamentally different and completely unrelated settings. Setting the trim by moving the c.g. will only result in increased drag and reduced handling qualities.
Maybe I explained my self wrong, I will try again I fine tune the cg so that the elevator is dead straight to minimise the drag.
savneo is online now Find More Posts by savneo
Reply With Quote
Old Apr 15, 2015, 12:52 PM
johnnyrocco123 is online now
Find More Posts by johnnyrocco123
You down with EPP?
johnnyrocco123's Avatar
Houston, Tx
Joined May 2009
6,928 Posts
CG on the ground is one thing

But the only way to find out how accurate it is would be to test it in the air.


If you trim to straight and level, but inverted it heads towards the ground at a pretty good rate, then it is nose heavy.


I cant tell you how many times I have went with recommended CG and it has been totally off. IT typically is a starting point, where on these planes it becomes way more important.


I have never flown inverted at 200mph lol I meant a throttle burst on a 45 degree up line and the n roll inverted.


This is how I used to check my VIP-1, Flash 2, Viper, Sunracer........

Its safe if you aren't reckless. Just needed a recommended starting point.
johnnyrocco123 is online now Find More Posts by johnnyrocco123
Reply With Quote
Old Apr 15, 2015, 01:32 PM
feefo is offline
Find More Posts by feefo
Registered User
feefo's Avatar
Staffs, UK
Joined Apr 2009
3,786 Posts
I can't remember the thread or who made the post (slope section IIRC, maybe it was Vespa? Might have been a different forum though....) but someone pointed out that flying inverted isn't a great test method unless the wing is symmetrical, therefore you might find that you still need to give some 'down' input to maintain level when inverted.

I've used the inverted method but tend to go with the dive test these days.
feefo is offline Find More Posts by feefo
Reply With Quote
Old Apr 15, 2015, 01:58 PM
vespa is offline
Find More Posts by vespa
El picudo del agave
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,712 Posts
Sav, my point was that moving the c.g. away from ideal in order to make the elevator straight will generally result in more drag, not less. Trim essentially just changes the camber of the airfoil and has a miniscule drag penalty for reasonable deflections. Moving the c.g. changes the loading of the tail which causes much larger changes in drag. It's possible that you could move the c.g. such that tail loading and drag are reduced, but most of these tails are negatively loaded so you would need to move the c.g. aft to see a drag reduction. And since we generally fly with a very small static margin, it's unlikely that you could move the c.g. far enough aft to effect a trim change without significant handling changes.
vespa is offline Find More Posts by vespa
Reply With Quote
Old Apr 15, 2015, 01:59 PM
johnnyrocco123 is online now
Find More Posts by johnnyrocco123
You down with EPP?
johnnyrocco123's Avatar
Houston, Tx
Joined May 2009
6,928 Posts
Quote:
Originally Posted by feefo View Post
I can't remember the thread or who made the post (slope section IIRC, maybe it was Vespa? Might have been a different forum though....) but someone pointed out that flying inverted isn't a great test method unless the wing is symmetrical, therefore you might find that you still need to give some 'down' input to maintain level when inverted.

I've used the inverted method but tend to go with the dive test these days.
Yeah, I was thinking dive test might work too.


It's just that the cg on these models when slightly tweaked changes the flight characteristics so much.

I know I want it nose heavy but not too nose heavy. I know I don't want it neutral


Maybe someone who is very experienced or has competed in F5D class could chime in for best practice with finding desired CG
johnnyrocco123 is online now Find More Posts by johnnyrocco123
Reply With Quote
Old Apr 15, 2015, 02:11 PM
savneo is online now
Find More Posts by savneo
Registered User
savneo's Avatar
Cyprus
Joined Mar 2012
201 Posts
Quote:
Originally Posted by vespa View Post
Sav, my point was that moving the c.g. away from ideal in order to make the elevator straight will generally result in more drag, not less. Trim essentially just changes the camber of the airfoil and has a miniscule drag penalty for reasonable deflections. Moving the c.g. changes the loading of the tail which causes much larger changes in drag. It's possible that you could move the c.g. such that tail loading and drag are reduced, but most of these tails are negatively loaded so you would need to move the c.g. aft to see a drag reduction. And since we generally fly with a very small static margin, it's unlikely that you could move the c.g. far enough aft to effect a trim change without significant handling changes.
I agree, I did not mention in my privious post that I always stay within manufacture CG recommendation.
savneo is online now Find More Posts by savneo
Reply With Quote
Old Apr 15, 2015, 02:45 PM
Mreyello is offline
Find More Posts by Mreyello
Registered User
Mreyello's Avatar
Deutschland, RP, Niederzissen
Joined Dec 2011
933 Posts
CG is the most important setup parameter for f5d planes.
As the most common so called F5d airframes are not used for competition the manufacturers recommendation are usually useless for the competition task.....

CG has to be adapted to personal preference,flying style.auw .etc.....every single change requires to adapt throws and incidence......

I personally tend to move CG backwards (with adapted incidence and adapted throws ) until i feel uncomfortable with stability at the straights..,, than probably move CG 1mm forward.

The final indicator for me to be close to the target is when plane does fly straigt in level with or whithout power (thrust angles all zero).

Yello
Mreyello is offline Find More Posts by Mreyello
Reply With Quote
Old Apr 17, 2015, 05:08 PM
lokomimu is offline
Find More Posts by lokomimu
Registered User
Joined Dec 2008
24 Posts
Swist F5D with dead battery. (4 min 41 sec)
lokomimu is offline Find More Posts by lokomimu
Reply With Quote
Old Apr 18, 2015, 10:57 AM
Darron is offline
Find More Posts by Darron
Registered User
Darron's Avatar
London
Joined Jul 2003
903 Posts
Quote:
Originally Posted by johnnyrocco123 View Post
Maybe someone who is very experienced or has competed in F5D class could chime in for best practice with finding desired CG
Apologies I've not read your earlier posts so not sure on what the issue is.

Competition pilots are not interested in trim with the power off. Dive test would be pointless.

Power off flying would need some up trim of course. You can't expect to fly the same trim power on/off.

CG will be personal taste, normally as far back as is comfortable.
But the changes will be tiny like 1mm makes a difference.
There are also other reasons to move the cg like flying straight while on knife edge.
Darron is offline Find More Posts by Darron
Reply With Quote


Thread Tools