Continuation of rational of trimming
02-15-2011, 03:48 AM
#26
Thread Starter
RE: Continuation of rational of trimming
ORIGINAL: BMatthews
But a 25% stability margin is a GROSS amount. Even trainer designs only use about 10 to 12% stability margin. Sport and competition aerobatic models use more like 0 to 5%.
But a 25% stability margin is a GROSS amount. Even trainer designs only use about 10 to 12% stability margin. Sport and competition aerobatic models use more like 0 to 5%.
1 A SM of 20% will need a stab that is app 11.25-11.5 half span. Gives a TVC of 0,61
2 A SM of app 5% will need a stab that is only 5 inches half span. Gives a TVC of 0,265
Both calc at 25% CG
I would suggest if one want to push CG aft, i would re-design stab to maintain a level of stability.
Best Regards
The stabs measures as in previous posts. The figure above is for illustrations only.
02-15-2011, 12:10 PM
#27
Join Date: Oct 2002
Location: Chilliwack, BC, CANADA
Posts: 12,425
Likes: 0
Received 22 Likes
on
19 Posts
RE: Continuation of rational of trimming
Well, you guys are confirming what I've suspected all along since I first learned way back when about what goes into designing pattern models. And that is that there's way too much going on and anyone designing their first model can't really expect to come up with a world beating design on the first go. Today's mixing radios can certainly help but in the end the model has to have the right stuff as well. And there's a huge amount more to a world class precision aerobatic model than first meets the eye and calculator.
06-07-2011, 05:34 AM
#32
RE: Continuation of rational of trimming
ORIGINAL: rmh
Having looked at this model -on line - I see nothing extraordinary about it
That is - looks like it should trim out fine as long as power is correct -and power makes a BIG difference.
On models such as these - I never used a bubble degree referrence for a simple reason
Too much error in these instruments as a matter of general findings. This is due to minute errors in LE and TE
Rather - I used a flat referrence plate (perfectly flat table)
Then I supported the model to level fore aft and side to side
Once I established this and a referrence line thru a zero thrust line , I had referrence for any adjusting which may be needed .
One degree is 1/4 '' in one foot -which means - any error on a measuring level can make a sizeable error.
Measurements always went from table UP to given points on the model-using a vertical, adjustable pointer which could be slid around to compare points anywhere
Further -when all is completed -I typically had zero -zero zero OR a measurable wing angle incidence of + 1/16'', Next I setup for 25%-27%% MAAC and flew the thing
This is not a secret procedure - it is very common as it gives a starting point.
The more relative angles you put in -the more compromises you will produce
Some flyers use little or no engine thrust deviance.
At cruise speed ( desired level ''hands off'') level flight speed -Typically THIS setup produced a slightly
nose heavy '' setup
again very common. and should be expected
At this point , common procedure is to shift CG to no more than 30% and note how the model changes by doing only cg changes
The amount of exponential in the radio and throw of surfaces can totally confuse this part of setup
Ideally upright (hands off) to inverted flight will result in a predictable slight down pressure on the elevator
These are the most important stages of a setup -in my book
The amount of thrust line change will depend on POWER and your habits
There is no magic setup which will yield hands off in any attitude IF speed remains the same .
Many guys I have designed for and flown with, do the setups in a similiar manner - No secret trim formula.
Having looked at this model -on line - I see nothing extraordinary about it
That is - looks like it should trim out fine as long as power is correct -and power makes a BIG difference.
On models such as these - I never used a bubble degree referrence for a simple reason
Too much error in these instruments as a matter of general findings. This is due to minute errors in LE and TE
Rather - I used a flat referrence plate (perfectly flat table)
Then I supported the model to level fore aft and side to side
Once I established this and a referrence line thru a zero thrust line , I had referrence for any adjusting which may be needed .
One degree is 1/4 '' in one foot -which means - any error on a measuring level can make a sizeable error.
Measurements always went from table UP to given points on the model-using a vertical, adjustable pointer which could be slid around to compare points anywhere
Further -when all is completed -I typically had zero -zero zero OR a measurable wing angle incidence of + 1/16'', Next I setup for 25%-27%% MAAC and flew the thing
This is not a secret procedure - it is very common as it gives a starting point.
The more relative angles you put in -the more compromises you will produce
Some flyers use little or no engine thrust deviance.
At cruise speed ( desired level ''hands off'') level flight speed -Typically THIS setup produced a slightly
nose heavy '' setup
again very common. and should be expected
At this point , common procedure is to shift CG to no more than 30% and note how the model changes by doing only cg changes
The amount of exponential in the radio and throw of surfaces can totally confuse this part of setup
Ideally upright (hands off) to inverted flight will result in a predictable slight down pressure on the elevator
These are the most important stages of a setup -in my book
The amount of thrust line change will depend on POWER and your habits
There is no magic setup which will yield hands off in any attitude IF speed remains the same .
Many guys I have designed for and flown with, do the setups in a similiar manner - No secret trim formula.
The reason I did things this way is quite simple :
You have to know whre you are FIRST- then pick away at any necessary change- however you choose.
BTW 1/16" (or 1/4degree) was done to allow for building error in wing
Why?
I found (after sending out hundreds of kits) people often change the wood thickness or change contour of part -for strength of personal taste,etc.
therefor - chances were - built to instructions, the wing would set zero or a tad positive to stab and engine
So -start from exact zero zero zero- - fly it and trim in necessary deflections
Then adjust for best handling
a fast model such as most pattern planes are, will respond in pitch NOTICABLY-to a couple of clicks of aileron or elevator trim - a few more clicks of rudder will also give a visual of yaw
The slower the plane -the greater the necessary changes (obvious)
The very latest "pattern"planes I have seen are straighter than many were years back.
why?
computer aided constructions and layups are VERY accurate.
The actual designs?
lateral area to horizontal (wing) area is quite different giving better roll /point etc..
all part of an evolutionary -not revolutionary design change.
This will continue to change -as more of these become electric powered.
