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Old Oct 09, 2012, 01:00 AM
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Ashford. Kent. England
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more on trimming... what I didn't mention..

With a more forward COG some "up" trim is often used to keep the model in level fight under power..
This we normally set on the elevator but do note that motor thrust lines can have the same effect
When you have up elevator trim, however slight it may be, as the model gets faster, usually when you add power and throttle up, this trim gets more effective and the model will climb ...
To reduce this ballooning/climb we alter the thrust line, more down thrust will cancel out the "up" trim.. and when you get the angle correct you wont get the model changing much from its level flight, it will just go faster..

Ballooning when turning into the wind is a result of speed, momentum and airflow.. Your model flies at a certain airspeed but it also has a ground speed... as it goes from cross wind to head on the momentum of the model tries to keep its ground speed constant ..so just for a while its airspeed is suddenly increased, now the airspeed is at ground speed plus the wind speed..the uptrim is more effective and the model balloons.. Then the model fairly quickly slows till the airspeed is back to where it normally flies at...
If you think about it, on a windy day your model will travel rapidly on its down wind leg, so will carry a lot of that speed around the turn.. and a heavier model will carry that speed better through the turn because it has a lot more momentum keeping it going
Momentum is what we use a lot when we do our aerobatics... some models require a dive to do a loop.. the dive builds the speed and the momentum , this is what carries the model through its maneuver
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Old Oct 09, 2012, 01:20 AM
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Ashford. Kent. England
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More? no...I will stop there, but be aware there are loads more factors that relate to trimming, such as the wing sections , wing and tail incidence angles. side thrust...and there are likely others I have missed..
So always do your research ,and please put me right where Ive got muddled

Whist messing this morning Ive knocked out a similar semi speed 500 model, heavily based on thomas's small version..
The lines were from what I remembered from his pictures its not identical, but not too shabby.
So I decided to start the build and, Two hours later I am at this stage of the build... all 6mm depron...KFm4.. simple direct laminations
Area 1.8sqft ..
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Old Oct 09, 2012, 05:57 AM
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Ashford. Kent. England
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I made a couple of changes as I progressed.. the model got a lower spline and the fins were brought forward a bit..
I have a tube mount let into the layers with zero angles for the thrust..

Still to do ...leading edge shaping...wing covering with tape..
The radio gear and battery will be sited on top of the wing in my usual fashion,.
I am not expecting too much speed as this wing is 18mm thick, being 3x6 mm layers, and having all the gear external will also add drag..But it should do ok as I have a small wizzy motor to use... time will tell
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Old Oct 09, 2012, 08:28 AM
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Germany, Nordrhein-Westfalen, Cologne
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Looks great, you got the style of the radius! Be careful with aileron throws, it should be very responsive. The c of g has to be quite forward, can`t tell you exactly because I am on holiday and the radius is at home. Will be back saturday then I can give you a % position.
Best
Thomas
Ps: found a photo on the laptop: the c of g is as far forward as the first black tape.
started at the more backward marking and that was a disaster.
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Old Oct 09, 2012, 10:02 AM
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nickchud's Avatar
Market Harborough
Joined Apr 2006
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thoughts on trimming

Just a little addition that might be useful..

Quote:
With a more forward COG some "up" trim is often used to keep the model in level fight under power..
I believe the Dive Test is used to get the COG right. For several reasons it's not a good idea to use trimming and thrust lines to correct for an incorrect COG. Get that right first.

So, trim for straight and level flight at half throttle. Then, when you cut the power and push the nose down, 45 deg say, the speed increases and the significance of any COG moment decreases. As a result, any up or down trim that has been used to correct for a forward or backward COG will become much more obvious. The motor has been cut so thrust line won't be a factor. Leave the elevator control alone (as long as it's safe) .

As Dave says, a little bit of forward COG is often used, it makes the plane more stable. If this is what you want, the trim for level flight should cause it to pull out of the dive. On the other hand, if you want a busy time and quick responses, the ideal COG will require some down trim for level flight, causing the dive test to get steeper.

Then have a look at the thrust line. Get downwind a good way to give yourself time; turn back upwind along the flightline; give it full throttle and hold it level. Then cut the throttle all the way. Does it suddenly climb up, or dive down? This will tell you if your elevator has been correcting for thrust. A bit of downthrust is usually wanted, so a bit of climb at this point would be normal. Not too much.

My humblest apologies to everyone who already knew all this stuff. And, if I've got anything wrong, please correct me.

That's my 2 penn'orth.
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Old Oct 09, 2012, 10:17 AM
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Thanks thomas...
Ive done my usual and made a glider at 9" span with the same profile.. adding weight till I got a flat but stable glide.... the scaled up COG comes in at 125mm from the front of the wing at the root... which is also spot on the KF step location at the root..
I suspect that this will be the sensitive spot on COG, so first flights will be slightly forward of that spot.
From the look of your picture that tape is about 1" in front of the COG I calculated... so its likely thats where I will start...
The motor going on is a blue wonder type 1700kv..I will be using 3 cells and a suitable small prop..
I will definately fly on low rated ailerons..small span models have a tendency to roll like mad...and I noted your roll rate in the video...
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Old Oct 09, 2012, 10:42 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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nickchud wrote:

"..I believe the Dive Test is used to get the COG right..."

The real question is probably what people want to view as being "COG right". The upper half of the image used at Polecat Aero CG location page summarizes it nicely. With the CG at neutral stability and no elevator trim, the plane continues the dive in a flat angle. With positive or negative stability that changes.

So you have it right as far as I can tell.

If you scroll down to the bottom half of that image it shows the pitch response to weak lift and link sink and what that will do to the plane. So there is some insight as to one of the subtle signals that glider pilots use in thermal hunting. It will be different for powered flight I guess but you might still see similar effects on the pitch.

Jack
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Old Oct 09, 2012, 11:18 AM
rcflyer
UK
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Quote:
Originally Posted by Thomas B. View Post
Hi Jim,
it is a new bat prototype, 84cm span and around 230 g with 2s lipo, M dihedral like the pterodactyl. Lighter wing loading and less power than the other ones, it is designed for small field flying.
It needs some further adjusting and finishing, but flies already well - made three flights in heavy wind conditions.
Thomas
Ok, will plans be up on Modell Avaitor when you have finished? I wouldn't mind having a go at that one seeing as the swift flies so nicely!

Cheers
Jim
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Old Oct 09, 2012, 11:29 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
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Quote:
the image used at Polecat Aero CG location page
Good link Jack... a picture speaks a thousand words.

And great planes Thomas.

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Old Oct 09, 2012, 04:55 PM
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United States, CO, Colorado Springs
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Quote:
Originally Posted by jackerbes View Post
nickchud wrote:

"..I believe the Dive Test is used to get the COG right..."

The real question is probably what people want to view as being "COG right". ... It will be different for powered flight I guess but you might still see similar effects on the pitch.
Look for a trimming article from a serious pattern pilot. As you mentioned, the Dive Test and other ideas from Polecat are specific to zero-thrust situations.

Depending on your prop diameter in relation to the control surfaces and the wing, you can get an effect very simlar to the power-off dive merely by increasing throttle. Also, since light airplanes with big props don't change speed much without changing throttle, the power-off dive test might be ignoring something important.

Being primarily a glider-guider, I don't know a lot about setting up a powered airplane, but I generally do the power-off dive test to get power-off trim and CG where I want it (that being neutral, so speed changes have a very small effect on attitude), and then adjust the thrust line until throttle has no effect on attitude, either.

I've found that I end up with a lot more side thrust and down thrust than I'd expect, which means either 1> this is a crummy method, or 2> I don't know what I should be expecting. Not sure which.

But back to my first suggestion-- the serious pattern pilots have a bunch of sayings and rules of thumb for setting up a neutral powered airplane that are unknown to me, so if you really want to understand it, I'd find one of them.
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Old Oct 09, 2012, 10:11 PM
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Thomas, what power system and prop did you use on the yellow swift? VERY nice looking plane! And great video of it flying...
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Old Oct 10, 2012, 04:16 AM
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Western Europe 52.7 N 8.9 W
Joined Jul 2007
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Quote:
Originally Posted by davereap View Post
Thanks thomas...
Ive done my usual and made a glider at 9" span with the same profile.. adding weight till I got a flat but stable glide.... the scaled up COG comes in at 125mm from the front of the wing at the root... which is also spot on the KF step location at the root..
I suspect that this will be the sensitive spot on COG, so first flights will be slightly forward of that spot.
From the look of your picture that tape is about 1" in front of the COG I calculated... so its likely thats where I will start...
The motor going on is a blue wonder type 1700kv..I will be using 3 cells and a suitable small prop..
I will definately fly on low rated ailerons..small span models have a tendency to roll like mad...and I noted your roll rate in the video...
Looking forward to your flight report(s) ......

will build one as soon as your happy with it .....
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Old Oct 10, 2012, 07:12 AM
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Germany, Nordrhein-Westfalen, Cologne
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Originally Posted by Flyboy Steve View Post
Thomas, what power system and prop did you use on the yellow swift? VERY nice looking plane! And great video of it flying...
Steve, the power system on the yellow one ( I call it radius):
Turnigy 2211 / 2300, 3s 850 mAh Lipo, 4,75x4,75 Graupner cam speed or 4,7x4,7 or 5x5 emc speed prop (no big difference here), 12 - 18 A esc.
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Old Oct 10, 2012, 07:15 AM
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Quote:
Originally Posted by rcjim11 View Post
Ok, will plans be up on Modell Avaitor when you have finished? I wouldn't mind having a go at that one seeing as the swift flies so nicely!

Cheers
Jim
Hi Jim, I will do the Radius first for Modell Aviator, but I will try to post drawings of the bat when I finished flight testing.
Any recommendations for simple and free drawing software? I am still working with pencil and paper.
Thomas
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Old Oct 10, 2012, 09:48 AM
rcflyer
UK
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Sounds good, what program do you normally use?
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