HobbyKing.com New Products Flash Sale
 
Thread Tools
Old Jan 12, 2003, 02:42 PM
Sailor
Guest
n/a Posts
Help with trim please???

Hi all,

I have an Alpha 160 (180??) and it flies OK but when trimmed out to fly
well, I noticed upon landing that there is a fair bit of down trim on the
elevator probably about 4 degrees

The CofG is already further forward than recommended by about 5mm but with
the elevator level with the tailplane it would just about stall.

Question - should I move the CofG even further forward (by shifting the
battery pack forward) or pack up the back of the wing - or the back of the
tailplane assembly(either way to achieve the elevator being level with the
tailplane)

Or am I just being picky i.e. does it matter if the elevator always has down
trim on it?????????

With down trim in it flies OK

All above references above are to when gliding - switching the motor on
really screws things up but I expected that!!!!!

Thanks for any help you guys are able to give me.

Please post responses to the group.

Cheers.





Old Jan 12, 2003, 03:42 PM
Dirtnap
Guest
n/a Posts
Re: Help with trim please???

When you power up, how are the flight characteristics screwed up.
It might help to decide.


"Sailor" <theyardarm@yahoo.co.uk> wrote in message
news:3e21bfab$0$234$cc9e4d1f@news.dial.pipex.com.. .
> Hi all,
>
> I have an Alpha 160 (180??) and it flies OK but when trimmed out to fly
> well, I noticed upon landing that there is a fair bit of down trim on the
> elevator probably about 4 degrees
>
> The CofG is already further forward than recommended by about 5mm but with
> the elevator level with the tailplane it would just about stall.
>
> Question - should I move the CofG even further forward (by shifting the
> battery pack forward) or pack up the back of the wing - or the back of the
> tailplane assembly(either way to achieve the elevator being level with the
> tailplane)
>
> Or am I just being picky i.e. does it matter if the elevator always has

down
> trim on it?????????
>
> With down trim in it flies OK
>
> All above references above are to when gliding - switching the motor on
> really screws things up but I expected that!!!!!
>
> Thanks for any help you guys are able to give me.
>
> Please post responses to the group.
>
> Cheers.
>
>
>
>
>



Old Jan 13, 2003, 05:32 AM
Sailor
Guest
n/a Posts
Re: Help with trim please???

When power is switched on it goes left and climbs hard - presumably because
the motor doesn't have enough downthrust and right thrust for the batteries
I am using - it is designed for 6 cells and I am using 8 so I half expected
this.


"Dirtnap" <dirtnap2001@hotmail.com> wrote in message
news:3bkU9.23615$nT1.485578@twister.kc.rr.com...
> When you power up, how are the flight characteristics screwed up.
> It might help to decide.
>
>
> "Sailor" <theyardarm@yahoo.co.uk> wrote in message
> news:3e21bfab$0$234$cc9e4d1f@news.dial.pipex.com.. .
> > Hi all,
> >
> > I have an Alpha 160 (180??) and it flies OK but when trimmed out to fly
> > well, I noticed upon landing that there is a fair bit of down trim on

the
> > elevator probably about 4 degrees
> >
> > The CofG is already further forward than recommended by about 5mm but

with
> > the elevator level with the tailplane it would just about stall.
> >
> > Question - should I move the CofG even further forward (by shifting the
> > battery pack forward) or pack up the back of the wing - or the back of

the
> > tailplane assembly(either way to achieve the elevator being level with

the
> > tailplane)
> >
> > Or am I just being picky i.e. does it matter if the elevator always has

> down
> > trim on it?????????
> >
> > With down trim in it flies OK
> >
> > All above references above are to when gliding - switching the motor on
> > really screws things up but I expected that!!!!!
> >
> > Thanks for any help you guys are able to give me.
> >
> > Please post responses to the group.
> >
> > Cheers.
> >
> >
> >
> >
> >

>
>



Old Jan 13, 2003, 01:12 PM
Dirtnap
Guest
n/a Posts
Re: Help with trim please???

OK. With that said, My first Idea is to decide if the incidence of the wing
is about right in relation to the fuse.
If it looks right then I would alter the angle of the stab.
Raise the leading edge of the horizontal stabilizer until when trimmed
for good glide, the elevator is at neutral.
Or...
If the wing looks to be a little too aggressive with it's angle of attack,
then raise the trailing edge of the wing.

I don't know if you were given a plan when you built the plane, I hope
you can refer to the side-view for the incidence of the wing and stab.

All this is said with the assumption that you have the CG at or near the
called out location.

Hope this helps,
John

P.S.
If I remember correctly, You have a glass fuse with a built-up wing.
Adding a little washout to the wing can reduce the effective angle of attack
(a little).
It helps with sudden stall problems too.
Might want to think about that as well.

It is always best to setup a plane to fly with trims at center, a flat
control surface goes through the
air more easily than one with an angle.
Therefore, you will (hopefully) increase your glide times.

Sorry if I am telling you things you already know,
I don't have any idea how much you have built and flown.
John


"Sailor" <theyardarm@yahoo.co.uk> wrote in message
news:3e22902f$0$228$cc9e4d1f@news.dial.pipex.com.. .
> When power is switched on it goes left and climbs hard - presumably

because
> the motor doesn't have enough downthrust and right thrust for the

batteries
> I am using - it is designed for 6 cells and I am using 8 so I half

expected
> this.
>
>
> "Dirtnap" <dirtnap2001@hotmail.com> wrote in message
> news:3bkU9.23615$nT1.485578@twister.kc.rr.com...
> > When you power up, how are the flight characteristics screwed up.
> > It might help to decide.
> >
> >
> > "Sailor" <theyardarm@yahoo.co.uk> wrote in message
> > news:3e21bfab$0$234$cc9e4d1f@news.dial.pipex.com.. .
> > > Hi all,
> > >
> > > I have an Alpha 160 (180??) and it flies OK but when trimmed out to

fly
> > > well, I noticed upon landing that there is a fair bit of down trim on

> the
> > > elevator probably about 4 degrees
> > >
> > > The CofG is already further forward than recommended by about 5mm but

> with
> > > the elevator level with the tailplane it would just about stall.
> > >
> > > Question - should I move the CofG even further forward (by shifting

the
> > > battery pack forward) or pack up the back of the wing - or the back of

> the
> > > tailplane assembly(either way to achieve the elevator being level with

> the
> > > tailplane)
> > >
> > > Or am I just being picky i.e. does it matter if the elevator always

has
> > down
> > > trim on it?????????
> > >
> > > With down trim in it flies OK
> > >
> > > All above references above are to when gliding - switching the motor

on
> > > really screws things up but I expected that!!!!!
> > >
> > > Thanks for any help you guys are able to give me.
> > >
> > > Please post responses to the group.
> > >
> > > Cheers.
> > >
> > >
> > >
> > >
> > >

> >
> >

>
>



Old Jan 14, 2003, 08:32 AM
Paolo Gelato
Guest
n/a Posts
Re: Help with trim please???

As john wrote, it seems to me that your problem is related to the difference
of the angle of attack between the wings and the stabilizer. There is
normally a slight angle to provide stability. But the greater the angle, the
more it will tend to nose-up when flying quicker than normal (i.e. when
running the engine), because at higher speeds the (usually) down thrust from
the stabilizer increases, which raises the nose, which in turn normally
makes the glider slow down.

I had a similar problem with a motorless glider, when performing high-speed
flight. Playing with the C.G. was useless (and somewhat dangerous), but
adjusting the angle of attack of the stabilizer solved it (I slightly raised
its leading edge). Make sure you proceed in small steps, or you might have
to rebuild it altogether... ;-)

John's remark about washout will not solve your particular problem, but it
has to be considered as "safe setting" for any conventional aircraft,
especially those which tend to snap-stall in a spin. The washout will make
the edges stall later, so the plane will start to sink *before* going out of
control, thus giving you the opportunity to speed up without spinning.

HTH

IceCream.


Old Jan 14, 2003, 01:22 PM
Dirtnap
Guest
n/a Posts
Re: Help with trim please???

I Agree Paolo,
Well stated.


Old Jan 14, 2003, 06:22 PM
Ray L. Minoke
Guest
n/a Posts
Re: Help with trim please???

"Paolo Gelato" <Paolo.DOT.Gelato@onlineautomation.DOT.ch> wrote:

>As john wrote, it seems to me that your problem is related to the difference
>of the angle of attack between the wings and the stabilizer. There is
>normally a slight angle to provide stability. But the greater the angle, the
>more it will tend to nose-up when flying quicker than normal (i.e. when
>running the engine), because at higher speeds the (usually) down thrust from
>the stabilizer increases, which raises the nose, which in turn normally
>makes the glider slow down.


A friend of mine has a Kranich (See http://www.nesail.com/kranich.html or
http://www.multiplexrc.com/kranich.htm) with a "flying stab". That is, the
entire horizontal stabilizer acts as the elevator. After reading your
explanation, it occurs to me that his arrangement may be superior to a
conventional tail for climbing under power. He always seems to be able to make
smooth controlled climbs while my Elegant (T-tail, fixed stab, see
http://www.hobby-lobby.com/elegant.htm) is difficult to hold steady under
power. I thought it was only a matter of flying skill, but maybe this is the
reason. Would you agree with that?
--
_____
\__________________|__________________/
(O)

Old Jan 14, 2003, 06:32 PM
Dirtnap
Guest
n/a Posts
Re: Help with trim please???

Yes that does make sense.
The full-flying tail is great but it must be setup carefully.
It is real easy to over-control with them.
Throws are critical.
FWIW


"Ray L. Minoke" <ray.l.minoke@GoFor21.com> wrote in message
news:3e2595cd.433017232@65.82.44.10...
> "Paolo Gelato" <Paolo.DOT.Gelato@onlineautomation.DOT.ch> wrote:
>
> >As john wrote, it seems to me that your problem is related to the

difference
> >of the angle of attack between the wings and the stabilizer. There is
> >normally a slight angle to provide stability. But the greater the angle,

the
> >more it will tend to nose-up when flying quicker than normal (i.e. when
> >running the engine), because at higher speeds the (usually) down thrust

from
> >the stabilizer increases, which raises the nose, which in turn normally
> >makes the glider slow down.

>
> A friend of mine has a Kranich (See http://www.nesail.com/kranich.html or
> http://www.multiplexrc.com/kranich.htm) with a "flying stab". That is, the
> entire horizontal stabilizer acts as the elevator. After reading your
> explanation, it occurs to me that his arrangement may be superior to a
> conventional tail for climbing under power. He always seems to be able to

make
> smooth controlled climbs while my Elegant (T-tail, fixed stab, see
> http://www.hobby-lobby.com/elegant.htm) is difficult to hold steady under
> power. I thought it was only a matter of flying skill, but maybe this is

the
> reason. Would you agree with that?
> --
> _____
> \__________________|__________________/
> (O)
>



Old Jan 15, 2003, 05:42 AM
IceCream
Guest
n/a Posts
Re: Help with trim please???

"Ray L. Minoke" <ray.l.minoke@GoFor21.com> a écrit dans le message news:
3e2595cd.433017232@65.82.44.10...
> "Paolo Gelato" <Paolo.DOT.Gelato@onlineautomation.DOT.ch> wrote:
>
> A friend of mine has a Kranich (See http://www.nesail.com/kranich.html or
> http://www.multiplexrc.com/kranich.htm) with a "flying stab". That is, the
> entire horizontal stabilizer acts as the elevator. After reading your
> explanation, it occurs to me that his arrangement may be superior to a
> conventional tail for climbing under power. He always seems to be able to

make
> smooth controlled climbs while my Elegant (T-tail, fixed stab, see
> http://www.hobby-lobby.com/elegant.htm) is difficult to hold steady under
> power. I thought it was only a matter of flying skill, but maybe this is

the
> reason. Would you agree with that?
> --


Yes, this may be the reason for the smooth flight. But (there is a *but*)
flying stabilizers are quite uncommon for the simple reason that it is more
difficult to obtain a safe aircraft (except with canard architectures, where
AFAIK flying stabilizers are always used).

The importance of this design appears when stalling. With flying stabs, the
CG is located *behind* the centre of lift. When the plane stalls, it then
tends to fall nose-up (and probably spin) and regaining flight speed may be
impossible (my grandfather died because of such a situation, back in 1935).
With conventional stabs, the CG is ahead of the centre of lift and the model
will dive when it stalls. Some plane designs with different profiles or
span/chord ratios for wings and stabs can fly, assuming that the stab's lift
decreases less than the wing's (proportionally). I have never made any tests
with flying stabs, but that's how I understand it.

Another chance to get a smooth flight with the engine on is what is used
with motor airplanes: angle the engine down a few degrees (2 to 5, depending
on the plane). You lose a little efficiency, but it is much safer than the
flying stab solution. I think this is the real reason for the Kranich's
superiority in that flight configuration.

Cheers

IceCream
Old Jan 15, 2003, 06:02 AM
Sailor
Guest
n/a Posts
Re: Help with trim please???

Thanks for your advices chaps - I will try the suggested mods and let you
know the results when the 130 kph wind dies down!!!!!!!!!!!!!!!


"IceCream" <icecream@fastnet.ch> wrote in message
news:548a4cbb.0301150238.1d5f2a1b@posting.google.c om...
> "Ray L. Minoke" <ray.l.minoke@GoFor21.com> a écrit dans le message news:
> 3e2595cd.433017232@65.82.44.10...
> > "Paolo Gelato" <Paolo.DOT.Gelato@onlineautomation.DOT.ch> wrote:
> >
> > A friend of mine has a Kranich (See http://www.nesail.com/kranich.html

or
> > http://www.multiplexrc.com/kranich.htm) with a "flying stab". That is,

the
> > entire horizontal stabilizer acts as the elevator. After reading your
> > explanation, it occurs to me that his arrangement may be superior to a
> > conventional tail for climbing under power. He always seems to be able

to
> make
> > smooth controlled climbs while my Elegant (T-tail, fixed stab, see
> > http://www.hobby-lobby.com/elegant.htm) is difficult to hold steady

under
> > power. I thought it was only a matter of flying skill, but maybe this is

> the
> > reason. Would you agree with that?
> > --

>
> Yes, this may be the reason for the smooth flight. But (there is a *but*)
> flying stabilizers are quite uncommon for the simple reason that it is

more
> difficult to obtain a safe aircraft (except with canard architectures,

where
> AFAIK flying stabilizers are always used).
>
> The importance of this design appears when stalling. With flying stabs,

the
> CG is located *behind* the centre of lift. When the plane stalls, it then
> tends to fall nose-up (and probably spin) and regaining flight speed may

be
> impossible (my grandfather died because of such a situation, back in

1935).
> With conventional stabs, the CG is ahead of the centre of lift and the

model
> will dive when it stalls. Some plane designs with different profiles or
> span/chord ratios for wings and stabs can fly, assuming that the stab's

lift
> decreases less than the wing's (proportionally). I have never made any

tests
> with flying stabs, but that's how I understand it.
>
> Another chance to get a smooth flight with the engine on is what is used
> with motor airplanes: angle the engine down a few degrees (2 to 5,

depending
> on the plane). You lose a little efficiency, but it is much safer than the
> flying stab solution. I think this is the real reason for the Kranich's
> superiority in that flight configuration.
>
> Cheers
>
> IceCream



Old Jan 15, 2003, 09:22 PM
Mitch Haley
Guest
n/a Posts
Re: Help with trim please???

IceCream wrote:
> The importance of this design appears when stalling. With flying stabs, the
> CG is located *behind* the centre of lift.


Are you referring to a one piece stab without a separate elevator, or are
you referring to a stab that generates lift in normal flight?
Mitch.
Old Jan 16, 2003, 03:12 AM
IceCream
Guest
n/a Posts
Re: Help with trim please???

Mitch Haley <mlh@voyager.net> wrote in message news:<3E2614AE.450C0C7C@voyager.net>...
> IceCream wrote:
> > The importance of this design appears when stalling. With flying stabs, the
> > CG is located *behind* the centre of lift.

>
> Are you referring to a one piece stab without a separate elevator, or are
> you referring to a stab that generates lift in normal flight?
> Mitch.

Oops (blush), you're right. I misunderstood Ray's post. HE wrote about a
one-piece stab, and I wrote about a stab which generates lift in normal
flight.

So, my (personal and revised) answer to Ray's post is: a one-piece stab
should behave similarly to a conventional one (fixed surface + moving part),
but it is much harder spotting the trim setting since there is no reference
(the fixed part which is missing). There is a difference, though, that is:
the one-piece stab is much more sensitive than a conventional one (usually),
as John wrote.

And for the problem mentioned, one may use a mixer which adds some nose-down
when the engine is switched on. Though this is not optimal, it helps a lot
(it is similar to adding down angle to the engine). Ideally, the down
command should be proportional to the air speed, not the engine speed, but
this cannot be done easily (need sensors and embarked electronics).

I wonder how Ray's Elegant behaves in the so-called "dive-test". I suspect
it will nose-up very quickly.

For those wo wonder what a "dive test"is, it consists in placing the plane
at a 45 degrees dive and letting the controls.
If the plane gently comes to a horizontal attitude, it's OK.
If it tends to dive more, it is nose-light (yes!), and if it tends ti climb
quickly, it is nose-heavy.
The explanation is related to what I explained in my post on the 14th. When
the plane dives, its speed increases, and the effect of the elevator
increases. With a nose-heavy plane, the elevator pushes down more than
required. This force increases with speed, but the weight in the nose does
not, which will tend to nose-up. Then, adjusting the CG is matter of
compromise between stability, speed range and personal taste.
When the CG is properly set up, if you have a conventional stab, you may see
that in neutral position the moving part is not aligned with the fixed part.
Now it's time to adjust the angle of attack of the fixed part (if possible).
One-piece stabs allow doing this by merely adjusting the lenght of the
linkages (here, they are definitely superior).

HTH

IceCream
Old Jan 16, 2003, 06:42 PM
Ray L. Minoke
Guest
n/a Posts
Re: Help with trim please???

icecream@fastnet.ch (IceCream) wrote:

>And for the problem mentioned, one may use a mixer which adds some nose-down
>when the engine is switched on. Though this is not optimal, it helps a lot
>(it is similar to adding down angle to the engine). Ideally, the down
>command should be proportional to the air speed, not the engine speed, but
>this cannot be done easily (need sensors and embarked electronics).


Presumably you meant motor when you wrote "engine".

As my friend with the Kranich pointed out to me, this can get you into trouble
if you have mixing on and your battery is weak or dead, particularly if you're
trying to abort a landing to go around again.
--
_____
\__________________|__________________/
(O)

Old Jan 17, 2003, 02:22 AM
IceCream
Guest
n/a Posts
Re: Help with trim please???

"Ray L. Minoke" <ray.l.minoke@GoFor21.com> a écrit dans le message news:
3e273f70.607524642@65.82.44.10...
> icecream@fastnet.ch (IceCream) wrote:
>
>
> Presumably you meant motor when you wrote "engine".
>


Yes (English is not my usual language...sorry <g>)

> As my friend with the Kranich pointed out to me, this can get you into

trouble
> if you have mixing on and your battery is weak or dead, particularly if

you're
> trying to abort a landing to go around again.


Therefore it's better to point the *motor* down a little, which does not
require a radio with mixers, and can be done with a few washers on the
mounting screws. Just make sure the propeller does not touch the fuselage
and folds easily.

IceCream
Old Jan 19, 2003, 11:32 AM
Registered User
Punta Gorda, FL
Joined Apr 2002
4,952 Posts
CG location affects the stability, the hands off glide speed and control sensitivity. The angle between the wing and tail affects glide speed and control response. Therefore:

1. The CG location should be used to adjust the stability. The dive test is useful here. See:
http://www.polecataero.com/
Go to articles and the article on CG by Dr. Mark Drela.

2. The angle between the wing and tail chord lines should be adjusted to get the desired glide speed.

3. The thrust line should be adjusted to minimize change in pitch attitude over the throttle range and to eliminate hands off turn.

4. The control surface throws should be adjusted to give the desired control response.

Because CG location affects so many things and because all but stability can be adjusted otherwise, CG location adjustment should be reserved for stability alone and set as the first priority.
Ollie is offline Find More Posts by Ollie
 


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
help with smart please bpu6999 High Performance 12 Jul 11, 2003 03:25 AM
help with Wingo please sbaug Foamies (Kits) 25 Mar 24, 2003 12:10 PM
Help with mixing please Zhack Electric Plane Talk 2 Aug 23, 2002 02:56 PM
Help with motor please rorywquin Electric Plane Talk 3 Jun 12, 2002 05:02 AM