Mechanics of Elevator Trim. In Detail.

Tina wrote:
> One point about the lift fairy sitting on the tail I'd like to
> understand is this -- actually a serious question. As I understand
> it, nearly aways the tail is exerting a downward force, since the
> center of lift is aft of the center of gravity on general aviation
> airplanes (that is true, isn't it -- that the cg is forward of the
> center of lift?). If so the tail really is imposing an increased load
> on the airplane, adding to its effective weight. The question I have
> is, how many pounds of weight is imposed aerodynamically for an
> airplane that might be loaded with its CG at the forward limit? I
> don't know where the center of lift is on ga airplanes -- a third of
> the way aft of the leading edge of the wing is an ok approximation,
> but a few inches error on an airplane weighing what ours does at max
> could make a huge change in the required force to overcome the nose
> heavy moment.

A rule of thumb is that the force on the horizontal tail
is 5 to 10 per cent of the wing lift. This translates
to a loss of 10 to 20 per cent of the raw gross lift
availbale from the horizontal airfoils.

> I'm obviously thinking about increased efficiency -- extra weight
> added because of either fat people, full fuel, or aerodynamically
> imposed, all cost horsepower (OK, watts for you purists) to move
> around.

This is the reason why modern military aircraft are designed
aerodynamically unstable, and the electronic gnomes of the
flight control system have to work all they can do.

The loss of gross lift is the proce to pay for simple and
safe longitudinal stability.

--

Tauno Voipio
tauno voipio (at) iki fi

On Jun 10, 1:09 pm, Tauno Voipio <tauno.voi...@INVALIDiki.fi> wrote:
> Tina wrote:
> > One point about the lift fairy sitting on the tail I'd like to
> > understand is this -- actually a serious question. As I understand
> > it, nearly aways the tail is exerting a downward force, since the
> > center of lift is aft of the center of gravity on general aviation
> > airplanes (that is true, isn't it -- that the cg is forward of the
> > center of lift?). If so the tail really is imposing an increased load
> > on the airplane, adding to its effective weight. The question I have
> > is, how many pounds of weight is imposed aerodynamically for an
> > airplane that might be loaded with its CG at the forward limit? I
> > don't know where the center of lift is on ga airplanes -- a third of
> > the way aft of the leading edge of the wing is an ok approximation,
> > but a few inches error on an airplane weighing what ours does at max
> > could make a huge change in the required force to overcome the nose
> > heavy moment.
>
> A rule of thumb is that the force on the horizontal tail
> is 5 to 10 per cent of the wing lift. This translates
> to a loss of 10 to 20 per cent of the raw gross lift
> availbale from the horizontal airfoils.
>
> > I'm obviously thinking about increased efficiency -- extra weight
> > added because of either fat people, full fuel, or aerodynamically
> > imposed, all cost horsepower (OK, watts for you purists) to move
> > around.
>
> This is the reason why modern military aircraft are designed
> aerodynamically unstable, and the electronic gnomes of the
> flight control system have to work all they can do.
>
> The loss of gross lift is the proce to pay for simple and
> safe longitudinal stability.
>
> --
>
> Tauno Voipio
> tauno voipio (at) iki fi

Thanks for the rule of thumb, Tauno. I have watched how busy the
flippers are on fighters when they are in the flare -- no human pilot
is working that hard for control. I knew the fighters are designed to
be aerodynamically unstable.

So the aerodynamic longitudinal stability the tail provides might
cost us 5 to 10%, The obvious question is, do canards buy back that
fraction? They would be offering positive lift, and if they stall
first would provide the same sort of longitudinal stability, wouldn't
they?


be

Tina wrote:
> On Jun 10, 1:09 pm, Tauno Voipio <tauno.voi...@INVALIDiki.fi> wrote:
>
>>Tina wrote:
>>
>>>One point about the lift fairy sitting on the tail I'd like to
>>>understand is this -- actually a serious question. As I understand
>>>it, nearly aways the tail is exerting a downward force, since the
>>>center of lift is aft of the center of gravity on general aviation
>>>airplanes (that is true, isn't it -- that the cg is forward of the
>>>center of lift?). If so the tail really is imposing an increased load
>>>on the airplane, adding to its effective weight. The question I have
>>>is, how many pounds of weight is imposed aerodynamically for an
>>>airplane that might be loaded with its CG at the forward limit? I
>>>don't know where the center of lift is on ga airplanes -- a third of
>>>the way aft of the leading edge of the wing is an ok approximation,
>>>but a few inches error on an airplane weighing what ours does at max
>>>could make a huge change in the required force to overcome the nose
>>>heavy moment.
>>
>>A rule of thumb is that the force on the horizontal tail
>>is 5 to 10 per cent of the wing lift. This translates
>>to a loss of 10 to 20 per cent of the raw gross lift
>>availbale from the horizontal airfoils.
>>
>>
>>>I'm obviously thinking about increased efficiency -- extra weight
>>>added because of either fat people, full fuel, or aerodynamically
>>>imposed, all cost horsepower (OK, watts for you purists) to move
>>>around.
>>
>>This is the reason why modern military aircraft are designed
>>aerodynamically unstable, and the electronic gnomes of the
>>flight control system have to work all they can do.
>>
>>The loss of gross lift is the proce to pay for simple and
>>safe longitudinal stability.
>>
>>--
>>
>>Tauno Voipio
>>tauno voipio (at) iki fi
>
>
> Thanks for the rule of thumb, Tauno. I have watched how busy the
> flippers are on fighters when they are in the flare -- no human pilot
> is working that hard for control. I knew the fighters are designed to
> be aerodynamically unstable.
>
> So the aerodynamic longitudinal stability the tail provides might
> cost us 5 to 10%, The obvious question is, do canards buy back that
> fraction? They would be offering positive lift, and if they stall
> first would provide the same sort of longitudinal stability, wouldn't
> they?

Yes - they do bring back some, and this is the reasoning behind
e.g. Rutan's Voyager,

The price is that the canard (front wing) has to stall first
unless you want to fall to ground in reverse when the thing
stalls. The rumours are that the canards are a PITA to land
nicely.

--

-Tauno

Thanks again. My intelligent but ignorant guess is designing canards
so that they stall first should not take a genius, but there may be
traps I don't see. The world is safe, though, since I don't design
airplane.

The landing issue you raised is pretty neat, since most of us --
especially Mooney drivers -- are careful about airspeed on final and
in the flare, and like to land with the wings almost stalled. But in
the case of a canard if that stalls first I think the airplane would
very enthusiastically want to pitch forward hard enough to bend the
nosewheel!

At least with the stabilizer still flying the nose might be able to be
put down more gently. You've provided some nice insights, thanks.

On Jun 10, 2:14 pm, Tauno Voipio <tauno.voi...@INVALIDiki.fi>
wrote:
> Tina wrote:
> > On Jun 10, 1:09 pm, Tauno Voipio <tauno.voi...@INVALIDiki.fi> wrote:
>
> >>Tina wrote:
>
> >>>One point about the lift fairy sitting on the tail I'd like to
> >>>understand is this -- actually a serious question. As I understand
> >>>it, nearly aways the tail is exerting a downward force, since theI
> >>>center of lift is aft of the center of gravity on general aviation
> >>>airplanes (that is true, isn't it -- that the cg is forward of the
> >>>center of lift?). If so the tail really is imposing an increased load
> >>>on the airplane, adding to its effective weight. The question I have
> >>>is, how many pounds of weight is imposed aerodynamically for an
> >>>airplane that might be loaded with its CG at the forward limit? I
> >>>don't know where the center of lift is on ga airplanes -- a third of
> >>>the way aft of the leading edge of the wing is an ok approximation,
> >>>but a few inches error on an airplane weighing what ours does at max
> >>>could make a huge change in the required force to overcome the nose
> >>>heavy moment.
>
> >>A rule of thumb is that the force on the horizontal tail
> >>is 5 to 10 per cent of the wing lift. This translates
> >>to a loss of 10 to 20 per cent of the raw gross lift
> >>availbale from the horizontal airfoils.
>
> >>>I'm obviously thinking about increased efficiency -- extra weight
> >>>added because of either fat people, full fuel, or aerodynamically
> >>>imposed, all cost horsepower (OK, watts for you purists) to move
> >>>around.
>
> >>This is the reason why modern military aircraft are designed
> >>aerodynamically unstable, and the electronic gnomes of the
> >>flight control system have to work all they can do.
>
> >>The loss of gross lift is the proce to pay for simple and
> >>safe longitudinal stability.
>
> >>--
>
> >>Tauno Voipio
> >>tauno voipio (at) iki fi
>
> > Thanks for the rule of thumb, Tauno. I have watched how busy the
> > flippers are on fighters when they are in the flare -- no human pilot
> > is working that hard for control. I knew the fighters are designed to
> > be aerodynamically unstable.
>
> > So the aerodynamic longitudinal stability the tail provides might
> > cost us 5 to 10%, The obvious question is, do canards buy back that
> > fraction? They would be offering positive lift, and if they stall
> > first would provide the same sort of longitudinal stability, wouldn't
> > they?
>
> Yes - they do bring back some, and this is the reasoning behind
> e.g. Rutan's Voyager,
>
> The price is that the canard (front wing) has to stall first
> unless you want to fall to ground in reverse when the thing
> stalls. The rumours are that the canards are a PITA to land
> nicely.
>
> --
>
> -Tauno

>
> Yes, the time my son spent with canard aircraft brought out all sorts
> of interesting information about canards and the history of trying to
> scale up Rutan's original concept Beech Starship. The smaller true
> canards like the Long-eze are pretty good aircraft. However there is
> a reason why we don't see large (six plus passenger) true canards.
> It's the relationship between CG, fuel load, payload and range.
> Apparently in the scale up process there is a point where it is no
> longer practical.
>
> Ron Kelley

Would it not also be true that passive stability is not as important
in larger modern airplanes? I would guess (again, I admit being
ignorant of the realities) that adding a 10% aerodynamically induced
increased load on bigger aircraft would be avoided for efficiency
reasons? It might be better to have enough fly by wire and computer
induced stability instead. I don't know enough about this stuff to
even find the back of an envelope, let alone do a calculation there.

Tina <tbaker27705@> wrote in news:54595019-a2b0-413e-aea8-
414ce92238c4@79g2000hsk..com:

>
>>
>> Yes, the time my son spent with canard aircraft brought out all sorts
>> of interesting information about canards and the history of trying to
>> scale up Rutan's original concept Beech Starship. The smaller true
>> canards like the Long-eze are pretty good aircraft. However there is
>> a reason why we don't see large (six plus passenger) true canards.
>> It's the relationship between CG, fuel load, payload and range.
>> Apparently in the scale up process there is a point where it is no
>> longer practical.
>>
>> Ron Kelley
>
> Would it not also be true that passive stability is not as important
> in larger modern airplanes? I would guess (again, I admit being
> ignorant of the realities) that adding a 10% aerodynamically induced
> increased load on bigger aircraft would be avoided for efficiency
> reasons? It might be better to have enough fly by wire and computer
> induced stability instead. I don't know enough about this stuff to
> even find the back of an envelope, let alone do a calculation there.
>

Yeah, that's one of the main reasons they're going for FBW. An aft cg also
improves buffet margins and allows a higher cruise altitude because of
that. Some airplanes pump fuel aft after takeoff to bring the CG well aft.
Even ones that aren't FBW. you have to have the autopilot engaged to
utilise this feature though.


Bertie

On Jun 10, 9:29 am, Tina <tbaker27...@> wrote:
> One point about the lift fairy sitting on the tail I'd like to
> understand is this -- actually a serious question. As I understand
> it, nearly aways the tail is exerting a downward force, since the
> center of lift is aft of the center of gravity on general aviation
> airplanes (that is true, isn't it -- that the cg is forward of the
> center of lift?). If so the tail really is imposing an increased load
> on the airplane, adding to its effective weight. The question I have
> is, how many pounds of weight is imposed aerodynamically for an
> airplane that might be loaded with its CG at the forward limit? I
> don't know where the center of lift is on ga airplanes -- a third of
> the way aft of the leading edge of the wing is an ok approximation,
> but a few inches error on an airplane weighing what ours does at max
> could make a huge change in the required force to overcome the nose
> heavy moment.

CG range for most typical lightplane airfoils is 25 to 33%
of the chord, while the centre of lift is around the 40% mark. The
load on the stab/elevator isn't all that big, but it's enough that
we'll teach you in groundschool that the aircraft's stall speed is
lower when loaded to the aft limit than when it's loaded to the
forward limit, and that the cruise speed is a little better at the aft
limit.

Dan

On Jun 15, 6:34 pm, "Ken S. Tucker" <dynam...@vianet.on.ca> wrote:
> I concur with Dan on his last two posts, yeah that's
> rare, but anyway...
> I designed and tested (models) of a fantastic plane,
> but when I chose between putting my wife and kids
> in my fantastic plane or into a proven (safe) C172,
> I chose the C172.
> Here's why: If my machine cracked up due to a fault
> in my design, and killed my family except for me, I'd
> feel obligated to shoot myself, though I wouldn't.
>
> That said, build your machine, put it threw it's paces
> then take on a passenger, who knows what the tag
> "EXPERMENTAL" means on the side of the A/C,
> and have fun.
> Ken-

Perhaps there will not be time in my life to see a design realized,
but if I were so fortunate, I would probably do just that...get in it
myself first.

But before doing that, I would let it fly itself over a desert, since
controls would be software anyway.

-Le Chaud Lapin-

On Jun 15, 9:16 pm, Le Chaud Lapin <jaibudu...@> wrote:
> On Jun 15, 6:34 pm, "Ken S. Tucker" <dynam...@vianet.on.ca> wrote:
>
> > I concur with Dan on his last two posts, yeah that's
> > rare, but anyway...
> > I designed and tested (models) of a fantastic plane,
> > but when I chose between putting my wife and kids
> > in my fantastic plane or into a proven (safe) C172,
> > I chose the C172.
> > Here's why: If my machine cracked up due to a fault
> > in my design, and killed my family except for me, I'd
> > feel obligated to shoot myself, though I wouldn't.
>
> > That said, build your machine, put it threw it's paces
> > then take on a passenger, who knows what the tag
> > "EXPERMENTAL" means on the side of the A/C,
> > and have fun.
> > Ken-
>
> Perhaps there will not be time in my life to see a design realized,
> but if I were so fortunate, I would probably do just that...get in it
> myself first.
> But before doing that, I would let it fly itself over a desert, since
> controls would be software anyway.

That sounds like a good idea.
A 1/4 scale is good, it can be powered by a cheap
chainsaw motor.
Do you have any general ideas for a lay-out yet?

> -Le Chaud Lapin-

If you lived close by, I'd give you some help.
Ken

On Jun 16, 10:08 am, "Ken S. Tucker" <dynam...@vianet.on.ca> wrote:
> On Jun 15, 9:16 pm, Le Chaud Lapin <jaibudu...@> wrote:
>
>
>
>
>
> > On Jun 15, 6:34 pm, "Ken S. Tucker" <dynam...@vianet.on.ca> wrote:
>
> > > I concur with Dan on his last two posts, yeah that's
> > > rare, but anyway...
> > > I designed and tested (models) of a fantastic plane,
> > > but when I chose between putting my wife and kids
> > > in my fantastic plane or into a proven (safe) C172,
> > > I chose the C172.
> > > Here's why: If my machine cracked up due to a fault
> > > in my design, and killed my family except for me, I'd
> > > feel obligated to shoot myself, though I wouldn't.
>
> > > That said, build your machine, put it threw it's paces
> > > then take on a passenger, who knows what the tag
> > > "EXPERMENTAL" means on the side of the A/C,
> > > and have fun.
> > > Ken-
>
> > Perhaps there will not be time in my life to see a design realized,
> > but if I were so fortunate, I would probably do just that...get in it
> > myself first.
> > But before doing that, I would let it fly itself over a desert, since
> > controls would be software anyway.
>
> That sounds like a good idea.
> A 1/4 scale is good, it can be powered by a cheap
> chainsaw motor.
> Do you have any general ideas for a lay-out yet?
>
> > -Le Chaud Lapin-
>
> If you lived close by, I'd give you some help.

I live in Austin, Texas.

Some locals and I have been toying around with the idea of renting a
garage for experiments, though I think that is premature. I would
rather use something like SolidWorks to create a model to verify that
the aircraft would fly first. I do have a vague vision of the form-
factor of the aircraft, but as I see it, there are two crucial things
that need to be determined before putting both feet in the water:

1. The PAV I envision relies on an unproven, unorthodox hypothesis of
the origin of lift. I say concept because there is no weird science
involved like anti-gravity machines or anything like that, but if it
were to fly, it would require a reaxmination of the prevailing
theories. This is the hardest part, and I have been concentrating
only on the lift elements. A lot of math and a prototype of certain
control surfaces would help.

2. The second problem is a problem that would be faced by any designer
of a PAV, and that is the power source. The PAV I envision would have
an extremely high reliance on electrial power (the lift engine itself
would rely on electrical power), and this is a hard problem.
Conventional fuels, in 2008, still appear to be the most pratical
approach, even if the fuel is only used to operate a generator.

As far as the engine, I cannot say what it is yet, but can say that it
doesn't use an ICE. ;)

-Le Chaud Lapin-