poor lateral control on a slow tow?

At 23:51 05 January 2011, ProfChrisReed wrote:
I'm still not convinced by those who propose that the wings of the
glider generate no extra lift (or even generate less lift) when
climbing on tow.

We know that on a winch launch the glider climbs because the wings
generate more lift than in level/descending flight. This must be true
because there is nothing pulling it up.

However, we are told that on aerotow the wings generate the same (or
less) lift as in level/descending flight and the tug just pulls the
glider up the slope.

Does this mean that the tug climbs in the same way, i.e. wings
generate only enough lift to carry the weight of the tug, and the prop
drags the tug up the slope? This doesn't match what I've read about
how aircraft work. L=W only in level flight. I think the tug's wings
generate more lift than its weight, and thus it climbs.

If this is true, the same must be true for the glider behind it.

Bring on an aerodynamicist to show me I'm wrong.


Chris

Yes, it does mean the tug climbs in the same way - the prop is dragging it
uphill.

In steady climb or descent the lift is actually slightly less than weight,
with the remaning very small component of the weight balanced by the drag
(in a glide) or the thrust (in a climb).

This can be demonstrated very easily with a vector diagram of the forces
acting on an aircraft, and you'll find this picture in every textbook on
aircraft performance or flight dynamics.

If you'd like to talk to an aerodynamicist, there are several
contributing to this thread - but more locally Dr Graham Dorrington in the
engineering department at Queen Mary has worked on an electric-powered
sailplane

http://www.flightglobal.com/articles...ane-flies.html

and should be able to convince you.

Doug

(aerodynamicst & flight dynamicist at City University)

At 05:22 06 January 2011, Eric Greenwell wrote:
On 1/5/2011 10:52 AM, Andreas Maurer wrote:
On Wed, 5 Jan 2011 09:23:29 -0800 (PST), Derek C
wrote:

Gliders appear to get near to the stall during slow aerotows at much
greater than their normal free flight stalling airspeeds. I would
suggest that aerotowing must increase the wing loading in some way.

I have to admit that I didn't bother to read all the 120+ postings
about this topic, so please forgive me if the things that I'm going
to
post have already been mentioned in this thread.


The main factor for the seemingly odd flying characteristics behind
the tow plane is the downwash of the latter.


Let me explain:
The downwash has a significant angle (the air is deflected downwards
behind the tow plane's wing to up to four degrees!), but due to the
larger span of the glider it only affects the inner part of the
glider's wing.

(big snip)

Andreas' posting was the clearest description for me of the wake effect.

I'd love to see "3-D" perspective view of the wake behind a towplane,
as
I doubt I'm visualizing it well.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

someone else posted this link before - there's a good picture half way
down of the tip vortex behind a typical towplane

http://www.sciencebuddies.org/scienc...Zoo_p057.shtml

At 03:57 06 January 2011, Derek C wrote:
On Jan 6, 2:20=A0am, Andreas Maurer wrote:
On Wed, 5 Jan 2011 15:09:39 -0800 (PST), Derek C

wrote:

Hi Derek,

The two most scary aerotows I have ever had we

1) 2 up in a K13 behind a Rotax engined Falke at about 50 knots
indicated airspeed

Well... 50 kts is pretty slow...

2) 2 up in a K13 behind a 150hp Piper Cub when we visited another
site. This tug wasn't very powerful anyway and its pilot seemed to
be
trying to demonstrate how slowly he could fly. Indicated airspeed
slightly under 50 knots.

Clear case: Low aspect ratio, wing loading twice of the ASK-13. How
much above the stall speed of the Cub? 10 kts at maximum? Scary...

I guess you had a word with the tow pilot afterwards.

In both cases the glider wallowed about and it seemed very difficult
to keep above the wake turbulence/prop wash.

Yes, the typical situation for a very slow aerotow.

I have not been towed by a Dimona or Katana, but they seem to be a
bit
faster than the above, so may not give the same problems. I think the
problem is more lack of airspeed than the type of the tug aircraft.

Well, I guess we both agree that this problem only manifests itself at
the low-speed area of the envelope, don't we?

The general consensus here in Germany (as well as my own experience)
is that an aerotow behind a motorglider is *much* easier to control
despite the fact that it takes place at 110-115 kph (60-63 kts)
instead of the 130-140 kph (70-75 kts) =A0that are typical for Morane
MS
893 and Robin Remorqeur.
Of course similar wing loadings result in similar reactions to gusts,
which helps to follow the tow plane.

Regards
Andreas

Yes, but 50 knots (93kph) should be OK for a K13 when its stalling
speed in free flight is only about 36 knots (67kph). But it clearly
isn't! Therefore either the wing loading or the angle of attack (or
both) must be greater during an aerotow than in free flight.

Cheers,
Derek C


... or the distribution of lift over the wing has changed ...

At 23:51 05 January 2011, ProfChrisReed wrote:
I'm still not convinced by those who propose that the wings of the
glider generate no extra lift (or even generate less lift) when
climbing on tow.

We know that on a winch launch the glider climbs because the wings
generate more lift than in level/descending flight. This must be true
because there is nothing pulling it up.

However, we are told that on aerotow the wings generate the same (or
less) lift as in level/descending flight and the tug just pulls the
glider up the slope.

Does this mean that the tug climbs in the same way, i.e. wings
generate only enough lift to carry the weight of the tug, and the prop
drags the tug up the slope? This doesn't match what I've read about
how aircraft work. L=W only in level flight. I think the tug's wings
generate more lift than its weight, and thus it climbs.

If this is true, the same must be true for the glider behind it.

Bring on an aerodynamicist to show me I'm wrong.


A winch launch is very different because (a) the angle between the cable
and the direction of motion of the glider is large, and therefore unlike a
tow the downwards component of the cable tension is no longer negligible,
and (b) the motion is not steady.

In this case the lift is greater than the weight because it is partially
counteracting the cable tension and weight. The precise balance depends
on pilot and winch driver technique. Even so, it is still the forward
component of the cable tension force that is doing the work required to
raise the glider to its release height.

The one (illegal in the UK I believe?) situation in which the glider wing
lift is doing the work would be a kiting launch - and here you are in fact
extracting the energy required from the wind ... something that a number of
researchers are trying to do

http://www.ampyxpower.com/PowerPlane.html

At 18:52 05 January 2011, Andreas Maurer wrote:
On Wed, 5 Jan 2011 09:23:29 -0800 (PST), Derek C
wrote:

Gliders appear to get near to the stall during slow aerotows at much
greater than their normal free flight stalling airspeeds. I would
suggest that aerotowing must increase the wing loading in some way.

I have to admit that I didn't bother to read all the 120+ postings
about this topic, so please forgive me if the things that I'm going to
post have already been mentioned in this thread.


The main factor for the seemingly odd flying characteristics behind
the tow plane is the downwash of the latter.


Let me explain:
The downwash has a significant angle (the air is deflected downwards
behind the tow plane's wing to up to four degrees!), but due to the
larger span of the glider it only affects the inner part of the
glider's wing.

Therefore, if the glider if lying laterally displaced, only one wing
is affected by the downwash of the tow plane - four degrees of AoA
difference between left and right wing need a lot of aileron to
correct.

Likeise, if the glider is flying straight behind the tow plane, the
downwash *decreases* the AoA of the affected inner part of the wing.
Getting the nose up by pulling back will restore the lift of the inner
part of the glider's wing, but now the outer parts of the wing have a
much higher AoA than they have in free flight.
Voila, meet the the conditions for poor alieron efficiency (high AoA!)
and tip stall.


The downwash is reduced by
- wingloading of the tow plane
- wing span of the tow plane

In other words: The more a tow plane looks like a motorglider (say, a
Dimona, or Katana Extreme), the less the flight characteristics of the
glider are affected.
Anyone who has ever been towed behind a motorglider or a microlight
will testify that problems like poor lateral control or running out
of elevator don't exist there, despite a far slower tow (55 kts
compared to a typical 70-75 kts behind a typical tow plane like
Reorqeur or Pawnee).


One interesting fact:
When Akaflieg Braunschweig flight-tested their SB-13 flying wing (with
a back-swept wing), they encountered a nose-down momentum after
lift-off that could not be recovered and usually lead to a crash
immediately after lift-off.

Explanation:
The downwash of the tow plane (Robin Remorqeur) hit the inner part of
the wing, decreasing its AoA (and lift) and therefore shifting the
center of lift backwards due to the sweepback.

Increasing the length of the tow rope helped.



Greetings from a snowy Germany
Andreas

Interesting experience with the SB-13.

There's a chapter in Eric Brown's book 'Wings of the Weird &
Wonderful' in which he describes flight tests of the GAL 56 flying wing
glider in 1946. This was a 28deg swept wing with an aspect ratio of 5.8
towed by a Spitfire IX* (!!!) to 20000ft (!!).

He describes the opposite effect, with a very strong (often
uncontrollable) nose-up pitch on take-off - this was thought to be due to
ground effect. In this case the tug span was similar (37ft) to the glider
span (45ft), so the wake/wing interaction would be different.

Interestingly he also reports that the GAL56 could be flown hands-free on
the tow - unless the tug slipstream was entered, in which case all lateral
and longitudinal control was lost. Robert Kronfield was later killed
spinning this aircraft.

On Jan 6, 9:50*pm, Doug Greenwell wrote:
A winch launch is very different because (a) the angle between the cable
and the direction of motion of the glider is large, and therefore unlike a
tow the downwards component of the cable tension is no longer negligible,
and (b) the motion is not steady. *

In this case the lift is greater than the weight because it is partially
counteracting the cable tension and weight. *The precise balance depends
on pilot and winch driver technique. *Even so, it is still the forward
component of the cable tension force that is doing the work required to
raise the glider to its release height. *

It's pretty easy to show that in the early part of full climb on a
winch launch when the cable is horizontal and neglecting cable weight:

tension in the cable = glider weight * tan(climb angle)

lift required from the wings = glider weight / cos(climb angle)

The latter is identical to the lift required in a turn with the same
bank angle as the winch launch climb angle.

Thus at 45 degrees the tension is the same as the weight and the lift
is 1.4
At 60 degrees the tension is 1.73 times the weight and the lift is
twice.

When using a tension-controlled winch, what tensions are actually
used?

On Jan 5, 8:45*pm, Andreas Maurer wrote:
On Wed, 5 Jan 2011 15:51:05 -0800 (PST), ProfChrisReed

wrote:
L=W only in level flight.

Wrong.

The correct term is

L=W in STRAIGHT flight.

If both forces are not equal, the resulting flightpath is going to be
a circular arc.

It's simple vector addition.

Regards
Andreas

Well to get anal......straight and level.....(unaccelerated flight.)

Also best to consider L+W+T+D=0 (Unaccelerated flight, climb, level
or gliding)

Once we have acceleration (change is velocity and or change in speed)
then L+W+T+D do not =0 We now have "unbalanced" force(s)

Cookie

On Jan 5, 6:51*pm, ProfChrisReed wrote:
I'm still not convinced by those who propose that the wings of the
glider generate no extra lift (or even generate less lift) when
climbing on tow.

We know that on a winch launch the glider climbs because the wings
generate more lift than in level/descending flight. This must be true
because there is nothing pulling it up.

However, we are told that on aerotow the wings generate the same (or
less) lift as in level/descending flight and the tug just pulls the
glider up the slope.

Does this mean that the tug climbs in the same way, i.e. wings
generate only enough lift to carry the weight of the tug, and the prop
drags the tug up the slope? This doesn't match what I've read about
how aircraft work. L=W only in level flight. I think the tug's wings
generate more lift than its weight, and thus it climbs.

If this is true, the same must be true for the glider behind it.

Bring on an aerodynamicist to show me I'm wrong.

Chris.

Winch towing is different than aerotowing on a couple of points. On
winch the glider goes through a couple of accelerations (change is
airspeed and or direction)..."arcing flight path". The glider also
must carry the weight and drag of the winch cable.

But the glider climbs on winch tow because the winch is imparting
energy to the glider. "thrust" if you will.
A glider climbs on tow only because of the energy imparted into it by
the tow plane.
A towplane climbs because of the energy inmparted into it by the
engine, from the fuel.

Its a study of potential energy (fuel) being converted into kenetic
energy (thrust) being converted back into potential energy (height)

A glider can only climb due to the following........

Tow plane
Winch
Thermal
Wave
Ridge lift
Its own power source (motorglider)
Being carried up a hill by a human (hang glider)

Etc

these are all external sources of power.

To climb we need "excess horsepower".....not excess lift

Excess lift will cause a change in flight direction and or change is
airspeed.

Cookie

On Jan 5, 1:52*pm, Andreas Maurer wrote:
On Wed, 5 Jan 2011 09:23:29 -0800 (PST), Derek C

wrote:
Gliders appear to get near to the stall during slow aerotows at much
greater than their normal free flight stalling airspeeds. I would
suggest that aerotowing must increase the wing loading in some way.

I have to admit that I didn't bother to read all the 120+ postings
about this topic, so please forgive me if the things that I'm going to
post have already been mentioned in this thread.

The main factor for the seemingly odd flying characteristics behind
the tow plane is the downwash of the latter.

Let me explain:
The downwash has a significant angle (the air is deflected downwards
behind the tow plane's wing to up to four degrees!), but due to the
larger span of the glider it only affects the inner part of the
glider's wing.

Therefore, if the glider if lying laterally displaced, only one wing
is affected by the downwash of the tow plane - four degrees of AoA
difference between left and right wing need a lot of aileron to
correct.

Likeise, if the glider is flying straight behind the tow plane, the
downwash *decreases* the AoA of the affected inner part of the wing.
Getting the nose up by pulling back will restore the lift of the inner
part of the glider's wing, but now the outer parts of the wing have a
much higher AoA than they have in free flight.
Voila, meet the the conditions for poor alieron efficiency (high AoA!)
and tip stall.

The downwash is reduced by
- wingloading of the tow plane
- wing span of the tow plane

In other words: The more a tow plane looks like a motorglider (say, a
Dimona, or Katana Extreme), the less the flight characteristics of the
glider are affected.
Anyone who has ever been towed behind a motorglider or a microlight
will testify that problems like poor lateral control or *running out
of elevator don't exist there, despite a far slower tow (55 kts
compared to a typical 70-75 kts behind a typical tow plane like
Reorqeur or Pawnee).

One interesting fact:
When Akaflieg Braunschweig flight-tested their SB-13 flying wing (with
a back-swept wing), they encountered a nose-down momentum after
lift-off that could not be recovered and usually lead to a crash
immediately after lift-off.

Explanation:
The downwash of the tow plane (Robin Remorqeur) hit the inner part of
the wing, decreasing its AoA (and lift) and therefore shifting the
center of lift backwards due to the sweepback.

Increasing the length of the tow rope helped.

Greetings from a snowy Germany
Andreas

Best explaination so far!

Cookie

On Jan 6, 5:24*am, "
wrote:
On Jan 5, 6:51*pm, ProfChrisReed wrote:





I'm still not convinced by those who propose that the wings of the
glider generate no extra lift (or even generate less lift) when
climbing on tow.

We know that on a winch launch the glider climbs because the wings
generate more lift than in level/descending flight. This must be true
because there is nothing pulling it up.

However, we are told that on aerotow the wings generate the same (or
less) lift as in level/descending flight and the tug just pulls the
glider up the slope.

Does this mean that the tug climbs in the same way, i.e. wings
generate only enough lift to carry the weight of the tug, and the prop
drags the tug up the slope? This doesn't match what I've read about
how aircraft work. L=W only in level flight. I think the tug's wings
generate more lift than its weight, and thus it climbs.

If this is true, the same must be true for the glider behind it.

Bring on an aerodynamicist to show me I'm wrong.

Chris.

Winch towing is different than aerotowing on a couple of points. *On
winch the glider goes through a couple of accelerations (change is
airspeed and or direction)..."arcing flight path". * The glider also
must carry the weight and drag of the winch cable.

But the glider climbs on winch tow because the winch is imparting
energy to the glider. "thrust" if you will.
A glider climbs on tow only because of the energy imparted into it by
the tow plane.
A towplane climbs because of the energy inmparted into it by the
engine, from the fuel.

Its a study of potential energy (fuel) being converted into kenetic
energy (thrust) being converted back into potential energy (height)

A glider can only climb due to the following........

Tow plane
Winch
Thermal
Wave
Ridge lift
Its own power source (motorglider)
Being carried up a hill by a human (hang glider)

Etc

these are all external sources of power.

To climb we need "excess horsepower".....not excess lift

Excess lift will cause a change in flight direction and or change is
airspeed.

Cookie- Hide quoted text -

- Show quoted text -

Oops! Forgot the most important part:

On winch the thrust is not along the direction of flight as it is in
aerotow.

The more the thrust is angled away from the direction of flight, the
more lift needs to be generated to compenstae.

Cookie