What is a "rear lift truss"?

I'm currently working on (re)certifying a German UL plane as an LSA
for the US market, and in the ASTM regulations I stumbled across:

"CS 23.369 Rear lift truss
(a) If a rear lift truss is used, it must be
designed for conditions of reversed airflow at a
design speed of –
V = 8·7 W/S + 8·7(knots)
where W/S = wing loading at design
maximum take-off weight (lb/ft2).
(b) Either aerodynamic data for the
particular wing section used, or a value of CL
equalling -0·8 with a chordwise distribution that
is triangular between a peak at the trailing edge
and zero at the leading edge, must be used."

This is from the CS 23, but the wording is the same in the ASTM-LSA
regulations.

What is a rear lift truss?

Oliver

"Oliver Arend" wrote in message
...
I'm currently working on (re)certifying a German UL plane as an LSA
for the US market, and in the ASTM regulations I stumbled across:

"CS 23.369 Rear lift truss
(a) If a rear lift truss is used, it must be
designed for conditions of reversed airflow at a
design speed of –
V = 8·7 W/S + 8·7(knots)
where W/S = wing loading at design
maximum take-off weight (lb/ft2).
(b) Either aerodynamic data for the
particular wing section used, or a value of CL
equalling -0·8 with a chordwise distribution that
is triangular between a peak at the trailing edge
and zero at the leading edge, must be used."

This is from the CS 23, but the wording is the same in the ASTM-LSA
regulations.

What is a rear lift truss?

Oliver

It is the wing struts.

Taken from:
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/229e08cffdb0a5f68625733e006178aa/d1a4060f22939f36862572eb0061ca3f/$FILE/AC%2023-19A.pdf

23.369 Rear lift truss (Amendment 23-48

70. What is the purpose of this regulation? Civil Air Regulation (CAR)
3.194 is
considered a special supplementary reversed airflow condition for design
of the rear lift
truss, which has no direct relationship to any of the points on the V-n
envelope. It has
been historically considered as a downwind taxi for a "tail-wheel" type
airplane. The lift
truss is the brace (frequently a “V,” sometimes “parallel” struts—one to
each existing
spar) running from the bottom of the fuselage to the lower spar cap(s)
of the wing. These
lift truss struts usually attach to the wing at midspan. “Wing struts”
are usually loaded in
tension (for positive load factor conditions)—except during negative “g”
maneuvers or
gusts, inverted flight conditions (aerobatic maneuvers), landing, and
taxi. In these latter
cases, the struts can be loaded in compression; therefore, they are
subject to Euler column
buckling phenomena. Even when on the ground, the airplane rear lift
truss (or strut) can
experience significant compression loads if the airplane has a tail
wheel. This is
especially true when the airplane is tied down or is taxiing downwind.

--
Anyolmouse

It is the wing struts.

Taken from:http://rgl.faa.gov/Regulatory_and_Gu...visoryCircular...

23.369 Rear lift truss (Amendment 23-48

[...]“Wing struts” are usually loaded in tension (for positive load factor conditions)—except during negative “g” maneuvers or gusts, inverted flight conditions (aerobatic maneuvers), landing, and taxi. In these latter cases, the struts can be loaded in compression; therefore, they are subject to Euler column buckling phenomena. Even when on the ground, the airplane rear lift truss (or strut) can experience significant compression loads if the airplane has a tail wheel. This is especially true when the airplane is tied down or is taxiing downwind.

So from what I understand the wing strut (as on most Cessna SEP, like
http://www.fly365.co.uk/images/cessna152-gbhwa.jpg) may under certain
circumstances be subjected to a higher compression load if the plane
is oriented away from the wind (especially with a tailwheel - higher
AOA) than it would be within the regular V-n flight envelope?

I know questioning FAA/EASA regulations is futile, but wouldn't that
mean that the wing itself would also be subject to higher loads than
considered in the flight envelope?

Oliver

On Apr 30, 3:18*pm, Oliver Arend wrote:

I know questioning FAA/EASA regulations is futile, but wouldn't that
mean that the wing itself would also be subject to higher loads than
considered in the flight envelope?

Oliver

Not necessarily. This rule seems to calls for consideration of the
chordwise lift distribution on the wing while the tail is low and the
relative wind is coming from behind. Under those circumstances, the
chordwise lift distribution will be biased towards the aft spars and
aft struts in a way that they'd never be exposed to in the (normal)
flight envelope.

Thanks, Bob K.

On Thu, 30 Apr 2009 15:18:38 -0700 (PDT), Oliver Arend
wrote:

It is the wing struts.

Taken from:http://rgl.faa.gov/Regulatory_and_Gu...visoryCircular...

23.369 Rear lift truss (Amendment 23-48

[...]“Wing struts” are usually loaded in tension (for positive load factor conditions)—except during negative “g” maneuvers or gusts, inverted flight conditions (aerobatic maneuvers), landing, and taxi. In these latter cases, the struts can be loaded in compression; therefore, they are subject to Euler column buckling phenomena. Even when on the ground, the airplane rear lift truss (or strut) can experience significant compression loads if the airplane has a tail wheel. This is especially true when the airplane is tied down or is taxiing downwind.

So from what I understand the wing strut (as on most Cessna SEP, like
http://www.fly365.co.uk/images/cessna152-gbhwa.jpg) may under certain
circumstances be subjected to a higher compression load if the plane
is oriented away from the wind (especially with a tailwheel - higher
AOA) than it would be within the regular V-n flight envelope?

we get accustomed to thinking about aerofoils blunt end forward.
sharp end forward still works to generate lift but with less
efficiency.

just mentally stand on your head and look at the wing in a tailwind.
it is sitting there in the breeze at a few degrees angle of attack,
and it will generate lift compressing the strut.
a taildragger sits with much more angle of attack (remember we're
upside down) and will generate more lift force.

back up the right way now...
generally the rear strut of the two is a smaller section than the
front strut and is more subject to buckling failure than the leading
strut. this is just a calculation verifying that there isnt a problem.

"Bob Kuykendall" wrote in message
...
On Apr 30, 3:18 pm, Oliver Arend wrote:

This rule seems to calls for consideration of the
chordwise lift distribution on the wing while the tail is low and the
relative wind is coming from behind.

The tailwheel folks might want to consider taking a lesson from the
glider folks and tie down with the tail elevated to get the wing closer to a
zero AOA. This greatly decreases the up force on the tiedowns when the wind
is from the front of the plane, as well as decreasing the downward force
when the wind is from the rear. Here in Florida, our summer afternoon
thunderstorm can easily create 50 MPH gusts, which can cause thousands of
pounds of force on your airframe and tiedowns. Why would you not want to
reduce that?

I have seen several varieties of custom stands used under the tail wheel,
as well as plastic milk crates, sand bags, and even spare tires.

Vaughn

This rule seems to calls for consideration of the
chordwise lift distribution on the wing while the tail is low and the
relative wind is coming from behind.

I realize the wing will create lift (or downward force) with relative
wind from the back. And the chordwise lift distribution is such that
(as described in the regulation) especially the torsional loads differ
greatly from the regular flight envelope. But if the airplane has only
one spar and hence just one strut (which would IMHO be the "forward
lift truss" or "main lift truss"), is this paragraph still to be
considered?

Oliver

Oliver Arend wrote:
This rule seems to calls for consideration of the
chordwise lift distribution on the wing while the tail is low and the
relative wind is coming from behind.

I realize the wing will create lift (or downward force) with relative
wind from the back. And the chordwise lift distribution is such that
(as described in the regulation) especially the torsional loads differ
greatly from the regular flight envelope. But if the airplane has only
one spar and hence just one strut (which would IMHO be the "forward
lift truss" or "main lift truss"), is this paragraph still to be
considered?

Oliver

Re-read "(a) If a rear lift truss is used, it must be designed for
conditions of..."

If you don't have a rear lift truss, then you don't have to account for
its strength under any condition....

Charles