The Murphy Builders Site

The wing is expected to stall at a stipulated airspeed and the given g
factor. When it stalls, the force on the wing is topped out. Thus the
lift at 1g is the weight of the plane plus the downward pressure on the
elevator. At 5.7g it would be 5.7 times as much. This gives the limiting
lift of the wing. Increase the weight, and you decrease the limiting g
factor proportionally. This assumes you keep the penetration airspeed
the same for different weights. If you want to fiddle the airspeed, lift
is proportional to V^2.

This gets a little more elaborate when dealing with Vne and turbulence
penetration but the idea above pretty much covers the basics.

Hope this moves you in the right direction. Not a simple question Drew.

Garry

On Sun, 2006-06-04 at 19:55 -0400, Drew Dalgleish wrote:

If the rebel has a G limit of +5.7, -3.8 at 1650 lbs How do I caculate what
the limit would be at a higher weight? TIA Drew
Drew



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Oh, allright - time to make a fool of myself !! I'm no engineer,
but,.... if:

"Increase the weight, and you decrease the limiting g factor
proportionally."

Then, for 2000 lb. gross, the number is:

(1650/2000) x 5.7 = .825 x 5.7 = 4.7025

.........still a pretty hefty airplane !!

Still, be careful in the bumpy stuff, slow down a bit, and
don't bend anything !! ;-)

As I warned, IANAE !!! :-)

--
......bobp
http://bpatterson.qhealthbeauty.com

-------------------------------orig.-------------------------
On Monday 05 June 2006 12:51 am, Garry Wright wrote:

The wing is expected to stall at a stipulated airspeed and the given g
factor. When it stalls, the force on the wing is topped out. Thus the
lift at 1g is the weight of the plane plus the downward pressure on the
elevator. At 5.7g it would be 5.7 times as much. This gives the limiting
lift of the wing. Increase the weight, and you decrease the limiting g
factor proportionally. This assumes you keep the penetration airspeed
the same for different weights. If you want to fiddle the airspeed, lift
is proportional to V^2.

This gets a little more elaborate when dealing with Vne and turbulence
penetration but the idea above pretty much covers the basics.

Hope this moves you in the right direction. Not a simple question Drew.

Garry

On Sun, 2006-06-04 at 19:55 -0400, Drew Dalgleish wrote:

If the rebel has a G limit of +5.7, -3.8 at 1650 lbs How do I caculate

what

the limit would be at a higher weight? TIA Drew
Drew

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I've done my math off list for Drew, as I don't like promoting this in
public, along with some other previously applied foot work to TC. Remember
also that is failure G loading so consider 3.1 maximum without permanant
deflection....still better than my old '58 C182.

----- Original Message -----
From: "Bob Patterson" <beep@sympatico.ca>
To: <rebel-builders@dcsol.com>
Sent: Sunday, June 04, 2006 10:23 PM
Subject: Re: [rebel-builders] math question

Oh, allright - time to make a fool of myself !! I'm no engineer,
but,.... if:

"Increase the weight, and you decrease the limiting g factor
proportionally."

Then, for 2000 lb. gross, the number is:

(1650/2000) x 5.7 = .825 x 5.7 = 4.7025

.........still a pretty hefty airplane !!

Still, be careful in the bumpy stuff, slow down a bit, and
don't bend anything !! ;-)

As I warned, IANAE !!! :-)

--
......bobp
http://bpatterson.qhealthbeauty.com

-------------------------------orig.-------------------------
On Monday 05 June 2006 12:51 am, Garry Wright wrote:

The wing is expected to stall at a stipulated airspeed and the given g
factor. When it stalls, the force on the wing is topped out. Thus the
lift at 1g is the weight of the plane plus the downward pressure on the
elevator. At 5.7g it would be 5.7 times as much. This gives the limiting
lift of the wing. Increase the weight, and you decrease the limiting g
factor proportionally. This assumes you keep the penetration airspeed
the same for different weights. If you want to fiddle the airspeed, lift
is proportional to V^2.

This gets a little more elaborate when dealing with Vne and turbulence
penetration but the idea above pretty much covers the basics.

Hope this moves you in the right direction. Not a simple question Drew.

Garry

On Sun, 2006-06-04 at 19:55 -0400, Drew Dalgleish wrote:

If the rebel has a G limit of +5.7, -3.8 at 1650 lbs How do I caculate

what

the limit would be at a higher weight? TIA Drew
Drew

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Thanks Garry, Bob and Wayne. My head was spinning before Garry got to me from

looking at math sites I googled. Bill Hayes at TC in London wants a letter
stating what I want to do and justifying it all. Hopefully Waynes math is
good enough. Looking in the archives I found mention of a rebel out west
with a gross of 3000lbs. So asking for 2000 shouldn't be to much :)

At 02:23 AM 6/5/2006 +0000, you wrote:

Oh, allright - time to make a fool of myself !! I'm no engineer,
but,.... if:

"Increase the weight, and you decrease the limiting g factor
proportionally."

Then, for 2000 lb. gross, the number is:

(1650/2000) x 5.7 = .825 x 5.7 = 4.7025

.........still a pretty hefty airplane !!

Still, be careful in the bumpy stuff, slow down a bit, and
don't bend anything !! ;-)

As I warned, IANAE !!! :-)

--
......bobp
http://bpatterson.qhealthbeauty.com

-------------------------------orig.-------------------------
On Monday 05 June 2006 12:51 am, Garry Wright wrote:

The wing is expected to stall at a stipulated airspeed and the given g
factor. When it stalls, the force on the wing is topped out. Thus the
lift at 1g is the weight of the plane plus the downward pressure on the
elevator. At 5.7g it would be 5.7 times as much. This gives the limiting
lift of the wing. Increase the weight, and you decrease the limiting g
factor proportionally. This assumes you keep the penetration airspeed
the same for different weights. If you want to fiddle the airspeed, lift
is proportional to V^2.

This gets a little more elaborate when dealing with Vne and turbulence
penetration but the idea above pretty much covers the basics.

Hope this moves you in the right direction. Not a simple question Drew.

Garry

On Sun, 2006-06-04 at 19:55 -0400, Drew Dalgleish wrote:

If the rebel has a G limit of +5.7, -3.8 at 1650 lbs How do I caculate

what

the limit would be at a higher weight? TIA Drew
Drew

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Drew



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A couple of points. On production airplanes, as Wayne says, the
limiting load factor has a 50% safety factor bulit in. So your 3.8 g
Cessna wouldn't expect to experience damage until 5.7 g. This is not
generally the case with numbers quoted by kit suppliers. Also, VA is
calculated based on an encounter with a hypothetical vertical gust of a
given speed and sharpness, and the g load resulting from such a gust
will be proportional to the square of speed, so unless you're prepared
to fly slower, there is no getting away from the fact that you're
sacrificing safety margin by flying at higher weights.

However, if you put the airplane on floats, it is undoubtedly going to
fly slower - about 25% slower - so the g load from a given gust at a
given power setting will be 50% less. That's how I've convinced myself
that, in the specific case of a float equipped airplane, the increased
load is acceptable.

Now gusts are not the only thing that applies g loads. Flying into the
ground (or water) hard also can cause noticeable stress on the airplane,
but more on the undercarriage and engine attachments than on the wing
(though the stress on the attachments will be negative, where they're
isn't so strong). And here, you don't have the V squared rule working
in you favour, because we're not talking about a lift-induced force. So
if you're going for a higher gross, the gear better be able to take it.
Again, in the case of floats, a lot of the extra weight tends to be all
in the floats themselves, but if you're planning to carry two 190
pounders and 60 gallons of gas, the gear better be strong enough.

Al





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I suppose they will want a similar letter for a new c of a application then.

I've never seen any numbers for the approved 1730 gross on floats.
What's the story there? The floats generate some lift and there is still
about 5.7g available at 1730? Or is it just assumed that the g rating
drops 5%. I know this is quibbling but 1730/2000 x 5.7 is slightly
better ;)

Ken

Drew Dalgleish wrote:

Thanks Garry, Bob and Wayne. My head was spinning before Garry got to me from

looking at math sites I googled. Bill Hayes at TC in London wants a letter
stating what I want to do and justifying it all. Hopefully Waynes math is
good enough. Looking in the archives I found mention of a rebel out west
with a gross of 3000lbs. So asking for 2000 shouldn't be to much :)

At 02:23 AM 6/5/2006 +0000, you wrote:

Oh, allright - time to make a fool of myself !! I'm no engineer,
but,.... if:

"Increase the weight, and you decrease the limiting g factor
proportionally."

Then, for 2000 lb. gross, the number is:

(1650/2000) x 5.7 = .825 x 5.7 = 4.7025

.........still a pretty hefty airplane !!

Still, be careful in the bumpy stuff, slow down a bit, and
don't bend anything !! ;-)

As I warned, IANAE !!! :-)

--
......bobp
http://bpatterson.qhealthbeauty.com

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IANAE either (nice one Bob!), but I remember from aerodynamics class that
Va (which is essentially an accelerated stall speed) changes with weight
just like stall speed, and uses the same formula, which is: the square root
of the weight ratio.
Square root of (weight 2 divided by weight 1).
For example, if a given airplane has a published Va of 150kts at a max gross
weight of 2000# and you are flying it today at 1500#, then Va needs to be
reduced to 130kts.
The square root of W2/W1 is .86
.86 x 150kts is 130kts.
The same could be applied in reverse to an increase in gross weight, but as
you can see from the above formula, that would actually give you a HIGHER Va
for a HIGHER than normal weight. The reason for this seemingly incorrect
proportional relationship between Va and gross weight, is simple: a heavier
airplane stalls at a faster airspeed. When you are talking about not wanting
the wings to fall off in turbulence, stalling is a good thing because a
stalled wing is not taking any load. Hopefully the ground is not near. By
this logic, I do not agree with the statement below that you have to fly a
heavier airplane slower to provide over-stress protection. It is actually
the opposite, a lighter airplane needs to be flown slower in turbulence.
Weather the stress comes from turbulence or doing a loop, the heavier
airplane has more protection. This is totally separate from Vne, which is a
different limitation with different reasons.
Thinking about it a little more, it is obvious that at a certain weight, the
airplane would be overstressed while just flying straight and level in
smooth air at 1G (although it would take a big engine or a long runway to
get in the air, and Vne would surely be busted). Obviously the original
question is fairly complex. I just wanted to clarify the Va thing.
BTW, what is the Va and Vne for a Rebel?

Jesse

From: "Alan Hepburn" <ahepburn@xplornet.com>
Reply-To: <rebel-builders@dcsol.com>
To: <rebel-builders@dcsol.com>
Subject: RE: [rebel-builders] math question
Date: Mon, 5 Jun 2006 07:52:41 -0400

A couple of points. On production airplanes, as Wayne says, the
limiting load factor has a 50% safety factor bulit in. So your 3.8 g
Cessna wouldn't expect to experience damage until 5.7 g. This is not
generally the case with numbers quoted by kit suppliers. Also, VA is
calculated based on an encounter with a hypothetical vertical gust of a
given speed and sharpness, and the g load resulting from such a gust
will be proportional to the square of speed, so unless you're prepared
to fly slower, there is no getting away from the fact that you're
sacrificing safety margin by flying at higher weights.

However, if you put the airplane on floats, it is undoubtedly going to
fly slower - about 25% slower - so the g load from a given gust at a
given power setting will be 50% less. That's how I've convinced myself
that, in the specific case of a float equipped airplane, the increased
load is acceptable.

Now gusts are not the only thing that applies g loads. Flying into the
ground (or water) hard also can cause noticeable stress on the airplane,
but more on the undercarriage and engine attachments than on the wing
(though the stress on the attachments will be negative, where they're
isn't so strong). And here, you don't have the V squared rule working
in you favour, because we're not talking about a lift-induced force. So
if you're going for a higher gross, the gear better be able to take it.
Again, in the case of floats, a lot of the extra weight tends to be all
in the floats themselves, but if you're planning to carry two 190
pounders and 60 gallons of gas, the gear better be strong enough.

Al





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If you are below max manoevering speed you can't generate enough lifting
force to overstress the airframe. The airframe doesn't care what the
weight or g load is only how much the lifting force is. That lifting
force is proportional to V squared. It does not change with weight.
There can be other issues but in general it does not change with weight
AFAIK.

If like many aircraft you are cruising above max manoevring speed then
slowing to that speed is the correct thing to do in turbulence
regardless of the aircraft weight. It will be the same speed regardless
of weight. My operating manual specifies Va to be 95 knots. So I would
conclude that 95 knots is safe at any weight from an airframe strength
consideration if the plane is built according to the manual. I think
I'll put a mark on the ASI for 95 knots. Indeed if you are worried
about an accelerated stall then perhaps you are better off at 95 than
going slower...

I don't see any reference to 5.7 g in my operating manual. I do see the
1650 lb manoevring load specified as 3.8 g though so that must refer to
the 95 knot Va. So where does the 110 knot (top of green arc) come
from? Is it the ultimate g limit - ie above 95 knots you won't bend it
in turbulence - above 95 it might bend but if you are below 110 it won't
fail catrostrophically from one bump?? And for more confusion there are
a few older Rebels that are 1500 lb as the 1650 lb was a very early
upgrade that not everyone did..

Great discussion and refresher!

Ken
IANAE either!

Jesse Jenks wrote:

IANAE either (nice one Bob!), but I remember from aerodynamics class that
Va (which is essentially an accelerated stall speed) changes with weight
just like stall speed, and uses the same formula, which is: the square root
of the weight ratio.
Square root of (weight 2 divided by weight 1).
For example, if a given airplane has a published Va of 150kts at a max gross
weight of 2000# and you are flying it today at 1500#, then Va needs to be
reduced to 130kts.
The square root of W2/W1 is .86
.86 x 150kts is 130kts.
The same could be applied in reverse to an increase in gross weight, but as
you can see from the above formula, that would actually give you a HIGHER Va
for a HIGHER than normal weight. The reason for this seemingly incorrect
proportional relationship between Va and gross weight, is simple: a heavier
airplane stalls at a faster airspeed. When you are talking about not wanting
the wings to fall off in turbulence, stalling is a good thing because a
stalled wing is not taking any load. Hopefully the ground is not near. By
this logic, I do not agree with the statement below that you have to fly a
heavier airplane slower to provide over-stress protection. It is actually
the opposite, a lighter airplane needs to be flown slower in turbulence.
Weather the stress comes from turbulence or doing a loop, the heavier
airplane has more protection. This is totally separate from Vne, which is a
different limitation with different reasons.
Thinking about it a little more, it is obvious that at a certain weight, the
airplane would be overstressed while just flying straight and level in
smooth air at 1G (although it would take a big engine or a long runway to
get in the air, and Vne would surely be busted). Obviously the original
question is fairly complex. I just wanted to clarify the Va thing.
BTW, what is the Va and Vne for a Rebel?

Jesse

From: "Alan Hepburn" <ahepburn@xplornet.com>
Reply-To: <rebel-builders@dcsol.com>
To: <rebel-builders@dcsol.com>
Subject: RE: [rebel-builders] math question
Date: Mon, 5 Jun 2006 07:52:41 -0400

A couple of points. On production airplanes, as Wayne says, the
limiting load factor has a 50% safety factor bulit in. So your 3.8 g
Cessna wouldn't expect to experience damage until 5.7 g. This is not
generally the case with numbers quoted by kit suppliers. Also, VA is
calculated based on an encounter with a hypothetical vertical gust of a
given speed and sharpness, and the g load resulting from such a gust
will be proportional to the square of speed, so unless you're prepared
to fly slower, there is no getting away from the fact that you're
sacrificing safety margin by flying at higher weights.

However, if you put the airplane on floats, it is undoubtedly going to
fly slower - about 25% slower - so the g load from a given gust at a
given power setting will be 50% less. That's how I've convinced myself
that, in the specific case of a float equipped airplane, the increased
load is acceptable.

Now gusts are not the only thing that applies g loads. Flying into the
ground (or water) hard also can cause noticeable stress on the airplane,
but more on the undercarriage and engine attachments than on the wing
(though the stress on the attachments will be negative, where they're
isn't so strong). And here, you don't have the V squared rule working
in you favour, because we're not talking about a lift-induced force. So
if you're going for a higher gross, the gear better be able to take it.
Again, in the case of floats, a lot of the extra weight tends to be all
in the floats themselves, but if you're planning to carry two 190
pounders and 60 gallons of gas, the gear better be strong enough.

Al

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I got the 5.7 number from the specs page at the murphy site. It doesn't
make much sense though cuz they give the same g loading specs for each of
the different gross weights.

At 09:04 PM 6/5/2006 -0400, you wrote:

If you are below max manoevering speed you can't generate enough lifting
force to overstress the airframe. The airframe doesn't care what the
weight or g load is only how much the lifting force is. That lifting
force is proportional to V squared. It does not change with weight.
There can be other issues but in general it does not change with weight
AFAIK.

If like many aircraft you are cruising above max manoevring speed then
slowing to that speed is the correct thing to do in turbulence
regardless of the aircraft weight. It will be the same speed regardless
of weight. My operating manual specifies Va to be 95 knots. So I would
conclude that 95 knots is safe at any weight from an airframe strength
consideration if the plane is built according to the manual. I think
I'll put a mark on the ASI for 95 knots. Indeed if you are worried
about an accelerated stall then perhaps you are better off at 95 than
going slower...

I don't see any reference to 5.7 g in my operating manual. I do see the
1650 lb manoevring load specified as 3.8 g though so that must refer to
the 95 knot Va. So where does the 110 knot (top of green arc) come
from? Is it the ultimate g limit - ie above 95 knots you won't bend it
in turbulence - above 95 it might bend but if you are below 110 it won't
fail catrostrophically from one bump?? And for more confusion there are
a few older Rebels that are 1500 lb as the 1650 lb was a very early
upgrade that not everyone did..

Great discussion and refresher!

Ken
IANAE either!

Jesse Jenks wrote:

IANAE either (nice one Bob!), but I remember from aerodynamics class that
Va (which is essentially an accelerated stall speed) changes with weight
just like stall speed, and uses the same formula, which is: the square root
of the weight ratio.
Square root of (weight 2 divided by weight 1).
For example, if a given airplane has a published Va of 150kts at a max

gross

weight of 2000# and you are flying it today at 1500#, then Va needs to be
reduced to 130kts.
The square root of W2/W1 is .86
.86 x 150kts is 130kts.
The same could be applied in reverse to an increase in gross weight, but as
you can see from the above formula, that would actually give you a HIGHER

Va

for a HIGHER than normal weight. The reason for this seemingly incorrect
proportional relationship between Va and gross weight, is simple: a heavier
airplane stalls at a faster airspeed. When you are talking about not

wanting

the wings to fall off in turbulence, stalling is a good thing because a
stalled wing is not taking any load. Hopefully the ground is not near. By
this logic, I do not agree with the statement below that you have to fly a
heavier airplane slower to provide over-stress protection. It is actually
the opposite, a lighter airplane needs to be flown slower in turbulence.
Weather the stress comes from turbulence or doing a loop, the heavier
airplane has more protection. This is totally separate from Vne, which is a
different limitation with different reasons.
Thinking about it a little more, it is obvious that at a certain weight,

the

airplane would be overstressed while just flying straight and level in
smooth air at 1G (although it would take a big engine or a long runway to
get in the air, and Vne would surely be busted). Obviously the original
question is fairly complex. I just wanted to clarify the Va thing.
BTW, what is the Va and Vne for a Rebel?

Jesse

From: "Alan Hepburn" <ahepburn@xplornet.com>
Reply-To: <rebel-builders@dcsol.com>
To: <rebel-builders@dcsol.com>
Subject: RE: [rebel-builders] math question
Date: Mon, 5 Jun 2006 07:52:41 -0400

A couple of points. On production airplanes, as Wayne says, the
limiting load factor has a 50% safety factor bulit in. So your 3.8 g
Cessna wouldn't expect to experience damage until 5.7 g. This is not
generally the case with numbers quoted by kit suppliers. Also, VA is
calculated based on an encounter with a hypothetical vertical gust of a
given speed and sharpness, and the g load resulting from such a gust
will be proportional to the square of speed, so unless you're prepared
to fly slower, there is no getting away from the fact that you're
sacrificing safety margin by flying at higher weights.

However, if you put the airplane on floats, it is undoubtedly going to
fly slower - about 25% slower - so the g load from a given gust at a
given power setting will be 50% less. That's how I've convinced myself
that, in the specific case of a float equipped airplane, the increased
load is acceptable.

Now gusts are not the only thing that applies g loads. Flying into the
ground (or water) hard also can cause noticeable stress on the airplane,
but more on the undercarriage and engine attachments than on the wing
(though the stress on the attachments will be negative, where they're
isn't so strong). And here, you don't have the V squared rule working
in you favour, because we're not talking about a lift-induced force. So
if you're going for a higher gross, the gear better be able to take it.
Again, in the case of floats, a lot of the extra weight tends to be all
in the floats themselves, but if you're planning to carry two 190
pounders and 60 gallons of gas, the gear better be strong enough.

Al

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Drew



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Ken, you are correct with your first statement, that if you are below
maneuvering speed you can't over-stress the airframe. The reason that is
true is because the wing stalls before it can harm itself. stall speed
does change with weight, and therefore so does the wing's ability to produce
enough lift to break off. At a lighter weight it will be able to produce
more lifting force prior to the stall for a given speed. An aircraft
manufacturer publishes a Va just as they publish a stall speed, but they are
only really applicable at one weight. Max gross I presume. When flying at a
different weight you are technically operating in an unpublished realm.
That's why bigger airplanes don't have any markings on the airspeed
indicator. They have little "bugs" that you set depending on your weight.
You know the standard rule of using 1.3 x Vso for your approach speed.
Technically this would change, even in small aircraft, for different
weights. You would use the formula I mentioned:
square root of (w2/W1)
In airliners we have a stack of flip cards that give all the pertinant
speeds (V1,Vr,V2,Vfs,Vman, Vref) for different weights. I have to admit
though, that for the airplane I fly, there is no Va, but there is a Vb which
is max gust penetration speed, and we don't adjust it for weight. It is so
low though, that I presume it has a big safety margin built in. This is like
the speed you would slow to if you blundered into a thunder storm.
I still maintain my argument above, that while Va is not published for
different weights, the wings' ability to over-stress itself does change with
weight. When I refer to accelerated stall speed what I mean is that at more
than 1G the wing will stall at a higher than normal speed (accelerated
refers to the increased load factor rather than the increased speed).

ITTSLAAE (I'm trying to sound like an aeronautical engineer)

BTW I have read several accident reports where airplanes went into extreme
turbulence and broke up in flight. In at least 2 of those the tail broke in
the downward (up elevator) direction before the wings broke.

Jesse

From: Ken <klehman@albedo.net>
Reply-To: <rebel-builders@dcsol.com>
To: rebel-builders@dcsol.com
Subject: Re: [rebel-builders] math question
Date: Mon, 05 Jun 2006 21:04:15 -0400

If you are below max manoevering speed you can't generate enough lifting
force to overstress the airframe. The airframe doesn't care what the
weight or g load is only how much the lifting force is. That lifting
force is proportional to V squared. It does not change with weight.
There can be other issues but in general it does not change with weight
AFAIK.

If like many aircraft you are cruising above max manoevring speed then
slowing to that speed is the correct thing to do in turbulence
regardless of the aircraft weight. It will be the same speed regardless
of weight. My operating manual specifies Va to be 95 knots. So I would
conclude that 95 knots is safe at any weight from an airframe strength
consideration if the plane is built according to the manual. I think
I'll put a mark on the ASI for 95 knots. Indeed if you are worried
about an accelerated stall then perhaps you are better off at 95 than
going slower...

I don't see any reference to 5.7 g in my operating manual. I do see the
1650 lb manoevring load specified as 3.8 g though so that must refer to
the 95 knot Va. So where does the 110 knot (top of green arc) come
from? Is it the ultimate g limit - ie above 95 knots you won't bend it
in turbulence - above 95 it might bend but if you are below 110 it won't
fail catrostrophically from one bump?? And for more confusion there are
a few older Rebels that are 1500 lb as the 1650 lb was a very early
upgrade that not everyone did..

Great discussion and refresher!

Ken
IANAE either!

Jesse Jenks wrote:

IANAE either (nice one Bob!), but I remember from aerodynamics class

that

Va (which is essentially an accelerated stall speed) changes with weight
just like stall speed, and uses the same formula, which is: the square

root

of the weight ratio.
Square root of (weight 2 divided by weight 1).
For example, if a given airplane has a published Va of 150kts at a max

gross

weight of 2000# and you are flying it today at 1500#, then Va needs to be
reduced to 130kts.
The square root of W2/W1 is .86
.86 x 150kts is 130kts.
The same could be applied in reverse to an increase in gross weight, but

as

you can see from the above formula, that would actually give you a HIGHER

Va

for a HIGHER than normal weight. The reason for this seemingly incorrect
proportional relationship between Va and gross weight, is simple: a

heavier

airplane stalls at a faster airspeed. When you are talking about not

wanting

the wings to fall off in turbulence, stalling is a good thing because a
stalled wing is not taking any load. Hopefully the ground is not near. By
this logic, I do not agree with the statement below that you have to fly

a

heavier airplane slower to provide over-stress protection. It is actually
the opposite, a lighter airplane needs to be flown slower in turbulence.
Weather the stress comes from turbulence or doing a loop, the heavier
airplane has more protection. This is totally separate from Vne, which is

a

different limitation with different reasons.
Thinking about it a little more, it is obvious that at a certain weight,

the

airplane would be overstressed while just flying straight and level in
smooth air at 1G (although it would take a big engine or a long runway to
get in the air, and Vne would surely be busted). Obviously the original
question is fairly complex. I just wanted to clarify the Va thing.
BTW, what is the Va and Vne for a Rebel?

Jesse

From: "Alan Hepburn" <ahepburn@xplornet.com>
Reply-To: <rebel-builders@dcsol.com>
To: <rebel-builders@dcsol.com>
Subject: RE: [rebel-builders] math question
Date: Mon, 5 Jun 2006 07:52:41 -0400

A couple of points. On production airplanes, as Wayne says, the
limiting load factor has a 50% safety factor bulit in. So your 3.8 g
Cessna wouldn't expect to experience damage until 5.7 g. This is not
generally the case with numbers quoted by kit suppliers. Also, VA is
calculated based on an encounter with a hypothetical vertical gust of a
given speed and sharpness, and the g load resulting from such a gust
will be proportional to the square of speed, so unless you're prepared
to fly slower, there is no getting away from the fact that you're
sacrificing safety margin by flying at higher weights.

However, if you put the airplane on floats, it is undoubtedly going to
fly slower - about 25% slower - so the g load from a given gust at a
given power setting will be 50% less. That's how I've convinced myself
that, in the specific case of a float equipped airplane, the increased
load is acceptable.

Now gusts are not the only thing that applies g loads. Flying into the
ground (or water) hard also can cause noticeable stress on the airplane,
but more on the undercarriage and engine attachments than on the wing
(though the stress on the attachments will be negative, where they're
isn't so strong). And here, you don't have the V squared rule working
in you favour, because we're not talking about a lift-induced force. So
if you're going for a higher gross, the gear better be able to take it.
Again, in the case of floats, a lot of the extra weight tends to be all
in the floats themselves, but if you're planning to carry two 190
pounders and 60 gallons of gas, the gear better be strong enough.

Al

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Jesse:

What you say is true, to a point. If you are flying at gross and pull
back you will stall just at the +ve g stress limit for the airplane if
you are flying at VA, whereas if you are below gross and try the same
thing, you will exceed the g limit before stalling. So, to avoid
exceeding the g limit, you have to fly slower when you're lighter. But
consider how much force is actually applied to the airframe. Let's say
the stress limit is 4g, and you're flying a 2000# airplane at gross and
at VA. You pull back and it stalls as it hits 4g, so the wings subject
to 4 x 2000 = 8000# (less the weight of the wings). So, the wings are
producing 8000# of lift as they hit the stalling angle of attack. Now,
let's try the same thing at the same speed, but at 1500#. Since the
speed is the same, the wings will again stall at the same angle of
attack, and again be producing 8000# of lift. So, as you say, we have
applied 8000/1500 = 5.33, and have exceeded the published g limit.
However, the actual stress applied to the airframe was still just 8000#,
so in fact you did not run a greater risk of actual damage than you ran
at gross. You may have broken the law, but you didn't run an increased
risk of breaking the airplane.

This is why the maximum allowable weight decreases as you move from
normal, to utility, to aerobatic category. Otherwise, you'd be safer
doing aerobatics in a heavy airplane than a light one, and you just need
to look at any flight manual for an airplane that's legal in more than
one category to see that's not the case. For example, the Zlin 242L
(which I used to own) grosses at 2400# in normal category, 2250# in
utility, and 2140# in aerobatic.

Now, VA is the speed at which you hit the load limit and stall
simultaneously. The top of the green arc, VNO , is something quite
different. It is the speed above which an encounter with the design
reference gust at gross weight will result in overstressing the
airplane. The more you exceed this speed, the less protection you have
from overstressing the airframe when you hit a "bump". Since God has
signed not contract with aviators that the design reference gust will
never be exceeded in nature, it's prudent to reduce speed below the
maximum structural cruising speed in rough air. Some manuals publish a
recommended rough air penetration spedd that is somewhere between VA and
the top of the green arc. It's a compromise between controllability
(avoiding indadvertent stalls) and stress avoidance,

Al







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Yup the tail issue is one of the things I was thinking of when I said
there could be other issues.

It's awhile since I studied any of this stuff but how can changing the
weight possibly affect the lift that the wing can produce at a given speed?

A given lifting force translates into higher g at light weight but the
lifting force is strictly a function of speed for a given aerofoil. That
is why the stalling speed rises at higher weights AFAIK. The half roe v
squared formula. No weight just V in the lift formula for a given wing.

I see I had a type in the last post. Should have been "BELOW 95 knots
you won't bend it in turbulence"

Ken

Jesse Jenks wrote:

Ken, you are correct with your first statement, that if you are below
maneuvering speed you can't over-stress the airframe. The reason that is
true is because the wing stalls before it can harm itself. stall speed
does change with weight, and therefore so does the wing's ability to produce
enough lift to break off. At a lighter weight it will be able to produce
more lifting force prior to the stall for a given speed.

snip




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Ahh - I should have kept reading before my last post as you covered the
g and load thing nicely Al.

thanks for the refresher below on where the top of green arc comes from.

Ken

Alan Hepburn wrote:

snip

The top of the green arc, VNO , is something quite
different. It is the speed above which an encounter with the design
reference gust at gross weight will result in overstressing the
airplane. The more you exceed this speed, the less protection you have
from overstressing the airframe when you hit a "bump". Since God has
signed not contract with aviators that the design reference gust will
never be exceeded in nature, it's prudent to reduce speed below the
maximum structural cruising speed in rough air. Some manuals publish a
recommended rough air penetration spedd that is somewhere between VA and
the top of the green arc. It's a compromise between controllability
(avoiding indadvertent stalls) and stress avoidance,

Al

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