Strength of high-wing versus low-wing single piston aircraft
Plus, if memory serves correctly, there is a 50% safety margin built in to these maximum loadings.
So an aircraft certified to withstand, say, 3.8g has to actually withstand the equivalent of 5.7g before things start to bend or break.
Practically, most light aircraft do not have "g" meters, unless aerobatic.
So the idea of approaching the limit loading has to be with the knowledge that the pilots interpretation of such loading is subjective and may be wildly inaccurate, especially when turbulence is involved.
It is quite possible to be loading the aircraft (in a semi-aerobatic or max. rate turn - type manoeuver) to below the maximum rated loading, but because of a gust or violent control action "spot" load part of the structure to above the limit.
I've always been a bit conservative when flying through turbulence. Anything more than light turbulence and I won't enter the yellow arc. Anything moderate or severe, (severe is rare) and I'll stay at or below Va. This, of course, is a subjective assessment, too. One mans' light is another mans' moderate, sometimes.
So an aircraft certified to withstand, say, 3.8g has to actually withstand the equivalent of 5.7g before things start to bend or break.
Practically, most light aircraft do not have "g" meters, unless aerobatic.
So the idea of approaching the limit loading has to be with the knowledge that the pilots interpretation of such loading is subjective and may be wildly inaccurate, especially when turbulence is involved.
It is quite possible to be loading the aircraft (in a semi-aerobatic or max. rate turn - type manoeuver) to below the maximum rated loading, but because of a gust or violent control action "spot" load part of the structure to above the limit.
I've always been a bit conservative when flying through turbulence. Anything more than light turbulence and I won't enter the yellow arc. Anything moderate or severe, (severe is rare) and I'll stay at or below Va. This, of course, is a subjective assessment, too. One mans' light is another mans' moderate, sometimes.
Join Date: May 2006
Location: Londonish
Posts: 779
Likes: 0
Received 0 Likes
on
0 Posts
Interestingly, despite being strutted, the aforementioned decathlon is certified for +6/-5 - that being considerably more negative than most aerobatic aircraft of similar performance (including many pitts flavours) which tend to be +6/-3.
There's also at least one strutted low wing aircraft I can think of.. just to really confuse the issue!
Oh, and the 50% margin bit is for a defined period of time (3 sec IIRC), it is allowed to bend the aircraft and break stuff; Although it has to be capable of continuing flight, you may not be able to use it again.
There's also at least one strutted low wing aircraft I can think of.. just to really confuse the issue!
Oh, and the 50% margin bit is for a defined period of time (3 sec IIRC), it is allowed to bend the aircraft and break stuff; Although it has to be capable of continuing flight, you may not be able to use it again.
Last edited by Mark1234; 13th Apr 2011 at 23:37.
Join Date: Sep 2006
Location: Los Angeles, USA
Age: 52
Posts: 1,631
Likes: 0
Received 0 Likes
on
0 Posts
Yes, they have a 1.5x safety factor for wood and aluminium construction. Carbon fiber or any composite however has to have a safety factor of 2x, which is the reason composite airplanes rarely are much lighter.
Is the extra safety factor required for composites due to the fact that (compared with aluminium or wood) they have a relatively short aircraft usage period? I imagine this will change over time if that's the case.
(Unless it proves to be not so durable, of course.)
(Unless it proves to be not so durable, of course.)
Join Date: Sep 2006
Location: Los Angeles, USA
Age: 52
Posts: 1,631
Likes: 0
Received 0 Likes
on
0 Posts
I think it's just because the materials are unproven still.
Also, much of the construction is hand lay-up and I have a feeling this is a factor as well. In essence; people gluing mats of carbon/kevlar/glass fibre into a mold and whetting it with epoxy. The thicknesses can be uneven and there's no automated way of creating the exact same uniform part each time. It also cures differently depending on if you bake it or let it dry at normal temps.
I'm sure this factor will come down in the future.
As a side - I'm just reading the Seneca POH for a checkride tomorrow and there it says that design load factor is 3.8G, but with the flaps out it's only 2.2G. I didn't know there was a different number for that, but it's worth keeping in mind. Obviously, on most aircraft you are well below Maneuvering Speed (Va) when flaps are out, so it's not a factor. But if you for some reason should have an aircraft that can deploy flaps at high speeds, this would probably not be a good idea to do in turbulence going fast.
Also, much of the construction is hand lay-up and I have a feeling this is a factor as well. In essence; people gluing mats of carbon/kevlar/glass fibre into a mold and whetting it with epoxy. The thicknesses can be uneven and there's no automated way of creating the exact same uniform part each time. It also cures differently depending on if you bake it or let it dry at normal temps.
I'm sure this factor will come down in the future.
As a side - I'm just reading the Seneca POH for a checkride tomorrow and there it says that design load factor is 3.8G, but with the flaps out it's only 2.2G. I didn't know there was a different number for that, but it's worth keeping in mind. Obviously, on most aircraft you are well below Maneuvering Speed (Va) when flaps are out, so it's not a factor. But if you for some reason should have an aircraft that can deploy flaps at high speeds, this would probably not be a good idea to do in turbulence going fast.
Join Date: Sep 2006
Location: Scotland
Age: 84
Posts: 1,434
Likes: 0
Received 0 Likes
on
0 Posts
Also, much of the construction is hand lay-up and I have a feeling this is a factor as well. In essence; people gluing mats of carbon/kevlar/glass fibre into a mold and whetting it with epoxy. The thicknesses can be uneven and there's no automated way of creating the exact same uniform part each time. It also cures differently depending on if you bake it or let it dry at normal temps.
Join Date: Sep 2006
Location: Los Angeles, USA
Age: 52
Posts: 1,631
Likes: 0
Received 0 Likes
on
0 Posts
Well, wood has a proven tensile strength that isn't uniform by any means, but they use very conservative numbers. They use the lowest of tested values as a design maximum. Also, one doesn't construct wood airplanes in the same way normally, although you sure could. Having thin sheets of veneer in a mold with epoxy - that's how all those 70's fruit bowls were made and they were sturdy as hell. Or the old cedar strip canoes that people still build. They last forever.
Soap box warning: People forget how strong wood is. Birch has 1.7x more tensile strength per weight compared to aluminium. Nobody believes that. It doesn't corrode and it doesn't fatigue. It's a great construction material if it gets sealed properly (to protect from water).
Soap box warning: People forget how strong wood is. Birch has 1.7x more tensile strength per weight compared to aluminium. Nobody believes that. It doesn't corrode and it doesn't fatigue. It's a great construction material if it gets sealed properly (to protect from water).
Show me a high wing strut braced monoplane competition aerobatic aeroplane.
I saw one win an international basic aerobatics competition once.
Thread Starter
Join Date: Jan 2011
Location: Pennsylvania, USA
Posts: 130
Likes: 0
Received 0 Likes
on
0 Posts
Ahhhh, the 152 Aerobat. I would buy one in a split SECOND if one existed (was for sale) and I had the funds. What an awesome custom Cessna. I'd die to get a right-seat ride in a 150 HP Cessna 152.
Are they stronger than the standard 152? I know they have more effective (longer) ailerons. Did they beef up the frame to take a higher 'posi and 'negi G load?
Are they stronger than the standard 152? I know they have more effective (longer) ailerons. Did they beef up the frame to take a higher 'posi and 'negi G load?
The aerobatic certification is +6/-3. Pitts & Extra's get the same certification as a Decathlon. However, Pitts are regularly used in competition at +8/-8. The weakness in Decathlon wings is not the strut bracing, but the rib to spar attach. Under the torsional load of rolling g, the wing ribs work loose. I have a dim recollection of spar issues at the strut attach point too, but my memory is dim. Both the C152 Aerobat and Decathlon are good training aircraft, but the world has moved on and I'm not sure that either could deal with sportsman level anymore, just as not even Neil Williams wouldn't be competitive in the Cosmic Wind "Ballarina" anymore.
The weaknesses of all these aircraft are well catalogued in the IAC tech papers. But the topic was are strut braced high wing aircraft stronger than unbraced low wing aircraft and there is no definitive answer. It depends on the design.
The weaknesses of all these aircraft are well catalogued in the IAC tech papers. But the topic was are strut braced high wing aircraft stronger than unbraced low wing aircraft and there is no definitive answer. It depends on the design.
Join Date: May 2006
Location: Londonish
Posts: 779
Likes: 0
Received 0 Likes
on
0 Posts
Old Akro - the minimum for aerobatic certification might be +6/-3, I'm not a certification expert. However, the manual and placards on both the vanilla (actually VH-BIK), and super (G-something) that I've played in are for +6/-5
I'm also lead to believe that different marques of pitts have higher limits but the S2A I fly is 6 and 3 (manual and placarding).
And yes Plasmech, the aerobat is modified (strengthened) though I don't know the exact details. Also gets windows in the roof and a jettisonable door.
I'm also lead to believe that different marques of pitts have higher limits but the S2A I fly is 6 and 3 (manual and placarding).
And yes Plasmech, the aerobat is modified (strengthened) though I don't know the exact details. Also gets windows in the roof and a jettisonable door.
The minimum positive g limits for a light aeroplane are:
+3.8g - normal category
+4.4g - utility category
+6.0g - aerobatic category
+2.0g - with flaps down.
Designers can use bigger numbers, but seldom do except in the aerobatic category. The negative g limits are normally (-)0.4 times the positive limit, or (-)0.5 times for aerobatic aeroplanes.
Microlights and gliders use slightly larger numbers: +4/-2 for microlights, +5.3/-2.65 for normal gliders, +7/-5 for aerobatic gliders.
These give what are called "limit loads" - which is what the aeroplane must be able to take indefinitely without any permanent damage.
You then start applying safety factors. The minimum safety factor is almost always 1.5 (+50% in other words), but other factors may come in depending upon the type of material.
Compared to metal, or even wood, composites vary both between examples, and through life. So, typically you use an additional 1.2 --> 1.5 factor for composites; 1.2 is the minimum and you can use that where you've got materials data for maximum temperature and humidity conditions, 1.5 is more normal, using "standard condition" data.
Other safety factors may also kick in depending upon what else is in there - there are factors for cables, castings, loaded hinges...
Take the limit loads, multiply by all the safety factors which apply, and you get something called the "ultimate load".
When you certify an aeroplane you have to prove that it will not fail catastrophically if loaded to ultimate loads for at-least 3 seconds.
If you break it, the loads to break it are the "failure loads", and the certification requirement is that the "reserve factor", which is the failure load divided by the ultimate load, is more than 1. Obviously, you want it as near to 1 as possible, to keep the weight down.
So, a typical metal light aeroplane is expected to take +3.8/-1.52 without any damage, but will be designed not to fail catastrophically within +5.7/-2.28g; however if it was made from composites using room temperature and low humidity test data the last numbers would be 8.55g/-3.42g. At-least at the start of the aeroplane's life - you can expect the failure loads to get worse through-life.
One other thing - a big well resourced company like Piper or Cessna will tend to get all their reserve factors as near 1.0 as possible. A smaller company however, say Europa or Vans, will probably use larger factors because they can't afford the high cost of really high quality testing and analysis. So, there's a likelihood that such an aeroplane, whilst probably a little heavier, will also be a bit stronger.
G
+3.8g - normal category
+4.4g - utility category
+6.0g - aerobatic category
+2.0g - with flaps down.
Designers can use bigger numbers, but seldom do except in the aerobatic category. The negative g limits are normally (-)0.4 times the positive limit, or (-)0.5 times for aerobatic aeroplanes.
Microlights and gliders use slightly larger numbers: +4/-2 for microlights, +5.3/-2.65 for normal gliders, +7/-5 for aerobatic gliders.
These give what are called "limit loads" - which is what the aeroplane must be able to take indefinitely without any permanent damage.
You then start applying safety factors. The minimum safety factor is almost always 1.5 (+50% in other words), but other factors may come in depending upon the type of material.
Compared to metal, or even wood, composites vary both between examples, and through life. So, typically you use an additional 1.2 --> 1.5 factor for composites; 1.2 is the minimum and you can use that where you've got materials data for maximum temperature and humidity conditions, 1.5 is more normal, using "standard condition" data.
Other safety factors may also kick in depending upon what else is in there - there are factors for cables, castings, loaded hinges...
Take the limit loads, multiply by all the safety factors which apply, and you get something called the "ultimate load".
When you certify an aeroplane you have to prove that it will not fail catastrophically if loaded to ultimate loads for at-least 3 seconds.
If you break it, the loads to break it are the "failure loads", and the certification requirement is that the "reserve factor", which is the failure load divided by the ultimate load, is more than 1. Obviously, you want it as near to 1 as possible, to keep the weight down.
So, a typical metal light aeroplane is expected to take +3.8/-1.52 without any damage, but will be designed not to fail catastrophically within +5.7/-2.28g; however if it was made from composites using room temperature and low humidity test data the last numbers would be 8.55g/-3.42g. At-least at the start of the aeroplane's life - you can expect the failure loads to get worse through-life.
One other thing - a big well resourced company like Piper or Cessna will tend to get all their reserve factors as near 1.0 as possible. A smaller company however, say Europa or Vans, will probably use larger factors because they can't afford the high cost of really high quality testing and analysis. So, there's a likelihood that such an aeroplane, whilst probably a little heavier, will also be a bit stronger.
G
Join Date: Feb 2007
Location: Amsterdam
Posts: 4,598
Likes: 0
Received 0 Likes
on
0 Posts
Ahhhh, the 152 Aerobat. I would buy one in a split SECOND if one existed (was for sale) and I had the funds. What an awesome custom Cessna. I'd die to get a right-seat ride in a 150 HP Cessna 152.
Or is this a rare case of sarcasm (a concept that seems to be virtually unheard of in the US)?
Nothing wrong with the 150hp aerobat.
Nothing wrong with the standard one, either. I did my aerobatic rating in one.
Not something the average pilot would choose for competition work, for sure! A tad underpowered, but (perhaps partly because of that) a pretty good trainer.
I believe the engine mounts are stronger than standard. I've heard that some of the wing and tailplane skins are thicker. And, of course, they have the double shoulder straps.
Nothing wrong with the standard one, either. I did my aerobatic rating in one.
Not something the average pilot would choose for competition work, for sure! A tad underpowered, but (perhaps partly because of that) a pretty good trainer.
I believe the engine mounts are stronger than standard. I've heard that some of the wing and tailplane skins are thicker. And, of course, they have the double shoulder straps.
Join Date: Oct 2002
Location: Job Centre
Age: 74
Posts: 169
Likes: 0
Received 0 Likes
on
0 Posts
Join Date: Sep 2002
Location: Daventry UK
Posts: 487
Likes: 0
Received 0 Likes
on
0 Posts
In purely practical terms, I'm sure that in the UK far more high wing Cessnas suffer wing buckling than ever do PA-28's.
That's because of what happens when they get blown upside down. I've seen several personally so it can't be a rare phenomenon. Of course it is an almost uniquely UK phenomenon as well because we seem to have some sort of religious objection to tying aircraft down here. Walk away from a Cessna in the US without tying it down and someone will come running after you.. (Or you wake up in the hotel in the middle of the night with palpitations and have to drive to the airport to check ... guess how I know that).
Quite why we have an attitude of bravado toward tie downs in a country that supposedly has the worst weather in the world is a total mystery to me. I've never seen a PA-28 blown over, but I have had mine twizzled round and tie it down much to the amusement of flying club members opposite whenever strong winds are forecast.
That's because of what happens when they get blown upside down. I've seen several personally so it can't be a rare phenomenon. Of course it is an almost uniquely UK phenomenon as well because we seem to have some sort of religious objection to tying aircraft down here. Walk away from a Cessna in the US without tying it down and someone will come running after you.. (Or you wake up in the hotel in the middle of the night with palpitations and have to drive to the airport to check ... guess how I know that).
Quite why we have an attitude of bravado toward tie downs in a country that supposedly has the worst weather in the world is a total mystery to me. I've never seen a PA-28 blown over, but I have had mine twizzled round and tie it down much to the amusement of flying club members opposite whenever strong winds are forecast.
Moderator
A few random thoughts...
The 150 Aerobat is what it is; an affordable and safe aerobatic trainer. Sure it's not the best aerobatic trainer ever, but it is an excellent stepping stone to learning, and if you choose to go on, them you find an aircraft even more capable - and expensive!
The aerobat has a number of different structural and system changes (and has a greater empty weight). A few of the improvements include changes to the structure of the horizontal stabilizer, and it's attachment to the airframe, this was a weak point of older 150's. Some of the Aerobat refinements were carried over to 150M's and 152's. It is, however, the same to fly as a 150, other than increased capability.
I have read remarks here about wings buckling. We have pointed out that 5.7G is the value at which the airframe might suffer really bad failure. How often does an aircraft of this type encounter 5.7G? Remind yourself of the concept on maneuvering speed. Below that speed, it is not possible for the wing to create enough lift to damage the airframe, it will stall first - and that's for 3.8G. If you were to attampt to subject the airframe to 5.7G, you'd have to be going much faster! Very few of us will ever blunder into that realm while flying safely.
Yes, if you blow the plane over on the ground, you're going to wrinkle it. That is a reaction fo the airframe to the ground, which has little "give". You could never get that sudden stop while reacting the airframe to the air - the air will move, and get out of the way, the ground won't. It's comparing apples to oranges.
Though we should be aware of stresses on the airframe, and avoid careless flying which can get you into a bad corner, we don't need to worry about the airframe and wings. They'll hold on!
The 150 Aerobat is what it is; an affordable and safe aerobatic trainer. Sure it's not the best aerobatic trainer ever, but it is an excellent stepping stone to learning, and if you choose to go on, them you find an aircraft even more capable - and expensive!
The aerobat has a number of different structural and system changes (and has a greater empty weight). A few of the improvements include changes to the structure of the horizontal stabilizer, and it's attachment to the airframe, this was a weak point of older 150's. Some of the Aerobat refinements were carried over to 150M's and 152's. It is, however, the same to fly as a 150, other than increased capability.
I have read remarks here about wings buckling. We have pointed out that 5.7G is the value at which the airframe might suffer really bad failure. How often does an aircraft of this type encounter 5.7G? Remind yourself of the concept on maneuvering speed. Below that speed, it is not possible for the wing to create enough lift to damage the airframe, it will stall first - and that's for 3.8G. If you were to attampt to subject the airframe to 5.7G, you'd have to be going much faster! Very few of us will ever blunder into that realm while flying safely.
Yes, if you blow the plane over on the ground, you're going to wrinkle it. That is a reaction fo the airframe to the ground, which has little "give". You could never get that sudden stop while reacting the airframe to the air - the air will move, and get out of the way, the ground won't. It's comparing apples to oranges.
Though we should be aware of stresses on the airframe, and avoid careless flying which can get you into a bad corner, we don't need to worry about the airframe and wings. They'll hold on!
In purely practical terms, I'm sure that in the UK far more high wing Cessnas suffer wing buckling than ever do PA-28's.
That's because of what happens when they get blown upside down. I've seen several personally so it can't be a rare phenomenon. Of course it is an almost uniquely UK phenomenon as well because we seem to have some sort of religious objection to tying aircraft down here. Walk away from a Cessna in the US without tying it down and someone will come running after you.. (Or you wake up in the hotel in the middle of the night with palpitations and have to drive to the airport to check ... guess how I know that).
Quite why we have an attitude of bravado toward tie downs in a country that supposedly has the worst weather in the world is a total mystery to me. I've never seen a PA-28 blown over, but I have had mine twizzled round and tie it down much to the amusement of flying club members opposite whenever strong winds are forecast.
That's because of what happens when they get blown upside down. I've seen several personally so it can't be a rare phenomenon. Of course it is an almost uniquely UK phenomenon as well because we seem to have some sort of religious objection to tying aircraft down here. Walk away from a Cessna in the US without tying it down and someone will come running after you.. (Or you wake up in the hotel in the middle of the night with palpitations and have to drive to the airport to check ... guess how I know that).
Quite why we have an attitude of bravado toward tie downs in a country that supposedly has the worst weather in the world is a total mystery to me. I've never seen a PA-28 blown over, but I have had mine twizzled round and tie it down much to the amusement of flying club members opposite whenever strong winds are forecast.
G