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 Post subject: Design G Load Limits
PostPosted: 05 May 2012, 06:18 
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Joined: 09/09/11
Posts: 58
Location: Concord, California
Aircraft: 1968 BE-36 Bonanza
Gentlemen:

I need some engineering explanations for the Bonanza aircraft. My aircraft is certificated in the Utility catagory and is rated at 4.4 g's for its maneuvering speed and I believe 6.6 g's at the ultimate airframe limit.

My question is simple: Why do these limits decline with flaps or gear extended? Wheel wells? Spar loads? What causes the airplane to want to bend or break under lower g forces?

I was attending a BPPP training clinic this past weekend in Concord, CA and when I asked an instructor this question, he did not know. I know that their is an answer to this question.

Mark Young
Concord, CA
1968 Model 36, SN E-72, N7604N

P.S. This was my first and initial Bonanza Pilot Proficiency Program attendance. Since I was able to review the materials from a friend's earlier attendance, I had modest expectations of how much new material I would gather. I was very surprised how much material I was able to gather. I would compare it to assembling 900 pieces of a 1000 piece jigsaw puzzel and then someone shows up with the missing 100 pieces. You get a great many "ah-hah" moments as the lone pieces drop into place.

I do not mean for this to be an advertisement for BPPP. I have no connections to them but it was a very solid presentation. The mechanical walk-around with one of their mechanics was excellent. You also get to ask questions of other motivated Bonanza pilots.


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 Post subject: Re: Design G Load Limits
PostPosted: 05 May 2012, 08:40 
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Company: Orion Endeavors Inc.
Location: Gulf Shores, AL (KJKA)
Aircraft: 1982 Baron 58P
Mark

Let me suggest you join the American Bonanza Society, or at least take advantage of their 1 month free trial. You will then be able to buy several of the excellent books they have by authors (Colvin, Eckalvar, Ball, etc) that give all sorts of details about the questions you are asking. They also have several mechanically oriented Guides that you would profit from having.


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 Post subject: Re: Design G Load Limits
PostPosted: 05 May 2012, 11:08 
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Location: Chicago, IL (KUGN)
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The short answer is when you change configuration the wing loading profile changes and stress concentrations occurr on spars, stringers, gussets, skins, tail surfaces, etc. when dirty in places they do not when clean. The structural engineer's task is to optimize the airframe's weight/strength (less structure = less weight = more useful load) against a defined performance envelope of known speeds and empannage configurations. Gear and flaps down at 165 kias generate drag and lift profile stresses that can be designed for, but the expense of useful load, CG, etc. For example Matt Younkin's aerobatic B-18 does things that would rip a standard 18 to pieces, but it is highly modified so it can survive the forces at the expense of other performance parameters.

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 Post subject: Re: Design G Load Limits
PostPosted: 05 May 2012, 15:03 
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Joined: 09/09/11
Posts: 58
Location: Concord, California
Aircraft: 1968 BE-36 Bonanza
Thank you. I am a member of ABS and have Colvin and Eckalvar's books. They speak around this issue but not directly as to what is happening inside the wing and airframe. I will check again.

Someone has suggested to me that it may be a certification limit--the FAA required the certification to a certain g load and the structures were not tested beyond this the required limit. Perhaps.

I have gone through a good number of ABS magazine back issues and there may be more information about forces are operating on aircraft structures are but I have not located anything more than reference to the basic limit numbers. With no search engine for the magazine, it is going to take a while.

I can see that the flaps seem the main generator of the lower g force limits. I can put the gear down at 152 kts and I am told that this limit is there to protect the gear doors. The flaps only come out below 122 kts. But this is just a guess on my part and these speed limits may exist for lots of other reasons.

There should be some very good data on these issues given the V-tail analysis done in the 1980's. Although they were looking at the empenage my guess is that they compiled lots of other data as well.

This may be stuff for engineers to talk amongst themselves but I am trying to translate these g load limit numbers into the real world of flying. I can easily comprehend V(a) speeds but it seems it whould be very easy to obtain excessive g loads in certain flap and gear confirgurations. What other type of operations put you in the neighborhood of these lower design limits?

Thanks again.

M. Young, Concord, CA


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 Post subject: Re: Design G Load Limits
PostPosted: 05 May 2012, 15:30 
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Username Protected wrote:
The short answer is when you change configuration the wing loading profile changes and stress concentrations occurr on spars, stringers, gussets, skins, tail surfaces, etc. when dirty in places they do not when clean. The structural engineer's task is to optimize the airframe's weight/strength (less structure = less weight = more useful load) against a defined performance envelope of known speeds and empannage configurations. Gear and flaps down at 165 kias generate drag and lift profile stresses that can be designed for, but the expense of useful load, CG, etc. For example Matt Younkin's aerobatic B-18 does things that would rip a standard 18 to pieces, but it is highly modified so it can survive the forces at the expense of other performance parameters.



Matt's airplane is not structurally modified. He flies it light and within the normal category speeds and load factor limits... The airplane doesn't happen to know that it's upside down... With all that said, I have flown his airplane quite a few times and any Twin-Beech Drivers who think they are going to imitate Matt, beware.... His airplane is a -C model, not a -D and it does not have the tail incidence mod, and it flies very differently than a -D or later model Beech. Do not try to fly his act in your C-45 H or D-18...

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 Post subject: Re: Design G Load Limits
PostPosted: 22 May 2012, 02:58 
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Mark,

Here is my attempt at an answer...

The G load for flaps extended operation is covered by FAR 23.345, which requires a positive load limit of 2 + a 25 fps gust at VF.

As you know, the FARs require a utility category aeroplane to withstand a load of 4.4G, and that is what Beech have put in the manual despite the airframe likely to be capable of more. I expect the flaps extended load limit is similarly a number to put in the manual that complies with the minimum required by the applicable FAR, rather than the actual limit of the airframe.

I suspect that the wing spars and airframe are still good for 4.4G with the flaps extended, however the flap tracks are likely the limiting factor considering the significant load on them at VF. (My SWAG for VF is 104 KCAS for the A36, with VF defined in FAR 23.345)

Simon


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 Post subject: Re: Design G Load Limits
PostPosted: 22 May 2012, 12:08 
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Location: Minneapolis, MN (KFCM)
Aircraft: 1970 Baron B55
First of all, AFaIK the landing gear position has no impact on the allowable g-forces. In the Bonanza the max positive G load is reduced when flaps are extended and I've always assumed that this is because extended flaps generate increased (localized) stress on some components of the wing and/or wing attach structure. And a related issue is the effects of flap extension on Va, something I've not seen mentioned in any POH I've read.

This post got me to check what the g-force limits in my B55 are in various configurations and I found that neither the POH nor the TCDS specifies any reduction with flaps or gear extended. The POH only provides the "Flight Load Factors" with "Flaps up" so one might assume they are different with flaps down but it looks like one is left to guess what that might be. I wonder if CAR 3 has a standard for this?

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 Post subject: Re: Design G Load Limits
PostPosted: 23 May 2012, 07:37 
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Username Protected wrote:
First of all, AFaIK the landing gear position has no impact on the allowable g-forces. In the Bonanza the max positive G load is reduced when flaps are extended and I've always assumed that this is because extended flaps generate increased (localized) stress on some components of the wing and/or wing attach structure. And a related issue is the effects of flap extension on Va, something I've not seen mentioned in any POH I've read.

This post got me to check what the g-force limits in my B55 are in various configurations and I found that neither the POH nor the TCDS specifies any reduction with flaps or gear extended. The POH only provides the "Flight Load Factors" with "Flaps up" so one might assume they are different with flaps down but it looks like one is left to guess what that might be. I wonder if CAR 3 has a standard for this?

I also think in addition to Lance's explanations, lowering the flaps, reduces the stall speed and in some situations of turbulence the stresses of vertical wind loads rely on the wings stalling to unload the load factors. If flaps are extended then the wing loads will be higher before a stall occurs. Thus the inbuilt protection against bending the a/c is compromised by adding more lift by using flaps. Think about the common complaint of airliners dropping suddenly during turbulence. This is the wings unloading the load factors by not creating lift(dropping). Airliners have a much lower G-Force limits than a Bonanza. I have seen the aftermath of a Cirrus pulled out of a 260 knot dive in IMC (out of control). Even though the wings did not rip off,the Flaps and Ailerons would have made a beautful "V " tails and were permanently bent. Amazingly the a/c was still controlable. Just my 2 cents worth.


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 Post subject: Re: Design G Load Limits
PostPosted: 23 May 2012, 09:34 
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One thing I read about maximum g limits was that they do not necessarily mean the aircraft is undamaged from a maximum g event. Only that it will keep flying. Not sure where I heard that, anybody else? :scratch:


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 Post subject: Re: Design G Load Limits
PostPosted: 23 May 2012, 11:27 
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Location: Hazelwood, MO (KALN)
Aircraft: BE35G
Design Limit Load (DLL) allows for non-permament deformation of the structure, i.e, application of the load bends the part but it returns to original shape after load is removed.

Design Ultimate Load (DUL) (DUL = 150% DLL in most cases) allows for permament plastic deformation but does not break/fracture. This provides 50% margin to DLL to account for manufacturing tolerances. Remember the test is done on one article and does not provide a statiscal average of results. If the test article has all high side tolerances it could be stronger than the production article that has average manufacturing tolerances. Aircraft are statically tested to demonstrate design load 100% and 150% capability. Sometimes they will take the structure to failure if it is above 150% to see where it will break. But breaking one a/c does not provide a statistically good assumption that your a/c would not break until reaching that load. Manufacturing tolerances will affect this result.

Bottom line, going over 100% DLL puts you into allowable plastic deformation region, you may likely bend parts and due to manufacturing tolerances you will be heading towards structural failure that may or may not occur before you get to 150% DLL.


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 Post subject: Re: Design G Load Limits
PostPosted: 23 May 2012, 20:10 
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Joined: 12/10/07
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Location: Minneapolis, MN (KFCM)
Aircraft: 1970 Baron B55
Username Protected wrote:
First of all, AFaIK the landing gear position has no impact on the allowable g-forces. In the Bonanza the max positive G load is reduced when flaps are extended and I've always assumed that this is because extended flaps generate increased (localized) stress on some components of the wing and/or wing attach structure. And a related issue is the effects of flap extension on Va, something I've not seen mentioned in any POH I've read.

This post got me to check what the g-force limits in my B55 are in various configurations and I found that neither the POH nor the TCDS specifies any reduction with flaps or gear extended. The POH only provides the "Flight Load Factors" with "Flaps up" so one might assume they are different with flaps down but it looks like one is left to guess what that might be. I wonder if CAR 3 has a standard for this?

I also think in addition to Lance's explanations, lowering the flaps, reduces the stall speed and in some situations of turbulence the stresses of vertical wind loads rely on the wings stalling to unload the load factors. If flaps are extended then the wing loads will be higher before a stall occurs. Thus the inbuilt protection against bending the a/c is compromised by adding more lift by using flaps. Think about the common complaint of airliners dropping suddenly during turbulence. This is the wings unloading the load factors by not creating lift(dropping). Airliners have a much lower G-Force limits than a Bonanza. I have seen the aftermath of a Cirrus pulled out of a 260 knot dive in IMC (out of control). Even though the wings did not rip off,the Flaps and Ailerons would have made a beautful "V " tails and were permanently bent. Amazingly the a/c was still controlable. Just my 2 cents worth.


While lowering flaps does generally decrease stall speeds that by itself (i.e. sans any loss of structural strength or force distribution) will not affect the tolerable level of g force. It does imply that any specific g-force can be achieved at lower speeds with flaps than without and that Va will be lower.
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Advice in this post may contain errors. Using said advice is totally at the risk of the user.


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