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28 Mar 2024, 07:56 [ UTC - 5; DST ]


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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 01:13 
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Thanks Chris. I just don't see how .6 can be L/D Max for landing and also best glide, but I see what the conflict is. I guess we need a Naval Aviator to explain it to us. :D

Yep, hence my question to the BT braintrust :)

Note that the Citation AoA does have a flap position indicator and that the gauge will "compensate" for flap position. So perhaps as Ian has stated, .6 could be the correct indication for both because the gauge is compensating for appr and landing flaps?

All that being said, if I read the words in the Ops Manual, .6 is listed as max endurance (time) and .35 as max range (distance). Doesn't that mean that .35 is max glide distance (which Mike C has backed us into with some math)?

According to aerodynamic theory, in an unpowered aircraft, best glide ratio happens at, and is equal to, the best L/D ratio. According to the Citation 501 manual, best L/D is at an AOA reading of 0.6.

In jets, best endurance, Vx, and best power off glide are all at best L/D. Best range for jets in still air is when the ratio of velocity to drag is at maximum, and this occurs at an AOA less than best L/D. You can download a pdf copy of Aerodynamics for Naval Aviators from the FAA website and read all about it.

As for Mike's mathematical formula, it assumes certain conditions that may not be true in practice as far as the way the Citation's AOA gauge functions.
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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 01:39 
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Vref and best L/D are not coincident on the 501.
Mike C.

The manual states that Vapp, not Vref, is at an AOA reading of 0.6, same as best L/D. Vapp is defined with approach flaps on. The AOA of 0.6 is not meant for a zero-flaps approach as far as I can tell. Instead, the manual calls for adding 20 knots to the normal V speeds in the event of a zero flaps approach and landing.

Pretty clearly for gliding purposes best L/D of 0.6 is meant for flaps up. Although the AOA gauge compensates for flap position, I'm not sure if this means best L/D with flaps down is also attained at an AOA reading of 0.6.

So strictly speaking perhaps best approach speed with flaps is not best glide speed with flaps. Another thing to flight test, but who wants to glide with flaps down?

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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 11:36 
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Username Protected wrote:
Vref and best L/D are not coincident on the 501.
Mike C.

The manual states that Vapp, not Vref, is at an AOA reading of 0.6, same as best L/D. Vapp is defined with approach flaps on. The AOA of 0.6 is not meant for a zero-flaps approach as far as I can tell. Instead, the manual calls for adding 20 knots to the normal V speeds in the event of a zero flaps approach and landing.

Pretty clearly for gliding purposes best L/D of 0.6 is meant for flaps up. Although the AOA gauge compensates for flap position, I'm not sure if this means best L/D with flaps down is also attained at an AOA reading of 0.6.

So strictly speaking perhaps best approach speed with flaps is not best glide speed with flaps. Another thing to flight test, but who wants to glide with flaps down?


In the Citation AOA video, the pilot cautioned that getting below about .35 would stop the climb or if at cruise go into a high drag, reverse command regime requiring a descent to regain speed. but yet we are to believe that .6 is best L/d Max which doesn't compute to me. The climb AOA is about what others do, but the .6 doesn't make sense, Edit, although that's what Cessna says.

Everybody seems to be convinced of their position here, so I guess we are :deadhorse: :D

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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 12:14 
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Username Protected wrote:
In jets, best endurance, Vx, and best power off glide are all at best L/D. Best range for jets in still air is when the ratio of velocity to drag is at maximum, and this occurs at an AOA less than best L/D. You can download a pdf copy of Aerodynamics for Naval Aviators from the FAA website and read all about it.

As for Mike's mathematical formula, it assumes certain conditions that may not be true in practice as far as the way the Citation's AOA gauge functions.

Ian,

Again, just reading the English in the manual, .6 is best lift/drag which would give you the greatest endurance (time) but not necessarily speed. .35 is listed as best velocity/drag which means most speed and the manual states is max range.

That goes against what you posted above (that all are at .6). So then what is .35 for and why is it called out in the manual?


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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 12:34 
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Username Protected wrote:
In jets, best endurance, Vx, and best power off glide are all at best L/D. Best range for jets in still air is when the ratio of velocity to drag is at maximum, and this occurs at an AOA less than best L/D. You can download a pdf copy of Aerodynamics for Naval Aviators from the FAA website and read all about it.

As for Mike's mathematical formula, it assumes certain conditions that may not be true in practice as far as the way the Citation's AOA gauge functions.

Ian,

Again, just reading the English in the manual, .6 is best lift/drag which would give you the greatest endurance (time) but not necessarily speed. .35 is listed as best velocity/drag which means most speed and the manual states is max range.

That goes against what you posted above (that all are at .6). So then what is .35 for and why is it called out in the manual?


I’m stating the obvious here but best range and best endurance are never the same.
To find best endurance in any aircraft just pull the power levers back to whatever speed you feel comfortable above stall. That will burn the least fuel per time unit=best endurance.

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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 14:54 
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>To find best endurance in any aircraft just pull the power levers back to whatever speed you feel comfortable above stall.

No not true....
Once you are below minimum sink it takes more power to maintain flight.
IE once you start getting really slow when practicing slow flight you need to add power to maintain altitude.
(Also some theoretical differences here between prop and jet, having to to do with the fact that the prop imparts the energy to a larger "Disk" of air vs the jet putting all the energy into a smaller disk. so the relative velocity of the accelerated air is lower and power imparted/lost to the air goes up as Velocity ^2)


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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 15:58 
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Username Protected wrote:
The rule in part 23 is "no less than" 1.3 Vso, stall speed.

So basically everyone sets it to 1.3 Vso.

That isn't best L/D.

Mike C.

Not all jets have Vref at 1.3 Vso. The Beechcraft Premier 1A, for example, is less than 1.3 Vso.

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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 18:31 
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Username Protected wrote:
In jets, best endurance, Vx, and best power off glide are all at best L/D. Best range for jets in still air is when the ratio of velocity to drag is at maximum, and this occurs at an AOA less than best L/D. You can download a pdf copy of Aerodynamics for Naval Aviators from the FAA website and read all about it.

As for Mike's mathematical formula, it assumes certain conditions that may not be true in practice as far as the way the Citation's AOA gauge functions.

Ian,

Again, just reading the English in the manual, .6 is best lift/drag which would give you the greatest endurance (time) but not necessarily speed. .35 is listed as best velocity/drag which means most speed and the manual states is max range.

That goes against what you posted above (that all are at .6). So then what is .35 for and why is it called out in the manual?

Chris, I don't believe I stated that best range is at 0.6. Per the manual, it is at 0.35.

I just had an email exchange with my friend who has a 501 and he confirms it's AOA 0.6 for approach (flaps down) and 0.35 for best range.
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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 19:02 
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Username Protected wrote:
Chris, I don't believe I stated that best range is at 0.6. Per the manual, it is at 0.35.

I just had an email exchange with my friend who has a 501 and he confirms it's AOA 0.6 for approach (flaps down) and 0.35 for best range.

Wait. Doesn't best range = furthest glide = best glide? So the answer to best glide is .35. :dancing:


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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 19:28 
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Username Protected wrote:
Chris, I don't believe I stated that best range is at 0.6. Per the manual, it is at 0.35.

I just had an email exchange with my friend who has a 501 and he confirms it's AOA 0.6 for approach (flaps down) and 0.35 for best range.

Wait. Doesn't best range = furthest glide = best glide? So the answer to best glide is .35. :dancing:

No, because gliding is power off, but for best range it's level flight, power on, and jets produce thrust approximately in proportion to fuel flow. For prop planes, however, best range is actually at best L/D.

Now to really complicate matters. It's clear to me from theory that best power off glide speed is at best L/D, and best L/D is at one specific angle of attack. Same for Vx. Stall clearly also happens at one specific AOA.

What is not clear to me from theory is why best endurance occurs at one specific AOA. According to a Boeing article I read, AOA will vary somewhat during a flight in which range is optimized. So I wonder if the 0.35 figure for best range is just what Cessna empirically determined it to be at a typical weight. I suspect for best range you're better off looking at ground speed and fuel flow.
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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 21:07 
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Username Protected wrote:
What is not clear to me from theory is why best endurance occurs at one specific AOA.


It doesn’t necessarily. It does if the thrust specific fuel consumption is constant. If you have the thrust curves there can be a correction applied to maximize endurance.

In other words, endurance is a function of 3 things: 1) directly to L/D, 2) inversely to TSFC and 3) directly to the weight of the fuel consumed (initial/final weights)

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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 21:35 
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Username Protected wrote:
What is not clear to me from theory is why best endurance occurs at one specific AOA.


It doesn’t necessarily. It does if the thrust specific fuel consumption is constant. If you have the thrust curves there can be a correction applied to maximize endurance.

In other words, endurance is a function of 3 things: 1) directly to L/D, 2) inversely to TSFC and 3) directly to the weight of the fuel consumed (initial/final weights)

Thanks, Tyler. That's what I gathered from the attached Boeing article.

I also understand that Vy for jets cannot directly be determined from AOA for similar reasons.

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 Post subject: Re: Citation 501sp
PostPosted: 04 Aug 2021, 01:22 
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Username Protected wrote:
Not all jets have Vref at 1.3 Vso. The Beechcraft Premier 1A, for example, is less than 1.3 Vso.

That seems contrary to certification regs which state not less than 1.3 Vso. This would also mean an AOA of greater than 0.6.

Can you give a numerical example at some landing weight and configuration?

Mike C.
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 Post subject: Re: Citation 501sp
PostPosted: 04 Aug 2021, 13:43 
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Username Protected wrote:
Can you give a numerical example at some landing weight and configuration?

Mike C.

I'll see if I can find it. A buddy of mine got typed in a Premier 1A and found the Vref calculation unique. Something like 1.26 Vso.

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 Post subject: Re: Citation 501sp
PostPosted: 04 Aug 2021, 18:18 
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