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 Post subject: Re: Citation 501sp
PostPosted: 01 Aug 2021, 23:38 
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As near as I can tell the debate with Mike C. and Ian is which AOA is best glide . .6 or .35.

Mike is saying best glide is at about 0.40.

And I still maintain 0.6 is correct for approach, with flaps in approach position.


I would say that you are both close enough. :thumbup:

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 Post subject: Re: Citation 501sp
PostPosted: 01 Aug 2021, 23:46 
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Testing the AOA vs airspeed in a best glide test won't be completely accurate because of the residual thrust of the engines at idle

That doesn't matter for finding best glide speed and best glide AOA. Those will still occur at minimum drag regardless of some residual thrust in the engines.

The residual thrust will make determining the engine out glide RATIO impossible (residual thrust will increase glide ratio above the true value), but it won't prevent finding the best glide SPEED or AOA.

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 Post subject: Re: Citation 501sp
PostPosted: 01 Aug 2021, 23:58 
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From a Citation study guide:

V2 is 0.7 AOA? Hmm, that seems high, not a lot of stall margin at that point.

Let's check it for C560V, 0 MSL, 15,900 lbs, 15C temps, flaps 15 degrees, data from AFM:

Vs, 88 knots, 1.00 AOA (by definition, stall occurs at AOA = 1.00)
V1, 96 knots, 0.84 AOA
Vr, 103 knots, 0.73 AOA
V2, 111 knots, 0.63 AOA
Venr, 168 knots, 0.27 AOA

So V2 being at 0.7 is a bit too high, should be closer to 0.6.

Best range of 0.35 AOA seems about right, which would also be very close to best glide.

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 Post subject: Re: Citation 501sp
PostPosted: 02 Aug 2021, 00:33 
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Now here's something else to add into the mix.

According to the 501 manual, maximum endurance occurs at best L/D, i.e. a reading of 0.6 on the AOA meter.

Take a look at this holding chart. At SL, 8500 pounds, best holding speed is 145 KIAS. I would have expected it to be closer to 120. Wonder if they bumped up the speed above 0.6 AOA to keep engine speed high enough to ensure proper operation of systems?


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 Post subject: Re: Citation 501sp
PostPosted: 02 Aug 2021, 12:05 
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Username Protected wrote:
Now here's something else to add into the mix.

According to the 501 manual, maximum endurance occurs at best L/D, i.e. a reading of 0.6 on the AOA meter.

Take a look at this holding chart. At SL, 8500 pounds, best holding speed is 145 KIAS. I would have expected it to be closer to 120. Wonder if they bumped up the speed above 0.6 AOA to keep engine speed high enough to ensure proper operation of systems?


Remember that holding speeds have to account for stall protection in 30 degree bank turns.

The .6 AOA is the approach angle of attack and wouldn't be the AOA for holding which is flown in the clean configuration when possible.

As an aside, I looked at the AOA for L/D Max for other planes and found them to be around .3 to .35. A T-38 article said .30. The F-15 is close to that, but differences in wing plan form create differences in AOA's for L/D/Max.

The image of the AOA in a previous post showed a note for AOA to climb to the next level. That's useful so that a climb isn't done too early, burning a lot of fuel just to hang on to the higher altitude; a common mistake resulting in overburning on fuel, and can be a range killer.

Besides the use of AOA, knowing target pitch attitudes on the ADI for the plane is helpful, as during the takeoff and climb, maneuvering is done on the ADI and it helps to know the pitch attitude for 250 knots, and after that the pitch attitude to accelerate to enroute climb speed, with some variations for weight..


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 Post subject: Re: Citation 501sp
PostPosted: 02 Aug 2021, 12:32 
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Remember that holding speeds have to account for stall protection in 30 degree bank turns.

The .6 AOA is the approach angle of attack and wouldn't be the AOA for holding which is flown in the clean configuration when possible.

Yes, maybe they added a turning stall margin. So the numbers for holding may be at an AOA less than 0.6. Or, I believe if you are in a turn and holding the same AOA, your airspeed has to increase anyway compared to wings-level flight.

In the manual, best endurance is given at 0.6 and I'm pretty sure that's in the clean configuration. 0.6 is also for approach, but with flaps in approach position, as that's how approach speeds are defined.

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 Post subject: Re: Citation 501sp
PostPosted: 02 Aug 2021, 12:49 
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Username Protected wrote:
Remember that holding speeds have to account for stall protection in 30 degree bank turns.

The .6 AOA is the approach angle of attack and wouldn't be the AOA for holding which is flown in the clean configuration when possible.

Yes, maybe they added a turning stall margin. So the numbers for holding may be at an AOA less than 0.6. Or, I believe if you are in a turn and holding the same AOA, your airspeed has to increase anyway compared to wings-level flight.

In the manual, best endurance is given at 0.6 and I'm pretty sure that's in the clean configuration. 0.6 is also for approach, but with flaps in approach position, as that's how approach speeds are defined.


The F-15 AOA below is on a 0-30 scale, where the Citation is on a 0 to 1.0 but can be compared to percentages of stall AOA.

In the F-15 AOA, cruise AOA is much less than approach AOA, and the .6 for best endurance in your manual which is also approach AOA doesn't compute. I could be missing something, can you show your manual page?

In the scenario of dual engine failure, is there any consideration for minimum airspeed for engine windmilling to run engine-driven hydraulic pumps? There is on the 747, 160 knots to have hydraulics for flight controls, even though a best glide speed could be lower.

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 Post subject: Re: Citation 501sp
PostPosted: 02 Aug 2021, 14:32 
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Hi Paul,

Ian posted a page from the Ops Manual in this post:
viewtopic.php?p=2923276#p2923276

The manual states 200KIAS for air restart. Nothing about hydraulics. Flight controls are manual so loss of hydraulics only kills gear (N2 blow down bottle for that), speedbrakes, and TRs (which are worthless w/o engines anyway).


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 Post subject: Re: Citation 501sp
PostPosted: 02 Aug 2021, 15:30 
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Username Protected wrote:
Hi Paul,

Ian posted a page from the Ops Manual in this post:
viewtopic.php?p=2923276#p2923276

The manual states 200KIAS for air restart. Nothing about hydraulics. Flight controls are manual so loss of hydraulics only kills gear (N2 blow down bottle for that), speedbrakes, and TRs (which are worthless w/o engines anyway).



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

Even this T-38 AOA shows a lower AOA for endurance and range than for approach. :shrug:


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 Post subject: Re: Citation 501sp
PostPosted: 02 Aug 2021, 15:42 
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Username Protected wrote:
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)?


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 Post subject: Re: Citation 501sp
PostPosted: 02 Aug 2021, 16:34 
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Username Protected wrote:
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)?


I think all AOA's of the type in jets run the the flap position through the computer to compensate for flap settings, but the .35 to .60 is more of a spread for max range and max endurance than other AOA's. Also max range and max endurance are considerations for speeds and fuel flows for an aircraft under power, but best glide is max range with no power.. That should confuse things even more. :D

Whatever one comes up with for a AOA for best glide, it isn't likely to be a constant in the descent as the headwind/tailwind component changes require a change in AOA (speed) to compensate; higher speed into a headwind for max range. Just adding to the confusion. :D

This would be a good discussion about AOA's at the bar after a day in ground school, but if it's brought up for debate in ground school, a half a day could be blown. ;)

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 Post subject: Re: Citation 501sp
PostPosted: 02 Aug 2021, 17:07 
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Username Protected wrote:
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?


Correct. All turbine equipment will have flap compensation. The term for that is "Normalized AOA", such that stall is always 1.0.


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 Post subject: Re: Citation 501sp
PostPosted: 02 Aug 2021, 19:19 
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Username Protected wrote:
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?


Correct. All turbine equipment will have flap compensation. The term for that is "Normalized AOA", such that stall is always 1.0.

Additionally, this video mentions that selecting anti-ice in the V, Ultra, and Encore will bump up the AOA reading a bit because it's presumed that you have ice on the wings, which will cause a stall at a lower angle of attack than normal:

[youtube]https://youtu.be/ZMHNdbZVlGk[/youtube]
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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 00:06 
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Even this T-38 AOA shows a lower AOA for endurance and range than for approach.

There is no rule that states approach speed must be at best L/D. In the case of the 501, it is a happy coincidence.

Approach speed must be a safe margin above stall speed but not so high as to require excessive runway length or excessive speed reduction prior to landing (unstabilized approach).

In the case of the T-38, in order to obtain a reasonably slow approach speed I suspect approach AOA is way above best L/D to get more lift from the thin, symmetrical wing at the cost of high drag, compensated by relatively high power settings on the engines (which also lowers spool-up time in the event of a go-around). Not to mention that the T-38 has twice the wing loading of the Citation.

If you flew an approach in a T-38 at best L/D speed you'd need a really long runway.

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 Post subject: Re: Citation 501sp
PostPosted: 03 Aug 2021, 00:47 
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Username Protected wrote:
There is no rule that states approach speed must be at best L/D.

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.

Quote:
In the case of the 501, it is a happy coincidence.

Vref and best L/D are not coincident on the 501.

Quote:
If you flew an approach in a T-38 at best L/D speed you'd need a really long runway.

That's true for a 501, too, you will be way too fast.

Mike C.

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