18 Apr 2024, 07:06 [ UTC - 5; DST ]
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Post subject: Re: Citation 501sp Posted: 28 Jul 2021, 21:23 |
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Joined: 11/06/20 Posts: 1298 Post Likes: +1290 Location: Tulsa, OK - KRVS
Aircraft: C501SP
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Hi Everyone,
I have my type rating scheduled for mid-October. Until then I am flying missions w/ my instructor/mentor. During our missions he has been quizzing me on systems and running me through some emergencies to see if I have the memory items down.
One of the things he asked last time was best glide speed in case both engines failed (fuel iced up due to no prist or something).
I read in the AFM that Maximum Glide is 120KIAS + 3 kts for each additional 500lbs.
It seems like that would be really hard to calculate in the middle of an emergency (or maybe I should be calculating it in real time as I burn fuel?). I got to thinking, is there a way to use AoA (assuming it was working) to get max glide? I was just reading in the Operating Manual that .6 is max L/D which is max endurance and max rate of climb. It then lists .35 as greatest velocity to drag and is max range in still air.
There was talk earlier in the thread about AoA and L/D ratios but I can't find it now.
Can I just fly .35 AoA for max glide?
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Post subject: Re: Citation 501sp Posted: 28 Jul 2021, 22:05 |
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Joined: 02/15/21 Posts: 2530 Post Likes: +1254
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Username Protected wrote: Hi Everyone,
I have my type rating scheduled for mid-October. Until then I am flying missions w/ my instructor/mentor. During our missions he has been quizzing me on systems and running me through some emergencies to see if I have the memory items down.
One of the things he asked last time was best glide speed in case both engines failed (fuel iced up due to no prist or something).
I read in the AFM that Maximum Glide is 120KIAS + 3 kts for each additional 500lbs.
It seems like that would be really hard to calculate in the middle of an emergency (or maybe I should be calculating it in real time as I burn fuel?). I got to thinking, is there a way to use AoA (assuming it was working) to get max glide? I was just reading in the Operating Manual that .6 is max L/D which is max endurance and max rate of climb. It then lists .35 as greatest velocity to drag and is max range in still air.
There was talk earlier in the thread about AoA and L/D ratios but I can't find it now.
Can I just fly .35 AoA for max glide? Chris, 0.6 is best L/D and therefore best glide. What you might look into is figuring out what nose down pitch angle gives you 0.6 on the AOA with power off and then in a real dual flameout situation you can initially pitch for that attitude (after trading speed for altitude if you are fast) and then look for 120kts and fine-tune it to 0.6 on the AOA. This is assuming you don't want to try an air restart, which per the manual needs 200 KIAS. By the way, I'm glad you didn't take my advice and use the DA42 for the MEL. I had my first flight scheduled the other day in a DA42 but we had a dual ECU failure on the right engine and had to scrub. No telling when the plane will be back in service. So looks like I won't be able to finish the MEL before my summer vacation ends. Everything happens for a reason. Happy at least that I can spend more time with my family.
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Post subject: Re: Citation 501sp Posted: 28 Jul 2021, 23:08 |
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Joined: 11/06/20 Posts: 1298 Post Likes: +1290 Location: Tulsa, OK - KRVS
Aircraft: C501SP
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Username Protected wrote: Chris, 0.6 is best L/D and therefore best glide.
What you might look into is figuring out what nose down pitch angle gives you 0.6 on the AOA with power off and then in a real dual flameout situation you can initially pitch for that attitude (after trading speed for altitude if you are fast) and then look for 120kts and fine-tune it to 0.6 on the AOA. This is assuming you don't want to try an air restart, which per the manual needs 200 KIAS.
By the way, I'm glad you didn't take my advice and use the DA42 for the MEL. I had my first flight scheduled the other day in a DA42 but we had a dual ECU failure on the right engine and had to scrub. No telling when the plane will be back in service.
So looks like I won't be able to finish the MEL before my summer vacation ends. Everything happens for a reason. Happy at least that I can spend more time with my family. Yeah, I guess that is my question for the BT braintrust. What is the difference between maximum endurance (.6 AoA) and maximum range (.35 AoA)? Sorry about your problems w/ the DA42. There is one that operates out of my FBO and I do like how quiet it is at idle. I bet it flies nice. The Seminole flew like a truck, but it is built like one too.
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Post subject: Re: Citation 501sp Posted: 28 Jul 2021, 23:12 |
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Joined: 12/03/14 Posts: 19252 Post Likes: +23622 Company: Ciholas, Inc Location: KEHR
Aircraft: C560V
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Username Protected wrote: Chris, 0.6 is best L/D and therefore best glide. I don't think that is correct. I think it is around 0.35 for Citations. 0.60 is Vref for landing, a speed that is 1.3 Vso, but that isn't best glide, just the slowest speed which is safe to fly an approach. Go fly the AFM 120 + 3 KIAS per 500 lbs speed and see what the AOA gauge reads. I suspect it will be quite far from 0.60. This would also be a good time to check AOA calibration by taking it to stall and see that happens at 1.0 on the gauge. I've seen a number of AOA systems which are miscalibrated. Mike C.
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Post subject: Re: Citation 501sp Posted: 28 Jul 2021, 23:48 |
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Joined: 02/15/21 Posts: 2530 Post Likes: +1254
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Username Protected wrote: Chris, 0.6 is best L/D and therefore best glide. I don't think that is correct. I think it is around 0.35 for Citations. 0.60 is Vref for landing, a speed that is 1.3 Vso, but that isn't best glide, just the slowest speed which is safe to fly an approach. Go fly the AFM 120 + 3 KIAS per 500 lbs speed and see what the AOA gauge reads. I suspect it will be quite far from 0.60. This would also be a good time to check AOA calibration by taking it to stall and see that happens at 1.0 on the gauge. I've seen a number of AOA systems which are miscalibrated. Mike C. In theory a power-off airplane should achieve max glide at best L/D. But good point, maybe in real life the AOA gauge reads other than 0.6 at best glide. Always test assumptions.
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Post subject: Re: Citation 501sp Posted: 29 Jul 2021, 00:43 |
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Joined: 12/03/14 Posts: 19252 Post Likes: +23622 Company: Ciholas, Inc Location: KEHR
Aircraft: C560V
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Username Protected wrote: In theory a power-off airplane should achieve max glide at best L/D. But good point, maybe in real life the AOA gauge reads other than 0.6 at best glide. I don't think AOA 0.6 is best glide on any airplane. Too slow, lots of induced drag. This may help: Attachment: Coefficients_of_Drag_and_Lift_vs_AOA.jpg The stall occurs at, say, about 18 degrees. This is AOA of 1.0, the stall angle of attack. Best L/D occurs about 6 degrees. This is 0.33 AOA. 0.6 AOA is about 11 degrees. Well off best L/D. It is a safe speed to go slow for landing, but is not an optimal glide speed. Max endurance will be at a higher AOA, slower airspeed. Max endurance is longest time aloft, but that's not best glide speed. Mike C.
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Post subject: Re: Citation 501sp Posted: 29 Jul 2021, 01:07 |
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Joined: 02/15/21 Posts: 2530 Post Likes: +1254
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Username Protected wrote: In theory a power-off airplane should achieve max glide at best L/D. But good point, maybe in real life the AOA gauge reads other than 0.6 at best glide. I don't think AOA 0.6 is best glide on any airplane. Too slow, lots of induced drag. This may help: Attachment: Coefficients_of_Drag_and_Lift_vs_AOA.jpg The stall occurs at, say, about 18 degrees. This is AOA of 1.0, the stall angle of attack. Best L/D occurs about 6 degrees. This is 0.33 AOA. 0.6 AOA is about 11 degrees. Well off best L/D. It is a safe speed to go slow for landing, but is not an optimal glide speed. Max endurance will be at a higher AOA, slower airspeed. Max endurance is longest time aloft, but that's not best glide speed. Mike C. I'm just going by what the manual says.
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Post subject: Re: Citation 501sp Posted: 29 Jul 2021, 02:27 |
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Joined: 02/15/21 Posts: 2530 Post Likes: +1254
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Username Protected wrote: The stall occurs at, say, about 18 degrees. This is AOA of 1.0, the stall angle of attack.
Best L/D occurs about 6 degrees. This is 0.33 AOA.
I don't think the numbers on the Citation AOA gauge refer to the present angle of attack as a fraction of the stall angle of attack. They are meant to represent proportion of available lift being utilized. But even then, maybe the scaling is somewhat arbitrary. I flew in a 501 a couple weeks ago and for a while we were at 5k at best range speed (works out to 500pph/side). Unfortunately I didn't take a look at the AOA to see if it read 0.35.
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Post subject: Re: Citation 501sp Posted: 29 Jul 2021, 07:32 |
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Joined: 03/09/13 Posts: 911 Post Likes: +449 Location: Byron Bay,NSW Australia
Aircraft: CE525,PA31
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Username Protected wrote: Chris, 0.6 is best L/D and therefore best glide.. I’ve always be thought .6 at FSI when we do dual engine outs. Andrew
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Post subject: Re: Citation 501sp Posted: 29 Jul 2021, 10:46 |
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Joined: 11/06/20 Posts: 1298 Post Likes: +1290 Location: Tulsa, OK - KRVS
Aircraft: C501SP
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Username Protected wrote: I don't think the numbers on the Citation AOA gauge refer to the present angle of attack as a fraction of the stall angle of attack. They are meant to represent proportion of available lift being utilized. But even then, maybe the scaling is somewhat arbitrary.
I flew in a 501 a couple weeks ago and for a while we were at 5k at best range speed (works out to 500pph/side). Unfortunately I didn't take a look at the AOA to see if it read 0.35. Thank you for posting the Ops Manual page. That is what cause me to ask the question. So if .6 is best L/D and best glide, what is .35? They listed it in the Ops Manual for a reason. When would you use .35? Maximum Range sounds like best glide, but that is why I am asking the question
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Post subject: Re: Citation 501sp Posted: 29 Jul 2021, 11:48 |
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Joined: 02/15/21 Posts: 2530 Post Likes: +1254
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Username Protected wrote: I don't think the numbers on the Citation AOA gauge refer to the present angle of attack as a fraction of the stall angle of attack. They are meant to represent proportion of available lift being utilized. But even then, maybe the scaling is somewhat arbitrary.
I flew in a 501 a couple weeks ago and for a while we were at 5k at best range speed (works out to 500pph/side). Unfortunately I didn't take a look at the AOA to see if it read 0.35. Thank you for posting the Ops Manual page. That is what cause me to ask the question. So if .6 is best L/D and best glide, what is .35? They listed it in the Ops Manual for a reason. When would you use .35? Maximum Range sounds like best glide, but that is why I am asking the question 0.35 per the manual is for best range in still air, i.e. best mileage.
In a prop plane, best range is achieved at best L/D. However, because of the way jet engines work, best range in a jet is achieved when the proportion of velocity to drag is maximum. This will be at an angle of attack less than best L/D.
Basically, in steady state flight, thrust=drag. In a jet, fuel flow is approximately proportional to thrust. So if you add power and your velocity increases at a greater rate than your drag, your fuel economy is increasing until the reverse happens. At the tipping point is max fuel economy.
Note: I'm not an aeronautical engineer but my father was. He used to test airfoil designs by making them out of engineering foam. We would then go out in the car and stick them out the side window as we drove. We used a kitchen scale to measure the lift as we tried different angles of attack and plotted them. Neighbors probably thought we were nuts.
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Post subject: Re: Citation 501sp Posted: 29 Jul 2021, 17:02 |
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Joined: 12/03/14 Posts: 19252 Post Likes: +23622 Company: Ciholas, Inc Location: KEHR
Aircraft: C560V
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Username Protected wrote: I'm just going by what the manual says. Well, the manual contradicts itself. 120 KIAS at 8500 lbs (per the checklist instructions) is well below 0.6 AOA, somewhere around 0.35 to 0.40. So is the manual wrong about 0.6 being best glide, or is the manual wrong about directing glide speed to be 120 KIAS at 8500 lbs? My bet is the former. I think the manual confused best L/D (best glide) with best endurance (most time aloft). Mike C.
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Post subject: Re: Citation 501sp Posted: 29 Jul 2021, 21:16 |
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Joined: 02/15/21 Posts: 2530 Post Likes: +1254
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Username Protected wrote: I'm just going by what the manual says. Well, the manual contradicts itself. 120 KIAS at 8500 lbs (per the checklist instructions) is well below 0.6 AOA, somewhere around 0.35 to 0.40. So is the manual wrong about 0.6 being best glide, or is the manual wrong about directing glide speed to be 120 KIAS at 8500 lbs? My bet is the former. I think the manual confused best L/D (best glide) with best endurance (most time aloft). Mike C. Mike, I don't think Cessna would make a mistake like that, let alone a mistake that has gone uncorrected for more than 40 years.
By the way, where are you getting "120 KIAS at 8500 lbs (per the checklist instructions) is well below 0.6 AOA, somewhere around 0.35 to 0.40"?
The manual does not state that the AOA reading is 0.35 to 0.40 in this situation, so the manual does not contradict itself.
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