Vy at high (for a NA piston) altitude

Jim K

Jim K

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I know that IAS reads low at altitude, and I think I understand that Vy decreases as well. Most of my flying is under 10k', so I largely ignore these factors. As the plane gets up around 12k, climb rate gets sluggish. On a flight last year topping a buildup, I figured out that you have to fly below the normal Vy speed to have any kind of a climb at these altitudes.

So the question: is there a rule of thumb or equation for this?

I'm going to be flying around some higher terrain and taking off from 5000' msl airports on our next trip. I did go out and practice a bit today and found that reducing my speed by about 10mph worked pretty well.
 
IAS does not read low at altitude. It is what it is. VX and VY do come together as you approach max altitude. I doubt at 12,000 feet you could quantify that with normal aircraft instruments and if you were in fact topping a buildup the data would be useless. If you are going to be doing mountain flying a mountain flying course would be a great idea. It would also help to know the type aircraft you were flying and what the density altitudes will be at the 5000 foot airport. Proper leaning for takeoff is critical at high DA airports. If slowing 10 Kts is needed to clear terrain I would suggest that’s not a place you want to be flying.
 
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Okay, IAS doesn't read low because it is what it is. Point being it reads lower than TAS, and Vy TAS increases as the air gets thinner:

vy-change-over-altitude.jpg

The question is if the IAS/TAS relationship changes at the same rate as Vy/TAS.

I'm surprised the difference is so small in the 172. I can't find anything to even hint at what I'm looking for in my POH. Here's a link to it, I'd love to be proven wrong:
https://drive.google.com/file/d/1jmRKiWLtVlOcYTj7h4QgNhIl5cdpvEfA/view?usp=drivesdk

With the very limited experience I have at those altitudes it seems like my plane (a Lance to answer the question) climbs very slowly at the book Vy of 105mph, but if I increase pitch to hold 95 I can get the VS back up to a reasonable rate. I'm trying to figure out what the best speed is going to be without burning a bunch of time, gas, and O2.

The trip planned is to Cody, WY. I wouldn't consider it mountain flying as it's in a pretty big valley,and while I do have to cross the Bighorns, there is also the option of going around if the weather isn't ideal. DA who knows, but we'll be flying in the mornings.
 
Most likely the lack of numbers means the difference is insignificant. Could be just a manufacturer choice not to publish it.

not sure I understand your question about IAS vs TAS as opposed to Vy(IAS) vs TAS.
 
midlifeflyer said:
Most likely the lack of numbers means the difference is insignificant. Could be just a manufacturer choice not to publish it.

not sure I understand your question about IAS vs TAS as opposed to Vy(IAS) vs TAS.
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As you know, as you climb if you hold a constant IAS, your TAS is increasing. Simultaneously, the TAS for best rate of climb is increasing. I always assumed these two curves canceled each other out (ie Vy IAS wouldn't change with altitude), but it would appear to not, at least in my experience in my plane. I suppose it's airframe specific.
 
aren't the published numbers for Vx, Vy, etc.. Calibrated Airspeed? = indicated corrected for instrument error. So True airspeed doesn't come into the equation.

Side note...thumbing through the POH you linked to...there's a huge chapter on conversions. WHY would the have conversion factors for acres, hectares, gallons per acre???? in relationship to an airplane operating manual?????

edit....scratch that. Not calibrated airspeed, but INDICATED airspeed
 
Jim, I think you should be spending more time researching density altitude takeoff than cruise climb.

Performance on the ground is degraded much more than performance in the air, and in the air, circling until needed altitude is reached is an option.

At Colorado Springs, altitude roughly 5,000 feet, Cessna 172, 150 HP, and just two of us and one medium bag.

Departure before 8AM to get a cooler air temperature, and I was fully prepared for the looooong takeoff run, as I had done all the calculations. That was still the first time that the end of a long runway seemed to be close before rotation, and not far below me as I crossed it. At that time, I had two calculators, the regular circular slide rule that every pilot had in those days, an one just for ground roll calculations at pre calculated DA. I donated that one to the Collège Park Aviation Museum, as it was from before 1945.

Horsepower is down, lift requires more speed, and tire drag is just as much as at sea level. A rejected takeoff will be with much hotter than normal tires, and that heat will be shared with the brakes, so the brake efficiency will be less. My commercial pilot ground school instructor recalled a rejected TO he had done at Denver that barely succeeded. He was in a 4 seat Piper, full gross, but in the winter. He off loaded HIS luggage, and successfully departed. The three other persons were paying for the flight.
 
if I increase pitch to hold 95 I can get the VS back up to a reasonable rate
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Did you wait long enough to be sure it can sustain the higher VS? Obviously there will be an initial increase in VS when you pitch for a speed below Vy. It takes awhile to settle in. Maybe you didn’t wait long enough?
 
In high density altitude I always fly on IAS. Nothing else matters. Vy and my plane is 74. It’s gonna be Vy whether I’m at sea level or 10000 feet. The only thing that matters is the what the plane is feeling. Not TAS. When you start thinking TAS is when you start reducing your safety margins.


Sent from my iPhone using Tapatalk Pro
 
I don't know why Piper doesn't publish different speeds at different altitudes for Vy and Vx. We know for a fact that Vx is less than Vy, except that they converge until they equal each other at the absolute ceiling, therefore it is impossible for the speeds to not change with altitude.

Here is a chart from a comparable aircraft, a C210L.

Screen Shot 2022-06-04 at 11.13.19 AM.png
 
dmspilot said:
I don't know why Piper doesn't publish different speeds at different altitudes for Vy and Vx. We know for a fact that Vx is less than Vy, except that they converge until they equal each other at the absolute ceiling, therefore it is impossible for the speeds to not change with altitude.

Here is a chart from a comparable aircraft, a C210L.

View attachment 107448
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Thank you! Interesting that vy drops 5-10 mph at altitude....

That chart also answers another question I had regarding how much weight effects climb rate and speed.

I wonder if Beech provided a similar chart.
 
MauleSkinner said:
Square root of the load factor (or weight reduction) applies to a lot of the operational speeds for the airplane.
Click to expand...

Just not the speeds being discussed in this thread.
 
Jim K said:
Okay, IAS doesn't read low because it is what it is. Point being it reads lower than TAS, and Vy TAS increases as the air gets thinner:

View attachment 107445

The question is if the IAS/TAS relationship changes at the same rate as Vy/TAS.

I'm surprised the difference is so small in the 172. I can't find anything to even hint at what I'm looking for in my POH. Here's a link to it, I'd love to be proven wrong:
https://drive.google.com/file/d/1jmRKiWLtVlOcYTj7h4QgNhIl5cdpvEfA/view?usp=drivesdk

With the very limited experience I have at those altitudes it seems like my plane (a Lance to answer the question) climbs very slowly at the book Vy of 105mph, but if I increase pitch to hold 95 I can get the VS back up to a reasonable rate. I'm trying to figure out what the best speed is going to be without burning a bunch of time, gas, and O2.

The trip planned is to Cody, WY. I wouldn't consider it mountain flying as it's in a pretty big valley,and while I do have to cross the Bighorns, there is also the option of going around if the weather isn't ideal. DA who knows, but we'll be flying in the mornings.
Click to expand...

Thinner is aviation is called less dense.
 
geezer said:
My commercial pilot ground school instructor recalled a rejected TO he had done at Denver that barely succeeded. He was in a 4 seat Piper, full gross, but in the winter. He off loaded HIS luggage, and successfully departed.
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I'd have departed too — to find a better job.
 
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That flight was on his own time, and he was making some private income, un taxed, on the Thanksgiving holidays.

His regular employment was a major air carrier, to remain anonymous.
 
FastEddieB said:
That doesn’t sound right. Are you sure? To the best of my recollection, the IAS for best rate does in fact decrease with altitude.
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Yeah. Vy IAS decreases and Vx IAS increases with altitude until they meet at the airplane's absolute ceiling (where the the vertical component of best rate and best angle are both zero. But the Cessna chart I posted above seems to be the exception rather than the rule, at least for single engine pistons. I fly about 30 different types of singles. I haven't checked them all but I think Cessna is one of the few one which bothers to publish them. I don't see it it Pipers, Mooneys. Bonanzas. Cirrus does publish them. It's not by much. Even in a 210, the difference between IAS Vy at sea level and at 10,000' is only 7 knots. Less than 1 KT/1,000'. Practically speaking it doesn't matter that much. Your vertical speed will be suffering a lot more than a few less knots of forward speed can make up for.
 
Vx increases as you climb, Vy decreases.
when Vx = Vy...that's the absolute ceiling, right.

In the POH link above, for sea level and max gross
vy = 106 mph indicated
vx = 100 mph indicated

so from an operating perspective true air speed doesn't need to be calculated, right?
so the number you're looking for is something between 100 to 106... so do you think its safe to say it's no more than 3 or maybe 4 MPH indicated swing all the way to the top?

Instead of altitude, would weight be a more significant variable?
 
midlifeflyer said:
I fly about 30 different types of singles. I haven't checked them all but I think Cessna is one of the few one which bothers to publish them. I don't see it it Pipers, Mooneys. Bonanzas. Cirrus does publish them. It's not by much.
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The Grumman Tiger POH publishes a couple of tables. Very roughly speaking, you subtract about 1kt per 1000ft plus another 1kt per 100lbs to 10000ft and then more than that up higher. Above 10500ft, the unwary could be pitching for 90kts when they should be aiming for 71kts. That's a pretty big jump.


upload_2022-6-5_12-32-36.png

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Anyone else want to identify as a 'Gulfstream American'?
 
@Salty & I have been talking back and forth about p32r performance at higher altitudes. I was out just screwing around turning money into noise yesterday and decided to test a little bit. I timed my climb from 11,500 to 12,500 at four different speeds. The plane was fairly light...maybe 2900 lbs, engine monitor told me I had 65% power at 11,500. It was around 0C, so ISA +7ish. Vy for my plane is listed as 105mph.

Speed (MPH).......time to climb 1000'
85...................2:21
95...................2:17
105.................2:15
115.................2:40

I need to try it again near max gross, and/or take my O2 bottle and go higher next time.
 
Jim K said:
@Salty & I have been talking back and forth about p32r performance at higher altitudes. I was out just screwing around turning money into noise yesterday and decided to test a little bit. I timed my climb from 11,500 to 12,500 at four different speeds. The plane was fairly light...maybe 2900 lbs, engine monitor told me I had 65% power at 11,500. It was around 0C, so ISA +7ish. Vy for my plane is listed as 105mph.

Speed (MPH).......time to climb 1000'
85...................2:21
95...................2:17
105.................2:15
115.................2:40

I need to try it again near max gross, and/or take my O2 bottle and go higher next time.
Click to expand...
Really cool experiment. I have to say I'm a bit surprised that you're already sub-500fpm being light in your airplane at those altitudes.
 
kaiser said:
Really cool experiment. I have to say I'm a bit surprised that you're already sub-500fpm being light in your airplane at those altitudes.
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Chart suggests ~450fpm if I'm reading it right, so she's matching book performance (which always makes me feel better). She's really doggy at max gross on a hot day. I can't imagine what a fixed gear 260 must be like. Someday I'll have a turbo...or less weight...or another engine...or a pt6...

upload_2022-8-17_8-54-21.png
 
I also played a bit on the way down...With the engine at idle, pulling the prop lever all the way back from where it was (set for about 2400rpm), brought the descent rate at best glide from ~1600'/min to around 1050. I knew it made a difference, but didn't dream it was that big. I've never played with it before at a high enough altitude to allow the plane to stabilize after pulling it back.
 
Jim K said:
....pulling the prop lever all the way back from where it was (set for about 2400rpm), brought the descent rate at best glide from ~1600'/min to around 1050...
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YOU'LL SHOCK COOL YOUR PROP!!!!!!!!
 
eman1200 said:
YOU'LL SHOCK COOL YOUR PROP!!!!!!!!
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Oh did I forget to mention that all six of my cylinders cracked and fell off? I opted for corn, even though I could've glided to the airport.
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Okay, that's obviously not true. Everyone knows a PA32 will not glide to any airport. Anywhere. Ever.

 
Jim K said:
@Salty & I have been talking back and forth about p32r performance at higher altitudes. I was out just screwing around turning money into noise yesterday and decided to test a little bit. I timed my climb from 11,500 to 12,500 at four different speeds. The plane was fairly light...maybe 2900 lbs, engine monitor told me I had 65% power at 11,500. It was around 0C, so ISA +7ish. Vy for my plane is listed as 105mph.

Speed (MPH).......time to climb 1000'
85...................2:21
95...................2:17
105.................2:15
115.................2:40

I need to try it again near max gross, and/or take my O2 bottle and go higher next time.
Click to expand...
If you're running on the light side (2900), try 100 mph. I am in a PA-32-300 (six) and my Vy is 105, like yours... but that's at Max. Vy at 2900 is 100. You probably won't see a major improvement, but that should be your target for Vy if you're running light. It'll probably only shave a couple/few seconds.
 
kaiser said:
Really cool experiment. I have to say I'm a bit surprised that you're already sub-500fpm being light in your airplane at those altitudes.
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In the perfect world I might expect a little more given it's a retract. Mine is a fixed gear PA32 and I made a chart that shows Rate of Climb for any given PA with any Outside Air Temp. Based on his info provided, my Six should climb at ~600 fpm up around a PA of 12k w/that OAT and at ~2900lbs. If I were at max gross (3400 lbs), I would be climbing a little shy of 300 fpm.
 
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