Question from Cessna POH

[uwochris]

Hey guys,

I am currently doing my training in 172s, with a bit of stuff in the Diamond Star. I fly 2 different types of 172s... a new "R" model, and an older "M" model. I have 2 questions regarding what's in the POH's.

First of all, for obstacle clearance (ie. 50 ft tree at end of runway), I was always taught to use Vx, the best angle of climb speed. The Cessna POHs, however, indicate that Vx for my plane is 64 kts... yet, it then recommends to use a speed of 59 kts at 50 ft to clear obstacles. Why do they quote 2 different speeds... should I use Vx, or should I use the 59 kts? This confused me because I always thought Vx is essentially used as an obstacle clearance airspeed, yet the POH shows 2 different speeds. My instructor recommends using Vx (I have my CPL flight test this week, so I want to make sure I use the correct speed), and I just want to see what you guys recommend.

Second of all, for the "R" model, it quotes Vx as being 60 kias at Sea Level, and 65 kias at 10000ft. For the "M" model, it quotes 64 kias at Sea Level, and 62 kias at 10000ft. Why does Vx increase with altitude in one model, and decrease in the other? I was always under the impression that Vx increases with altitude, and Vy decreases with altitude... am I wrong?

Anyway, thanks in advance.

 

[Dutch]

Not sure about the second part but here's a stab at the first question.

When you rotate at 48 knots (172S) you will set a pitch attitude for Vx and as you pass through 50' will be accelerating through the barrier speed of 54 knots. Contributing to the different speed is the acceleration of the aircraft and the configuration of the flaps (10 degrees). No guarantees if this is completely correct but is my best explaination.

 

[midlifeflyer]

First of all, for obstacle clearance (ie. 50 ft tree at end of runway), I was always taught to use Vx, the best angle of climb speed. The Cessna POHs, however, indicate that Vx for my plane is 64 kts... yet, it then recommends to use a speed of 59 kts at 50 ft to clear obstacles.

Vx is the best angle of climb with =no flaps=. The recommended speed is Vx with 10 °flaps. Take a look at the short field takeoff performance charts. You'll also see that the 59 is at max gross weight and that it goes even lower as weight on board decreases.

Second of all, for the "R" model, it quotes Vx as being 60 kias at Sea Level, and 65 kias at 10000ft. For the "M" model, it quotes 64 kias at Sea Level, and 62 kias at 10000ft. ...I was always under the impression that Vx increases with altitude, and Vy decreases with altitude... am I wrong?

Your impression is correct. Either you read the two lines backwards in the POH or there's a mistake in it. I have a PIM for the S model, and the two numbers are in the correct relationship.

 

[Lrjtcaptain]

back in the day at ERAU in the 172R we rotate at 55 and do VX on obsticle clearence with flaps at 10'.....once clear pitch for 69 and flaps up which was VY back then. VX however is based on no flap condition in the 172 so im sure that is why you have a different number. I never completly belived my old CSEL DE when told me that in a Flap10 situation you werent adding any drag whatsover to the airplane, just adding lift. He may be right but if that were the case then theoretically in a 172 you could do cruise with 10' of flaps, pitch the nose over and get a few more knots, however VFE comes into play. Thats another story. Our 172s at riddle, the R models however had different V speeds then they do know i believe due to the engine problems they had when i was there. The speeds have slightly changed.
As for question 2....dont understand why the 2 172's would change in reverse order. Must be a mistake.

 

[midlifeflyer]

I never completely belived my old CSEL DE when told me that in a Flap10 situation you werent adding any drag whatsover to the airplane, just adding lift.

I think you were right in not believing it. I'm not sure what the 172 R & S book says (don't have it handy), but you'll notice that in the expanded normal operations section, older 172s with a 10º flap short takeoff procedure say to not use the additional flaps in high density altitude situations. I'm pretty sure it's because in the thinner air, the benefit of the increased lift do not compensate for the increased drag.

 

[kaufhaus]

I never completly belived my old CSEL DE when told me that in a Flap10 situation you werent adding any drag whatsover to the airplane, just adding lift. He may be right but if that were the case then theoretically in a 172 you could do cruise with 10' of flaps, pitch the nose over and get a few more knots, however VFE comes into play.

I've heard the same sort of story before, and had trouble swallowing it, too. I believe that any increase in lift comes with an increase in (induced) drag - that's just the way things work. Also, if 10 degrees of flaps = free lift, why didn't Mr. Cessna make that the normal shape of the wing to begin with?

 

[a-v-8er]

About Vx and Vy and how they change with increase in altitude.

Vx increases and so does Vy if we're talking true airspeed (TAS)

If we're talking about indicated airspeed (IAS), then Vx generally increases, but Vy decreases. This may vary how much depending on airplane type.

Here's a link that might be helpful:
http://142.26.194.131/aerodynamics1/Performance/Page10.html

 

[SkyWestCRJPilot]

One more thing, when using the charts in the POH for the takeoff and landing distance to clear a 50' obstacle, remember not all trees are the "FAA approved" 50' tall trees. When I instructed back east the Airport Facility Directory noted that there were trees at the ends of the runways of many of the airports we used that were 60', 70', 80' tall. Just something to keep in mind.

 

[dmspilot00]

The reason these change is due to the current takeoff weight.

I think that some of you misunderstood the question.

Uwocrhis, I suspect that the difference may have to do with one manual being in calibrated airspeed and another manual being in indicated airspeed. If this is the case, then try converting the M and R speeds to either both calibrated or both indicated and see how they compare. Check it out and let us know.

 

[Mickey]

The difference in speeds for the take off are not due to different weights. The POH speeds are for max TOW in both cases. The difference is the obstacle clearance speed is with flaps set to 10 degrees while the Vx in the POH is for flaps zero. So, if you are doing a short field takeoff with flaps set to 10 and you are at max TOW use 59KIAS to clear an obstacle. If you are at max TOW doing a normal takeoff with flaps set to zero and in your climbout decide you need to clear an obstacle use 64KIAS.

Sorry I don't have time to check on why the Vx is going down with altitude. Maybe tomorrow.

Take care

 

[Booker]

172N

From my 172N PIM:

"Best Angle of Climb, Sea Level.....59 KIAS"

50' Barrier Speed, Flaps 0, MTOW.....59 KIAS

There are lower speeds published for 2100 and 1900 pounds, adjusted for weight as previously mentioned. No data for Flaps 10 is published. Maybe check your POH/PIMs closely to make sure you're comparing apples and apples.

Best,
Booker

 

[midlifeflyer]

Re: 172N

From my 172N PIM:

"Best Angle of Climb, Sea Level.....59 KIAS"

50' Barrier Speed, Flaps 0, MTOW.....59 KIAS

They're the same in the 172N because the short field takeoff configuration for the N model is, as you indicate, no flaps. It's different in later models.

uwochris is flying a R model in which the short field configuration is 10º. That';s what accounts for the difference.

 

[Booker]

Re: Re: 172N

They're the same in the 172N because the short field takeoff configuration for the N model is, as you indicate, no flaps. It's different in later models.

uwochris is flying a R model in which the short field configuration is 10º. That';s what accounts for the difference.

OK, thanks. It's been a long time since I've flown a 172. We used Flaps 10 for short takeoffs, but I had forgotten that the older models only had Flaps 0 data published.

Out of curiosity, if someone has the info handy, what does Piper publish as a barrier speed at max gross with Flaps 0 in the Warrior or Arrow?

Thanks,
B

 

[dmspilot00]

Re: Re: Question from Cessna POH

This is the answer to what I'm interpreting uwochris is asking.

Then I think you are misinterpreting what uwochris is asking.

I don't know where you get this stuff with weight? You said that it "isn't Vx calculation," but in your prior post you talk about calculating Vx for various different weights in the Seminole? It doesn't make any sense.

Also, in my post, I was responding to the part about why Vx decreases in one plane and increases in another (one must be indicated and the other something other than indicated being my guess).

Booker: I have a Warrior POH, but what does barrier speed mean? Never heard that term before!

 

[Booker]

Re: Re: Re: Question from Cessna POH

Booker: I have a Warrior POH, but what does barrier speed mean? Never heard that term before!

Barrier speed = obstacle clearance speed.

B

 

[dmspilot00]

Re: Re: Re: Re: Question from Cessna POH

Barrier speed = obstacle clearance speed.

The Warrior II manual says Vx is 63KIAS.

Now, here is where it gets interesting:

Under the Normal Procedures: takeoff section, for a short field, it says to maintain 44-57KIAS (depending on weight, I'm assuming 47 is MGTW) until obstacles are cleared. It uses the same numbers, 44-57KIAS, for obstacle clearance speed for both flaps 0 and 25 degrees.

Then, under soft field with 25 degrees of flaps, it says to maintain 52 KIAS to climb past obstacle height. This time they don't give a range of airspeeds nor do they say "depending on weight," they just give one speed.

Now why would obstacle clearance speed be the same for 0 and 25 degrees of flaps, and different depending on whether you're doing a soft field takeoff or short field takeoff?? And why does Piper specify a range of speeds for different weights for short field and no range for soft field?

 

[Booker]

The Warrior II manual says Vx is 63KIAS.

Now, here is where it gets interesting:

Does the Warrior II POH have spaghetti charts that show liftoff and barrier speeds for different weights? That's actually what I was curious about. Here's why:

If memory serves, the Warrior III POH chart for "Flaps 0 Takeoff Distance [over a 50' obstacle]" states at MTOW the barrier speed (that's Piper's nomenclature, by the way) is in the high 50s - about 59 KIAS. However, Vx is published at 63 KIAS. So, at MTOW, with Flaps 0, the published climb speed is four knots below Vx. Anybody know why?

B

 

[dmspilot00]

Does the Warrior II POH have spaghetti charts that show liftoff and barrier speeds for different weights? That's actually what I was curious about. Here's why:

If memory serves, the Warrior III POH chart for "Flaps 0 Takeoff Distance [over a 50' obstacle]" states at MTOW the barrier speed (that's Piper's nomenclature, by the way) is in the high 50s - about 59 KIAS. However, Vx is published at 63 KIAS. So, at MTOW, with Flaps 0, the published climb speed is four knots below Vx. Anybody know why?

Yes, it does actually. At MTOW the "barrier speed" is 57 (which is the same as it said in the checklist section), and it is the same for both flaps up and at 25 degrees, and Vx is 63 also for the Warrior II also. So, that's a whole 5 knot difference.

I was thinking about this last night and the lightbulb in my head went on, so to speak. I think the reason that both Cessna and Piper has published a different speed for obstacle-clearance/barrier speed than Vx may be this: the process of accelerating to Vx from liftoff speed causes you to lower the pitch and results in a less-than-ideal climb angle.

For example, in the Warrior II, lifting off at the given 52KIAS and keeping the nose low enough to accelerate to 63 must not result in as steep a climb as keeping the nose high and the airspeed at only 57. Since the obstacle is only 50 feet, the process of accelerating to Vx is evidently counter-productive enough that the aircraft manufacturers decided to publish different speeds.

Somewhere out there, there must be a hypothetical "obstacle height" at which accelerating to Vx would produce better results, but trying to clear the obstacle of a lower height would call for keeping the nose high, the airspeed close to the liftoff speed, and not accelerating to Vx.

Piper only publishes data for a 50-foot obstacle, but if another manufacturer publishes one speed to clear a 50-foot obstacle, another to clear (for example) a 75-foot obstacle, etc., we can confirm this hypothesis.

 

[Booker]

I was thinking about this last night and the lightbulb in my head went on, so to speak. I think the reason that both Cessna and Piper has published a different speed for obstacle-clearance/barrier speed than Vx may be this: the process of accelerating to Vx from liftoff speed causes you to lower the pitch and results in a less-than-ideal climb angle.

Thanks for confirming the speed discrepancy. To add, a Vx climb would likely give a better angle, but the plane would need to accelerate to a faster liftoff speed to make it worthwhile and not experience the performance degradation you mention. That ultimately means a longer takeoff ground roll and distance. And usually it's the takeoff distance that goes into the sales brochures, not the actual angle of climb.

B

 

[Mr. Irrelevant]

I was thinking about this last night and the lightbulb in my head went on, so to speak. I think the reason that both Cessna and Piper has published a different speed for obstacle-clearance/barrier speed than Vx may be this: the process of accelerating to Vx from liftoff speed causes you to lower the pitch and results in a less-than-ideal climb angle.

For example, in the Warrior II, lifting off at the given 52KIAS and keeping the nose low enough to accelerate to 63 must not result in as steep a climb as keeping the nose high and the airspeed at only 57. Since the obstacle is only 50 feet, the process of accelerating to Vx is evidently counter-productive enough that the aircraft manufacturers decided to publish different speeds.

This may all be true, I'm not really sure. However on rotation in a short field TO, I'm looking for pitch angle right away and not worrying about accelerating to the recommended or Vx indicated airspeed. By the time a pilot gets to that airspeed in a short field TO, they'd blow right by it in anything with ok power. In the short field TO, we're looking to not hit anything right? I know if I get my pitch angle correct (in VFR), I can just cross check with the ASI and rarely when I have the pitch angle I want am I more than 2-3 knots off. By that time I'm always at 50 feet AGL. Of course I haven't flown a piper warrior in at least a year and may be too used to high performance singles.

I suppose I disagree with your assumption but can't offer any better reason for that few knots discrepency. Nice thread.


Mr. I.