PA28-140/160 v-speeds

ircphoenix

ircphoenix

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ircphoenix
Alright so I have a 1970 Cherokee 140 C. It has the 160 HP STC upgrade.

Now, I was under the impression that my v-speeds are all going to be the same as a regular 140. My performance numbers would definitely be different based on the upgraded power available, but the aerodynamics of the aircraft remain the same.

Am I wrong? The CFI doing my insurance checkout says that I am, but I don't understand the logic behind it.

Can someone tell me if I'm wrong, and if so, why?
 
The additional weight of the larger engine may change the V speeds slightly but I doubt it would be significant. The engine weight difference is probably minuscule too. V speeds are calculated at gross weight so as weight changes so would they.

Just fly it by the numbers & you'll be fine.
 
What's your Vy in the POH? 77 kts? My Archer with the 180hp engine is 76. Probably not a lot of knot difference with the engine. A 160 Warrior should be about Vy of 79. All in the neighborhood. Probably not helpful.
 
Pilawt said:
Is there a Supplemental Airplane Flight Manual for the engine change STC? It should set forth any changes from the original; anything not changed, use the numbers in the original book.
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That's what I was thinking. Don't they have to document new stuff like this when they do a conversion??
 
saddletramp said:
The additional weight of the larger engine may change the V speeds slightly but I doubt it would be significant. The engine weight difference is probably minuscule too. V speeds are calculated at gross weight so as weight changes so would they.

Just fly it by the numbers & you'll be fine.
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It's mods to the original 150 hp engine to upgrade it to 160. Weights don't change to any significant degree.

V-speeds stay the same. If you use 75 mph IAS for Vx and 85 mph Vy you'll be fine.
 
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Pilawt said:
Is there a Supplemental Airplane Flight Manual for the engine change STC? It should set forth any changes from the original; anything not changed, use the numbers in the original book.
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There is. The only speeds it addresses are performance other than V-speeds (climb rate). It does say talk about modifying book performance by an increase of 2%... so maybe that's what I'd go with. Vy is 85... so ... 87 with the STC? Seems more of a power on mod than an engine out consideration... aerodynamics and max gross are the same, regardless of increase in horsepower.

The book for a 1970 plane is woefully lacking. I have like... TWO performance charts... compared to a 1975 Warrior book I saw with a section of performance charts that has the same number of pages as my entire manual.
 
ircphoenix said:
Alright so I have a 1970 Cherokee 140 C. It has the 160 HP STC upgrade...
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Surely you should just check the POH and/or other performance paperwork that's required to be in your possession when operating said aircraft.
 
The STC would have any book changes. Does it change them? If not, you're right. If it does, you're wrong. That simple :)
 
ircphoenix said:
It does say talk about modifying book performance by an increase of 2%... so maybe that's what I'd go with. Vy is 85... so ... 87 with the STC? Seems more of a power on mod than an engine out consideration... aerodynamics and max gross are the same, regardless of increase in horsepower.
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That's not what it means. V-speeds are not "performance" -- they are reference speeds that enable you to achieve optimum performance (e.g., rate of climb, angle of climb, etc.). You likely get better performance with the more powerful engine, but the V-speeds don't change unless the supplement says so.
 
Pilawt said:
That's not what it means. V-speeds are not "performance" -- they are reference speeds that enable you to achieve optimum performance (e.g., rate of climb, angle of climb, etc.). You likely get better performance with the more powerful engine, but the V-speeds don't change unless the supplement says so.
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I agree.

Plus I would think Vx and Vy would DECREASE not increase slightly with more power (but I'm no expert).
 
I looked at the manual for the 1964-67 Piper "Cherokee C", which included the Cherokee 150 (same engine as a stock Cherokee 140), Cherokee 160 and Cherokee 180. All three are aerodynamically the same as a stock Cherokee 140, though gross weights vary from 2150 lb to 2400 lb.

Vy and Vx were quoted exactly the same for all three models -- Vy 85 mph (74 kt); and Vx 74 mph (64 kt).

Not saying you should rely on that old Cherokee C manual -- use your airplane's manual with the STC supplement. Only saying that V-speeds don't necessarily change with minor differences in power.

Does your engine STC boost max gross weight? MGW of the stock Cherokee 160 was 2200 lb.
 
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If the STC didn't change them then they didn't change. The reference speeds in your owners manual are still your official reference speeds.

Now, if you want to have someone go out with you on a severe clear ego-air day and verify the speeds are actually the optimal speeds, I think that would be a fine idea. I suspect any differences that you find will be very tiny and not worth worrying about.
 
Power required isn't going to change just because you added 20HP. I suspect the shape of the power available curve isn't going to change either as most of that curve is determined by the prop efficiency. Now if you repitched the prop, it might change the v-speeds, but not by much.
 
ircphoenix said:
Alright so I have a 1970 Cherokee 140 C. It has the 160 HP STC upgrade.

Now, I was under the impression that my v-speeds are all going to be the same as a regular 140. My performance numbers would definitely be different based on the upgraded power available, but the aerodynamics of the aircraft remain the same.

Am I wrong? The CFI doing my insurance checkout says that I am, but I don't understand the logic behind it.

Can someone tell me if I'm wrong, and if so, why?
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The answer is simple, if there was no document in the STC package that is a supplement to the existing airplane performance data, its negligible...
 
Yay! I love being right.

And no. The STC doesn't increase the max gross. For my plane it is 2150.

I think he's looking at a -161 POH and using that as a reference.

The only other data I see in the supplement is the same info that refers to modifying speed and distance performance. But that info on the high end wouldn't affect V-speeds imho.

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SoCal RV Flyer said:
It's nice to find it in writing, all spelled out and such!! :D
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Which was my confusion to begin with. I know my v speeds... They're kind of important... But I was presented with new information, so I decided to research instead of succumbing to cognitive dissonance and retreating into my "rightness." Luckily I have you fine people to tell me if I'm stupid.
 
It is an article of faith that pilots should know or have reference to a card with V-speeds for the aircraft they are flying. Brushed over is the fact that "the numbers" are based on a sea level airport at max gross takeoff weight and that both Vx and Vy change with altitude: IAS for Vy goes down while IAS for Vx goes up. Makes those V-speed cards far less important.

Bob Gardner
 
I don't think that an engine HP increase changes Vx or Vy at all. If you increase HP, you increase the excess HP (beyond that needed to hold the aircraft in the air at zero climb rate at min drag speed) which totally goes into increased climb rate. Vy corresponds to a tangent point on the drag curve with a straight line originating at origin of the speed vs. drag graph (0,0) and adding HP doesn't change either of those. Airframe mods that reduce drag would change V speeds.

I have been puzzled by slightly different V speeds published by Piper in different years for essentially the same short fuselage, Hershey bar wing, non retract Cherokee.

The max gross weight for the Cherokee 160 is 2200 lbs.vs. 2150 for the Cherokee 140 but the STC I had for my Cherokee 140/160 didn't provide any MGW relief so legally, you can only regard it as an extra 50 lbs. safety margin.
 
bobmrg said:
It is an article of faith that pilots should know or have reference to a card with V-speeds for the aircraft they are flying. Brushed over is the fact that "the numbers" are based on a sea level airport at max gross takeoff weight and that both Vx and Vy change with altitude: IAS for Vy goes down while IAS for Vx goes up. Makes those V-speed cards far less important.

Bob Gardner
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In addition, there is some dependence on balance.

Some POHs list Vs at max forward and max aft CG. The difference is not negligible. For instance, on a clean Cessna 172, the stall speed is 3 KIAS lower with max aft CG (it's 5 knots indicated), nearly a 10% effect. Because approach speeds are tied to stall speed, you'll need to approach ~5 knots slower at aft loading than at forward. It also affects maneuvering speed.

Particularly on Cherokees, Vy seems to have a strong dependence on load. If you're light, you can get MUCH better climb rate at slower speeds. Solo in a Warrior, I find 70 KIAS works substantially better than 75 KIAS, especially at intermediate altitudes (say, 5000 MSL). That's rather relevant to IFR, as the "book" says you can't climb at 500 FPM up there. You can (at least solo), if you use the correct Vy.
 
ircphoenix said:
I think he's looking at a -161 POH and using that as a reference.
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A CFI looks at a -161 manual and assumes the speeds apply to a Hershey-bar-winged -160? ¡Caramba! :confused:

The only other data I see in the supplement is the same info that refers to modifying speed and distance performance. But that info on the high end wouldn't affect V-speeds imho.
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That manual is not artfully written, but yes, you're right.
 
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