Why Stall Speed Increase for Cessna 180 HP Conversion?

[eetrojan]

My school has a 1984 Cessna 172P. When it left the factory, it had a 160 HP Lycoming IO-320-D2J engine. In 1991, the owner installed a 180 HP Lycoming O360-A4M, using "Air Plains" STC SA4428SW.

Looking at the POH supplement, the Vso/Vs stall speeds increased from 33/44 KIAS to 40/50 KIAS. Why?

Is it simply because the larger engine is heavier and the CG moved forward?

Since the POH approach speeds didn't change (still 60-70 KIAS), what practical affect if any does the increased stall speed have on the way you fly the plane?

Edit: fixed date from 1999 to 1991.

 

[Pilawt]

Does that airplane also have STC SA2196CE, which increases gross weight from 2400 lb to 2550 lb?

 

[eetrojan]

Does that airplane also have STC SA2196CE, which increases gross weight from 2400 lb to 2550 lb?

Just looked, and yes it does.

In that STC, it says "increased gross weight to 2,550 lbs, per Mike Kelley Aircraft, Inc. Drawing GW172861." What did they change?

 

[OkieAviator]

Limited flaps to 30

 

[eetrojan]

Limited flaps to 30

Ohhh. Interesting. I would never has figured that out since the plate next to the flap switch lever still shows 0-10-20-30-40. Thanks.




Follow up question. Are you saying that the limited flap travel explains the increased stall speeds?

I ask, because the POH for a 172R with a stock 160 HP engine shows stall speeds of 33/40, but the POH for a 172S has with a 180 HP engine shows stall speeds of 40/48. However, they both have flaps that stop at 30.

 

[flyingcheesehead]

Follow up question. Are you saying that the limited flap travel explains the increased stall speeds?



I ask, because the POH for a 172R with a stock 160 HP engine shows stall speeds of 33/40, but the POH for a 172S has with a 180 HP engine shows stall speeds of 40/48. However, they both have flaps that stop at 30.

No, it's the increased gross weight. The difference in lift between 30 and 40 degrees of flaps is minimal, but part of the difference there will be due to the difference in IAS vs CAS at 30 vs 40 degrees of flaps. For the clean stall speeds, I bet if you convert 40 KIAS to KCAS, multiply by sqrt(2550/2400), and convert back to KIAS you'll be very close to 48.

In any case, I think the difference in KIAS stall speeds here is mostly due to calibration error, and the difference in KCAS stall speeds is likely significantly less. The approach speeds probably didn't change because it's a range and 1.3Vs0 KCAS probably was within that range both before and after the change. Since airspeed indicator error will be lower at approach speeds than at stall speeds, the difference isn't as big.

 

[PPC1052]

No engineer, here, but it strikes me that it is a combination of all of the above (more gross weight, forward CG, less flaps).

 

[OkieAviator]

Are you saying that the limited flap travel explains the increased stall speeds?

As mentioned above the increased weight increases the stall speed. The flap limiting allows the extra gross weight because of the added HP. The thought is I guess you would have issues on a go around with 2550 lbs and 40 degrees flaps.

 

[poadeleted20]

As mentioned above the increased weight increases the stall speed. The flap limiting allows the extra gross weight because of the added HP. The thought is I guess you would have issues on a go around with 2550 lbs and 40 degrees flaps.

Which is why the old 40-flap 172's with 2300 lb MGW can get a 100 lb increase to 2400 MGW just be limiting the flaps to 30. The issue is the balked landing requirement in 14 CFR 23.77. Those older 160HP 172's can meet that requirement with 40 flaps at 2300 lb but not 2400 lb. By limiting the flaps to 30, drag is reduced enough they can meet it at 2400 lb, but it still takes that 180HP engine before they can meet it at 2550 lb. However, even with the 180HP conversion, you can't do it at 2550 lb with 40 flaps, so to get that MGW, you have to both limit the flaps and put in the bigger engine.

 

[Pilawt]

In that STC, it says "increased gross weight to 2,550 lbs, per Mike Kelley Aircraft, Inc. Drawing GW172861." What did they change?

Limited flaps to 30

The 172P (1981-86) only had max 30° flap extension from the factory. Thus it was able to have a 2400 lb gross weight, 100 more than the 172D-172N models, and still meet minimum FAA balked-landing performance with full flap. Cessna was motivated to eke out some more useful load to compete with the Piper Warrior at that time, and to offset the weight of the optional air conditioning system offered in the 172P. As Ron mentioned, owners of 172N aircraft with 160 hp can get that extra 100 lb MGW by installing the flap limiter per STC SA2196CE.

Curious why the label next to the flap switch in your airplane has a '40°' marking on it. The 172P didn't come that way from the factory (see photo from 1981 brochure, below).

I wonder if the Mike Kelley drawing also deals with changing the color arcs on the ASI to reflect the revised stall speeds.

As mentioned above the increased weight increases the stall speed. The flap limiting allows the extra gross weight because of the added HP. The thought is I guess you would have issues on a go around with 2550 lbs and 40 degrees flaps.

That's it -- it wouldn't meet FAA go-around criteria with full flap. I have the 180 hp engine in a 172N (STC SA4428SW), and purchased but never installed the flap-limit (STC SA2196CE). So I still have 40° of flap but still only 2300 lb MGW.

Which is why the old 40-flap 172's with 2300 lb MGW can get a 100 lb increase to 2400 MGW just be limiting the flaps to 30.

Not all of them. The STC SA2196CE increase of MGW to 2400, when the 180 hp engine is not installed, only applies to 172N (1977-80).

 

[Ghery]

Which is why the old 40-flap 172's with 2300 lb MGW can get a 100 lb increase to 2400 MGW just be limiting the flaps to 30. The issue is the balked landing requirement in 14 CFR 23.77. Those older 160HP 172's can meet that requirement with 40 flaps at 2300 lb but not 2400 lb. By limiting the flaps to 30, drag is reduced enough they can meet it at 2400 lb, but it still takes that 180HP engine before they can meet it at 2550 lb. However, even with the 180HP conversion, you can't do it at 2550 lb with 40 flaps, so to get that MGW, you have to both limit the flaps and put in the bigger engine.

And when you do both you get a pretty good load hauler. 755 pounds in the cabin with full long range tanks in the club's C-172N. Sweet.

 

[eetrojan]

Curious why the label next to the flap switch in your airplane has a '40°' marking on it. The 172P didn't come that way from the factory (see photo from 1981 brochure, below).

Weird. It looks factory, but the original POH pages in the plane only mention flaps 30. Also, consistent with your comment, everything I've read also says that the 172P, first introduced in 1981, is when Cessna reduced flap extension from 40 to 30 degrees.

This plane's airworthiness certificate was first issued in June 1983, but it is a 1984 model year. I believe it was purchased with the float plane options in case that makes a difference.

Here's a photo of the actual flap switch I took the other day:

I wonder if the Mike Kelley drawing also deals with changing the color arcs on the ASI to reflect the revised stall speeds.

The ASI was changed/updated. The increased stall speeds are mentioned in the supplemental POH pages, but I don't see anything in the legalese of the STC itself that requires the revised airspeed indicator. Is that so-called "Mike Kelley" drawing readily available?

Interesting. I just noticed that the ASI has a red line at 140 and a yellow line at 158. I guess that relates to Vne being 140 when configured as a sea plane and 158 when configured as a land plane?

 

[MAKG1]

Waitaminit….

That extra red line is a big hint that there is another modification to this aircraft.

 

[eetrojan]

Waitaminit….

That extra red line is a big hint that there is another modification to this aircraft.

I added an edit above, speculating that the red line at 140 KIAS relates to the float plane configuration, but I'm just guessing.

The generic float plane supplement doesn't support my guess regarding a reduced Vne, so I'm stumped.

 

[Pilawt]

Here's a photo of the actual flap switch I took the other day:

Curious. It has 40° printed on it, but the slot for the indicator only goes down to the 30° mark.

Here's the one in my '78 172N -- note the indicator slot goes all the way down.

Notice the social commentary? In 1978 Cessna labeled the power socket next to the flap switch "LIGHTER". In your 1984, it's "12 VOLT DC". :wink2:

 

[eetrojan]

Curious. It has 40° printed on it, but the slot for the indicator only goes down to the 30° mark.

Here's the one in my '78 172N -- note the indicator slot goes all the way down.

Thanks Pilawt. Great observation.

But, that begs the question: "If all 172P Cessnas are limited to flaps 30 out of the factory, and this particular one was already flaps 30 max to begin with (even though it has 40 on the plate), then what was changed in accordance with STC SA2196CE to increase the MTOW to 2550?

 

[Pilawt]

But, that begs the question: "If all 172P Cessnas are limited to flaps 30 out of the factory, and this particular one was already flaps 30 max to begin with (even though it has 40 on the plate), then what was changed in accordance with STC SA2196CE to increase the MTOW to 2550?

Nothing, other than the engine and the markings on the airspeed indicator. The POH for the stock 172P (page 2-5) says white arc is 33-85 KIAS, and green arc is 44-127. Your ASI has been changed.

 

[MAKG1]

Notice the social commentary? In 1978 Cessna labeled the power socket next to the flap switch "LIGHTER". In your 1984, it's "12 VOLT DC". :wink2:

Not social commentary.

It's 24V in that 1978. I have the dead tablet to prove it. A pretty nice Air Plains 172N put it out of its misery.

 

[Pilawt]

Not social commentary.

It's 24V in that 1978. I have the dead tablet to prove it. A pretty nice Air Plains 172N put it out of its misery.

Either way, it's no longer PC to call it a "lighter" ...

 

[poadeleted20]

Either way, it's no longer PC to call it a "lighter" ...

Nah, it's just about the money -- saves 'em a buck or two to deliver the plane without the lighter in the socket as standard equipment.