Are 172s created equally?

Well I threatened to write an essay on this, but in reality its more like a couple of paragraphs. I've often wondered why the Aero Clubs 172s have markedly different performance. Yes, WAM is 20 years newer than JGP, but the basic design did not change a great deal since the M model was released in 1973. Skimming the model changes between the M and R models, the main aerodynamic change is a reduction in max flap deflection (although JGP only has a maximum flap setting of 30 degrees similar to WAM). Most of the other changes are internal, and there are quite a few.

OK so what are the differences that matter? Well lets look at the vital statistics. JGP is powered by a Lycoming O-320-D2J with the STC (supplemental type certificate) modification to 180hp, as compared to WAMs IO-320-L2A, also uprated to 180hp. She has a two bladed Sensenich propellor up front which is of similar size and mass to the two bladed McCauley propellor that WAM has. I forgot to get the exact model specs so I could do a more precise comparison of pitch angles and other relevant details, so watch this space, I'll add it later.

I won't go into the perceived performance differences in the air because you can simply read through my blog to get a sense of that.

I think the most significant difference between the two aircraft is the weight. I found in the technical manuals, the pages where the aircraft was most recently weighed.

WAM: 1666.1 lbs
JGP: 1549.1 lbs

That is a 117lbs difference, which I'd say is pretty significant. That equates to the weight of a smallish person or child.


Julien said…
I'd say most of the difference in weight went into the avionics. Is one 172 certified for IFR and the other one not? You may also pay a weight penalty for a backup vacuum pump. Happy Flying!
Flyinkiwi said…
A good point Julien, WAM has the 3 axis autopilot while JGP doesn't have one.
suomynona said…
I don't know any specifics to WAM and JGP but here's a few differences over time:

-Older models have 40 degrees flap, this was removed on the 180hp versions (and stc'd models) to allow the increase in MTOW as at 40 degrees it didn't meet FAA standards in full-flap climb requirement at MTOW.

-Newer models have a renewed seat design, in particular the rails, which no longer 'sink' into the vertically-oriented holes (and wear out with use). Also vertical and tilt adjustment is standard (and heavier by design).

-Newer models have strobes lighting and a wing-mounted light panel, all adding weight.

-Avionics of course. Backup pumps and autopilot servos adding a lot of weight. Same for any model though.

-Interiors. More plush seats, leather fabrics, denser more sound resistant sidewalls and inertia reel seatbelt systems all adding weight. Thicker windows too.

-Bigger IO-360/O-360 engines. Extra 40 cubic inches, even when flat rated to 160hp, still heavier.

-Compared to pre-N, the tail section, in particular the dorsal fin, is significantly more reinforced.

-From about J models onwards, the wing tips had more flare/droop.

-About L models onwards, the gearspan went a tad wider.

-N models (and after) had rudder trims. (Not all N models though)

-Leading edges. Later models had more droop in the wing's leading edges. Heavier.

-Newer, about R models onwards, have a different engine cowl mount. Lighter. But the offset is that it's not held on as well so cracks are common.
Julien said…
In addition to the full-flap climb requirement at MTWO mentioned by suomynona, another issue with the 40 degrees of flaps was the increased risk of stalling on a poorly-executed go-around.

Full flaps on older 172s really act as a big speed brake and create a big pitch-up moment when applying full power, which requires a fair bit of pressure on the yoke to counteract.

I can easily imagine a student pilot forgetting to keep his eyes on the airspeed indicator in the excitement of a go-around.
suomynona said…
& further to last, the 40 degrees flaps was also dropped to allow 2400lbs MTOW instead of 2300lbs on later 160hp models.
suomynona said…
& sorry to beat this to death. But WAM has 360 engine.

Refer CAA Supplement Notes: STC SA2196CE which increases the gross weight to 2550 lbs and STC SA4428SW which installs an IO-360 Lycoming 180 HP engine VFR GPS/Autopilot Interface-Hamilt. Aero Mainten. HAM/VFR/001 - Supplement for POH for Reims/Cessna 172 R&S equipped with the TAE 125 Installation when installed in accordance with STC EASA A.S.01527 or LBA STC SA 1295.
Flyinkiwi said…
No no this is great stuff, thanks for posting it.
Flyinkiwi said…
Julien, I have about 12 hours in 172N's, I know all about the pitch up moments when applying full power. I think it also said something in the manual about not slipping them with 40 degrees of flap deployed because the disturbed airflow over the tail may dangerously reduce elevator authority, but that's another post.
suomynona said…
No-slipping with flap is more to do with the structural load placed on the flaps when doing so. That and putting flaps down at too-high a speed accelerates the wear hugely, in particular on the flap tracks and rollers.
PropellerHead said…
Well, well, the WAM vs JGP debate you threatened to start a few posts ago, Euan. I believe I commented that time it had to be power to weight - 20Kg or so for the fuel injection, at least that and a bit more for the seats and inertia reel belts plus the engine - easily 50+Kg (110+lb)
I have 43.8 hrs in 172s in my logbook - 10.5 in JGP and 33.8 in WAM so easy to see where my preference lies.
Not quite as simple as that but I like WAM for:
- Comfortable seats, height adjustable (I am not very tall) and inertia reel belts
- Simple but effective autopilot
- Fuel injection - one less "knob" to worry about
- Looks much nicer if you are "showing off" the club's aircraft to a friend who knows next to nothing about aviation.
- Easier to handle on a go-around; nowhere near the pitch-up of JGP.

Having said that, though, JGP is much nicer with its recent upgrades and for pure performance cannot be beaten (except by DQV and the twin but that's not a fair comparison!!)
Keep up the posts - always good to read.
Cheers - Barry
Flyinkiwi said…
Barry: I'd speculate that comparative climb performance at sea level would see JGP outclimb even DQV because of the superior power/weight ratio JGP enjoys. It might be fun to test it out one day.