I'm not a design engineer, just a low time pilot who flys a PA-22 most of the time. I've been looking at the Glastar recently and cannot figure out how it performs as claimed. For example:

Pa-22 150 = 1,100 empty weight, 147sq ft wing area, 29' span, 13.5lbs wing load = 1,150 ground roll to takeoff

Glastar 150 = 1,100 empty weight, 128sq ft wing area, 35' span, 15.5lbs wing load = 400 ground roll to takeoff

How does the glastar get off the ground in 1/3 the roll? I'm interested in learning.

Dan

MAGIC

I've never flown in a Glastar, but I do find them to be a very cool design and there are probably many factors to why they perform differently (airfoil, prop, etc.) than the Pacer. If you look at the two side by side, it is apparent that the Glastar is a much sleeker (lower drag) design, this is part of the beauty of composite-over-tube fuse. Another biggie is the aspect ratio. Look at the two from the top down, or as they fly over, you'll see the Glastar's long skinny wings, versus Pacer's short and stubby. A higher aspect ratio will produce a better lift to drag ratio at the low speeds we fly.

Also, a Pacer does not usually need nearly that much runway... but loading it up does have a large effect on T/O performance

Although I tend to think Cropduster has the best perspective I do think that the longer, narrower wing probably has a big impact. Yes, when I'm light the tripe does get off quicker but the numbers for both aircraft were at gross so I assume the Glastar would have a shorter roll relatively too. I like the glastar because the stall to top end envelope is so good.

So really chauk it up to wing airfoil and aspect ratio. Interesting.

There are performance numbers out there that are published by marketing folks and others that are published by engineering folks. The marketing numbers are typically optimistic and the engineering numbers are generally conservative. Marketing wants to sell airplanes and engineering wants to avoid responsibility for any incidents based on miscalculations or erroneous measurements. Web-site vs Pilot Operating Handbook comparisons usually yield rather large discrepancies. Owners sometimes embellish what their machines will do as well. Experimental manufacturers usually leave actual performance determination to the specific aircraft builder to avoid encounters with lawyers.

Everything in aviation seems to be a give and take when it comes to aircraft performance, you give a little here to gain a little there. I imagine as you start to look at some of the other aircraft performance numbers you'll find where it had to sacrifice in order to gain that take off performance. What about slow flight numbers and stall speed? Or rate of climb, Vx, Vy, endurance, Fuel burn, ???

Disclaimer: This is not a scientific study, just a result of looking at a lot of airplane numbers trying to find the perfect one only to realize that the perfect aircraft is the one that sacrifices more in the areas that I don't care as much about and gains more in the areas that are important to me and my mission! Have fun!

CW

Are you using the original performance data from the 125 hp PA-22?

Are you using the takeoff performance of the Glastar with the 180hp engine?

I think you may find the answer to your question somewhere in there.

Squash wrote:Are you using the original performance data from the 125 hp PA-22?

Are you using the takeoff performance of the Glastar with the 180hp engine?

I think you may find the answer to your question somewhere in there.

Agreed! They can all be rockets off the ground with enough hp but it doesn't shorten the landing. For US experimental class there is that whole responsibility thing for the builder to establish the performance numbers. They are not copies coming off an assembly line, they can vary plane to plane and so do not come with a POH. If you are building an established kit with recommended engine and make no changes then you should come close to manufacture performance numbers but most manufacturers numbers will still err on the side of a sale. If you are considering investing that much time in building, do it just like if you were spending a pile buying one and go take a test flight to find out. You don't want to be disappointed later.

I initially thought the same of the power discrepancy, but OP lists in his comparison that they are both 150 variants, taken to mean an O-320 powered version of either, I assume.

The only reason the glastar performs so much better is because it costs 5x as much!

Yes, they both have 0-320-150's. that's why i was interested. You have the same hp, same empty and gross weight, glastar has less wing area, so it is interesting to me to know why it performs as it does in ground roll and cruise. Although cruise is a funtion of cleaner aerodynamics to a large extent.

dplunkt wrote:Yes, they both have 0-320-150's. that's why i was interested. You have the same hp, same empty and gross weight, glastar has less wing area, so it is interesting to me to know why it performs as it does in ground roll and cruise. Although cruise is a funtion of cleaner aerodynamics to a large extent.

I'd say you have to look closely at the parameters of the performance testing. Loaded light or loaded to max gross? Without the book in hand, the numbers you're quoting don't mean much.

My 170 could get off the ground in just a few hundred feet with only me and 1/4 tanks, especially on a cool day. That's my anecdotal performance report. The numbers for max gross from the book gave values MUCH longer than that, maybe upwards of 4-5x, but they stated all performance recorded at the max gross of 2200 lbs. So like others have said, it's marketing vs engineering, the optimist vs the pessimist, much more so than the aerodynamic differences of the wing design.

PS: Does anyone know if the Part 23 certification requires the published AFM number to be at max gross? Or is it at the discretion of the manufacturer?

One might have a thicker wing . That will make a big difference.

The Glastar has an aspect ratio 9.6:1, Pacer 5.8:1, for example the Supercub is 7:1.
I believe this and the difference in airfoil choice are mainly responsible for the different takeoff characteristics. The Pacer using the USA35B, similar to the Clark-Y, and the Glastar using NASA GA(W)-2. You can see a big difference in laminar flow on each. Also, the Glastar has long span fowler flaps, and Pacers have shorter plain hinged flaps, which could again favor the Glastar for creating more lift at takeoff.

Plane and pilot published an article where a 150hp pa-22 can clear 50' with a 600' takeoff roll.
Glasair publishes that their glastar can takeoff in less than 400' at gross, but they don't specify the horsepower, and one can only assume that they would use data from their 180hp version.

Wingspan, airfoil, chord, and aspect ratio are all important, but I'm sticking with my instinct that the performance specifications given for these two planes at 150 hp are not accurate.

I personally would like to see verification of the performance at 150 hp between these two airplanes. Please prove me wrong.

The differences between the two most likely can be attributed to the flaps if they are used for takeoff and a more modern airfoil. The glastar has a modern designed slotted flap with fowler motion. The tri pacer flaps are not the best design for high lift and I believe the manual calls for no flaps on a normal takeoff.

As others have said, aspect ratio will also be a factor for some improved performance, but would not make up all the difference.

One could do the math and figure it out I suppose if they had all the right numbers.

A guy at my airport built & owns a Glastar Sportsman, he once got on my case when he read something I'd posted (probably here) saying that Glastar should have increased the wing area for 2+2 airframe design-- it's the same 130-ish sq ft as the 2-seater. I would think that an increase of about 20% (like the diff between a 2-place & 4-place Cessna) would have been a good idea, but I do have to admit that his airplane (originally with an O-360 & now with an O-375 "Bart" engine) performs better than you might think after looking at the power loading & wing loading figures. Maybe the fancy modern airfoil design & high aspect ratio wing has a lot to do with that, but stick that O-375 and c/s prop into a Pacer & those stubby old "Clark Y" wings would perform pretty good too.