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Talk to me about empty weight and flight characteristics

Cardiff Kook

Cardiff Kook

PATRON
Sisters, OR
I am low time working towards PPL. I have been looking at J3's, PA-11's, and PA-18's to purchase.

"Get one as light as you can" seems to be the theme.

Assuming useful weight is not the issue- how big of a difference does weight make in the "joy to fly" characteristics of a low horsepower airplane.

As an example: I have been flying a j3 cub with what I believe is a 750# EW with a c85 engine- all my time has been dual. I had been looking at a PA-18-95 with a C90- EW is 950# which I think is pretty reasonable for a PA-18-95.

So that is a 200 lbs difference on empty weight- which seems signficicant with what I believe are pretty comparable engines.

Can anyone contrast what these two flight experiences might be? (flying the pa-18 involves a long commercial flight or I would just do it myself.)

The nibmleness of the j3 is great. Would that carry over to the -18 despite 200# ew increase? Would climb rate be significantly less in the -18?

Thanks an advance!
 
I learned to fly in my 150 hp PA12. when I soloed without a 250 pounder in the back seat it was night and day better. 200 some pounds lighter in a lower powered plane must be incredible. Lighter is much better, but so is a good cg.
 
I have an Experimental PA11 that weighs about right in the middle of the 2 you describe. I have an O200 in mine and it performs well….it doesn’t climb as well as my 800lb 90hp Taylorcraft though….I have some things to do as I’m not making full HP on takeoff….need higher rpm, so after I repitch my prop, I’ll report back. Until then, I’m at about 80-85 hp on takeoff with the prop I have….it’s still great, and feels nimble and right….I just want more. Lol…my 750 lb J3 is great too, but it feels all of the bumps vs my 11 which only feel almost all of the bumps�� you may be able to put that 18-95 on a diet and loose at least some weight….worth a look
 
Cardiff Kook said:
Interesting.

Why is it better?

How does CG play?


Sent from my Pixel 3 using Tapatalk
Click to expand...

In a nutshell.....

Weight = Performance
CG = Feel

When the CG and Weight are in a good spot the performance, control "harmony", and "feel" of the airplane is noticably better.
 
Small engine cubs are not all created equal, not just from a weight standpoint but engine makes a huge difference too. Comparing just a C85 powered J3 to an O-200 powered PA18-95 is a massive difference in power. It may only be a 10-15hp gain but in relative terms that's almost a 20% increase in power between those two planes. That means a bigger prop with more pitch and in the case of the O-200 a lot more RPM so you can run a flat prop and still have a decent cruise. That alone can make up a fair amount of difference in weight.

In terms of weight, IMO 800-850lbs is a sweet spot. Lighter than that and you really get beat up in rough air and heavier than that and the plane falls off in performance quite a bit with a small engine. Aside from legality, imagine having a 950lb cub with 95hp and it's like flying a J3 around with an adult passenger full time. Climb rate is cut drastically and depending on where you're at that is a big deal. Most of the strips I fly into have trees or hills at both ends so climbing out over obstacles is a big deal.
 
750lb to 800 is the sweet spot. That's what I think works best. I have flown 3 different pa11 with a C90 and 1 with a 0200. 1 18 with a C90 and 1 with a 0200. Sweetest handling best flying of them all, 18, 11, J4, J3, J2 was a 554 lb E2 37hp Cub.

Glenn
 
A C-90 engine in a PA-11, modified J3c or a Pa-18-95 really generates 95 horsepower at take-off and a true 90 horsepower at moderate RPMs.
The J3C converted to a PA-11 in this photo weighed about 780 empty. So 8.2 pounds per horsepower. She was a dream to fly.

My old converted J5 weighed 1005 pounds empty and had a O-290 125 horse. So 8.0 pounds per horsepower. She got off pretty fast when light.

My current PA-12 weighs 1220 pounds and has 150 horse. So once again 8.1 pounds per horsepower. She is also fairly fast off the ground with just me and 3/4 fuel. ( about 150 ft in dead wind. )

But the old J3C / PA-11 hybrid climbed better after lift off. She had stock PA-11 wings and no flaps. But she climbed at 1200 fpm off the runway. Something the other two cubs cannot.
All three had or have flat pitch climb props.
 

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Alex Clark said:
A C-90 engine in a PA-11, modified J3c or a Pa-18-95 really generates 95 horsepower at take-off and a true 90 horsepower at moderate RPMs.
The J3C converted to a PA-11 in this photo weighed about 780 empty. So 8.2 pounds per horsepower. She was a dream to fly.

My old converted J5 weighed 1005 pounds empty and had a O-290 125 horse. So 8.0 pounds per horsepower. She got off pretty fast when light.

My current PA-12 weighs 1220 pounds and has 150 horse. So once again 8.1 pounds per horsepower. She is also fairly fast off the ground with just me and 3/4 fuel. ( about 150 ft in dead wind. )

But the old J3C / PA-11 hybrid climbed better after lift off. She had stock PA-11 wings and no flaps. But she climbed at 1200 fpm off the runway. Something the other two cubs cannot.
All three had or have flat pitch climb props.
Click to expand...
All those lbs/hp seem about the same or no?

Is the superior climb of the j3 due to the 0.1 lbs/hp difference? (Serious question.)

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Do not hesitate to capture that 18-95 if you get the chance. You will be happy. It is a slightly better aircraft for cross country, and just as much fun in the pattern. There is no serious off-airport opportunities in SoCal.
 
Everything in aviation is a compromise. When you kick that instructor out of the J3 you will see the difference weight makes. To me nothing flies better than a light J3 but load two guys in it in Texas summer and it is horrible. I prefer the PA18-95 because the front seat is comfortable, I am not hitting my head on the spar carry-thru and it is still a hoot to fly. If I didn't like to land as short and get out as short as possible and carry gear I would have another one but for my mission right now I like my 160 hp Super Cub. Flew a J3 with an 85 a few weeks ago and it was a blast but my feet are cramped against the front seat and it juts isn't that comfortable to me for hours at a time.
 
What is the mission for the plane solo trainer time builder? Short local flights or long distance? Do you want to carry passengers/fuel/camping gear? Are you flying off runways 1,000 ft or more? useful load is very important, as is power. The reason everything is compared to a Supercub PA18 is the overall performance heavy or light is hard to beat. A J3 is a fun plane to fly but limited when it comes to carrying 2 people/fuel/gear, PA 11 is in the same boat. The empty weight of the plane is somewhat determined by the mission. Most working Cubs in Are 1200 lbs give or take. Lots of heavy gear/tires/wings/interior to fit the mission. They fly just fine light or heavy.
DENNY
 
Quote Originally Posted by Alex Clark View Post
A C-90 engine in a PA-11, modified J3c or a Pa-18-95 really generates 95 horsepower at take-off and a true 90 horsepower at moderate RPMs.
The J3C converted to a PA-11 in this photo weighed about 780 empty. So 8.2 pounds per horsepower. She was a dream to fly.

My old converted J5 weighed 1005 pounds empty and had a O-290 125 horse. So 8.0 pounds per horsepower. She got off pretty fast when light.

My current PA-12 weighs 1220 pounds and has 150 horse. So once again 8.1 pounds per horsepower. She is also fairly fast off the ground with just me and 3/4 fuel. ( about 150 ft in dead wind. )

But the old J3C / PA-11 hybrid climbed better after lift off. She had stock PA-11 wings and no flaps. But she climbed at 1200 fpm off the runway. Something the other two cubs cannot.
All three had or have flat pitch climb props.
All those lbs/hp seem about the same or no?

Is the superior climb of the j3 due to the 0.1 lbs/hp difference? (Serious question.)

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Cardiff:
I believe that the lighter PA-11 climbed better , ( at least for the first 1500 ft off the runway at sea level.) Because of the weight per square foot of wing.
The PA-11-90 had stock wings of 178.5 square feet. So it was lifting 4.3 pounds per square foot.
The J-5-125 had droop tips. so about 180 square feet. It was lifting 5.6 pounds per square foot of wing.
My PA-12-150 has extended square wings. About 183 square feet; It is lifting 6.6 pounds per square foot of wing.
 

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I routinely dump in full camping gear, (and I don't rough it too much these days, a big tent, real plush mattress pad, a camp kitchen) my 55 pound e bike with extra batteries, and 37 gallons of fuel in my RANS S-7S (760 pounds) BUT, I don't take a passenger. No rear seat for one thing, at least when carrying all that gear. With a geared 78" prop and 106 HP it does real good even loaded (better with all the spark plug wires connected, which caused me to turn back from JC last weekend, a very rare glitch). Ahhh, but flown totally empty it is a whole different animal, real "lifty", several hundred FPM any time you want it, no need to advance the throttle either, it just always wants to go up.

But that isn't the way I fly it much, it's just fun to unload all the gear after a trip and go all the way, take it ALL out, but after a flight or two I put my usual min gear as far back as I can get, about 20-25 pounds worth, that way I can land shortest. One thing often left unsaid when talking light planes of any ilk: a 760 pound plane landing real slow will always beat a 1100 plane landing at the same speed, most often anyway. The bigger plane, slow though it may be, just has more kinetic energy to dissipate.
 
Let us look at the above, but at gross weight for all three:
PA-11-90 at 1220 lbs. Is 6.8 pounds per square foot of wing.
J5-125 at 1450lbs , is 8 pounds per square foot of wing. ( It was always over this gross weight if you wanted any fuel )
PA-12-150 at 1935 pounds , is 10.5 pounds per square foot of wing. ( this PA-12 has a gross weight increase STC and extra tubing)
 
Cardiff Kook said:
All those lbs/hp seem about the same or no?

Is the superior climb of the j3 due to the 0.1 lbs/hp difference? (Serious question.)

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I guess what I'm saying is that with small engined cubs you can't take the weight numbers in isolation. The engine plays a big part of total performance and a numerically small power difference can mean a lot under 90hp. I would say those planes would perform equally given the numbers.

That being said, I wouldn't trade my J3 for the world. Lightweight does make for really wonderful light controls and a very responsive plane. But really aside from all that the aesthetics are the bigger part of the equation for me and you just can't beat a J3 for looking good. After all looking cool is 90% the point of flying an airplane (actual statistic, didn't make that one up).
 
For this class of aircraft, pay a lot more attention to empty weight and legal useful load. They fly fine overloaded, and if something were to happen and it was proved that you were overweight that neat little clause in your expensive insurance policy may bite you.

They all fly fine except for the truly heavy ones, and they fly like heavy airplanes. My least favorite Cub was a factory new 180 CC Cub. Flew like a 1946 Autocar.
 
bob turner said:
..... There is no serious off-airport opportunities in SoCal.
Click to expand...

With both the Mojave Desert & the Colorado desert, that seems hard to believe.
Guess it depends on what you call "serious off-airport".
If nothing else, there's places like the Chicken Strip & Bacon Strip Ranch.
I've never flown in California, but I spent the first 35 years of my life there.
So am (or used to be) familiar with some of the remoter areas.
 
I like to fly a J-3 when it is warm out and I have nowhere to go.
I like to fly a PA-18-95 when I am by myself and just have minimal camping gear.
I need to fly a PA-18-150 when I have two people and camping gear.
I need to fly the 206 when I have five people and camping gear.


Buy what will fill your mission.
 
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