Just reduced my H9 Cap 232 from 26.0 lbs. to 25.0 lbs. This is a 4% reduction in weight.

Alum tube to CF tube: 3.9 oz. savings
Alum gear to CF gear: 7.1 oz. savings
Heavy to Light Pilot: 4.3 oz. savings
15.3 oz. total savings

Wing Loading before: 34.2 oz / sq. ft.
Wing Loading after: 32.9 oz / sq. ft. 4% reduction in wing loading.

I will fly the new weight / wing loading tomorrow hoping to gain overall performance for IMAC style flying, especially vertical performance.

Question: What does everyone think will be the outcome?
No change in performance
Slight change in performance
Large change in performance

Bob...

I took 11.75oz out of an 80" Extra and I noticed a really nice performance increase. Both vertical performance and wing loading.

The plane weighed 16.25 pounds before I started trimming weight, and I got it down to 15.5 pounds.

A pound is a pound. I'll bet your going to be really happy with it.

That's not a drastic change, but I feel you will definitely feel the difference in flying characteristics. It should still penetrate well. You'll see more agressive vertical performance. Also, your rolling & snap maneuvers should be more crisp, exits more definitive & less mushy. It will feel a bit lighter on the wing. My .02.

You are on the right track - and at 22-23 lbs this thing really changes performance -
your change is on the order of 1/2 tank of fuel -weightwise .
for best vertical performance - go to smaller prop - try a 26x10 Mejzlic - let it rev
it does not matter what the engine is - larger quieter props are not as good at verticals -
IF you have an engine which will swing this prop at close to 7000 you will see very good verticals.

Think in terms of "excess thrust" to get a feel for the vertical benefits. If your plane weighed 26 lbs and had 30 lbs of thrust, you would have 4 lbs of excess thrust - thrust additional to weight that is available to accelerate the mass upwards. Take a pound of airplane away, and now you have 5 lbs of excess thrust, which is a gain of 25%. That's why it seems like gains are better than 4%.

No quite. For vertical, excess thrust is Thrust minus weight minus DRAG. Can't forget drag, which is considerable at IMAC airspeeds. You would get much much less than a 25% performance increase in your example.

OK, I was definitely neglecting drag, I was thinking in terms of low speed maneuvers. Like a punch out from a harrier. But true, this is an IMAC discussion. However, drag just helps make my point, and the gains would be higher than 25%, not lower.

My example, 26 lb airplane, with 30 lbs of thrust, and you are climbing at whatever airspeed causes 3 lbs of drag. You have one pound of excess thrust. Take a pound of weight from the plane, and in the same situation, you have 2 lbs of excess thrust. A gain of 100%. My point being, in a vertical that's what you percieve, not the 4% weight difference.

In reality, on a downline, drag equals aircraft weight. On an upline it is less, but it is significant... much more than 3 lbs. That's why it takes a 2:1 thrust to weight to get good IMAC performance... the little extra pulls you through snaps and other higher drag maneuvers.

Good point though... the ratios do work out so that you should have a greater percentage of excess power if you include drag at speed.

Meant to fly with new lower weight, but bad servos and then bad weather got in the way. Was able to get some flying in today. New lower weight was great. Greatly improved vertical. Not too noticable in speed during level flight. But, vertical is a lot better. Before, it would slow during the upper part of IMAC maneuvers making control mushy and required lots of input to fly a line. Now, the plane drives through the upper parts of maneuvers and is much easier to control. I can now go vertical almost unlimited, whereas before it would slow to almost nothing after approx. 300 feet.

I can't put any math on this reply, just that 1 pound less in this case has made a big difference. I guess one factor is how overweight was the CAP in the first place. In this case, I feel the CAP was just overweigt and losing that 1 pound made a difference. If the CAP was 2 or 3 pounds overweight, I probably would not have gotten the same results.

Thanks for all the reply's, Bob...