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praterdj

Well Known Member
I was in an earlier post "Myths and Choice of IO-360 Lycoming engines and read a reply by Milt where he listed a report from a Cherokee owner -

(Copied from previous post)

On the website noted below a Cherokee 140 owner (over the span of 2 web pages) does an elegant job of comparing climb performance in his Cherokee with an old muffler having a displaced baffle and a newer more efficient muffler.

He calculates for a 7hp gain he derived a 114fpm climb improvement. If things are proportional that means the extra 20 hp would give an improvement of 300 fpm off the deck.

7 HP increase gives >100fpm climb improvement


My question is whether or not any similar type tests have been performed by changing weight as opposed to horsepower?

I know there would be many factors that would make it difficult, if not impossible, to provide dead on performance results. (engine performance from one run to the next, density altitude changes, etc.) but it would be very nice to know the performance gains possible by saving X number of pounds.

I read an article in one of the aviation magazines where a guy went through his plane to reduce as much weight as possible. Even to the point of shortening all of his wiring and eliminating unnecessary wiring loops to reduce the weight. He mentioned that overall he saved several pounds but I don?t think he mentioned what performance gains was achieved.

Has anyone ever experimented with this? Sounds like a nice project for those fortunate flying RV owners out there to take on.

It would certainly motivate folks still building to be weight conscious.

Regards,

Donald
RV-8 Empennage
N-2845 DP Reserved
 
A certain amount of power is required to maintain level flight at the speed for best rate of climb. The power required for level flight is equal to the drag times the TAS. If the engine has some power left over, the excess power can be used to climb. The excess power required to climb is equal to the aircraft weight times the rate of climb.

So, as a first order approximation, the rate of climb is inversely proportional to the weight. If you reduce the weight by 10%, the rate of climb would go up by a ratio of 10/9, or 11%.

But, if you reduce the weight, the drag at Vy goes down, and Vy goes down a bit as well. So the power required to maintain level flight at Vy goes down, and the excess power available to climb goes up. So the actual improvement in rate of climb will be a bit more than this first order approximation.
 
Formulas

:D Here is a ball park formula for HP

Increased climb rate increase = HP x 33,000 x 0.80 / GW

(GW = gross weight; units are in: HP, lbs and ft/min)

In the Piper example with 7HP, lets assume fuel fuel and solo about 1600 lbs.
That is about right since empty wt is 1200 lbs, gross is 2000 lbs and fuel fuel is 36 gal. Therefore 1200 + 200lb pilot + 36 gal (x6) = 1616 lbs

increased climb rate increase (with 7 HP and 1600 lbs) = 115.5 ft/min

increased climb rate increase (with 20 HP and 1600 lbs) = 330 ft/min

Assumption, gross wt and prop efficency (assumed 80%). The 33,000 is a constant with units = [(ft-lb/min)/hp].

LOOK at rate of climb specs from Van's data for a RV-7 180HP vs. 200 HP at gross and solo weights the differnce for 20 HP is:

+350 ft/min (solo 1400 lbs)
+250 ft/min (gross 1800 lbs)

My formula gives 377 ft/min and 293 ft/min respectfully, a pretty good fit. :D

G
 
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Right on, the gift that keeps giving

vanplane said:
I've noticed that additional horsepower usually costs money and fuel, while reduced weight usually saves both.

.........increase the efficiency of the engine without increasing its weight you win on both ends, but that's much more difficult to do on an RV than a Cherokee. We've already got efficient cowlings and exhausts, so large improvements are difficult to come by.

Weight on the other hand -- both on ourselves and our airplanes -- is an enemy in all regimes of flight. We can't do much about Van's airframe -- it's already pretty darn light for the job it's got to do -- but we can refuse to make it heavier than it needs to be.

.............determined that my airplane will have the least amount of "stuff" possible that will allow it to do the job I want to do. No extras, no "just in case" stuff like dual batteries/alternators/brakes/radios. One of each please and I'll not put myself in a situation where a failure will really hurt.
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Right FREAKING ON. :D Excellent points.

Van for decades has told builders to keep it light. They still do but I think they are burned out since many don't listen. Some never say no to an extra pound of airframe weight.

It seems with the proliferation of DUAL everything vogue in RV's, that puts an air transport jets to shame, encourage by experts like Bob N, RV's have been getting fatter. Also heavy auto engines, fancy panels, paint and interiors has also pushed weights even higher. These are sport planes and its their light construction that makes them what they are. A heavy over gross weight RV is not that much fun to fly and gets closer and closer to a Cherokee.

The only exception I take with what you wrote is "We can't do much about Van's airframe". You also say large improvements in RV's are harder to come by, which I also wholeheartedly 100%, but there are improvements to be had.

The cowl and cooling can be improved. Details can be faired in. All these things can make up (documented) max gains of 8-10 mph in top speed. That's like adding 15 to 20 HP. However climb will only increase slightly due to drag reduction, simply because we climb at much lower airspeed's than cruise (parasitic drag). Increase in climb from drag reduction is more like 4 HP than 15-20 HP. Still that's good for another 50 or 60 ft/min. Subtract 100lbs and get another 100 fpm. It all counts. However excess HP is the big dog in the climb game. That is where the constant speed prop shines, allowing higher RPM (HP) and better prop efficency (HP actually turned into thrust).

Bottom line weight, drag, HP all work together, but keeping it light and reducing drag are gifts that keep giving and don't cost gas, like more HP does.

George
 
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I'm all about keeping it light (day vfr), but in fairness to Electric Bob it should be pointed out that his recommendations for redundant charging systems and batteries are for ifr planes with electric panels.
 
Differnt Approaches.

Donald,
Can't believe how many differnt approaches there are to this query.
I took this approach erlier when pondering the problem.

This will not be absolutely accurate as the change in ROC with change in AUW will not be linear. At least I think Kevin would say it isn't.
But it will give you an idea of what to expect.

From Vans Website Performance, an RV-8 with 200hp has a Rate of Climb of:
2700fpm @ 1800lb
1900fpm @ 1400lb
So, for a difference of 400lb the ROC differs by 800fpm
Therefore for each 100lb increase in AUW, the ROC will decrease by 200fpm.(800/400).

Or for every 1lb weight you save in the airframe the ROC will increase by 2fpm.

Pete.
 
fodrv7 said:
Donald,
Can't believe how many differnt approaches there are to this query.
I took this approach erlier when pondering the problem.

This will not be absolutely accurate as the change in ROC with change in AUW will not be linear. At least I think Kevin would say it isn't.
But it will give you an idea of what to expect.

From Vans Website Performance, an RV-8 with 200hp has a Rate of Climb of:
2700fpm @ 1800lb
1900fpm @ 1400lb
So, for a difference of 400lb the ROC differs by 800fpm
Therefore for each 100lb increase in AUW, the ROC will decrease by 200fpm.(800/400).

Or for every 1lb weight you save in the airframe the ROC will increase by 2fpm.

Pete.
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I'm going on a diet.
 
kevinh said:
You joke, but I actually think that's a pretty good idea. Good for the plane and good for the pilot. ;)
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Funny you all mention this. The useful load has gone up on our 9A by 81 pounds over the last few months. :eek: Between my bride and me we have made the effort to down size and have a healthier life. The benefit is showing when I fly solo, but forget the climb. The really good part is how roomy the airplane is now when we fly together! :D

Blue Skies,
 
szicree said:
I'm all about keeping it light (day vfr), but in fairness to Electric Bob it should be pointed out that his recommendations for redundant charging systems and batteries are for ifr planes with electric panels.
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Please do! Keep it simple, keep it light were my two guiding principles and I ended up with an RV-4 that weighed 864 lbs. In the spirit of healthy competition see if you can best me. :)
 
Hmm, think the tail of my -7 weighs more than that... j/k

Seriously though, there is no way I'd best that.
 
szicree said:
Boy, that is light. What motor you got? Instruments? You got a wicker seat in that thing?? :p
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I probably shouldn't share this secret, but two words: helium bladders
 
I'd bet the scales are lying... that's close to 100lb less than 950ish number quoted for most 'keep it light' RV4's. It's lighter than Dave Ander's cafe champ RV4 (975 0360 w/CS prop and very light elsewhere). I'd be very interested in an article on how a 10% weight reduction was achieved.

Chuck

PS Helium is only worth a few pounds and you have to deal with expansion at altitude (don't ask why I know...)
 
chuck said:
PS Helium is only worth a few pounds and you have to deal with expansion at altitude (don't ask why I know...)
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Probably the same way I know you don't put a cooler (with a drain plug) full of Florida fish and shrimp on the back seat of a RENTED 172. :eek:
 
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