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  #1  
Old 10-27-2010, 03:37 PM
Darren S Darren S is offline
 
Join Date: Apr 2009
Location: Canada
Posts: 439
Default Here's a question for all you smart aviation guys

I pm'd the resident RV-6 speed demon looking for ways to tweak my -7. I wanted to know how weight reduction affected cruise speed. He said as a rule of thumb, a 100 lbs reduction in weight will increase speed by 1 knot.

Does anyone have a rule of thumb for climb performance ? I know engine size, speed, air density, altitude etc.... will all have an effect on climb performance. I'm looking for something like : as a rule of thumb, at 4000 feet ASL in standard conditions a 25 lb. reduction in weight will yield a 100 fpm climb increase.

Or something like that.

I'm curious because I think I can save about 15 - 20 lbs by switching from steam gauges to a glass EFIS. It's a bit of work and I'm looking for an increase in performance. Just curious as to how much difference it will make overall and whether it's worth it.

Thanks,

Darren
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  #2  
Old 10-27-2010, 04:26 PM
elippse elippse is offline
 
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Location: Arroyo Grande, CA
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Default

Your question can't be answered as asked. Obviously, 25lb means absolutely nothing to a 747, but quite a bit to an ultralight. So as far as additional drag is concerned you need to express the weight increase-decrease as a ratio of the new to the old. New, 1550lb, old, 1500 lb, ratio 1.033 or 3.3% more. Since for induced drag, which is where weight has its effect, induced drag goes up by the square of the weight increase, or 1.068, 6.8% more. But that means the most when flying high or slow, and you still must know the total drag to see what it will do to you. But with ROC, that's a little easier. Basically ROC results from excess power over the power required to keep the plane flying S&L. Let's use sea-level power and ROC. So you have 160 HP turning rated rpm and an 80% efficient prop, which gives 128 prop HP. Your plane requires 35 prop HP to fly S&L; that leaves 93 prop HP for climb. If your plane weighs 1500 lb, that's 93 HP X 550 lb-ft / sec-HP = 51150 lb-ft / sec. Divide that by your plane's weight and you have 34.1 ft/sec, 2046 ft/min. Now increase your weight to 1550 lb and you have 33 ft/sec, 1980 ft/min, so the 50 lb increase, +3.3%, decreased your ROC -3.2%. Simple, no?
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  #3  
Old 10-27-2010, 05:12 PM
Kyle Boatright Kyle Boatright is offline
 
Join Date: Aug 2005
Location: Atlanta, GA
Posts: 4,218
Default

20-30 lbs won't make a measurable change in your speed. However, your climb rate will increase. So if you currently fly at 1500 lbs and reduce that to 1470 lbs, your climb rate will increase to:

(1500/1470) x your original climb rate
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Last edited by Kyle Boatright : 10-27-2010 at 05:25 PM.
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  #4  
Old 10-27-2010, 05:20 PM
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hevansrv7a hevansrv7a is offline
 
Join Date: Jul 2005
Location: Detroit, MI
Posts: 1,587
Default An example

First, just so I am not misunderstood: Paul is 100% right and said it well, except that he still needs to learn to break paragraphs. Engineers, what can you do?

Example:
First, high cruise:
CAFE's RV6A flying at 12,500', weighing 1600 pounds needs 70.00 THP to go 170.35 mph TAS.
Increase the weight to 1650 pounds and it needs 71.03 THP. Slow down until the THP is back to 70.00 and the speed is now 169.12 mph TAS.

The 50 pounds of weight slowed the airplane in that configuration by 1.23 mph TAS or approx. 0.72%.

Now for same altitude climb:
If the airplane weighs 1650 pounds its best L/D speed (CAS) is 106 mph for which it requires 40.58 THP. At 1600 pounds, its best L/D speed (CAS) would change to 104.038 and the THP required to 38.88. The THP difference is 1.7. Since we know the THP difference and we assume the same total available power, we know that the increase in climb will be 1.7 x 33000 / 1600 = 35 f/m. This tells us nothing about the percentage difference, just actual climb rate difference in this configuration.

For different altitudes, results will differ, especially in TAS. I did not go into the detail, but the best L/D TAS varies with altitude and even the CAS will be affected. This is a minor potential error in these calculations.

I did the above in about 2 min. on my new, simplified spreadsheet which I will put on the website in a day or two when I've cleaned up the interface and double-checked the results against more examples. Then you will be able to do it, too.
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We can lift ourselves out of ignorance, we can find ourselves as creatures of excellence and intelligence and skill. We can be free! We can learn to fly!" -J.L. Seagull
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  #5  
Old 10-27-2010, 09:18 PM
Darren S Darren S is offline
 
Join Date: Apr 2009
Location: Canada
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Default

Dang, that's some complicated sums Thanks for chiming in. I like Kyle's simple equation
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  #6  
Old 10-27-2010, 09:27 PM
nucleus nucleus is offline
 
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Location: Bozeman, Montana
Posts: 858
Talking CG is a bigger factor

My plane goes faster if the extra weight is in the baggage compartment.


Hans
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  #7  
Old 10-27-2010, 10:03 PM
gciampa gciampa is offline
 
Join Date: Dec 2008
Location: Fuquay Varina, NC
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Mine does as well, but, the wifee always makes me stop after an hour, because she gets a cramp in her legs from seating on the luggage.

:-)
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  #8  
Old 10-27-2010, 10:33 PM
wrongway john wrongway john is offline
 
Join Date: Jul 2007
Location: TX & CO
Posts: 465
Default

For estimating rate of climb, I use Dave Anders very simple rule of thumb that he used for his RV-4, that for ever pound less, it increases the roc by 3 fpm.
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  #9  
Old 10-27-2010, 10:55 PM
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David Shelton David Shelton is offline
 
Join Date: Oct 2007
Location: Belvidere, IL
Posts: 169
Default Trim Drag

As others noted, you will reduce trim drag and cruise faster if your airplane is balanced near the aft CG limit. Modern sailplanes feature a ballast box in the tail to allow pilots to fine-tune their CG. Consider keeping track of your CG and finding a place to bolt some lead at the tail?
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  #10  
Old 10-28-2010, 05:19 AM
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hevansrv7a hevansrv7a is offline
 
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Location: Detroit, MI
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Default I question this

Quote:
Originally Posted by wrongway john View Post
For estimating rate of climb, I use Dave Anders very simple rule of thumb that he used for his RV-4, that for ever pound less, it increases the roc by 3 fpm.


I don't want to be in the position of arguing with Dave Anders. His accomplishments are testimony to his expertise. But, this rule, if it is what he said, seems too high to me.

If you add 100 pounds to your weight (16 gallons of fuel) do you see your ROC go down by 300 feet per minute? I don't think that I do.

Maybe my calculations are wrong, but I got 50 pounds and 35 feet per minute for a 6A at 12,500 feet. Dave's plane is lighter, but not that much lighter.

Paul or Kevin - what do you think?
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H. Evan's RV-7A N17HH 240+ hours
"
We can lift ourselves out of ignorance, we can find ourselves as creatures of excellence and intelligence and skill. We can be free! We can learn to fly!" -J.L. Seagull
Paid $25.00 "dues" net of PayPal cost for 2015, 2016, 2017 and 2018 (December).
This airplane is for sale: see website. my website

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