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

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?

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

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.

Dang, that's some complicated sums Thanks for chiming in. I like Kyle's simple equation

CG is a bigger factor

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


Hans

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.

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.

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?

I question this

wrongway john said:

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.

Click to expand...

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?

nucleus said:

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


Hans

Click to expand...

This is true and the reason many planes fly faster after they are painted. The reason being is that most of the paint is aft of the CG and that aft weight helps unload the HS.

Engineering school was a long time ago, but the formula I always use for Rate of Climb (ROC) is:

ROC - 33,000 x (Excess HP/Weight) with ROC in fpm and weight in lbs. A little algebra can be used with the formula to compare the ROC's for two different weights. While you should know the HP(required and available) from your Phase 1 flight testing, the actual values of Excess HP and the constant drop out when you are looking for Delta ROC.

As my professors used to say, "The derivation is left as an exercise for the student."

Paul

wrongway john said:

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.

Click to expand...

That's the same thing Dean Wilson (designer of the Avid Flyer) told me many, many years ago. Is a rough estimate, but works!

Ironflight said:

Engineering school was a long time ago, but the formula I always use for Rate of Climb (ROC) is:

ROC - 33,000 x (Excess HP/Weight) with ROC in fpm and weight in lbs. A little algebra can be used with the formula to compare the ROC's for two different weights. While you should know the HP(required and available) from your Phase 1 flight testing, the actual values of Excess HP and the constant drop out when you are looking for Delta ROC.

As my professors used to say, "The derivation is left as an exercise for the student."

Paul

Click to expand...

I would modify your formula a bit:

ROC = 33,000 x (Excess HP/Weight) x eta

where eta is the propulsive efficiency of the propeller. For a propellor at optimum efficiency, eta is typically about 85%. But if you are climbing at an off-design speed (i.e, climbing with a cruise prop), efficiency can be significantly lower.