Interesting (?) Climb Speed results
 

Hi all, Question for the collective wisdom,

I flew a couple of sets of Sawtooth Climbs yesterday, in fairly smooth air, and two different altitudes in my -8. I ran speeds in 5 knot increments from 70 knots to 115 knots. I was amazed at how little "curve" there was - while there is a slight peak at 100 knots (which is what I was told to expect to be the best rate of climb speed), the difference between all of the speeds is remarkably small - a range of difference of about 3 seconds for a 1000' climb. This tells me one of two things:

1) That within a speed range of about 80 to 110 knots the RV-8 will climb at about the same rate, so the climb speed isn't critical at all (and you might as well climb at a faster speed for engine cooling).

or

2) I have no idea what I'm doing, and my data has got to be screwed up!

I flew standard sawtooth runs, timing from 500 feet below test altitude to 500 feet above it, after stabilizing airspeed and power on target.

So has anybody else seen this lack of sensetivity to climb speed?

(I figure I don't really need to worry about Best Angle speed - this thing climbs so fast that I can't imagine havign to worry about short field procedures!) ;)

Paul
RV-8, 180 Hp carbed, with Blended Airfoil C/S Hartzell

I wonder if this observation changes between C/S and F/P props (asked in genuine ignorance)?

-mike

With my Turbo Subie and IVO 76 inch adjustable pitch prop, my 6A climbs best at 85 knots IAS. At 80 knots, the VSI drops off 200-300 fpm, at 90 knots about 100-200 fpm less, at 100 knots about 500 fpm less. I notice quite a lot of sensitivity with speed vs ROC with this combination. This is a heavy brute at 1150 empty.

I was up yesterday early morning -5C, almost zero temp/dewpoint spread, 4000 feet MSL field, half fuel just me, 35 inches, 4600 rpm, 2100-2200 fpm. The controllers usually comment on a performance like this. The deck angle is pretty impressive. On takeoff, the prop tips were shedding some pretty dense vortices. :D :cool: :cool:

Hi Paul,
First off congrats on your new bird!, second can I ask what prop length you have?
Thanks

Hi Kirk - I am running the 74" Hartzell. When I went to place my order, I called Van's to see if they recomended that or the 72, and got zero help...so I called Hartzell, and we decided that there was no harm in getting the 74, as I could always shorten it to a 72 if I neded to for some reason...

Paul

Climb speed
 

I have an RV6A with the same pwr setup, I had much the same results last year during my test flights. I did climbs at 80kias, 90, and 100, saw the same elapsed time and moved onto another flight test card. I use 110kias now for best rate with good cooling.
Jon

Thanks Jon - that is reassuring.

We had a round-table at lunch with some aero guys and a few test pilots.

The aero guys figured the curve is flat becasue this is a pretty low-aspect ratio design. When I pointed out that the aspect ratio is almost the same as my Grumman, and it had a nice climb rate/speed curve, they excused themselves to go do "real work"...

The test pilots all said they would with-hold judgement until they get to fly the airplane(rightttt.....) ....I said they'd have to wait - and prove to me that they have some taildragger time! (not many taildragger jet fighters these days...) ;)

I'm moving on to the next test series...

Paul

We found 103kts is best climb ... but 120kts is best climb to cooling.

So we climb to pattern atl at 103kts and then climb to cruse at 120kts or higher.

heres how i did my testing;

1. climb to 4000 msl
2. level, trim, and lean
3. full power
4. climb to 5000 msl and record the time
5. start over

did this at 60, 70, 80, 90, 95, 100, 103 (after alot of testing), 105, 110, 120, 130, 140.

Below 90, we had a really hard time keeping the engine cool enough to get to 5000.

the day was 86 deg F.

It's your airplane, of course, but if it were mine, I would've said "Yes, I see your point...I'll believe the F-16's climb rate figures when I fly one." Or "I'm sure the roll rate on the F-5 is as high as you say, but I really can't judge it until I roll one myself."

Paul - I'm guessing you were seeing climb rates on the order of 2000 ft/min, which means about 30 seconds for a 1000' climb. A difference of 3 seconds is a 10% difference in rate of climb over that speed range. I haven't done these tests on my aircraft yet, as it is still parked in the garage. But, a 10% difference in rate of climb over that speed range seems reasonable to me.

The rate of climb depends on the weight and the excess power (excess power is the difference between the power available, and the power required to maintain level flight at the climb speed). I don't know what weight you were at, but at 1,600 lb, a 200 ft/min variation in rate of climb implies a difference in excess power of 1600 * 200 = 320,000 ft-lb/min. There are 33,000 ft-lb/min per hp, so that means the excess power varied by about 320,000/33,000 = 9.7 hp over that speed range. In other words, your results imply that the horsepower required to maintain level flight would vary by 9.7 hp over that speed range.

I took a look at the drag data published in the RV-6A Aircraft Performance Report done by the CAFE Foundation. 70 kt to 115 kt represents a speed range of 80 mph to 132 mph. They show a drag of 160 lb at 80 mph, and a drag of 145 lb at 132 mph. Power required = drag * speed. Doing the unit conversion gives me 34.4 hp at 80 mph (70 kt) and 51.2 hp at 132 mph (115 kt), or a difference of 16.8 hp, which is a bit more than you saw. One explanation could be that the prop efficiency improves quite a bit as the speed increases, so the power available increases (power available = power produced by the engine times the prop efficiency). The data that I've seen for my prop shows a significant variation in prop efficiency with speed in the low speed range - 50% prop efficiency at 50 kt, and 70% prop efficiency at 100 kt, for example.

This is rather a long way to say that I think your data is plausible.

The real test is to see if the results are repeatable on another flight. For extra points you would do each speed twice, on reciprocal headings, averaging the rates of climbs on the two headings. Doing the same speed on headings 180 degrees apart helps cancel out any effect of changing wind with altitude.