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Steve Sampson

Well Known Member
Am I right in assuming best climb speed is the same as best L/D? I presume so. Presumably it is pretty much weight, engine and prop type independent?

So what approximate speed is considered best climb for a -4?

Thanks
 
Steve Sampson said:
Am I right in assuming best climb speed is the same as best L/D? I presume so. Presumably it is pretty much weight, engine and prop type independent?
Click to expand...
I assume that by "best climb speed", you mean the speed for best rate of climb (Vy), not the speed for best angle of climb (Vx).

To a certain extent, the speed for best rate of climb depends on the prop. If you have a constant speed prop, the engine can produce maximum power at all airspeeds. With a fixed pitch prop, the engine rpm at full throttle increases as you go faster, so the power available increases as the airspeed increases. Thus the airspeed for best rate of climb will be a bit higher than with an aircraft with a constant speed prop. There is also an altitude effect - Vy will decrease slightly with altitude, eventually becoming the same as Vx at the absolute ceiling.

On my RV-8, with a 74" diameter Hartzell constant speed prop (the older one, with 7666 blades), Vy at 1580 lb at 8000 ft was about 90 kt CAS. Data plot here.
 
Dayton/Kevin, thanks for the feedback.

I will get to plot my own polar it time, but testing is slow I find.

Kevin, the implication of what you wrote in the link, as I understood it, was that best L/D, or Vy changes with weight. Surely this is an insignificant change? As a glider pilot you expect L/D to be almost constant, just the speed you achieve it at, to vary with the amount of water you carry.

Have I got this wrong or is it a function of our short stubby wings and Reynolds numbers?

Thanks.
 
Steve Sampson said:
Kevin, the implication of what you wrote in the link, as I understood it, was that best L/D, or Vy changes with weight. Surely this is an insignificant change? As a glider pilot you expect L/D to be almost constant, just the speed you achieve it at, to vary with the amount of water you carry.
Click to expand...
Ignoring the small effect of how prop efficiency changes with airspeed, the speed for best rate of climb will be at a constant angle of attack as you vary the weight. So, if you reduce the weight, Vy will reduce by the square root of the change in weight. E.g., in my case I found a Vy of 90 kt CAS at an average test weight of 1583 lb. If I reduced the weight to 1400 lb, I would expect Vy to be = 90 * square root of (1400/1583) = 85 kt. Not a huge change, especially considering that a plot of rate of climb vs speed has a very flat top. So if you are 5 kt fast or slow from Vy, the reduction in rate of climb is extremely small.
 
I suspect the best rate-of-climb would be very close to the best L/D with a constant speed prop. With a fixed prop the horsepower increases as the rpm increases with higher airspeed so the rate of climb speed is somewhat higher then best L/D.
I consider best angle is 84mph with 1/2 flaps and best rate to be 110mph clean. The rv4 climbs so well that I normally use 120 just to see better.
Chuck Ross 160hp RV4 72X72 wood prop 750 tt
 
Concur with CROSS on climb speed for viz. About 80 mph for angle seems to work well plus or minus a couple for heavy or light which concurs with the RV-4 POH I downloaded from a Google search last night.
 
Cruisn'

Steve,
Great posts above and good info. Watching 1500 hours come and go on my RV4 gave me some pretty good numbers for pretty much all of the flight envelope for a light, 0-320 powered RV4.
I cruise climbed at 120-140 MPH which gave me 1000 fpm to 5000' with good cooling, where I transitioned to VVI, 500 fpm to higher alts normally 120 kts TAS. Initial ROC on T/O was 1500 fpm at 100 mph, all of course with a FP prop.

Cheers!
Smokey
HR2
 
2200 fpm

Testing is proving difficult: the air wont stay still. Also I find holding an accurate speed quite difficult because the nose is so high its hard to get a reference to hang on to.

However what I have established is that for my 160hp -4, with a constant speed 2-blade MT prop, best climb is not slower than 120mph. At that speed, with a gross of 1300lbs (the aircraft me and 2/3rd fuel) I am getting between 2150 and 2200fpm. The prop is governed to 2670, which is 30rpm short of full power. The climbs are being measured between 2000' and 3000' starting from 1000'

Clearly the c/s prop is a huge advantage over FP for this.

One other data point is the new bigger jet is letting me climb to 8000' at full power with the highest CHT getting to no more than 380F. Oil temps remain a little low at 177F a very rare maximum, but then we have had lots of cold air.

I wonder if my jet is now slightly over rich but I am just going to operate for a while and see how everything works out in warmer weather.
 
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