next »

[carlmill]

Hi,

I have a 2+ month old Legend Cub with the Continental O-200 and the Sensenich 72" ground adjustable prop. I've mostly been running it with the 46" pitch cartridge but recently tried the 50" cartridge.

Here is the, so far, limited data @ 75 degree temp:

                                   46 vs 50
rpm with 60 mph climb   2525/2300
rpm full power level      2750+/2600
true speed at 2400 rpm      92/98

First the Theory part:  I realize that speed vs pitch change will be very non-linear and will need testing with various power, prop pitch, altitude variations to determine the point of diminishing return as far as trading off climb/runway length is concerned. My question is, will variations in climb rpm, max rpm, and fuel flow per a particular rpm be more linear with prop pitch change than speed? I'm trying to interpolate/extrapolate with limited data

Now the Practical: In June I plan to put this Cub on Baumann 1500's.  How much will the additional float drag reduce the above rpm #'s with the 46" pitch?  In other words, should I be ordering a 44" cartridge?

Thanks,

Carl

[DaveG]

The RPM at 60 mph will not change with more drag, just the climb rate.
The max level flight rpm will reduce because the max speed will be lower with more drag.
And..of course the speed at 2400 will be lower with more drag.
Less pitch will obviously improve climb at 60 mph at the expense of cruise speed, and will make it easier to overspeed in level flight (which is easy to manage...)
DaveG

[JimC]

My question is, will variations in climb rpm, max rpm, and fuel flow per a particular rpm be more linear with prop pitch change than speed?

Neither.  You also need to factor in manifold pressure and several other things.
And, the rpm at 60 mph WILL change with drag changes, as will climb rate.

You most likely will want to get a 44" cartridge for the floats.  I run a 7142 with an O-200 J3 landplane and often wish it were a bit flatter (the 7535 is too flat).  On an experimental, I think a 7440 would be a really good choice.
JimC

[DaveG]

Jim
You're right, of course...The RPM at 60mph will change when drag increases.....
I had to think about it a bit more..... 
As the drag increases the blade AOA will need to increase since more thrust will be needed, which will load the engine more, lowering the rpm. The interaction between the prop's characteristics and the engines torque curve will muddy the waters a little, but the basics still hold, I think....
It's sort of a double whammy, since the weight and drag of the floats reduces the climb, and the engine looses some power when it can't turn as fast, which hurts the climb also....
I prefer to optimize the climb performance for my Cub, since there really isn't any thing you can do to make it any useful amount faster..... 92 mph vs 98 mph is "in the noise" for me......

I blame it all on it being Friday.....It's been a LOOONG week.....
DaveG

[JimC]

I also prefer optimising climb performance.  The fastest cub in the world is dead slow.
JimC

[newcub]

What is the rate of climb at 60 mph with the two cartridges?

[carlmill]

What is the rate of climb at 60 mph with the two cartridges?

Newcub,

In order to give you an accurate answer to your question I would have had to do timed climbs, record density altitude, had smooth air and record weights.  I've been having too much fun with it (just got it mid June) to do all those test pilot details.  My Legend does have an ADI horizon which acts like an instantaneous VSI.  Unfortunately, the feet/min markings are somewhat crude.  A VERY rough estimate would be 775 ft/min with the 46" and 625 ft/min with the 50" on a 75 degree day with two aboard.  I'll try to get better numbers in the future but the subjective difference in take off and climb is obvious.

I also prefer optimising climb performance.  The fastest cub in the world is dead slow.
JimC

JimC,

Can't argue with your first sentence when flying out of short field, fields with obstructions, heavy on a hot day or off the water. As for your second sentence, Cubs are indeed relatively slow, but I would be glad to have the option of an extra 10-15% in speed if I were flying to visit you, DaveG or Newcub, as long as my fuel stops had reasonably long strips.  The cartridge switch takes five minutes and the spare cartridge costs $58. 

Thanks, guys,

Carl

[DaveG]

There is a common misconception showing here... It is, that having the same RPM on 2 different fixed pitch props means equal power is being supplied by the engine. This is simply not true...

Let's use the 92 mph vs 98 mph at 2400 RPM data point. This speed delta implies that about 13.5% more THRUST horsepower is being generated. That's how much more power it takes to go 6 mph faster at 92 mph.

The higher pitch prop may be slightly more efficient at the the higher speed, but most of the speed difference is because it takes more MAP and thus more engine power to turn that prop at 2400 rpm. If a MAP gauge was used along with the tach, and plotted on the engines power map, you would find that at a constant POWER (and thus fuel flow) that the 2 props will be essentially the same speed. The lower pitch prop will be slightly less efficient but it would take a darn good pilot and some very careful measurement to tell the difference.

For airplanes like the Cub, with it's limited speed range, biasing the prop pitch towards the climb end of the speed range has very little down side. If you want to go fast with a climb prop, you just spin the engine a bit faster, it's a little noisier but it doesn't really cost any more in fuel then the cruise prop would burn.

That plot of power versus MAP and RPM is really a required piece of information, along with a manifold pressure gauge and a good tach, to properly assess the differences between props.

Example for the O-200...
2550rpm/23.7 in  and 2350rpm/25.8 in (this is about wide open throttle at 4Kft!) both make 75 horsepower!
The prop that is turning 2525 at WOT and 60 mph is making about 92 hp at SL
The prop that can only turn 2300 is only making about 85 hp SL.....
AND....
The prop that turns 2550rpm can make 75 hp up to roughly 6500 ft.....
But...this is a little misleading.... since the prop/engine/airplane combination that can turn 2550/23.7 at SL will need to be throttled back a bit to remain under 2750 at 6500 ft......nothing is perfect....this is why they invented CS props......

My O-200/Cub(PA-11 style cowl) with a Mac 75/38 goes 90mph true at 3500 ft, at 2550/23.5in. I wouldn't change a thing....
DaveG

[carlmill]

DaveG,

Strangely, I understand what you're saying.  Thanks for the review.  Essentially, there's no free lunch.

95% of the time Cub flyers are happy as clams putting along at 80 mph and will select a flat prop for it's ability to get off fast and climb well.  On the other hand, depending on your "mission",  there might be a time where you'd like a Mac 75/35.  For instance, getting out of that smallish lake on a warm summer day.  Likewise, even in a Cub, there might be an instance where getting to your destination faster (accepting fuel flow penalty) could be more important than enjoying the low and slow. In that circumstance you'd want a coarser prop.

What the Sensenich prop does is give you both in one prop, and at only 12 lb complete.  What it comes down to is, what do you do with your plane.  I intend to be on floats June/July and doing some X/C in the spring and Fall but I don't want to own two props.

You mentioned CS props.  Anybody have any experience with the Aeromatic on a Cub?

Carl

[DaveG]

"Likewise, even in a Cub, there might be an instance where getting to your destination faster (accepting fuel flow penalty) could be more important than enjoying the low and slow. In that circumstance you'd want a coarser prop."

Actually what I was trying to show is that a coarser pitch prop could very well be slower then the climb prop under some circumstances.
For example, using the previous data points.... If you want to fly above 4K feet at 75% power.

The optimum CRUISE prop would turn 2750/23.2 in (Max Continuous) at wide open throttle at about 6000 ft.This would yield 75% power at the best True Airspeed. It would be interesting to know what combination fits this set of circumstances.
.
DaveG

[David Johnson]

Those floats don't have enough dents.

[carlmill]

Those floats don't have enough dents.

Guess I'll have to work on that, starting next June.

[JimC]

.........Let's use the 92 mph vs 98 mph at 2400 RPM data point. This speed delta implies that about 13.5% more THRUST horsepower is being generated. That's how much more power it takes to go 6 mph faster at 92 mph.

You seem to be under the impression that cruise speed increases with the square root of the power ratio, since you assume that  (98/92)^2=1.1347 for a 13.47% change.

However, cruise speed actually increases with the cube root of the power ratio, not the square root.
So, the actual equation is (98/92)^3=1.2087, and you have to increase the thrust horsepower by 20.87% in order to increase your speed from 92 to 98 mph.
JimC

[willyb]

My Legend 0-200 works just fine with a Mac 75-35.Cruise is 92mph/2550 and 5gph when leaned.If I were to try anything different I would wait to buy the long version of the Sensenich ground adjustable.I think the 76'' blade is going to be the best set-up yet.I am expecting a call this morning from Ed Zercher at Sensenich regarding a release date.
  Bill

[JimC]

Bill, what's your top speed and corresponding rpm with the 7535 at low density altitude?  I've tried the 7535 on an O-200 J3 and topped out at about 110 mph at 1100 feet DA.  I'd guess that you'd probably be at least 10 mph faster, maybe more.   I liked the 7535 performance, but was leary of it due to the high probability that someone might overtach the engine to the point of requiring a teardown.  As an aside, a McCauley 7440 produces greater static thrust than the 7535 up to the crossover point at about 2750 rpm.  I do think the 7535 would probably outpull the 7440 in climb if one were to let the 7535 run up to about 2850 rpm during the climb.

Please keep us posted on the 76" Sensenich ground-adjustable.  I'm very much interested in it.  Is it a true 76", or is it clipped to 74" like the wood Sensenich's?
JimC