Prop pitch vs. cruise speed

[FastEddieB]

I think I brought this up once before, but I want to re-solicit some opinion...

 

I posted about my Warp Drive prop blades going back to the factory re: loose leading edge strip. I should get them back tomorrow.

 

As background, my Sky Arrow prop has never been adjusted in the time I've owned it (since new). I checked it once, and the pitch was exactly what the AMM called for - about 13.5º, IIRC.

 

My performance has always been OK, but on takeoff I only see around 5,000 rpm, but this again is what the book calls for. Initial climb is just under 5,000 rpm, and here is a full power climb through 10,000':

 

 

(check out the groundspeed on the Garmin!)

 

Here's full power cruise at 13,500':

 

 

 

Having last owned a plane with a constant speed prop, I can't help but think of the wasted performance. That is, our engines (ROTAX 912ULS2 in my case) are rated for 100hp, but that's at 5,800 rpm. IOW, if our engines never see 5,800 rpm, we virtually never see 100 hp, hence the waste.

 

I'm toying with the idea of setting a little less pitch when reinstalling the blades. I'm Experimental, so its OK to "experiment".

 

I know that less pitch will result in better performance in the climb - I'm wondering what effect it will have on cruise. My analogy is that many cars and bikes will go faster in something other than top gear - lower gears let the car/bike rev higher and hence go faster.

 

I'm sure this is something you guys have experimented with.

 

1) How did a bit less prop pitch affect climb vs. cruise?

 

...and...

 

2) Did you find that full power cruise was hurt or helped?

 

...and...

 

3) Do I remember correctly that 1º of prop pitch change equals roughly 200 rpm? If that's the case that's probably where I'll start.

 

PS - I'd love to go to a constant speed prop, but even as an Experimental I believe the regs prohibit that to stay in the Light Sport category.

[Roger Lee]

Hi Eddie,

 

Those are nose bleed altitudes. I'm usually no more than 100' - 1000' off the deck.

 

1. Less pitch will give you better climb, but you will loose cruise speed. You can cruise at 5500 rpm all day if you chose to do so. I personally like 5100-5200 and my WOT flat and level is 5600.

 

2. If you fly at those high altitudes all the time then a flatter pitch will help maintain some of the rpm and HP. If you fly from sea level and set the prop to get 5500 WOT in cruise then at 10K+ it will be a dog.

 

Example:

If you fly most of the time at 9K' - 10K' then you should set the prop up to see 5500-5600 WOT at that altitude. You will however get much higher rpms when you take off at low elevations and loose some cruise speed at low elevations due to the flatness of the pitch.

Set you prop to get 5500-5600 at your average cruise altitude for best all around performance.

 

3. 1 degree is about 200 rpm, it is dependent on the prop some.

With a carb, air aspirated and ground adjustable prop we will never see 100 HP and certainly less at altitude.

 

 

p.s.

How did you get that little degree symbol by your one?

[Jim Meade]

The degree symbol on my computer with Windows 7 Pro is alt 0 (zero) 1 7 6.

 

°

 

Hold down the alt key and press 0 1 7 6 on number pad then let up the alt key. Maybe someone will come up with a macro for it.

 

 

[FastEddieB]

The degree symbol on my computer with Windows 7 Pro is alt 0 (zero) 1 7 6.

 

°

 

Hold down the alt key and press 0 1 7 6 on number pad then let up the alt key. Maybe someone will come up with a macro for it.

 

I'm on a Mac, where its just "option - 0 (zero)".

[FastEddieB]

Hi Eddie,

 

Those are nose bleed altitudes.

 

That was pretty much a test. I guess my normal altitude is between 1,500' and 8,000' AGL, depending on terrain. I'm 62, and a long day above about 6,000' leaves me pretty tired the next day. I enjoy flight at 1,000' agl or even less, but with the terrain in my neck of the woods and no chute that leaves me with limited options if something goes wrong.

 

1. Less pitch will give you better climb, but you will lose cruise speed.

 

I can see that for any given rpm, but my point is that if I can pull more rpm I may be able to actually cruise faster, albeit at a higher fuel flow. Right?

 

I've got pretty good base numbers logged post-annual last year, so I should be able to get a pretty good handle on the difference 1º makes.

[Runtoeat]

Eddie, I went with Roger's advice of 5550 rpm set at normal cruise altitude. For me, this is around 2500 msl. I was at 5800 rpm before resetting the pitch to get 5550 rpm. I picked up airspeed going to the lower rpm cruise setting. I haven't looked at the HP curve for our 912 ULS lately but I believe it shows more HP at 5800 rpm than at 5550 rpm. I never have fully understood why torque curves don't always follow HP curves so there's magic somewhere here which allows the 912 ULS engine to pull my CTSW faster at a lower rpm than the max hp rpm of 5800 - maybe due to better torque at lower rpm? I'm sure the gearheads will jump in here and provide informative information on this subject.

 

P.S., I didn't happen to see any oxygen lines in your picture.....:-)

[FastEddieB]

 

 

P.S., I didn't happen to see any oxygen lines in your picture.....:-)

 

That's why I didn't spend more than 30' above 12,500' and why I didn't continue to 14,000'+, which I could have. But thanks for looking out for me!

 

BTW, after Jan 1 I'm flying under light sport limitations, so I think I'm limited to 10,000' now regardless.

[Roger Lee]

Hi Eddie,

 

Our issue for a ground adjustable prop is to strike a balance between climb and cruise. The 5500-5600 WOT at your average cruise altitude will do that for you. You can get too flat and too course. Too flat your spinning your wheels where you spend the most time (cruise), unless you need the extra climb to get out of higher elevations, high density alt and or fly high all the time.

Try the 5500-5600 mark (balance) and see if you don't like it. I haven't had a single owner ever want to go back to a higher or lower rpm.

[FastEddieB]

Update...

 

I should be on for Sunday to have a friend from the EAA help me with the pitching.

 

I was way off on the pitch I measured. This from the AMM:

 

 

My plan is to probably pull back from 23º to 22º as a first step and see if I gain 200 rpm in cruise.

 

BTW, the instructions from Warp Drive say to always measure pitch at the tip. My manual calls for measuring at 68% of the prop radius. Where do you measure it on a CT?

[Roger Lee]

Hi Eddie,

 

Warp is different. It doesn't really have a twist or curvature like some props. It's pretty flat and straight on the back side. If you have tapper tip blades measure back about 6"-8" from the tip. If you have square tip blades the tip is fine. It also doesn't really matter where you choose to measure the blades and I don't usually care what the pitch really is. Some blades I measure back 8" or so to place my protractor. Just do it the same each time and you don't need to level the plane either. It doesn't matter. All you care about is what the final result is regardless of pitch. Set your first pitch guess and then do a static with the Warp. If it is close to where you want it go fly it then make you final adjustment then. Just add or subtract what ever degrees you want to achieve that final WOT RPM at your average altitude.

If you want call me for details. You'll be a pro the first time and do it in 40 min. or less.

[FastEddieB]

I'll be doing the deed tomorrow.

 

The blades arrived back yesterday, looking almost new. I got everything assembled and I'm pretty sure I've got the protractor thing worked out:

 

 

 

Looks like my hub is no more than about 1º off from vertical, so the math should be pretty easy.

 

Still, my EAA friend has done this before and I never mind a second set of eyes and hands to make sure I'm on the right track.

 

I'll let you know how it works out!

[FastEddieB]

Eureka!

 

We mounted the blades at 22º, one less than before, looking for an extra 200 rpm in the takeoff and climb.

 

Sure enough, right to 5,200 rpm on takeoff and throughout the climb, when before I barely saw 5,000 rpm, if that:

 

 

That was taken shortly after takeoff, before retracting takeoff flaps,

 

I climbed to 9,500', showing 5,200 rpm all the way up. Full throttle and level at 9,500, I saw about 5,500 rpm and about 102k TAS:

 

 

This is a knot or two better than I used to get at full power when I'd see about 5,100 or 5,200 rpm and about 98 or 99k. It remains to be seen what effect this all has on fuel consumption. At other altitudes, I was going a tad slower at any given rpm, which makes sense with less pitch.

 

Overall, I'm very happy so far.

[Runtoeat]

Good info Eddie. Along with running at optimum rpm, the other advantage of pitching to 5500 to 5600 rpm WOT is you will not overspeed should you break a throttle cable and the engine defaults to full throttle.

[FastEddieB]

Good point.

 

Got me thinking that if one was cruising along at partial power and the engine suddenly got real rough, one checklist item should be to go to full power. If the roughness was caused by a broken throttle cable, that should smooth things right out.

[J L M]

OK, please give me a little input. My 2007 CTSW was getting 5500 RPM on s&l flight, 3000msl. It went for an annual condition and came back with max rpm 0f 5280 on s&l flight. It also had a piece of duct tape on the front cooler. I just adjusted the blade angle and took it up yesterday. 3000msl at s&l flight I saw 5500rpm's and it would flash yellow on occasions, not consitantly. My oil temp seemed to jump a little, around 235 and would also flash yellow on climb out. I landed and pulled the duct tape off and tryed it again. Oil temp stayed arond the 160's. BTW, the OAT was 52F at the time. As I started the decent to land my engine light came on and I noticed the oil temp was reading 115 and turned yellow. I added a more narrow piece of tape back to the cooler and tryed again. Now my oil temps are staying in the 195 degree area. Do these numbers sound about right? Tks for any input.

[Roger Lee]

Hi Jerry,

 

You are right to keep the WOT setting at 5500-5600. The tape will help. We are running about the same temps as you in the morning (52F) and I have a piece of 2" aluminum tape across the top part of the radiator. My temps for the CHT and oil stay around 200 -210F.

235F for oil temp is no big deal so don't worry about it especially if it were intermittent. Nothing you can do about temps when at idle or a landing approach and the lower temps are ok for that interim. You just don't want to fly there all the time.

[J L M]

Thanks Roger for all your help on the phone and online. This forum has been a hugh help to me. I do have another question, sorry if it has been posted some where else. When the fuel measuring stick says 'Right' on one side and 'Left' on the other, does the right side correspond with the right wing as you sit in the plane or as you face the plane? I know its such a newbie question, but thats what I am

[Runtoeat]

Hi Jerry, since this thread originally started out as talking about prop pitch, I have replied to your question about fuel dip sticks and put this reply in "Airframe" section of the Forum.

[sandpiper]

Jerry,

 

I do my own maintenance so I don't have to worry about it but, if I took my plane in for work and the mechanic changed my prop pitch without my approval I would be a little bit upset.

[Roger Lee]

The easy way to remember which way the dip stick faces is to always put the "L shape " or tab facing to the outside of the wing you are measuring fuel on. Aircraft orientation should always be in the direction that the plane faces. i.e. the pilot seat is on the left side, the passenger on the right.

[J L M]

Hi John,

I was told that one blade was a little off and was corrected to match the other two. I just know that it was very different when I got it back. Anyway, seems to be back where it was after taking 1.2 degrees out. Back at 5500 WOT rpms now.

 

Tks guys for the info.