Trim Question

[Doug Moon]

Hello,

This weekend I made a change to the plane and it was quite an eye opener.

I have been flying my PA75 on a 13" 4 blade for a loooong time. One of our club members was selling off some props. I picked up a new never flown 14 1/8" eather 2 blade, red tint if that matters.

I threw it on plane for a test and WOW! It was like taking 2 oz out of the nose.  The changes were dramatic. Inverted flight required constant attention. On the 4 blade it was so locked in you could check your phone and order a pizza. BUT the corner with the 2 blade was drastically improved. It was effortless. I was way more precise on my angles as well. I can imagine alot of that was gyro procession. But I did notice over the top it was lighter on the lines. Not loose or anything but lighter.

To get back some of the tension up top do I go back on leadouts like I would if I took out nose weight?
Or do I add back in some nose weight?
Or a little of both?
Or add tip pitch for a little more boost?
Or push the pipe in?

I am curious how others would attack this situation...

Or do I build an electric plane and adjust the accelerometer?  I know Mike Scott would like that option. 

So many questions.... 

2 blades are just effortless in the corners. Even at that length it would just flip through the corner.

[Matt Colan]

My first guess is the lighter prop moved the CG back which brought your leadouts further forward in relation to the CG. So, I’d try either moving the leadouts back or add the equivalent in nose weight back in from the four blade back in to maintain the same CG.

What I’m curious about is how your motor burned more fuel on the 2 blade than the four blade when your needle setting barely changed. That was a frightening four leaf clover hoping it was gonna get through!!

[Doug Moon]

>>>snip<<<<
What I’m curious about is how your motor burned more fuel on the 2 blade than the four blade when your needle setting barely changed. That was a frightening four leaf clover hoping it was gonna get through!!

YES forgot about that part. It went from 3-4 extra laps to beeping prior to the clover on pretty much the same needle setting. I think the longer blade length did that.

[Mike Palko]

Did the lap time change? Slower lap time means running out of fuel earlier in the pattern and possibly less tension overhead.

Mike

[Joe Gilbert]

Doug it could of been a few more rpm also which would make it burn more fuel   Gut feeling the engine unloaded with two blade a bit more so more fuel is needed and that loss of load could change line tension up top. Two blade should be lighter so lead out kiss back.

[Dan McEntee]

  Just curious as to what pitch the four blade was?
  Type at you later,
   Dan McEntee

  OK, just re-read the original post. You don't say what the two blade pitch was, though. As hard as it might be to believe. If they were both 4 inch pitch, the four blade made have been a bit of an over load and leaning the engine out. The two blade let it free up and work, thus using more fuel. Too late to go back and listen to the engine runs to see if the four blade was a little more monotone, and the two blade may have been breaking a bit. You might want to try that four blade with a bit less pitch since you are experimenting.
   Type at you later,
   Dan McEntee

[Sean McEntee]

Hope you get it sorted out.  I'm currently enjoying the more liner trim process of FF models.  Nose weight and incidence adjustments cures most ailments  . I'll get back to CL stunt one of these days...probably after college.

[Doug Moon]

Mike,
I didn't get a time on it. I decided to make the switch on the 2nd round of a contest and shapes were more important. It felt almost the same I couldn't really tell. It was close enough to where my flight scores went up considerably even with a mid 20s take off and inverted flight.

Joe,
I would think the opposite? Loaded engines work harder to keep the applied setting so they use more fuel. One truck pulling a boat vs the same truck without a boat. The one without the boat gets better mileage. Same even holds true in electric. Electric helicopter hovering on a 12s 10000 mAh will set there for a solid 10-12 minutes vs full flight maneuver loads that use up the same battery in 4 minutes.

Dan,
The 4 blade should be 4. I have not checked that one in about 5 years. It's an Eather the pitch is ground in so they don't change much if at all over the years. The new one I got appears to be 4-4.25 depending on where you pitch it.

The really odd thing about that prop change was I added over 1" of diameter but the corner increased a substantial amount. I guess the gyro effect of 2 more blades is more than the overall diameter. But the overall diameter sucked more fuel....??

[Joe Gilbert]

Check RPM of two blade I would guess more and not loaded as much so engine running faster and getting more revelations per lap so. sucking fuel more times per flight. On a truck pulling trailer rpm may be same but carb open more to make the same rpm we have fixed air flow. It does surprise me the one inch diameter is allowing the plane to turn faster. I kinda felt the four blade gave somewhat better drive in corners than two blade. So many variables. Tons to learn.

[Mike Palko]

The really odd thing about that prop change was I added over 1" of diameter but the corner increased a substantial amount. I guess the gyro effect of 2 more blades is more than the overall diameter. But the overall diameter sucked more fuel....??

The weight of the prop and where the weight is located (near the tip or hub) also affects the turn.

Look out when you get it to lock in again!

Mike

[Ken Culbertson]

I know Mike Scott would like that option. 

Maybe you should retrim when the wind is not in your face on all 360 degrees of the circle at the same time!

Ken

[Dan McEntee]

Mike,
I didn't get a time on it. I decided to make the switch on the 2nd round of a contest and shapes were more important. It felt almost the same I couldn't really tell. It was close enough to where my flight scores went up considerably even with a mid 20s take off and inverted flight.

Joe,
I would think the opposite? Loaded engines work harder to keep the applied setting so they use more fuel. One truck pulling a boat vs the same truck without a boat. The one without the boat gets better mileage. Same even holds true in electric. Electric helicopter hovering on a 12s 10000 mAh will set there for a solid 10-12 minutes vs full flight maneuver loads that use up the same battery in 4 minutes.

Dan,
The 4 blade should be 4. I have not checked that one in about 5 years. It's an Eather the pitch is ground in so they don't change much if at all over the years. The new one I got appears to be 4-4.25 depending on where you pitch it.

The really odd thing about that prop change was I added over 1" of diameter but the corner increased a substantial amount. I guess the gyro effect of 2 more blades is more than the overall diameter. But the overall diameter sucked more fuel....??

  Well, by my simple reasoning, with the four blade, you have approximately twice the prop that you do with the two blade, even with the extra 1/2" on each tip, if they are similar in pitch. In my playing around with 4 blade props, mainly on sport and small scale models, the pitch needed to be about half of what a two blade would be or the engine would be over loaded. Now granted, I wasn't playing with a PA.75, but every engine has it point where you can go over board, and maybe you got in that neighborhood with yours. Would be interesting to see how that prop would work on anything other than a PA.75??? But that's my barnyard guess. You cut t he prop load in half, and I  didn't even think about the balance issue, bit that sure would come into play on the corner.
   Type at you later,
    Dan McEntee

[dale gleason]

Could it be that a longer prop blade would be sending a larger "disc" area of flow over the wing and tail, which means more airflow over the flap and elevator, making them more effective in corners... ?

dg

[Dave_Trible]

I recently put on the Eather 4 blade where I usually run my own or a Mejlik three blade-all at 12.5" (trimmed the four blade down).  It would fly very well but I'd have to retrim the airplane since it felt like I'd put on an ounce of nose weight.  Sure the prop is a little heavier but not to that degree.  I assumed it was caused by adding weight to the gyro effect of the prop.  I have a few Eather 2 blades but never thought I got the thrust from them I get from more blades.  I do pull quite a bit more pitch than you are using and lower rpm.  Maybe that's giving up too much blade area for the rpm.
I'd be interested to know the rpm difference you are getting between props.  When I go set up a new airplane I set three or four props about the same with the pitch gage.  Then I fly the first prop, check and record the rpm at a given needle.  WITHOUT touching the needle I go through each prop and repeat the measurement.  They will all be a little different-sometimes 200 rpm different.  The slowest one is generally the best flying and keeps the engine in a four cycle most generally.  The added load does consume more fuel.  I'm betting Doug you have a good measure more pitch on your two blade than the four but may still need more rpm to make up for the loss of blade area.

Dave

[Doug Moon]

Check RPM of two blade I would guess more and not loaded as much so engine running faster and getting more revelations per lap so. sucking fuel more times per flight. On a truck pulling trailer rpm may be same but carb open more to make the same rpm we have fixed air flow. It does surprise me the one inch diameter is allowing the plane to turn faster. I kinda felt the four blade gave somewhat better drive in corners than two blade. So many variables. Tons to learn.

I will have to get a tach... 

There is more drive in the corner with a 4 blade. Lots MORE!!  But that can cause the corner to appear rounded off. It does give the feeling of flying through the corner. A good two blade has that feeling of "backing off" right in the corner. I can't really explain it but I can feel it. Then you setup the motor for a really good exit off of the corner. It's hard to explain but the feeling with the two blade in the corner is the plane is completely unloaded so it flips through the corner. 4 blades are super awesome on pull etc but the corner is loaded more and can be softer because of it. One of the judges at the contest told me it was very noticeable change in corner between the two flights.

[phil c]

Doug,
The size of the prop is more than just blade number.  Since they are both 12.5in diameter, the 4 blade has TWICE the load of the 2 blade.
Have you actually measured the turn radius from a video.  The extra load of the prop will take more effort and the turn actually is larger.
When judging the larger turn is easy to see, regardless of how it occurs- prop or pilot.

Phil Cartier

Alive and Kicking!

[Doug Moisuk]

With a major change like that wouldn’t one go back to basic trimming?

[Jim Benzinger]

I will have to get a tach... 

There is more drive in the corner with a 4 blade. Lots MORE!!  But that can cause the corner to appear rounded off. It does give the feeling of flying through the corner. A good two blade has that feeling of "backing off" right in the corner. I can't really explain it but I can feel it. Then you setup the motor for a really good exit off of the corner. It's hard to explain but the feeling with the two blade in the corner is the plane is completely unloaded so it flips through the corner. 4 blades are super awesome on pull etc but the corner is loaded more and can be softer because of it. One of the judges at the contest told me it was very noticeable change in corner between the two flights.

Doug,

There is a good chance that the 2-blade in question has just a little bit of undercamber. Check the back. Even a slight bit of undercamber will greatly increase the load on the prop. It also has great pull which may explain why it is so comparable to the 4-blade. But, undercamber's typically aren't very speed stable in the wind. It will wind up.

I've found that the Eather 3-blade 13" flat back, (Blue), at about 4.2" is a very nice compromise between pull and corner. Any more diameter and the corners get all wonky.

[Steve Helmick]

In the Brian Eather color coding, I think red = undercambered. They work well for overweight planes. I'd add about 1/4 deg. pitch at the tips to gain overhead line tension. Burning more fuel is a good thing, IMO.    Steve

[Doug Moon]

I went flying this weekend. 2 weekends in a row!!  BOOM!

I moved the LOs back .25" and the plane has much more line tension now. It flys really well now.

Now onto figuring out the fuel usage. I check the undercamber with a straight edge its almost non existent.

[PJ Rowland]

Hi Doug..

I ran the .75 with many props.. including a few eather 4 blades.. the findings you experienced are fairly normal.

The 2B will give a tighter corner.. the 4 blade will have alot of level flight "pull"..but suffers in the vertical on some.designs..

There is alot more going on with respect to prop wash over the wing than people give credit to.. the 2b is more chaotic over the wing so will produce more usable turbulent boundary  exposure. The 4 blade smoothes this out ( which isn't good for us ) and id expect certain airframes to be more prone to showing when lift is as a premium ( such as up top going vertical  )

There isnt *really* a trim setting you can do that will cure this..  its more about fuse side area than anything else..

Specifically "load factor" ... 

You * can * dial in more outthrust if thats an option.. by 1/2 deg but it won't solve it completely..

You * can * move leadouts back a touch but again not really solving the original problem of why this difference occurs.

[Steve Fitton]

What lap times were you running Doug? I have a guess, but am curious if it was measured.