Engine unloading and needle setting.
 

When setting the high speed needle how exactly should I account for unloading when the plane flies?

Normally I back it off about 500 RPM. That is for a 46fx, using low pitch props (4 and 5).

With a higher pitch (7,8,9) I'd expect more unloading. Say.. a 1000-2000 rpm increase for the sake of argument.

Does that mean I should back the needle off 1000-2000 rpm to accommodate the expected unloading?

What do people normally do?

Unload
 

How much it unloads has a lot more to do with how slippery the plane is than the pitch of the prop.

Engine unloading and needle setting.
 

Thanks. How about the needle setting?

Engine unloading and needle setting.
 

The only time I go a little extra rich on the needle is when the engine is on a fun-fly. Hovering and vertical maneuvers generally require a richer high-speed needle.
For my sport airplanes I usually set the high-speed needle a little rich (300-500 RPM). Also, with the help of a friend, I have him/her give the engine full throttle while I firmly hold on to the airplane. Once the engine is running strong I point the airplane straight up. If the engine sags I richen her up a little. I've seen people tune their high speed needles this way with no ill effects.
Make a few trial runs in the air and go from there.
If you're unsure, start a bit too rich and slowly lean it out through flight testing.
Is there an airframe and engine in mind?

Engine unloading and needle setting.
 

Thanks Jazzy. That is the rough procedure I have followed to date. Although usually with low pitch props.

I've read accounts of rpm increasing by 10 or 15% in the air with higher pitch props. That's a whole bunch IMO, and I just wonder how the engine can do that if it wasn't running well rich to begin with.

The plane is a Great Planes Tracer with an OS46FX, Macs quiet pipe, and currently an 11x7 APC is the prop choice.

Time will tell.... I'll have to listen carefully when it flies.

Unloading
 

Guys,

I would have to disagree on that.

A prop with a higher pitch would need the plane to be going much faster, to unload to the same degree as a lower pitched prop. At that same speed, the low pitched prop will already be "turbining" even faster.

The reason high pitched props are not recommended, is their relatively low static thrust, when compared to low pitch, large diameter props that impose the same static load on the engine.

I set the high speed needle while the plane is "nose up".
If the plane can only pull itself up vertically, or cannot hold its own weight up, just peaking the engine is enough. I richen it only about 100 RPM, just to keep safe.

If the plane can actually accelerate vertically, I richen it by 200-300 RPM, but it equates to more when the plane is set to a horizontal attitude.

If the plane is capable of 120 MPH, or more, I make it 400 RPM rich, with the nose 'straight up'.

This way I cater for the effort the engine must make to draw fuel, in the specific plane.

Sincerely,

High Speed Needle Setting
 

I'll typically set it with the engine at full throttle and the plane pointing straight up. I get it to a solid lean run, but not full lean and then put it in the air. This ends up with a barely lean or 2 cycle, run in the air. There is a nice oil trail in the exhaust so I know the engine is being lubed well. IMO, there is no point of squeezing every single RPM out of an engine...other than to greatly shorten its life.

Engine unloading and needle setting.
 

Dar, So, in fact, you do richen more when you expect more unloading, correct? You disagree on the amount of unloading that is normal?

Engine unloading and needle setting.
 

The setting I referred to in my first reply was a typical starting point. Sometimes after initial flights you may have to tweak it a click or two one way or another to get it just right depnding on how much it unloads in the air (due to different planes/props/engines). You might say 'trial and error' method.

Engine unloading and needle setting.
 

300 rpm shy of max lean rpm with about a 1/4 tank of fuel left in tank should be fine.

Unloading
 

Gene, Jim, Homebrewer,

Unloading is not a constant and Gene, you are right, it is a 'trial and error' afair.
You do have to take into account (predict) the amount of unloading, from the sleeknes of the plane (a Clark-Y trainer's engine will most likely unload much less than a QM pylon racer.

Also, the higher the prop pitch, the less it will unload at any given speed. With a low pitch prop; 5, or 6 and a sport engine, many sport planes may reach the point of "pitch speed".
Any further increase in speed; diving, the prop will become a turbine and spin even faster. But this cannot happen, unless your plane is a very aerodynamic, pylon racer, pseudo-jet, or the like,

With an 8, or 9 inch pitch prop, you will be lucky to reach "pitch speed", even if your plane is a pylon racer. But when these speeds are reached, the unloaded engine will spin significantly faster than the static numbers, even 3k more is not theoretical. This, because it was heavily laden with excessive pitch, on the ground.

So if you are into QM, or Q500... do give it more needle on the ground.

Sincerely,

Engine unloading and needle setting.
 

My impression of unloading was that it was due to a reduction in the angle of attack, hence reduction in drag, of the prop blade as forward airspeed increased. Reduced drag, hence increased engine speed. With the decrease in angle of attack also comes a decrease in lift (thrust), unless the prop blade was statically stalled (i.e. operating past the peak of the lift versus aoa curve when the plane is at rest). Hence, higher pitch = higher potential unloading. In general, I epected it to be rougly proportional, ie. increased pitch meant increased unloading even if "fully" unloaded is never acheived. None of which is meant to make any particular point.

When a plane exceeds the pitch speed. Is that what is called "wind up"?

So... for practical purposes (not racing) the take home message seems to be: no special treatment for the needle just because the pitch is on the high side - tune it normally, and see how it behaves in the air.

Fully unloaded...
 

Jim,

I would define fully unloaded as flight at pitch speed, at the fully unloaded RPM. It is a function of a rising variable, which defines itself.

If an engine tachs at 12K on the ground and unloads in flight to 14K. And at that speed the engine manages to "screw" the prop through the air; the blade's angle of attack is 0, relative to the surrounding medium. I would call this a fully unloaded condition.

Prop blades have drag even at 0 degrees and it is far from zero. With the blade tips exceeding 0.7 Mach, it is very tangible.

A high pitched prop is statically stalled to a higher extent, because more of its blade disk area is at an excessive angle of attack. This is the reason such a prop is quite low on static thrust.

You do need to richen the main needle before flying, as I described in post #6 in the thread, or your own way. After flying the plane and listening, you can fine tune the richness.

Sincerely,

Engine unloading and needle setting.
 

Jim, isn't the Tracer a Sport/Pattern type plane?
I wouldn't be concerned about fully unloading an 11x7 on a 46 given the airframe.
Unless of course your TX's throttle is one of those that is only a two position switch. All or nothing.:D
Set it a few hundred RPM rich and fly.
It'll be just fine.

Engine unloading and needle setting.
 

Tests done on an F3D pylon racer showed a gain of 2000 revs at 176MPH. But there's more involved with normal sports type engines than just the prop unloading. Almost all of them are run well below peak HP so when the prop unloads the engine develops more HP to drive the prop even faster. This isn't a continuous cycle of course because the drag of the model builds up very quickly to balance any extra thrust developed. But another effect comes into play as engine revs increase. The airflow through the carb rises which means the depression (ability to draw fuel) increases. This should go a long way to counteract any tendency to lean out.

Engine unloading and needle setting.
 

Thanks downunder. The increased carb fuel draw explains part of what was nagging me about the needle setting and how the engine was going to run at a higher power level without leaning.

Jazzy, you're correct about the plane.

Another situation
 

If an engine is propped below its RPM of maximum torque, it is going to unload a whole lot at high speed.

That prop is the size when going one size up in the props, you lose a lot of RPM.

For example:
You change from 11x4 to 11x5; you lost 500 RPM.
You change from 11x5 to 11x6; you lost 400 RPM.
You change from 11x6 to 11x7; you lost 500 RPM.
You change from 11x7 to 11x8; you lost 1,400 RPM.

I used increasing pitch numbers, but increasing diameters will act about the same, with 0.5" changes and the same pitch.

Lets just say you use the 11x8 prop. As speed is gained, not only is the load reduced, but also the torque is increased. So the gain in RPM will be larger than if you used, say, an 11x6.

BTW, as much as I admit Downunder is right, you still have to open the needle to allow for even more fuel, the consequence is a lean run.

Sincerely,

Engine unloading and needle setting.
 

Hideho and good morning,

Some terrific replies thus far, we are lucky here to have such terrific information at our fingertips.
That said, I have a "method" that works for me, kind of a SWAG.
(seat-of-the-pants wild arse guess).
I use the pinch test to "simulate" the unloading of the prop.
While running on the table at the field, I shoot for max rpm.
Then, while at full throttle, I "pinch" the fuel line for a second as close to the NV as possible.
I listen for a increase in RPM, if I don't get it, I richen it up a click or two. Then I will from time to time lift the plane vertical and repeat the same procedure too.
This accomplishes two things for me.
As the fuel is used, and the tank empties, it compensates for the leaner run. When the prop "unloads" there is still enough mixture to go lean without going over-lean.
This is done without the use of a tach, or formula.
It has just been the way I always do it, and although clearly not scientifically accurate, works just fine to me. As Gene stated it's always trial and error, or trial and record.
I learned this method from a knowledgeable "old guy" years ago, (may he RIP).
BTW, this method rarely works in a cowled engine, although if you have access, a pair of needle nose pliers works too.

Nice thread,
Thanks to all who participated.