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-   -   Pitch up with new prop! (https://www.rcuniverse.com/forum/aerodynamics-76/3282722-pitch-up-new-prop.html)

ec121 08-20-2005 10:50 PM

Pitch up with new prop!
I was experimenting with my Funtana90 today and replaced the 16/4 APC prop with a 16/6 of the same brand. I am running a Magnum 1.08 2c engine. With the 16/6 the plane flew like it was tail heavy. It pitched up and started to climb as soon as it left the ground with neutral elevator. Climbed with throttle increase. Couldn't trim it out because it would dive at low throttle. Is the thrust angle that sensitive to speed. I put the 16/4 back on and it flew fine. Had a lean run and got to glide it in. Glide was fine with the 16/4 do it isn't tail heavy. Almost landed itself. What am I seeing here?

WS 08-20-2005 11:07 PM

RE: Pitch up with new prop!
I believe those 16 x 4 props have wide blades? (heavier?)

Put the two props on one of those kitchen scales. A few ounzes in weight difference acting several inches from the CG could make the change in stability that you saw.

ec121 08-21-2005 11:49 AM

RE: Pitch up with new prop!
I just weighed the 16/6. It weighed 3.25oz. The 16/4 is on the plane, but it is thinner than the 16/6. I had to cut out the spinner for the 16/6. It is a lot heavier in the hub and has thicker blades to hold the added thrust. I don't believe there is enough weight difference to cause what I saw. It also had a full fuel load near 16oz at takeoff. I was flying it 4oz. tailheavy then I added 4oz of lead to the firewall to get it to land without dropping the tail. It didn't fly like I saw yesterday before I added the weight. I'm still pondering on iy.

BMatthews 08-21-2005 01:32 PM

RE: Pitch up with new prop!
I suggest you try doing a dive test with the props on the plane. For something like a Funtana you should be able to push over to a vertical dive and then neutralize the elevator stick and the model should continue in a pure vertical line or pull the nuse back up VERY slowly..... like maybe pitch up only 5 to 10 degrees over a 200 foot dive. Set the elevator trim such that it flies at 2/3 to 3/4 of max speed and dead level hands off for a good time and then do the test. If it tucks and tries to loop negatively you're tail heavy with down trim to compensate. If it pulls nose up faster than indicated then you're nose heavy and flying with too much up trim to compensate. Move the CG to fix whichever way the model reacts and re-adjust the elevator trim to restore that level mid power cruise and test again.

I'm guessing that you're nose heavy and the extra speed of the higher pitch prop is inducing a climb due to the elevator trim. If not then you have thrust line issues.

With the CG and resulting elevator trim set according to such a dive test the model should show only a very slight or no tendency to nose up with added power. If it still acts up then I suggest you check the thrust line to wing centerline angle for any upthrust. It should be 0-0 or perhaps just one degree of downthrust.

ec121 08-21-2005 09:14 PM

RE: Pitch up with new prop!
I'll try the dive test but right now it flies with just about zero trim on the 16/4. Not more than a click or two of up. Might be enough to do it with the 16/6. I have the weight screwed to the motor box. Wouldn't be hard to take a couple of ounces off. I put the weight on because every time I cut the throttle to land it pitched up like it was tailheavy. Some folks say that is a characteristic of the Funtana90s, but it shouldn't be unless it is tailheavy or the incidence is off.

BMatthews 08-21-2005 10:10 PM

RE: Pitch up with new prop!


I'll try the dive test but right now it flies with just about zero trim on the 16/4. Not more than a click or two of up. .....
With all due respect this doesn't mean much. The elevator may be trimmed level to the stabilizer but if the incidence angles are out by a bit you may have some unwanted incidence anyway. The only way to tell what the model's trim is like is to do a dive test. It also sounds like there's something odd about your description of the tail dropping during the landings unless you have some noseweight in the model. As you say that suggests tail heavy but it could also be due to some unwanted downthrust. In the end it may be worth setting up some incidence and thrustline guages that you can be sure of and measure it all up to the nearest degree of angles for all the key elements.

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