Plane Nose Dives

[freed2003]

Hello, recently I have built a plane reminiscent of the Vanguard P-18. However when I launched it(with 900 winds on a 3/32 motor) it just nose dived(and came down really hard). I have added some clay to the back since the weight was below 7.5 anyway, but are there any other adjustments I should make?(Also I forgot to make the wind adjustable.) Would a bad wash-in cause this problem?

[jander14indoor]

Multiple possibilities.
Did you wind the motor backwards?
Do you have to much down angle in the horizontal stabilizer (leading edge high)? The tail should be level or leading edge down with respect to the wing.
Is your balance point correct? I seriously doubt you've built a plane that needs weight in the tail unless you've moved the wing WAY back. Almost NEVER happens.
What is the total weight?
How does it glide with a motor only wound enough to tighten it on the hooks? It should glide from shoulder height with a gentle push to the floor 20+ feet away in a smooth path, no stalls.
Unless the plane rolled hard as it dove to the ground I doubt it was a wing wash problem. Not that you have that optimal, just it didn't cause the dive.
How much down thrust do you have on the prop, you need some, but excess can cause a dive.

Give us some description on your trim adjustments and we may have other ideas.

General starting point for trim.
Prop pointing down and left about 2 degrees.
Left wing longer than right and left tip leading edge about 1/8 inch higher than right tip leading edge.
Wing angle of attack - level to leading edge elevated 4 mm
Tail boom, offset about 1/2 inch left (this will be on of your controls of turn circle size).
Horizontal stabilizer tilted so left side tip 1/2 inch higher than right (the other main control of circle size).
Horizontal stabilizer level to trailing edge elevated 4 mm with respect to wing.
Center of gravity a couple of cm in front of trailing edge to a couple of cm behind depending on the size of your stab. Larger means further back.

Jeff Anderson
Livonia, MI

[freed2003]

When I launched it the center of gravity was closer to the leading edge than the trailing edge. The leading edge was higher by about 2 cm. The vertical stab has been tilted for turning. the horizontal stab has been tilted about 1 cm for a left turn.The horizontal stab should be level at zero degrees, so the trailing and leading edge should be the same height(at least it was designed this way, might be a problem) the propellor is slightly pointing up and going straight forward.

[jander14indoor]

2 cm high on the leading edge sounds excessive. If the stab is level, I'd expect no more than 5 mm or so on the wing leading edge. I wonder if you are going straight into a deep stall...

CG at leading edge is WAY to far forward. That will cause the plane to dive. You need to be near the trailing edge. But instead of adding weight, let me strongly recommend you remove and remount the wing further forward (PS, you ARE checking the balance with a rubber band attached, of course). Adding tail weight is seldom a good thing.

In addition, if the cg is that far forward, you have very short coupled plane (tail and wing too close together) and it will be VERY sensitive to pitch adjustments and VERY sensitive to pitch disturbances. So again, let me recommend you fix the balance by moving the wing.

Yes I know you said you didn't make the wing adjustable, I STILL recommend moving it.

Prop up and straight didn't cause the dive, but it WILL cause problems later. When you get the basic glide right, you'll start power stalling something fierce. Point that prop DOWN. Straight might work, but I suspect you will want it offset to the left a little. If your circle size is steady, no issue. If it starts large and then tightens, point the prop left. Don't point it right. If your circle starts small and then opens up, add some stab tilt.

Now, after you've fixed the basic trim stuff, GLIDE test the plane first before you add power. With only enough turns on the motor to stretch it between the hooks you should be able to adjust the plane to a gentle, left turning glide from a shoulder height launch with a gentle push. DON'T throw it to get it to go further, just gets things off on the wrong foot. A gentle push is all you need if properly trimmed and balanced.

Jeff Anderson
Livonia, MI

[freed2003]

Ok I made a mistake. The leading edge is only a few mm taller than the trailing edge. The weight is 6.6 so I don't know how to add weight

[JasperKota]

Buy some sticky tack or blu tack (clay can fall off easily) and then wrap it around the CG, if it's a trimmed plane. You can also move the clay to move the CG. Attaching some extra balsa wood next to/on the motor stick to ensure that it is stiff is also a good way to add weight.

[freed2003]

Just to be clear, if the leading edge of the horizontal stabilizer is elevated that's bad?

[jander14indoor]

Pretty much. In almost all cases proper trim will have the wing at a higher angle of attack than the tail. If you have the stab with an elevated leading edge it probably caused the dive.

Jeff Anderson
Livonia, MI