Effect of Engine Nose Down Angle on Performance

[Canadian_JOY]

I've been burning too much midnight oil working on our project airplane and, being a little foggy and sleep-deprived, haven't been able to "science out" this question in my head, so I'm putting it to the great minds gathered here.

Let's take a hypothetical homebuilt airplane where the designer specified the engine mount to have 1 degree of nose-down angle. Sometime during the fabrication of the engine mount something got tweaked and the engine ended up with 2 degrees of nose-down angle.

What's the net effect on aircraft performance and handling resulting from this additional nose-down angle on the engine? Would 1 degree of difference have a noticeable effect? Or would it take perhaps 2 or 3 degrees of additional nose-down angle before the effect on performance and handling would be noticeable?

If anybody has real-world examples where they have flown the same airplane with different nose-down angles on the engine I'd be very curious to learn what the real vs calculated effects are.

As always, your well-considered replies are appreciated.

[bret]

Hum...I thought they built in prop shaft up? does this question have anything to do with maybe forgetting to install that one set of extra washers with the engine isolators?

[Canadian_JOY]

Bret - this is a question based on the hypothetical case suggested in the original post. I'm trying to understand the effect of nose-down angle on the engine, and to what precision that angle must be set. Or, in other words, to what level of precision does the designer's goal need to be met - is a degree of tolerance ok, or would the difference of a degree produce substantially different handling or performance?

This question started to rumble around in my mind as I worked late in the night on firewall-forward items. I just haven't been able to come up with a good answer on my own, hence the question posed here.

[Ed_Wischmeyer]

One of the standard reasons for thrust line offset (down and to the right) is so that as power as increased, the increased thrust acts to reduce the amount of trim required for the increased power.

The Cessna Caravan, for example, has a bunch of engine offset to make it easier to fly. The P-51 has no offset, and is notorious for needing to be re-trimmed all the time. Same for the Lancair IV.

As for 2 degrees vs 1 degree, and I'm blowing smoke here, but I doubt that any "normal" pilot could tell the difference.

Ed

[Toobuilder]

It is common among the rag wing Piper group to remove some or all of the significant down thrust some of those airplanes have. I believe there is even an STC. It is reported to make a small, but noticable increase in cruise speed. A search for "Piper thrust line modification" will turn up some discussion

[rv7charlie]

What Ed said sounds like what I've read about the subject. And I've also read that it has a penalty at cruise, as Toolbuilder said. I do know that a lot of alt engine guys have built their mounts with no offset, and no ill effects (other than a bit more rudder needed in climb).

Charlie

[Canadian_JOY]

Thanks for the great responses. I'm still too foggy to fully think through the thrust vectors etc so these explanations are much appreciated.

[rvbuilder2002]

RV's don't have any built in down thrust. Only side thrust to the right.

[flyboy1963]

...since nobody mentioned it, this should keep you up at nite Mark!

over time, as the engine mounts sag, the down-tilt increases.
This probably almost exactly compensates for the oil, dirt, wrenches and bugs that accumulate in the rear fuselage, zero-ing out the nose-up effect.

....so I'd worry more about the cowling to spinner fit as the hours and wear increase, than the exact thrustline.

but the enjunear in me thinks that if you put in long bolts, with a nut between the firewall and mount, you could rig up a little 'trim' wheel to adjust the thrust line while in cruise, to optimize the AoA and prop P-effect, right?!?!

[6 Gun]

That's been on my mind also as the offset on my RV6 has at the tip of the prop an 1.5 "closer to the right wing than the measurement to the left wing that just sounds like a lots but it is feet on the floor in cruise and ball centered any one who can measure and post to tip of prop to wing on their planes would help answer this question.
Bob