More downthrust ?

[First time Juke]

Hi !

What good does downthrust do ? I recall Tsunami was added more downthrust. Bearcat had a lot of it, but Rare Bear has less, why and how much is enuf ? Is it for compensating something ?

regards,

Juke T

[StanleyR]

Re: More downthrust ?

My uneducated layman guess from modeling experience is that downthrust is needed when:

1. The aircraft tends to climb from level when more speed is realized.
or in most cases 2. The wing incidence is too positive or perhaps the horizontal stabilizer is too negative to the datum or flying line.

Sounds like a bad fix, eh? So my other uneducated guess as to why do it at all would be that it must be easier to change the engine thrust angle than to change the incidence of the wing.

When is it enough? Probably when the aircraft can remain level throughout the desired speed envelope without major trim changes.

I have a friend who firmly believes in setting all angles of his models at 0 thrust, 0 wing, and 0 stab'. He swears by its efficiency and stability, especially for racing scale warbirds, and has ignored all the airfoil theories that I have read about lift, angles of attack, etc. He made me modify an F8F model to his settings years ago, which eventually turned into a RareBear, and that required a BIG change to the wing saddle. It kind of looked funny on an F8F. I kept asking him if he was sure about this, and he absolutely insisted! Well, I was surprised to see how well the model flew and that he didn't have to make any elevator trim adjustments, or hold back pressure, etc. It must have something to do with the weight (lack of) of the model the wing lift has to sustain as opposed to the full scale aircraft.

Don't know if this is gospel, but I thought I'd share it anyway

[First time Juke]

Re: More downthrust ?

Thanks Stanley,

I did get same sorta answers on an other site too. I just need some reassurance. I recall later model Spits had down thrust too. Possibly the stronger engines were pulling them airframes vertically and elevator trim did not compensate the nose-up tendency.

In Me 109 the cannon was firing through the hub and this nose down would have been impossible to arrange.

later,


Juke T

[flyingjibus]

Re: More downthrust ?

no not impossible, The amount of downthrust built into engines is so small; you could easily mount a cannon barrel through 3 degrees of downthrust.

[First time Juke]

Re: More downthrust ?

no not impossible, The amount of downthrust built into engines is so small; you could easily mount a cannon barrel through 3 degrees of downthrust.

I doubt that. The barrel in 109 went through the engine ( between the cylinder rows ) the and fired through the hub.
Also Yak-9 had the cannon firing through hub and Airacobra, but both had no apparent downthrust.
If the downthrust was already taken into consideration at the design phase of the armament then maybe it could be arranged if the downthrust was like 1 degree or such. 3 degrees is way too much...it would need at least a 3 inch hole to fit a 30 mil cannon.

with regards,

JT

[mayday51]

Re: More downthrust ?

Juke,

......how about moving the rear mount too?....

.....it works in my mind......

....keep 'em kicking