Multiplex FunJet - Page 102 - RC Groups
Nov 11, 2006, 10:44 PM
Registered User

# Down thrust.

Quote:
 Down thrust means the motor is pointing down at the back where your prop is, thus making the nose go downward in flight. It is the same effect as an elevator
It is my understanding that the conventional interpretation of "down thrust" means the motor is thrusting downward not the airplane i.e.

. . . . . . . . . . . /

And this is where I work

Is that not the way it works in the RC world?

Thanks a bunch,

Joe
 Nov 12, 2006, 07:26 AM psykolefse.jorgis.com "Cause I'm a pilot, I only care about me"...
 Nov 12, 2006, 11:00 AM 'FPV'er...not a "LOS'er You know, I have no problem with understanding thrust angle when the motor is up front. Now swap ends and make it a pusher and I get confused. Maybe looking at the motor angle from the front of the motor would make sense. If the front of the motor is higher than the back of the motor then it has upthrust, and visa-versa. Would that be correct? Of course the effect it has on the plane is totally different. Up thrust on a tractor tries pull the nose up (climb), but up thrust on a pusher tries to pull the tail up (pitching the nose down).
 Nov 12, 2006, 12:56 PM Part-Time Super Hero It certainly makes a difference when something is at the back or the front. You have an elevator at the back that goes up to give lift but if you have the elevator at the front, ie canard, then it goes down to give lift, however, if your motor at the back (pusher mode) is pointing down then the nose of the plane will get pushed down and if the motor is pointing up, the nose will get pushed up. If your motor is up front and is pointing down the same will occur, exept the nose will get pulled down rather than pushed. Mark
 Nov 12, 2006, 03:07 PM almost slackin Just got back from a wonderful morning of battling the wind. Flying into the wind at part throttle and pull up and throw full aileron and watch the Funjet spin on its tail while hovering, briefly. Had the nearby gang at the soccer field entertained too. Using extreme control throws to get it doing some awesome aerobatics, just too cool. Tried all this with the Stryker and mono Twinjet and just not nearly as fun. Don't mean to gush but it's the first flights in a couple days and it was great!
 Nov 12, 2006, 04:29 PM so i rewired it... Okay, so I can't perceive relative speed well... we radared the FJ once on the 500sh/4s/4.75x4.5 and got 101 mph! Also, I upped the elevons to 150% movement with 60% expo... holy cow it spins like a top just like the microjet! It's hilarious... Last edited by xorbe; Nov 13, 2006 at 02:43 AM.
Nov 12, 2006, 06:53 PM
If it floats....sail it!
Sometime ago, Jurgen commented that the older Thunder Power PL cells I was was using and posted in a graph might not be up to the task of the Medusa 028-040-2500, so I ran it today with two different props on a new MaxAmps 3S1P 2100HV and used the digital in-flight recorder.

The graph left of the vertical gray line is an APC 6.5 x 2.9 prop and right of the line is the APC 6x4e.

The in-flight readings indicate the pack will put out 35+ amps in bursts and holds voltage at least enough to fly at half to 3/4 throttle, which is how I fly. While these packs are about 5 grams heavier than the TP's they do not heat up as much as the TP's - not an issue in the FJ that has excellent air flow, but is a real issue in some of my other planes.

At \$50/pack vs. around \$75+ for TP's, on a 3 for 2 cost basis, I am sold on them. YMMV.

FC

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Nov 13, 2006, 02:17 AM
RC Bum
Quote:
 Originally Posted by 2Sunny And this is where I work
Looks like you're working on a crossword puzzle.

Eric
www.ke6us.com
 Nov 13, 2006, 03:18 AM Xtreme Nut Flying and doing crosswords? Hmmmm ... I think I better start taking the BUS!!! I took the FJ out and tried to fly her slow and in a small area ... this baby can really fly in a large gym if you are good ...
 Nov 13, 2006, 03:53 AM so i rewired it... Has anyone dug the cabin's foam out to fit in a wider battery?
 Nov 13, 2006, 04:23 AM Registered User So it looks like we still have some contention over the motor thrust angle. I originally thought it was about whether the motor was thrusting downwards, but the first reply indicated that it was the effect on the airframe that denoted up or down thrust. Can we please get some clarity on this? With regards to the elevon travel, I had to use the standard throws from the instructions to avoid fouling on the vertical fins with full deflection on both channels (one up and one level). My servos are easily capable of double this amount so do I just hack away at the elevons till they clear, or will this have negative consequences that I haven't thought of?
 Nov 13, 2006, 04:26 AM Registered User I just saw your post Mark, are you 100% about that? It doesn't sound right to me edit - unless you're calling ------/ the motor pointing up Last edited by gt35r; Nov 13, 2006 at 04:32 AM.
Nov 13, 2006, 05:34 AM
Registered User
Quote:
 Originally Posted by gt35r So it looks like we still have some contention over the motor thrust angle. I originally thought it was about whether the motor was thrusting downwards, but the first reply indicated that it was the effect on the airframe that denoted up or down thrust. Can we please get some clarity on this? With regards to the elevon travel, I had to use the standard throws from the instructions to avoid fouling on the vertical fins with full deflection on both channels (one up and one level). My servos are easily capable of double this amount so do I just hack away at the elevons till they clear, or will this have negative consequences that I haven't thought of?
Rather than argue the semantics, perhaps it is better to discus the effect of the motor thrust line.

The FunJet instruction say "The new design of motor
mount is very helpful , as it enables you to adjust the
downthrust easily, and the set thrust line can simply be
read off on a scale. Start by flying at full-throttle, and trim
the model accurately, i.e. straight and level “hands-off”. If
you now switch the motor off, the FunJet should make the
transition to a steady glide. If the model climbs and slows
down, the downthrust is too great - reduce the downthrust
angle. If the model goes into a dive, the downthrust is too
low - increase the downthrust slightly and add a little uptrim
before flying again. Repeat this procedure until the
model goes into a smooth descent when you close the
throttle, and responds to an open throttle by spontaneously
flying straight and level, with a slight tendency to climb."

Start off by having the marker, attached to the motor mount, at the neutral point on the scale. If the model climbs on the reduction of power as above, reduce the downthrust by rotating the adjustment scew clockwise to move the marker back towards the prop.

I had to cut away the Elevons to clear the fins.

If that is a true photograph in flight why is the pilot not fully strapped in as the only flight crew at the controls.

Tony
Nov 13, 2006, 05:58 AM
Registered User
Quote:
 Originally Posted by ModelTony -------- reduce the downthrust by rotating the adjustment scew clockwise to move the marker back towards the prop. Tony
Ok then, you are saying ------\ = downthrust

Thanks.

On this point I think the instructions are a little confusing and the various comments here indicate that it isn't as simple as one might first think.

Perhaps it would have been clearer for them to say 'adjust trim to glide level, then apply full throttle and if model climbs too steeply add downthrust by turning adjustment screw anti-clockwise'
Last edited by gt35r; Nov 13, 2006 at 06:30 AM.
Nov 13, 2006, 06:11 AM
Registered User
Quote:
 Originally Posted by 2Sunny It is my understanding that the conventional interpretation of "down thrust" means the motor is thrusting downward not the airplane i.e.
Quote:
 Originally Posted by MarkJayne It certainly makes a difference when something is at the back or the front.
Quote:
 Originally Posted by Vantasstic You know, I have no problem with understanding thrust angle when the motor is up front. Now swap ends and make it a pusher and I get confused.
Guys, I got a really easy answer for ya...
it's the same no matter what end it's on!

Point the motor down, the plane goes down!! (front or rear mounted)
Point the motor up, the plane goes up!! (ditto)
The effect is the same no matter which end you put it on.

SO...

DOWNTHRUST = Motor pointing DOWN
UPTHRUST = Motor pointing UP