I have never owned a Kingcat or Bobcat or Reaction but I have always wondered why the engines are pointed so severly downward ? I think I know why but I'd like to hear why from those in the know.

On many models, like the KC and Reaction, the engine eflux is so close to the horizontal stab that the accelerated airflow causes a low pressure area underneath, pitching the nose up more and more as power is applied. When the exhaust is pointed down the effect is to create a thrust-variable counter'force to this problem.

That's what I hear too, but my Reaction X and a friend's Reaction 54 (kit) have a single empennage (sp?).... unless Bruce snuck a twin boom out on the market without telling the public.

Ya, good point, and because I posted before my coffee kicked in this morning, I have done a minor edit.

It's the same physics on the Reactions as it is the KC's and smaller Boomers.

Two theories have been banded about on this:

1. High speed air under the tail???? Benoli's

2. Semi semetrical airfoil section increases lift at faster speed, therefore downthrust needed to counteract.

Don't ask me, I have no down thrust on my Falcon 120, or on the Boomerang Sprint and never did on the Elan, but everyone told me I needed downthrust??? Mick Reeves Reapear and my C-ARF Rookies did not need any thrust adjustment either, the one thing all of these have in common is no thrust tube and a turbine stuck on the back of the fus, the other thing they have in common is a rearward C of G, because that's how I like it.

Mike

Don't like popcorn can I have some peanuts. LOL

I wonder why this doesnt seem to be true with the Boomerangs ?

it was on my boomerang.... i had about 6 washers and still had the effect.... finally just made some new rails

I have no down thrust on my Elan and it flies just fine. In fact if anything I have noticed at the bottom of high speed power on dives it feels almost like it wants to tuck a little bit and I seem to need an unusually high about of up elevator to make the pullout. having said that I initially flew it with a Wren Supersport (17lbs thrust engine) and never noticed this tendancy but I know have the equivilant of a P 120 sized ( 28-30 lbs) and it has only been with this engine on there that I have noticed this.

I recently shimmed my horizontal stab up about 1/8" so the airplane no longer needs a bunch of up elev trim to fly level and I havnt noticed it tucking anymore, but I havnt really done much in the way of high power high speed dives and pullouts since shimming it.

Turbine is below the CG causing a pitch up moment without the downthrust. Same for Reaction or Shockjet.

Hi guys,
Now , changing pitch trim on a airplane at different speeds is nothing more than logical !!
Every airplane can normally just be trimmed for a certain airspeed (assuming same configuration concerning flaps /slats/gear)
So if you change speed lets say from 80mph to 120mph , there must be a trim change ! This is true not just for model airplanes but even more for the real ones
up to an A380 !
In fact , the trim position of the horizontal stabilizer is THE SPEED INDICATOR ! That is exactly what you learn and experience during your first lessons flying a real aircraft
like a Cessna 150 for example !And as mentioned above it will be the same even on an an Airbus 330.Accelerating in straight and level flight will always result in a more
aircraft nose down (AND) trim ,while reducing speed will always result in a more aircraft nose up (ANU) position !
So, to have a throttle to elevator mixer activated on our model aircraft has nothing to do with a bad aerodynamic lay out but much more with basic aerodynamic behaviour
of a flying machine !
Of course , this behaviour is the reason why all the real aircraft have not only elevators to control the pitch axis but also a full flying horizontal stabilizer for the pitch trim function !

Now,trying to compensate for that behaviour over the whole speed range using an angled engine thrust alone is an almost impossible task and really matching it is just
pure luck !Theoretically it can`t be reached for the most extreme ends of the envelope ( say min speed/max thrust versus max speed/min thrust )

My JMP Hustler (still to be built)is a twin boom aircraft as well and has the engine mounted in line with the fuselage-axis !It will of course be flown with a throttle to elevator
mixer activated already for the first flight !My experience shows that such a mixer is normally set proportionally to the region of 5 to 8% AND over the throttle range when operating
fast model jets like my Savex L-39 EDF or my JMP Firebird . Firebird has the advantage of having a full flying stab , so those trim changes do not result in an aerodynamically
less than optimum deflecting elevator !

see you
Hans

On Elan, I have seen several reports of stab servo covers melting from the turbine heat. On mine, I shimmed the engine down a bit to avoid this and still got some very light heat damage on the covering (it needed some aluminium tape on the leading edge). I suspect that BVM decided to use a lot of down angle on the engine to resolve this problem.<div>
</div><div>Arnaud</div>

The stab airfoil sections on the a/c I am familiar with are all symmetrical.

The issue is less prevalent as the distance between the stab and efflux and stab increases, to the point of zero on larger Boomers like the XL. The KC H-stab is a lot closer to the efflux because it's mounted mid vertical stab. The Boomer XL's h-stab is mounted on top of the v-stabs, and it needs NO down thrust. Each of these a/c has the same thrust -vs- Cg configuration, and only the KC needs down thrust.

The thrust line is NOT below the CG. It is, in fact well above it.

It is not a trim-vs- speed issue; the pitch varies wildly with thrust on the models being discussed, and very little with speed.

The trim -vs- speed issues do not apply on "all aircraft". Those with aft CG's plus symmetrical stabs set to zero incidence, the exact configuration of many RC models and some full-size aerobats like the Su-26, are exempt. The Sukhoi, in fact, had NO provision for pitch trim installed.

I dunno, my Reaction X has a 'major league' amount of downthrust.

Another way to answer the original question is "that's what is required to make it fly right". The thrust angle minimizes pitching with the application of power.

The question that everybody is answering is "why do certain designs pitch when power is applied?"

I personally agree with highhorse AND C185Pilot. It's probably a combination of the high speed jet efflux below the stabilizer and the position of the thrust line in relation to the true CG. Don't know which is the more powerful effect. But I do know that when I took the prototype Reaction 54 out for its maiden flight, it had zero thrust angle and pitched nose up severely when power was applied at low speed. Elevator trim was the same at different speeds, so it was obviously a thrust line issue. The correct angle was found using trial and error. I imagine other designers went through the same process...

That was C185Pilot's point. He also has a Reaction and a ShockJet. They all have a lot of downthrust.

Chad,

Ahhh, I get it... I 'read' it both ways and couldn't figure it out...

... BTW, in my defense, that post was prior to my first coffee of the day![:-]

I should have said Wing Semi Semetrical airfoil section, as the speed increases so does the lift, those that have a pronounced SS airfoil Like the Falcon 120 need a lot of downthurst to compensate, the Boomerang range do not have a pronounced SS airfoil in conparason, therefore need less compensation, combine all this with the high speed air passing under the tail and we have a pitch up, Look how close the tailplane is to the efflux on the Falcon and others that need the up thrust and how far away the tail is in comparison on the Boomers

Mike

It aint to dry the runway out on days like today then or blow leaves off the runway, during the winter then? Laura

Sticking close to the original subject, I have a situation where I'm converting a prop powered North Star amphibian to a 125% Turbine powered North Star seaplane. The original design has the prop thrust line 1 1/2 to 2 degrees down thrust. Keep in mind that the engine is in a pod mounted on the vertical stab well above the fuselage. My conversion has the turbine mounted on the top of the fuselage, just behind the CG. I'm thinking the down thrust was intended to provide a nose up moment in flight. In the conversion I have built a twin boom tail with the horizontal stab near the top of the vertical stabs ala the Boomerang and Bob Cat. I plan to make the thrust line parallel with the fuselage center line as a starting point. Being a delta wing, is this plane likely to want some up thrust?

First of all, what do you consider "up thrust" - nose of turbine pointed upwards or exhaust end tilted upwards? I've always been confused about that when it comes to turbines.

Hard call on what your plane might need. The thrust line might be above the CG, causing a pitch down, and the efflux might cause a bit of pitch up. If you're lucky, they might cancel each other out! Wouldn't count on it though... you should probably make it easy to adjust, and plan on a bit of trial and error.

Bruce, my interpretation is, up thrust wants to raise the nose. In the case of the North Star, the engine will be behind and above the CG so aiming the nozzle up (Inlet down) will tend to push the tail down. The plane, rotating about the CG will thereby lift the nose. My R-54 does the opposite in that it pushes the tail up and pushes the nose down.

Off Subject: Bruce, I'll be contacting you regarding the rudder and elevator cables in the R-54. They work so well in my R-54, I want to use the same system in the North Star. I'll bury the cable housing in the vertical stabs and exit the cable near the rear of the verticals to control rudders and elevator.

Consider the turbine wheel as a prop, does not matter if its in front or behind the C of G the thrust line stays the same whether pushing or pulling the reaction is also still the same.

If the turbine wheel and thrust line is pointing down at the rear, the reaction is for the nose of the airframe to Lower, now put that same thrust line on the nose of the airframe, still pointing down and still the reaction is for the nose of the airfame to lower, but the prop in conventional terms is pointing towards the sky, so its up thrust in conventional teminology with tractor propulsion which is the nomuculture.

Mike

Good you brought that up, on the Elan thread its been named the "Wooly elevator syndrome" lots of disscusion on why it happens, but no real conclusions, I suspect too much Expo and or C of G can possibly be the culprit in some circumstances.

Mike

Mike, if I read you correctly then I disagree with your thrust vector analysis. I agree that the nozzle facing down with the engine at the rear of the plane will push the nose down because the plane is rotating about the CG. If I keep that same thrust line and move the engine forward of the CG the nose will rise.
Efflux has been mentioned in several responses. Someone please explain to me what efflux is. I think it refers to the effect on the horizontal stab with the high speed nozzle discharge air passing under the stab and elevator. The result, high speed - low pressure air below the stab and relatively slow speed lower pressure air moving above the stab forces the stab down.

Thrust is a reaction force so it is the direction it makes the turbine go is the way it is pointed, so if the turbine is pointed up (starter at the top) it is up thrust, and if pointed down, it is down thrust, simple as that. Think on it as a firework or rocket.

Seems to be a lot of confusion over this and discussed many times on forums before, if your prop engine is at the front and the engine is pointing down then everyone knows this as down thrust, as it is now being pushed at a downward angle by the thrust, forget for now that it is bolted to an aeroplane.
It is the exact same with a turbine, think of the air coming out of the back, being the same as the air coming out of the back of a prop, replace the engine with a turbine with the same angled down mount and you have the same thing.

NOTE
Up thrust is always UP thrust, and DOWN thrust is always DOWN thrust no matter where it is mounted on an aeroplane.

What an aircraft needs to trim it for normal straight flight will depend on the aircraft design. Engines, we will call them thrust/push producers mounted at the back can react differently when behind the CG and whether the thrust produced exists above or below the tailplane. (benoli effect)
In general it is found for thrust producers mounted on the front of an aircraft the trust direction is down and on the rear of an aircraft the thrust direction is UP. The thrust producer may be seen to be angled down at the back but the thrust it produces is UP. What this then does to an aircraft is a different matter and may cause it to pitch down which is where the confusion lies.

On many twin boom aircraft fitted with a turbine, in order to keep the hot gases away from the tailplane the exhaust is angled down but may have no effect at all to the trim of the aircraft.
I found by increasing this rear UP thrust (exhaust angled further down) my Bobcat (not bvm) would now fly off the runway on its own where as before with the turbine level it needed to be hauled off and used to jump into the air.