Xairflyer

My Feedback: (1)

Point the turbine vertical (exhaust down) which way would the model go

BaldEagel

Relate the angle of thrust to the AOA of the wing if at 0degs the aircraft climbs at WOT then by pointing the thrust line down this decrease the AOA of the wing and reduce the tendancy to climb, but Bernoullies therom at the tail also applies, but then we have to think about all the other pod and boom airframes that don't require this amount of up thrust, hmmmmmmm.

Mike

Xairflyer

My Feedback: (1)

Mike go to your bed, your as bad as me

BaldEagel

I'm off now.

Mike

joeflyer

My Feedback: (48)

Hey Wocketman,

I'm sure you know more about this than I. I've heard the Bernoulli explanation but don't fully buy into it. I think it would depend on how close to the stab the exhaust stream is located. Not all twin boom aircraft exhibit this phenomenon.

It seems to me that it is largely due to the Falcon 120's flat bottom airfoil. The faster it goes the more lift it creates, just like my old 40 trainer. Perhaps there are two forces at work here. What do you think?

Joe

Woketman

My Feedback: (6)

For most of these airframes, the model would go no where. Just sit on the ground, as the thrust to weight ratio is < 1.0

Woketman

My Feedback: (6)

Well, sure. A lot depends on geometry, moment arms, etc. But that was the reason for the down thrust on the Bobcat and the Kingcat. But it would affect this Falcon design too, perhaps more or less than others.

Modellbau USA

My Feedback: (5)

Woketman is 100% correct the reason that you need down thrust on the engine is do to Bernoulli and also another phenomenon called the Coanda Effect. The coanda effect is the tendency of a fluid to be attracted to a nearby surface. This effect is exaggerated when are is moving at a high velocity i.e. Jet Exhaust near a flight surface i.e. stabilizer. The reason that this effects the Falcon 120 , Bob Cat, King Cat and our new Tornado is do to the distance of the horizontal stabilizer from the high speed air from the turbine. Even though you might think the rest of the twinn boom jets are alike aerodynamically there very different. Its is not only the lift the wings and tail produce it is the total aerodynamic forces that are produced by all the flying surfaces including the fuselage. Conversely the same aerodynamic reason we need down thrust on our turbine is what gives theses airplanes there superb flying characteristics at slow and high speed.

Xairflyer

My Feedback: (1)

Sounds like a cop out to me, so what if it was 1:1 or even more, my Bobcat (smaller version) is 10.5lb with a 13lb turbine, which way would it go?

JetCatJimmy

Why can't you just accept that this time you're wrong. Put your turbine on the front of your jet with the tailcone pointed down and you've got up thrust. Put that same turbine on the back in the same configuration and it is now down thrust. This has been discussed over and over. No need to get your feathers ruffled.

Woketman

My Feedback: (6)

Upwards for a brief period of time, then flop over and hit the ground due to lack of stability! [8D]

Instructor

My Feedback: (16)

Maybe I'm reading your drawings wrong, or you and I don't agree on thrust angles. If the thurst of a turbine is pointed down, is it down thrust or up thrust? What I'm trying to say is this: When the thrust is applied, what happens to the nose of the plane? It pushes down, dosen't it? So wouldn't this be down thrust? We don't fly the plane from the tail, but from the nose. When the elevator is raised or in the up position, what happens to the nose? It rises, right? Same for the thrust of a turbine engine. We all know for every action, there's an opposite reaction. Based on the CG of the wing, the nose will be lowered when the turbine thrust is applied if the outlet of the turbine is poing down and oppisite if the outlet is pointing up. That's the way I see it........

Larry

BaldEagel

It does not matter if its a turbine or a prop driven motor, the thrust line is the direction in which the thrust is acting, on a conventional tractor driven airframe down thrust is with the prop wash pointing up over the top of the wing in effect, thrust works in horizontal terms so this puts a positive AOA on the wing and will cause the aircraft to climb if excessive, now put the same tractor motor on the back of the airframe at the same thrust line (for argument sake don't turn the motor around) the thrust line is still pointing up over the wing or in this case the tail, it is still downthrust and will cause the aircraft to climb due to the AOA of the wing.

Mike

Instructor

My Feedback: (16)

So what you are saying is this: If I install a tractor prop engine with 5º of down thrust and apply the throttle the plane will climb? I don't know about you, but everytime I try that, the plane dives. When the AOA of the wing is positive and the power is applied, the plane will climb, that I agree with. If the wing, stab, and AOA of the engine is 0 and the power is applied the plane will continue on the same path. Ask BVM Why they have about 7º of up thrust if you want to call it that on their Bob Cat or King Cat. I know what you are talking about when you say the faster the planes flies the more lift the horz stab produces. So you are saying that they use up thrust in the turbine to overcome this lifting action. If this is the case, then why are the horz stabs using a lifting surface in the first place???


Larry

BaldEagel

How would you test an airframe to see if it needs alteration to its thrust line?

Mike

Xairflyer

My Feedback: (1)

So a turbine pointed up at the front will go up but if bolted at the same angle on the back it will go down?

You are getting things mixed up Thrust is a Reaction force, it is the direction it makes it travel, up/down/fwd/back so 'up thrust' is 'up thrust' no mater if it is at the front or the back of an aeroplane.
Thrust is the direction the push that the prop or turbine develops.

Again back to the rocket, point it up and let go and it will go up, because the reaction of the jet/power/air/thrust (whatever you want to call it) out the back pushes the rocket up, this is called up thrust.

If we bolt our rocket on the front of an aeroplane and point it straight up, and to keep woketman happy, the rocket produced more power/thrust than the weight of the aeroplane, the aeroplane would go up, whether it was bolted on the front middle or back.
Point it up at 45 deg it would still go up at 45 deg. This is the direction of the thrust.

Whether a model needs up thrust or down thrust is a trimming issue, and depends on each plane, but thrust direction dos'nt change just because it is at the back, the engine angle may do, but the engine angled down at the back is not producing down thrust unless the propeller, jet blast is going out the other way in which case the aeroplane would fly backwards

JetCatJimmy

With all due respect, that us simply not correct. When you have a wing that is now directing which direction you travel, each increment you move forward, the turbine is lifting the rear of the jet. This rotates along the wing moment and in turn the plane rotates downward.

I don't wish to get into an argument with you about this. You feel you're correct, that's fine. I however feel that your idea of thrust makes sense only on a wingless craft. Last time I checked this wasn't the rocket forum.

BaldEagel

Chaps

Before we go any further we must establish that thrust acts horizontaly to the the earth's axis in a stable airframe with all things being equallibrum, thus the AOA is dictated by the angle of thrust (up or down) in turn the direction the airframe will travel is a direct result of the wing angle in relation to the thrust line, a positive AOA will produce lift and the airframe will climb, a negative AOA will cause the airframe to dive once we understand that simple premise the up or down thrust conumdrum becomes easily understood.

Mike

JetCatJimmy

So I won't say that I'm correct or not, but I would like the definitive answer. In a Bobcat, when I angle the tail cone down to counteract the tendency to lift under throttle, regardless of Bernoulli effect, is the layman to call that the addition of up thrust or down thrust?

From my experience, BVM advises to apply approx 5 degrees of down thrust. That just makes sense to me.

BaldEagel

The confusion comes when we try to relate pushing or pulling the airframe through the air, even BVM can be wrong especially when explaining to the layman who would not understand the difference between the back or front of a turbine and therefore the difference between down or up thrust.

Mike

Countryboy

My Feedback: (25)

Too bad we can't get Eddie Weeks to chime in here. I think the man knows a little about thrust angles. God, I miss his postings here on RCU.

http://www.guitarsolos.com/videos-vt...fg0R5Dc%5D.cfm

Woketman

My Feedback: (6)

JetCatJimmy is basically correct. But instead of calling it up thrust or down thrust, let's just say turbine pointed up or down (turbine front). If the turbine is well behind the CG and the aerodynamic center, and it is pointed turbine up, guess what? You will get a nose down piching moment: the nose will dive. Xairflyer: yes, the turbine's thrust will try to push upwards, however, since it is behind the CG and the aero center, it will create a nose down moment.

Eddie has left the turbine scene. He is an all electric guy now doing some cool stuff. We talk every now and then. I am supposed to go visit him soon.

VF84sluggo

My Feedback: (55)

You must consider the location of thrust line/vector from the turbine in relation to the vertical location of the jet's CG.

For example, with the A-4 and F-4, the thrust line is pretty much right through the vertical CG. On a carrier approach you could trim for the proper AOA, then move the throttle for glideslope control and the pitch attitude (thus AOA/speed) would not be disturbed, or only minimally so (adding power is normally destabilizing, ie nose pitch-up, so some nose movement is unavoidable.) You could almost purely apply the "control speed with pitch, glideslope with power" technique.

Contrast that with the F-14 (and T-2) with low-slung engines: each time a power change was made for g/s correction, it impacted AOA/speed as the nose would noticeably want to rise or fall...much more difficult to bring aboard speed-wise. The pitch/speed, power/glideslope was not so black and white. More coordination of pitch and power was required as compared to the A-4/F-4.

As far as Coanda Effect, you can demonstrate this easily. Blow across the top of a piece of notebook paper held tightly between your thumbs and forefingers just under your bottom lip, and it will rise and 'flag' almost straight out. A moving fluid field will induce a pressure drop, so you now have a higher pressure on the bottom side of the piece of paper than on the top. You can now impress the kids at birthday parties, and win bets, that you can make a piece of paper rise by blowing along the top side!

VF84sluggo

My Feedback: (55)

...hit quote instead of edit...oops!

BaldEagel

OK Guys, I think if we wish to continue this debate we ought to take it to the aerodynamic forum and let this one get back on track, post a link here if anyone does.

Mike