A-10 thrust angle

[KenSt]

I posted something in the jet section but didn't get a lot of feedback, perhaps this is a better place.

I have read many threads complaining about the nose down tendency of the GWS A-10 when applying power, due to the high thrust line and low wing. After looking at that model and the real A-10, it became apparent to me that the model's thrust angle is wrong.

I think that it would handle better if the exit of the fan ducts were directed slightly upward instead of down over the elevator. Any high engine pusher plane I've seen uses this kind of thrust angle.

Am I wrong? Do I have this backwards? I get confused sometimes about thrust angles.

[vintage1]

I think you are most likely right.

But I wouldn't stake my reputation on it ;)

[MarkusN]

Angling the thrust line upward brings the thrust vector closer to the center of drag of the plane if the engine is aft and high. So, yes, that should reduce those power effects on pitch.

[KenSt]

Thanks guys! C'mon Vin stick your neck on the track! The train won't be here for awhile! :rolleyes: :D

[podavis]

You will be adding a vertical force component acting down rearward of the cg without changing the location of the forward thrust force component relative to the location of the net drag force very much if the angle of the drag component does not change, i.e. the AOA is the same. You might get more nose up with increasing thrust because of the additional downward thrust line aft of the cg. The close proximity of the high velocity airstream over the elevator might also be causing a drop in air pressure above the horz stab. No matter which of these helps, I think a small angle change may have a good effect.

[KenSt]

Usually when adding thrust as noted above, you probably want the aircraft to go faster, and/or up anyway, at least I will. :D I would think 2 or 3 degrees up and away from the stab should make a big difference over the stock set up. Some have also complained of an overly sensitive elevator.

I would like to set up the nacelles more to scale. That is, nacelle centerlines parallel and in line with the top of the fuse, and then angle the thrust up and to the rear at the exhaust cone to avoid nose down and sensitive stab.

[Brandano]

Well, the engines are above both the CG and the center of drag, so it's quite likely they are angled upwards on the full size to avoid the nose pitching down with increased thrust. I believe they'll be slightly angled outward as well, to improve the behavior on single engine, though it will be few degrees since they're fairly close to the centerline. I'd check the thrust angles on similar amphibian designs, where the engines are mounted high to keep them out of the water

[Thomas B]

Even with the A-10 nacelles properly angled up at the front, you can still get nose down force when addling a lot of throttle at low air speeds.

However, the effect is still less than having the same situation with the nacelles horizonal.

Having the nacelles installed with upthrust is the only way to go.

The previous poster is correct in comparing it to some high thrust line seaplane designs, such as the Lake.

I have a little park flying boat called a Skimmer, laid out like a VJ Sportsman or similar flying boat.

Early on, it did not have enough upthrust and even with some flying speed, it wanted to pitch down with throttle increases. I eventualy tripled the upthrust from about 2.5 degrees in the beginning to about 8 degrees.

Completely cured the pitch down under power issues.

[KenSt]

Tom, I am not familiar with this plane. Does the thrust blow up and to the rear or down and toward the rear? Where is the prop in relation to the CG.? Is the prop close to the wing or further back? I intend to curve the exhaust cone of the nacelle. The nacelles will be mounted so that the front is higher than the back, BUT the exhaust will exit at an upward and rearward angle within the nacelle. All this is what I was referring to inmy first post, to me it gets confusing sometimes.

[Thomas B]

Tom, I am not familiar with this plane. Does the thrust blow up and to the rear or down and toward the rear? Where is the prop in relation to the CG.? Is the prop close to the wing or further back? I intend to curve the exhaust cone of the nacelle. The nacelles will be mounted so that the front is higher than the back, BUT the exhaust will exit at an upward and rearward angle within the nacelle. All this is what I was referring to inmy first post, to me it gets confusing sometimes.

My little Skimmer has a pylon mounted motor above the wing and the prop is in front of the pylon, pulling, just ahead of the CG. The motor and prop is thrusting up at the front, making the thrust line angle up. The A-10 also needs to angle up at the front and the thrust tube needs to angle down at the back, not up.

It's all about the thrust line.

[MarkusN]

The situation for a tail mounted engine is quite different.

With the engine above the wing you have no chance in hell to have the thrust line go through the CG or the CD. (If you do you get a Helicopter.) By having the thrust line pointing up (or back down if you will) you can have it blow at the stab, compensating the pitching down effect it would otherwise have that way. Different concept, more or less same result.

With the engine mounted aft you can have the thrust line go through CG / CD 1), thus avoiding undesired pitch effects without having to resort to aerodynamic tricks involving additional lifting surfaces.

1) Or, more precisely: go by the CG close enough to have the desired order of magnitude of the compensating pitching moment.

[Brandano]

By upward I mean that the tail end of the engines will be higher than the inlet, and the jet exhaust will be angled upward.it makes sense, add more thrust and it pushes the tail down, contrasting the pitch down tendency

[MarkusN]

By upward I mean that the tail end of the engines will be higher than the inlet, and the jet exhaust will be angled upward.it makes sense, add more thrust and it pushes the tail down, contrasting the pitch down tendency

I was replying to Thomas B directly above.

Basically you want the thrust line to go slighly above the CG / CD. As with normal downthrust of a nose mounted prop a slight pitching down moment is required with higher thrust (to compensate for the pitching up tendency of the stab at higher speed.)

You dont want "real" pitching up action with higher thrust, just less pitching down. (meaning: I basically agree with you, just that you don't really want to go into the "pushes the tail down" range.)

[KenSt]

Is there something recognized in the aerodynamic world as the center of drag? I would think that any shape, in this case being pushed throught the air would have a neutral point where the drag is equal and balanced. Is it as simple as cutting out a paper or cardboard shape which is actually the front view of the aircraft, and suspending it from two points to find the center of area? AutoCad or Inventor should also work, I have both.

[MarkusN]

Is there something recognized in the aerodynamic world as the center of drag? I would think that any shape, in this case being pushed throught the air would have a neutral point where the drag is equal and balanced. Is it as simple as cutting out a paper or cardboard shape which is actually the front view of the aircraft, and suspending it from two points to find the center of area? AutoCad or Inventor should also work, I have both.
I just coined the term for the sake of this discussion (PIDOMA, so to speak), as I thought its meaning would be immediately clear. It wouldn't be as easy as obtaining the CG of the frontal cross section, that's for sure. Just imagine wing and tail, which both produce drag but may well be hidden behind each other. It will also vary wildly with AoA (because main contributing surfaces change their level significantly).

[KenSt]

Good points. I guess my idea can't be that far fetched since I saw in another post about a review in FLYING MODELS MAGAZINE where the author added thrust deflectors to his A-10 nozzles to accomplish the same as I want to do. I believe it is the June or July issue of 2005 which of course I don't have :mad: . The A-10 is on the cover.

[macboffin]

Tom, I am not familiar with this plane. Does the thrust blow up and to the rear or down and toward the rear? Where is the prop in relation to the CG.? Is the prop close to the wing or further back? I intend to curve the exhaust cone of the nacelle. The nacelles will be mounted so that the front is higher than the back, BUT the exhaust will exit at an upward and rearward angle within the nacelle. All this is what I was referring to inmy first post, to me it gets confusing sometimes. Your proposed set-up sounds right.