How much pitch control would elevons provide
04-25-2013, 04:34 AM
#28
RE: How much pitch control would elevons provide
OK thanks.
I have seenone very similar on this website
http://www.geistware.com/rcmodeling/cg_super_calc.htm
They may evenbe identical, but on the adamone site it doesn't specify what CG calculator they use or where the equation came from.
I have seenone very similar on this website
http://www.geistware.com/rcmodeling/cg_super_calc.htm
They may evenbe identical, but on the adamone site it doesn't specify what CG calculator they use or where the equation came from.
04-25-2013, 07:10 AM
#29
RE: How much pitch control would elevons provide
ORIGINAL: countilaw
The LOWERING of ''flaberons'', flaps or Elevons will pitch the nose down. Not up. The LOWERING of the elevator will also pitch the nose DOWN.
EXAMPLE: Upon landing, the flaps are lowered and the added lift pitches the nose down. If lowering the flaps pitched the nose UP it would put the aircraft in a stall.
Frank
The LOWERING of ''flaberons'', flaps or Elevons will pitch the nose down. Not up. The LOWERING of the elevator will also pitch the nose DOWN.
EXAMPLE: Upon landing, the flaps are lowered and the added lift pitches the nose down. If lowering the flaps pitched the nose UP it would put the aircraft in a stall.
Frank
There are numerous fixes such as airplanes that have leading edge devices that deploy with flaps. When I checked out as an Instructor Pilot, USAF T-38, I was surprised at the amount of pitch-up when the flaps were lowered. One had to push the stick forward quickly and with "push" to hold the altitude until the thumb-button trim took over.
I have flown a number of flap equipped RC models that all nosed up with flap deployment.
When The flaps are deployed on a trimmed machine, said flaps NOT being in smaller increments, the chord line also moves down from wing LE to flap TE which is a super-short time frame acting as a fast change of AOA. The elevator or slab-H-stab, is acting as just that - a LOT of UP elevator in that instant.
I have flown airplanes that do have a system to provide for flaps down and auto trim with several different systems. Only ABSOLUTE is that there are NO ABSOLUTES for all cases.
05-02-2013, 02:44 AM
#30
RE: How much pitch control would elevons provide
Hossfly,
One rule we CAN pretty much depend on is that lowering a flap will cause a nose down pitching moment on the wing. Oh Yes, that's a given!
What is uncertain is the effect that has on the tailplane. If the aeroplane has NO tail then the effect is nose down. End of.
For aeroplanes with tails then, deflecting a flap down applies that nose down pitch to the wing, and it increases lift.
The effect of a plain flap on a wing is illustrated in Figure 2 below. At all angles of attack the lift coefficient is increased resulting in a greater lift. Notice that when you put the flaps down, the airspeed is initially the same and lift still equals weight, so you are shifting left from the red line to the purple one, not up to it. When the flaps go down, the nose goes down too.
Since weight has not increased the pilot tilts the aircraft nose down a few degrees to get lift equal to weight again. The tail gets tilted LE down as well (since it's connected) which gives a nose up pitch to the aircraft, so we have a bit of nose up, and a bit of nose down.
Added to that, deflecting an inboard flap down distorts the spanwise lift distribution and so the downwash over the tail is altered.
Figure 4 illustrates the view from behind an aircraft, and you can see that the tailplane is flying in the downwash of the wing. As you know, any wing which produces lift must deflect the air downwards behind it. Lift and downwash go hand in hand. The downwash in normal flight has been allowed for when setting the tail angle.
When flaps are used, the tip vortices are reduced and we start a vortex at the outer end of each flap. Next time you fly in an airliner, sit at a window aft of the wing and look out for the flap vortices: they show up well as condensation trails in damp air conditions. The total lift remains the same, equal to weight, so the total downwash is the same, but it is concentrated inboard behind the flaps as illustrated in the lower drawing of Figure 4.
The extra downwash produced by the flaps pushes down on the tailplane, raising the nose, so although the flaps themselves tend to push the nose down (like elevons on a flying wing), this may be partly offset or even overwhelmed by the downwash effect on the tail. It depends on the tail area and position. The smaller the tail area, the less thisnose-up effect & v v..
As illustrated in Figure 5, a small tail well aft of the wing (tail B) or a high-set tail (C) will suffer less downwash than tail A, close to and level with the wing.
So the variables we need to consider are:-
One rule we CAN pretty much depend on is that lowering a flap will cause a nose down pitching moment on the wing. Oh Yes, that's a given!
What is uncertain is the effect that has on the tailplane. If the aeroplane has NO tail then the effect is nose down. End of.
For aeroplanes with tails then, deflecting a flap down applies that nose down pitch to the wing, and it increases lift.
The effect of a plain flap on a wing is illustrated in Figure 2 below. At all angles of attack the lift coefficient is increased resulting in a greater lift. Notice that when you put the flaps down, the airspeed is initially the same and lift still equals weight, so you are shifting left from the red line to the purple one, not up to it. When the flaps go down, the nose goes down too.
Since weight has not increased the pilot tilts the aircraft nose down a few degrees to get lift equal to weight again. The tail gets tilted LE down as well (since it's connected) which gives a nose up pitch to the aircraft, so we have a bit of nose up, and a bit of nose down.
Added to that, deflecting an inboard flap down distorts the spanwise lift distribution and so the downwash over the tail is altered.
Figure 4 illustrates the view from behind an aircraft, and you can see that the tailplane is flying in the downwash of the wing. As you know, any wing which produces lift must deflect the air downwards behind it. Lift and downwash go hand in hand. The downwash in normal flight has been allowed for when setting the tail angle.
When flaps are used, the tip vortices are reduced and we start a vortex at the outer end of each flap. Next time you fly in an airliner, sit at a window aft of the wing and look out for the flap vortices: they show up well as condensation trails in damp air conditions. The total lift remains the same, equal to weight, so the total downwash is the same, but it is concentrated inboard behind the flaps as illustrated in the lower drawing of Figure 4.
The extra downwash produced by the flaps pushes down on the tailplane, raising the nose, so although the flaps themselves tend to push the nose down (like elevons on a flying wing), this may be partly offset or even overwhelmed by the downwash effect on the tail. It depends on the tail area and position. The smaller the tail area, the less thisnose-up effect & v v..
As illustrated in Figure 5, a small tail well aft of the wing (tail B) or a high-set tail (C) will suffer less downwash than tail A, close to and level with the wing.
So the variables we need to consider are:-
- [*]Flap size - chord and spanwise extent[*]flap spanwise position[*]tail area[*]tail moment arm[*]tail height[/list]
05-02-2013, 03:07 AM
#31
RE: How much pitch control would elevons provide
That bit above about flap spanwise position might need explanation.
Most models are like my JayTee and have the flaps inboard ahead of the tail. When flaps go down I need downtrim to hold the nose down.
However some model jets have twin booms, and the flap position varies. On the Baby Boomerang the flap is between the booms, right ahead of the tail. It needs down trim with down flap.
However on the BVM KingCat the flaps are placed outboard of the booms (so they are outboard of the tail as well). It needs UP trim with down flap. Some owners use up aileron (crow) to help counteract the flaps' nose down pitch.
On the Falcon 120 the flaps are partly inboard of the boom and partly outboard. The result is a negligible pitch effect from flaps. No trim change.
Most models are like my JayTee and have the flaps inboard ahead of the tail. When flaps go down I need downtrim to hold the nose down.
However some model jets have twin booms, and the flap position varies. On the Baby Boomerang the flap is between the booms, right ahead of the tail. It needs down trim with down flap.
However on the BVM KingCat the flaps are placed outboard of the booms (so they are outboard of the tail as well). It needs UP trim with down flap. Some owners use up aileron (crow) to help counteract the flaps' nose down pitch.
On the Falcon 120 the flaps are partly inboard of the boom and partly outboard. The result is a negligible pitch effect from flaps. No trim change.
