Tail Rotor / Fuselage Attitude Relationship
 

Hello,

I'm new to this forum so I apologize if I'm breaking any rules. I've stumbled across the answer to many helicopter aerodynamics questions that I've had in these forums but didn't make an account until now.

I have an aerodynamics question that has me stumped which I'm unable to find a good answer for. Hopefully someone can help me out?

I fly a Bell 212 (CCW blade rotation). I noticed that in straight and level, trimmed cruise flight with level fuselage attitude and constant collective and cyclic position if you bring the aircraft out of trim by applying right pedal the fuselage will yaw right and pitch down, and if you apply left pedal it will yaw left and pitch up.

Where do these nose up/down tendencies come from? Maybe the answer is very simple and I'm just missing it, but I am stumped. Thank you in advance for the help!

It's the effect of the horizontal stabilisers as you are changine the air flow across them and probably changing how much the main rotor downwash impinges on them.

You should also see roll in the same direction as the yaw as it is a secondary effect. If you are masking this inadvertently with cyclic, you may well be applying some small fore and aft cyclic as well without realising it.

Very astute observation, Chucklehead!
The downwash from the main rotor is skewed by the rotation, and tends to press downward on the right horizontal tail more than the left (for a US rotation rotor.) This is especially true of helos with low stabilizers, where the MR wash is stronger than if the tail is high up. If you could see the column of air, it would show a sweep to the right and down as viewed from behind, at high cruise speed.
The asymmetrical downflow means that if you press right pedal and slide the tail under the left side if the wash, there is much less downforce. This makes the nose dive downward. Left pedal moves the tail to the right where the downforce is higher, so the nose pops up.
On some helos the pedal at high speed is as powerful a longitudinal control as the cyclic!

Follow-Up
 

Hi Nick. You always remembered everything, and I cannot recall the following, so maybe, since you are still on " active duty " so to speak ( still working ), here is a question:

The UH-60 attacked this aero mode by feeding a lateral accelerometer ( sideslip ) signal to the stabilator electronics. Did the Army give the UH-60 electronics to the AH-64 so as to copy this feature? ( I do know they used stabilator actuators with an S-70 part number, because I saw that on aircraft at Ft Rucker ).

Way, way off topic, but the next AHS South Florida Meeting on Nov 12 will feature the Miami Red Bull Flugtag two-time winning team from the Sikorsky Development Flight test Center. Imagine these helicopter guys winning a fixed wing gig! Come on down!

John,
I believe they did, as I have heard my dad mention it numerous times. Still bothers him that the Apache was given his stabilator.
Bryan

S/S to Pitch
 

Given that your dad and pals had that FBW tail in the air just 30 days after the meeting that decided to do it, not much sleep was had by that group. Stands out as one of the most amazing straight engineering feats in the helo business.

Crab and Nick,

Thank you for the replies! I did not realize that the MR downwash still had that much of an effect on the horizontal tail in cruise flight, but that explanation makes complete sense. It actually explains some other things that I didn't realize I was thinking about incorrectly, such as fuselage attitude in a loss of TR thrust situation.

Would you say that the MR downwash is the dominant force on the horizontal tail in cruise? I suppose it would depend on airspeed and power, but for some reason I had previously been imagining the lift being the main factor with the MR downwash not really being a factor much beyond ETL.

Thanks again for the help! I really appreciate it.

Chucklehead,
The horizontal tail is mainly there to stop the aircraft from doing a loop at high speed. It fights the rotor's very strong tendency to pitch and keep going when disturbed (because the rotor reacts to an angle of attack change by increasing its moment in the same direction, a function of how the advancing blade sees the angle of attack increase, and flaps in the direction of the disk angle).
For example, if the nose is pitched up by a disturbence, the advancing blade sees the angle increase and gains lift, so it starts up, and gets to peak flapping over the nose, which in effect makes the nose up disturbence worse.
The horizontal tail sees the pitch up attatude change as a change to its angle of attack that increases its up load. That way, the up force from the tail fights the main rotor and re-balances the helo. If your horizontal tail suddenly disappeared in high speed cruise flight, your helo would almost immediately do a loop, either nose up or down depending on what the first disturbence was.
The horizontal tail is sized to exactly fight the main rotor at the highest speed and the worst CG (usually aft CG). The tail size must be increased if the speed is increased, or if the CG is moved aft, or if the gross weight is increased.
You can tell if the tail is too small during pullup maneuvers at high speed. If the nose wants to go further up, and it takes forward stick to stop it, the tail isn't strong enough to balance the main rotor, and you have to help it. We call that "negative maneuvering stability" and usually make the designers fix it.
That is the dominant force the horizontal tail creates, and its reason to exist. Old helicopters (H-13, H-55, H-34) have tiny tails because the have low Vne, so the instability of the main rotor is milder.

John, I am pretty sure the Apache has the hardware but not the electronics and software, but I will check. I know that the Army got tired of Hughes messing around with the Apache problems in pitch and just made them buy the Black Hawk's tail and be done with it. In fact, they also tried to get Sikorsky's chief developer (our friend and mentor, Bob Zincone) to get assigned to Hughes to fix the Apache's early woes. Bob simply refused!

And don't forget that on the 212, the horizontal stab has slightly different angles between the left and right sides....

These discussions between Coyle, Lappos and others should be valued. The are not mere reminiscing, this is the summation of experience made simple for the rest of us. Gold.

Peter:
Thanks. Nice to know the other reader has been found...