HTL Question for the Experts

[NoWingsAttached]

I have a lowrider Air Command, with a HUGE cruciform tail, like twice the surface area of an RFD flying tall tail. It flies just fine the way it is.

Question: If I point the motor down 5 degrees, why wouldn't this eliminate any undesireable HTL characteristics? Seems like the only acceptable solution I've read here to "low rider", HTL designs, is to rebuild the airframe and basically put the pilot's belly button in line with the thrust line of the prop.

[Timchick]

Are you talking about a tandem or single place gyro?

[NoWingsAttached]

In this case it is tandem. But either way, doesnt' pointing the thrustline down do the same thing as raising the seats up?

[birdy]

Yes, as far as the TL orientation goes.
But then you have the thrust pushn the machine down too, addn load to the rotor.
Just like addn a down loaded stab, it costs energy.

[BEN S]

Take a look at the Sportcopter when it is sitting on the ground!
Ben S

[Udi]

Greg - pitching the engine to make the thrust line pass thru the (auto)gyro CG was done before by a guy by the name of Juan de la Cierva. The difference between your gyro and his autogiro is that your engine is in the pusher position and his was a tractor. Pointing the engine down will indeed make the thrust line closer to the gyro CG, but it will also have 2 undesirable consequences.

1. The down-angled prop thrust line will add a permanent down force to your gyro - like dead weight. This will reduce your rotor efficiency. If you point it down by only a few degrees, the loss won't be too bad though. This is easy to calculate. For example - a down angle of 5% with a prop thrust of 400 lbs will add 35 lbs to your gyro "weight" (calculate: sin(5)*thrust).

2. This one may be more significant. Since your stab is inside the prop wash, angling the thrust line down would cause the prop wash to be angled up - pushing the horizontal stab from below, again causing a nose-down pitching moment similar to the HTL that you are attempting to circumvent. That will obviously be counter productive. The only way to avoid this consequence is to change the angle of your stab in the same amount you angle the engine. If you do that - pointing the stab down by 5 deg - your net contribution to stability would be positive.

If your thrust line currently 6" above the gyro CG than you will need to point the engine down about 9-10 degrees (calculate: arcTan(6"/36")) to get it exactly CLT but I think that pointing it down 5 degrees and correcting the stab angle accordingly will be enough. Have you measured your gyro CG and thrust line offset?

Udi

[Passin' Thru]

The stock Bensen B-8M always had 3° nose down thrust line. Look at the plans drawings.
The rest of the story is that a stock Bensen with Bensen metal rotor blades cruised about 3° nose up.

[NoWingsAttached]

Originally Posted by birdy

Yes, as far as the TL orientation goes.
But then you have the thrust pushn the machine down too, addn load to the rotor.
Just like addn a down loaded stab, it costs energy.

I suspected there might be a trade off as you mention. [scratches head, worries about drilling holes in new Yamaha conversion attachment points... but seriously wants to get the thing put together]

[NoWingsAttached]

Originally Posted by Udi

Greg - pitching the engine to make the thrust line pass thru the (auto)gyro CG was done before by a guy by the name of Juan de la Cierva. The difference between your gyro and his autogiro is that your engine is in the pusher position and his was a tractor. Pointing the engine down will indeed make the thrust line closer to the gyro CG, but it will also have 2 undesirable consequences.

1. The down-angled prop thrust line will add a permanent down force to your gyro - like dead weight. This will reduce your rotor efficiency. If you point it down by only a few degrees, the loss won't be too bad though. This is easy to calculate. For example - a down angle of 5% with a prop thrust of 400 lbs will add 35 lbs to your gyro "weight" (calculate: sin(5)*thrust).

2. This one may be more significant. Since your stab is inside the prop wash, angling the thrust line down would cause the prop wash to be angled up - pushing the horizontal stab from below, again causing a nose-down pitching moment similar to the HTL that you are attempting to circumvent. That will obviously be counter productive. The only way to avoid this consequence is to change the angle of your stab in the same amount you angle the engine. If you do that - pointing the stab down by 5 deg - your net contribution to stability would be positive.

If your thrust line currently 6" above the gyro CG than you will need to point the engine down about 9-10 degrees (calculate: arcTan(6"/36")) to get it exactly CLT but I think that pointing it down 5 degrees and correcting the stab angle accordingly will be enough. Have you measured your gyro CG and thrust line offset?

Udi

Definitely the tailfeathers should match the new AOA of the prop.

[raghu]

Originally Posted by Udi

1. The down-angled prop thrust line will add a permanent down force to your gyro - like dead weight. This will reduce your rotor efficiency. If you point it down by only a few degrees, the loss won't be too bad though. This is easy to calculate. For example - a down angle of 5% with a prop thrust of 400 lbs will add 35 lbs to your gyro "weight" (calculate: sin(5)*thrust).

Udi

Good to see you around Udi!

....Of course the dead weight is not a given; you can always change the hang angle to compensate.

[Udi]

True, and also an effective and properly adjusted stab will tend to align the prop thrust line with the direction of flight, even when the hang angle would otherwise be 5 deg nose down.

Udi

[Timchick]

Udi, glad to see you back on the forum again.

[NoWingsAttached]

Originally Posted by Udi

True, and also an effective and properly adjusted stab will tend to align the prop thrust line with the direction of flight, even when the hang angle would otherwise be 5 deg nose down.

Udi

OK, so angling the motor seems like a good idea.

[Redbaron]

Greg the best way to determine if your gyro needs changes is to fly in some strong thermals. If you come out alive I reckon your gyro is stable or you got some damn good reflexes :-)

[Steve McGowan]

Originally Posted by NoWingsAttached

I have a lowrider Air Command, with a HUGE cruciform tail, like twice the surface area of an RFD flying tall tail. It flies just fine the way it is.

Question: If I point the motor down 5 degrees, why wouldn't this eliminate any undesireable HTL characteristics? Seems like the only acceptable solution I've read here to "low rider", HTL designs, is to rebuild the airframe and basically put the pilot's belly button in line with the thrust line of the prop.

IF IT AIN'T BrOkE .......