Increase the Moment Arm!

[Dean_Dolph]

I've been poking around in the pile of pictures I've taken over the years and came across one I had been looking for for sometime. It shows the horizontal stab on a Art Evans (Agent Orange!) machine where he extended the the mount for the stab back and up behind a tall tail. I Thought some of you that never had the opportunity to see it might find it of interest. And you engineering types might have a comment or two.

[Mike Hook]

The Falcon two place recently sold on ebay had a similar setup.
Mike

[Udi]

This is, in my opinion, the best possible placement for the stab in a pusher gyroplane - as far back as possible and in the center of the prop wash.

Udi-

[Hognose]

Those wires have got to be draggy as all get-out. But then, hey, it's a gyro, it's already draggier than a barn door.

Without the wires it would be structurally untenable. Even with them I have my doubts. Far cry from an ideal truss... but then, loads on it aren't too high.

Udi, mildly OT, but are you familiar with the Wren and Petersen conversions for the Cessna Skylane? They get remarkable performance using a fixed canard embedded in the prop wash.

cheers

-=K=-

[RICK MARTIN]

Kevin,
You guys are makin' me feel good. Most people building Gyrobees these days have shortened the tail boom, but I have left mine longer as in the original design.

Don't you ever sleep?

[Udi]

Udi, mildly OT, but are you familiar with the Wren and Petersen conversions for the Cessna Skylane? They get remarkable performance using a fixed canard embedded in the prop wash.


The Petersens have installed a canard (using flaps, BTW) in the prop wash for the same reason we want our stab in the prop wash - to improve low airspeed performance. The Peterson mods have added 10-15 knots on both sides of the Skylane flight envelope!

Rick - it makes no sense to shorten the tail boom. The only practical consideration should be keeping rotor clearance. With that in mind, you want your stab as far back as possible.

Udi

[automan1223]

I wonder at what point though the flow changes from a smooth laminar flow from the prop to a turbulent wash. You would not want your control surfaces so far back that they are trying to work in a turbulent environment .......

Wind tunnel anyone ?

Jonathan

[RICK MARTIN]

Udi,
Thanks. I had planned to leave it where it is unless I see a conflict once I put the Dragon Wings on.

[Dean_Dolph]

Jonathan, what is your definition of laminar? The airflow from a prop is a swirling flow that in my limited understanding, will be far from laminar to start with. Then, if there is a tall tail involved as in this case, the flow gets straightened a little but I suspect it is turbulent for some distance behind the gyro. The degree of turbulence is the question and then does it make any difference in this application.

I have attached a picture from a PRA convention in Brookville, OH. where Frank Black was testing his VW powered ducted fan. The fan was pointed out over a soybean field so the pattern of the airflow is visible and also the length of the high velocity flow. It is surprising how far back the flow extended. How much of this, if any, can be attributed to the fan is a guess.

Before anyone asks, apparently Frank wasn't happy with the fan because the fan disappeared after that. I know one thing, that was the loudest machine I've ever heard! Look at the guys standing around covering up their ears when Frank is performing the thrust test. I think the VW was using more power to make sound than it was to make thrust! I've included a picture of the ducted fan for those who are interested. Frank owned (still does as far as I know) a machine shop in the Cincinatti area. He made every piece on this machine.

[ymmv]

The airflow from a prop is a swirling flow that in my limited understanding, will be far from laminar to start with. Then, if there is a tall tail involved as in this case, the flow gets straightened a little....

Per Wolfgang Langewiesche [1], the swirling isn't much straightened by the tail; rather, the tail is pushed over by the flow. The craft feels it as a roll moment. Most pilots attribute the roll to torque, but it isn't. He says (I wouldn't know) that tails are (were) cocked a little off center to counteract this.

Whether Langewiesche is still considered an authority I don't know, but he seems to make sense.



[1] Stick and Rudder, An Explanation fo the Art of Flying, Wolfgang Langewiesche, McGraw Hill, 1944. Do people still read this book?

[Dean_Dolph]

Bart, I don't think anyone is going to say that the swirling airflow from the prop isn't going to impart a force on the vertical stab. But my response was addressed toward the issue of laminar flow and turbulance.

I think Wolfgang and I are saying pretty much the same thing, 'gets straightened a little' (DD) and '..swirling isn't much straightened...."' (Wolfgang). He didn't say it didn't get straightened at all. Also, I doubt if he was thinking about a vert stab that is totally immersed in the prop wash like a tall tail is. It is my understanding that the the tall tails reaction to the prop wash pretty much counters engine torque on a pusher gyro so the roll effect is less noticeable than it would be otherwise.

I have the book but haven't looked at it lately and certainly don't remember the exact words so I'll take your word for it. This isn't really rocket science since it is easy to visulize the swirling flow impacting on the vertical stab. If I can do it, anyone can!

[ymmv]

I have the book but haven't looked at it lately and certainly don't remember the exact words so I'll take your word for it.

Just to keep the record straight, I wasn't quoting Langewiesche, I was paraphrasing. It's a useful book, though not everybody considers it gospel. But you're right, Dean, his point is pretty easy to visualize.


It is my understanding that the the tall tails reaction to the prop wash pretty much counters engine torque on a pusher gyro so the roll effect is less noticeable than it would be otherwise.

Beats me. Sounds sane. Also I agree with you that any tail is going to be in well scrambled air, no matter where you put it. But this isn't a subject I really know anything about, so I'll un-delurk now.

[Doug Riley]

Like computers, open-air props work on the "garbage in, garbage out" principle. The air entering the front of a pusher prop disk is downstream from all sorts of turbulence-making stuff. At various stations on the prop blade, the blade will see bits of air that approach it at low angle of attack, high AOA and stalling AOA.

What's more, AOA at any given station will be changing constantly as the blade travels around. That's a big part of the extra racket that pushers make, and the even greater racket when you fly the prop somewhat edgewise (as in a slip). The trajectory of the air leaving the prop is affected by the trajectory of the air approaching it, so a pusher gyro's slipstream can be expected to be very un-laminar, even coming right out of the chute.

The ducted fan ought to have cleaner outflow simply because the inflow IS ducted. The inflow is less affected by interference from the fuselage. Even with the ducted unit, though, I'd bet that the slipstream wouldn't be as pretty as the pictures, once the aircraft was moving along. (A static test isn't completely representative of what you get in flight).

Increasing the moment arm of a tail surface is very helpful. It not only gives the tail surface a greater lever arm (like a longer crowbar) -- it also increases the damping effect MORE than proportionally. The problems with long tails on pushers are a combination of structural limits and clearances from the rotor and the ground. Art Evans's wildly cantilevered HS looked like it might fatigue off the gyro eventually -- to me, anyway. Ditto for huge V-tails that are supported only by the 2x2 tail tube. You have to consider the possibility that you'll twist the 2x2 right off after awhile...