How Fast Will It Flip?

reelmule

Reelmule
Joined
Aug 19, 2005
Messages
388
Location
Cave Creek, AZ
Aircraft
Beech Baron C55; Piper PA 18 Super Cub; RAF 2000 gyroplane
Total Flight Time
5,000+ hrs;
Chuck, I marvel at your ability to "suffer fools" such as myself. Thanks for all the math but I'm with Birdy when it comes to all the math.
When I first started college I was anxious to go into aeronautical engeneering but found my math skills didn't go beyound Calculus 101. I did have 3 slipsticks and a pocket protector for the leaky pens! You really didn't have to be a rocket scientist to do medicine and ortho was kind of like a 5 yr shop course using bones instead of precious lumber like Stan puts in his staircases!
As you can see from my Avitar I do fly a RAF with a 5 ft width HS with endplates set at -2* incidence. I know its out of the prop blast but is much wider than my cabin so it is out of the draggy air and at speed over 40 it feels very stable. PPO and Bunts are generally a high speed phenomena and the protection of the HS improves with speed. I avoid all low speed high power scenereos as your just as dead from a torque over. I agree with you on the desirability of placing the HS behind the rudder and if I had known of it when shopping it would be my first choice. I really have the hots for a tractor design and have bee in touch with Ron Herron--he sends you his regards and accolades for all of your contributions.
 
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C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,656
Location
Florida
Walt, I don’t suffer arrogant fools very well but don’t mind at all assisting people who are sincere in their quest for knowledge.

Most of the math associated with Newtonian mechanics can be reduced to simple arithmetical expressions that are no more complicated than calculating gas mileage.

But reducing the math to simple form does not eliminate the requirement for cognitive ability.

I once read that fully 50% of US adults can’t calculate the fuel mileage of their automobiles. Nearly everyone can divide 300 miles by 10 gallons and come up with 30 mpg but get hung up when it comes to deriving the raw data; topping off the tank, recording odometer readings and subtracting what from what and dividing what by what?

I once rented a house to a young lady, who had graduated from high school just a few years previously, and asked her; “Chris, what sort of gas mileage do you get from that old Ford?”

Chris: “I don’t know exactly but it’s real good. I can put in $5 worth of gas and make 2 round trips to Tampa.” (Gas was much cheaper at the time.)
 

Doug Riley

Platinum Member
Joined
Jan 11, 2004
Messages
6,313
Chuck, I did some measurements at B-Days 2-3 years ago on Pat's RAF. It's the one with the HS out on an aft "stinger," right?

I don't have my notes with me here, but, roughly, the HS just PPO-proofed the machine at about 70 MPH... assuming that the HS "saw" 70, that is. It had somewhere around 10 sq. ft. of area. I used the best lift coefficient I'd been able to get in my own tests of airfoil H-stabs: C.L. of 0.8 at -14 degrees AOA.

The problem with most RAF H-stab installations is that the stabs aren't set at a significant negative incidence. As a result, they won't necessarily generate a PPO-stopping counter-force until the gyro has pitched down somewhat. By then, the bird has started to accumulate angular momentum, necessitating a larger counter-force than you'd otherwise need. And, as you pointed out, it has lost some more rotor AOA as it pitches over (unless the pilot is a skilled stick-floater).

If you do add a large H-stab with negative incidence, it's necessary to re-hang the machine. The original RAF hang specs call out a very tail-heavy number; something around seven degrees. Once you counteract the effects of the HTL using a down-loaded stab, the machine WILL be way tail-heavy (Harry Sieckmann ran into this issue). You'll need to move the head aft to re-establish a normal Bensen hang spec of around 11.5 degrees.
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,656
Location
Florida
Yes, Pat’s stab is the one on a stinger.

Pat wasn’t convinced a stab was necessary, having grown accustomed to the built in bobble. However, during his early hours in an RAF, Ernie had flown a few laps with him and said that Pat was on the verge of tumbling out of the sky. Pat might not agree with Ernie’s assessment.

After installing the stab, Pat admitted that he hadn’t realized the effort required to keep the thing upright and how much it reduced the workload.

Pat put his thoughts on paper which I converted into English and posted on Norm’s old forum.
 

Aussie_Paul

A reforming stirrer!!!!!
Joined
Oct 30, 2003
Messages
4,441
Location
Ballarat Australia
Aircraft
None at the moment.
Total Flight Time
Since 1982 Gyro 5000+ mostly instructing, and approx. 200 fixed wing in the late 1960s.
If you do add a large H-stab with negative incidence, it's necessary to re-hang the machine. The original RAF hang specs call out a very tail-heavy number; something around seven degrees. Once you counteract the effects of the HTL using a down-loaded stab, the machine WILL be way tail-heavy (Harry Sieckmann ran into this issue). You'll need to move the head aft to re-establish a normal Bensen hang spec of around 11.5 degrees.

In the attempt to have the keel flying level we found that RAFs needed to have the head in the most forward position but still the keel was flying 3 degrees nose low at 70 mph. We started with our stabs at zero incidence. We did not know much back in 2000!!! After trying negative AoA on the h/stab we realised that it was an improvement.

Next we decided to try and build a CLT Raf. That machine was called Hybrid. We used a gearbox with the offset down, stepped the keel 4" and raised the engine 7". Of course the prop turned the opposite direction to the Raf belt drive so a new fin and rudder were required. We already knew that there was insufficient vertical surface area on a Raf with the doors on so we made the fin and rudder 30% larger. We also ended up having to shift the main wheels back a couple of inches to have suitable rotation.

We were a bit lucky as Hybrid turned out to be almost CLT with the above mentioned mods. I could fly complete circuits at 60 mph in gusty conditions with the stick locked. Above 60 mph Hybrid was less stable. I would lock the stick as soon as the aircraft left the ground and not use it again until I began the first flare. We just used throttle, rudder and roll. This was very exciting for us.

The next challenge was to see how we could improve a Raf without the expense of using a gearbox and new tail feathers. This required stepping the keel to allow the engine redrive combo to be lowered 6”. The rear keel after being lowered was angled back up towards the tail wheel at approx 3 degrees. This kept the tail wheel in its original position to allow correct rotation. The main gear was also moved back a couple of inches. Now the Raf would almost fly keel level with the head in the most forward position, and with a larger h/stab than Hybrid would fly with the stick locked in gusty weather at 60 mph.

We decided to design Firebird using the experience gained during the above experimenting and produced a side by side, easy to get into, extremely stable gyroplane.:first:

Aussie Paul. :)
 
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