Strong Tail?

Abid said:

So I would like to understand what the counter is logically to Martin's analysis.

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Martin basically removes the minimum floor from the fixed wing equation and suggests that is a good decent speed (~240 ft/min) for gyros. This is not conservative. As Doug says the maximum would be the vertical decent rate which is around 1200 ft/min to 1600 ft/ min . This may be too extreme to mandate as a minimum for all gyros. Section T mandates about half or third of a vertical decent- in effect 480 ft / min . This seems reasonable to me particular given the possibility of behind the curve decent where it is not possible to flare effectively.

If it were up to me I see no reason not to go with the Section T criteria. Simple to assess and relatively conservative- will not save you from a vertical decent but will absorb pretty much any landing that has some forward speed.

Doug Riley said:

In my experience, the small, stubby H-stabs typically found on gyros have a real-world maximum CL of under 1.0. A friend and I did some tests on one having an accurate 0012 airfoil, 20-inch chord and 42-inch span. We used an instrumented boom on my truck. Stall was at AOA of about 14 degrees. IIR, the maximum CL we could get was 0.8. Aspect ratio matters!

Still, a pusher gyro's tail lives a very rough life. Besides the ground-handling abuse, it is constantly battered by the turbulent propwash and the mechanical vibes from the powerplant. If it's made of aluminum, it has no infinite-cycle fatigue life. So I like the idea of over-design for tails.

For a gyro to survive a landing out of a stabilized vertical descent (18-20 mph straight down), its landing gear would need a vertical travel of 2-3 feet. How close to that "gold standard" should you be required to get, at minimum? It's a tough call, but saying that a well-executed landing does not involve a stall-and-free-drop is not adequate. Any amount of too-high flare results in the beginnings of a vertical descent, involving some decent fraction of that 18-20 mph drop. A sport gyro (not to mention a trainer) should be quite forgiving of drop landings, IMHO.

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That isn't a gold standard Doug. That is a crash. Pure and simple. That is about the same as an airplane coming down under a Ballistic Chute. No one pulls a ballistic chute in an airplane (like Cirrus) and worries about the airplane being able to fly again. They only care about them getting out with light bruises and alive.

raghu said:

Martin basically removes the minimum floor from the fixed wing equation and suggests that is a good decent speed (~240 ft/min) for gyros. This is not conservative. As Doug says the maximum would be the vertical decent rate which is around 1200 ft/min to 1600 ft/ min . This may be too extreme to mandate as a minimum for all gyros. Section T mandates about half or third of a vertical decent- in effect 480 ft / min . This seems reasonable to me particular given the possibility of behind the curve decent where it is not possible to flare effectively.

If it were up to me I see no reason not to go with the Section T criteria. Simple to assess and relatively conservative- will not save you from a vertical decent but will absorb pretty much any landing that has some forward speed.

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Well do not fly a Calidus at above 1100 pounds. I don't personally have a problem with this view but I think if the test is done properly with that criteria a couple of models may have an issue and yet in the field they seem to have a decent record at 1200 pounds MTOW.

That picture was shown at todays meeting between the LAA Engineering team and Francis Moyle (LAA Inspector) of Phoenix Aero (Cornwall England) and myself, and certainly raised smiles.

As Denis has posted Phoenix Aero, of Cornwall England, is now the UK distributor for Gyro Technic products in the UK/EU market!

Francis Moyle was the agent/applicant to the LAA for the Section-T certification of Razor Blades for use in the UK, which has now been granted,
and Francis now has Phoenix Aero ready to take UK/EU orders for the full Gyro Technic product line.

In the meantime we had arranged the meeting today with the LAA HQ at Turweston Airfield. There we initiated the start of talks for the Section T requirements and test procedures that will be required for approval for the GT-VX1/GT-VX2 kits that would be marketed here by Phoenix, if approved.

A lot of question were asked by them on the kits, plans, fabrication, materials used, quality control. Also discussed were how various test are likely to be structured for them to satisfy themselves that the kits structure/s will meet the various stress requirements.

Amongst these were the possibility of some tests being allowed to be done in the US under supervision of accredited/official observers eg (Designated FAA designees) who can verify the authenticity of photos/videos of tests being carried out.

The LAA charges hourly fees for the work they do on going through the various proposals made to them, so this will entail expenses that we together with the LAA will now be trying to get an idea of, rather then simply plunging into an open ended money pit.

As many will probably be aware the LAA Razor Blade approval was a rather protracted affair that has however finally been successful, It is hoped that this prior exposure to Gyro Technic's methods, the quality control of the product, and the fact that there are now increasing numbers of Gyro Technic rotors out flying together with both GT-VX1's and 2's, will have given them some yardstick by which to get an idea of the viability of the products that Denis is now producing and Phoenix Aero is hoping to market here.

Abid, we'll have to disagree about using ability to land out of a vertical descent as the "gold standard" of landing-gear design. Long-stroke gear was the norm in the 1930's -- because airstrips were cow pastures. Many of the Cierva-era gyros could land straight out of a vertical descent. The ability to do so puts the gyro just a little closer to a helicopter's flight envelope. It sort of compliments jump takeoff.

The late great Butterfly series had long-stroke gear and could land out of a fast, behind-the-power-curve sink. They didn't have the requisite 3-ish FEET of travel and, AFAIK, could not withstand a landing out of a fully-developed vertical descent. But they could take a landing that represented a fair percentage of a full vertical. So it's not as ridiculous a standard as all that.

A skillful gyro pilot can kiss a gyro on so softly that it's a 1-G landing. At that rate, the Bensen rigid axle actually is fine. The question is how much allowance we should make for student errors and emergency landings. I don't have a strong opinion about setting the minimum gear capacity at half a vertical, or a quarter. Just trying to get calibrated.

A modern gyro having the old-time ability to land vertically WOULD be fun. That ability isn't necessary as a minimum, though.

P.S. on the Butterfly: a former student of mine has one. I've flown it several times. He thought the long-stroke gear would be good insurance against his own rough beginner landings. In practice, though, we both ended up flying it with the gear intentionally locked up in the fully-retracted position! IOW, we deliberately disabled it. Why? This model does NOT have a long-travel nose gear, only mains. You could not immediately dump the rotor upon touchdown, because the gear needed time to complete its stroke. You had to hold the stick back, lest the machine capsize forward-and-sideways if there was a crosswind.

I wondered about the Butterfly geometry for a different reason; With that much main gear travel, was there a noticeable difference in handling due to the VCG difference between raised and extended positions? Did the extreme lowered position pose any drag-over tendencies?

Doug Riley said:

Abid, we'll have to disagree about using ability to land out of a vertical descent as the "gold standard" of landing-gear design. Long-stroke gear was the norm in the 1930's -- because airstrips were cow pastures. Many of the Cierva-era gyros could land straight out of a vertical descent. The ability to do so puts the gyro just a little closer to a helicopter's flight envelope. It sort of compliments jump takeoff.

The late great Butterfly series had long-stroke gear and could land out of a fast, behind-the-power-curve sink. They didn't have the requisite 3-ish FEET of travel and, AFAIK, could not withstand a landing out of a fully-developed vertical descent. But they could take a landing that represented a fair percentage of a full vertical. So it's not as ridiculous a standard as all that.

A skillful gyro pilot can kiss a gyro on so softly that it's a 1-G landing. At that rate, the Bensen rigid axle actually is fine. The question is how much allowance we should make for student errors and emergency landings. I don't have a strong opinion about setting the minimum gear capacity at half a vertical, or a quarter. Just trying to get calibrated.

A modern gyro having the old-time ability to land vertically WOULD be fun. That ability isn't necessary as a minimum, though.

P.S. on the Butterfly: a former student of mine has one. I've flown it several times. He thought the long-stroke gear would be good insurance against his own rough beginner landings. In practice, though, we both ended up flying it with the gear intentionally locked up in the fully-retracted position! IOW, we deliberately disabled it. Why? This model does NOT have a long-travel nose gear, only mains. You could not immediately dump the rotor upon touchdown, because the gear needed time to complete its stroke. You had to hold the stick back, lest the machine capsize forward-and-sideways if there was a crosswind.

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I think you should check Martin Hollman’s calculations where in his side by side design areas drop for about 5 inches is a 2.5 to 3 G landing. Now this depends on certain things but a dead drop from about 11 to 12 inches for most would represent more Gs for a limit load.
What you are suggesting is that gyroplanes would be way past Part 23 standards for airplanes to land. If you want to land vertically from a height of 70 feet in a complete vertical descent that is what you are asking for. In other words a tank. Tanks are heavy. It is well known that current production gyroplanes which are usually made with training in mind like models M-16 from Magni, MTO Sport from AG, ELA G8 or AR-1 open cockpit from us. They all take the student beating quite adequately. There isn’t a problem there to solve. But none of them would be guaranteed to take that vertical descent without any damage.
Yes I know the vertical landing gear on the Butterfly. Which trained pilot lands like that. It couldn’t do a full vertical descent over a fifty foot obstacle so it was not fulfilling any actual function to me. Yet you got the ugliest design landing gear to show without any true functional gain. Not that Butterfly type of gyroplane was looking for any aesthetic or cruise speed efficiency anyway. But certainly even todays trainers can fly at a fast cruise of 90 - 100 mph in a XC while being able to do training and landing at as slow as 0-5 knots. Soft field landing can be done in todays gyroplanes with power on at almost 0 knots from a height of 4 feet without an issue.

Brian, the extended-gear position did put you in somewhat greater danger of tipping over on the ground. After all, the gear tread (width) was about the same as any other gyro, while the masses were higher. Tippy. Tricycle vehicles are on the tippy side already, without adding height.

Moreover, even the Butterfly gear didn't have quite the right mechanical geometry. There's been much criticism of "swing axles" on gyros, since they push outward as they compress, shoving the gyro sideways if one wheel hits first. This is the origin of the infamous Dominator "duckwalk." The Butterfly uses swing axles and does feature some out-swing of the wheels as the gear compresses.

The Butterfly tail has a "T" configuration, with a large (flat-plate, not airfoil) H-stab on top of the ruddervator, in the middle of the propwash. This gyro is also CLT, with a seat that you "hop up" into. It should be resistant to uncommanded pitch-overs, but completely immune? I don't know. I've never flown Dan's Butterfly at more than 65 or so.

BTW, dan converted his Butterfly from that half-height stock ruddervator to an RFD tall tail. This got rid of the need to hold a lot of pedal against prop-induced yaw on takeoff. No more leg cramps!

The "mechanically correct" way to build long-stroke landing gear is to ditch the swing axle entirely. The compression struts should be exactly vertical, and mounted on non-moving outriggers. That's the way the oldtimers did it in the 1930's. This setup involves a minimum of five tubes and four attachment points on the frame, per wheel. I guess this complexity has discouraged modern-gyro designers from copying the classic setup -- but it did work well, with no "duck walk" or other peculiarities.

As to how really useful a full vertical-landing capacity is -- probably not much, unless it's coupled with jump takeoff. It's more of a party trick, albeit a fun one. The vertical-landing gear would simply allow you to land in a bunch of places that you couldn't get back out of without strong jump ability. But then you're back to collective pitch, cost, complexity, and no LSA status.

The Butterfly-Cartercopter jump gyro did spectacular jumps, but at a severe cost in controllability because of the massive blades, as has been discussed on this Forum. Design is all about compromise.

Higher seating position raises the CG higher and makes the tipping over tendency higher in a noticeable way even if your landing gear width remains the same as other machines. There is a term for this tipping over tendency in engineering and there is a formula that escapes me right now. To equate you have to make landing gear wider than other machines. There is no free lunch.

Edit to add: I believe the term is called static rollover threshold and engineers calculate that and experimentally determine that for tall vehicles by putting it on a platform that can be tilted slowly in roll simulating a lateral force due to shift in CG till the vehicle hits the point of no return. The more you can tilt, the better it is. A higher CG as you can imagine will generally tip you over much earlier than a lower CG. Don't even need a formula to understand that.

Here is an old picture of a trike called Delta Jet that I co-designed and we did ASTM compliance testing on it. That is 5600 pounds roughly on that carriage. This isn't the dead drop test. I do not have pictures or video of that on hand right now. This is the hang test for ultimate load. AR-1 as you can see from the looks is nothing but a take on from that trike, except its stainless steel Tig welded frame instead of 6061-T6 bolt together frame. It is stronger than the trike here. The landing gear on AR-1 is more heavy duty than this trike although of same 7075-T6 single piece spring formation. It should handle an extra 1800 pounds compared to the trike as limit load.
Once you start demanding minimum standards to handle more than this limit load landings, you really need to start looking somewhere else because that isn't the right track. These structures (and similar) are well proven with 10's of thousands of hours in the field. Engineering is simple and fairly well proven conservative. Even so I saw the accident that Greg Spicola had with the Saudi trike pilot where the gyroplane did fall from a height of I'd say 20 to 30 feet straight down as the guy froze on controls all the way back and came vertically down hard. It still damaged the gyroplane landing gear, its frame, and even cracked the engine case. The G's there were likely quite high. The fact that the people survived and Greg even walked away from that is the best we can hope for.
Most airplanes are rated for a hard landing of around 600 FPM max. Beyond that you can have major damage.

The maddening aspect of designing gear around a high CG is that widening the mains' stance ALONE doesn't do that much good. Tri-gear vehicles capsize sideways-and-forward; the "forward" bit is determined by the nosewheel. So you also have to move the nosewheel away from the center of the aircraft (i.e. forward) to get much improvement.

That said, even a wide-stance gyro is subject to tipover in a crosswind, because a fast rotor will pull it up-and-over. (Yes, I've managed this charming little trick in both high-stance and low-stance gyros.)

This comes with the territory. It's something that rotorcraft pilots just have to know about and guard against.

Doug Riley said:

...

That said, even a wide-stance gyro is subject to tipover in a crosswind, because a fast rotor will pull it up-and-over. (Yes, I've managed this charming little trick in both high-stance and low-stance gyros.)

This comes with the territory. It's something that rotorcraft pilots just have to know about and guard against.

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If by that you mean badly managed rotor thrust direction can flip you over on a really bad landing, then for sure. That is pilot error.

Abid said:

I think you should check calculations where in his side by side design areas drop for about 5 inches is a 2.5 to 3 G landing.

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Can anyone give me a link to the Martin Hollman’s calculations of the required spring sinking?
Thanks

Jean Claude said:

Can anyone give me a link to the Martin Hollman’s calculations of the required spring sinking?
Thanks

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It is in post #16 in this thread.
Raghu's view is that it is not conservative enough. Martin in my view is giving rationale for why his calculation given gyroplane's behavior at touchdown is "equivalent" to an airplane limit landing load whose behavior is different. It is a legitimate point deserving of proper discussion in my view not just something to throw aside. I believe ASRA in Australia uses Martin's analysis as compared to BCAR Sec T. Please feel free to correct me if I am wrong. ASTM standards are supposed to be "minimal" industry consensus standard not maximum. So it is important to keep that in mind. Manufacturers may do a lot more than the standard requires but the ticket to entry is to be set at the minimum consensus.

raghu said:

For the strength condition the load factors assume 2/3 rotor lift to give inertial relief. This does not require assuming stall. Now tell me if we assume full lift of the rotor what would be the reaction load at the wheels upon landing? Exactly .

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According to my calculations, to keep 3g max on the landing gear, I get 20% less sag when the rotor is supposed to carry 100% during impact instead of 2/3.
Note that his method does not apply in the case of a hydraulic shock absorber, or when stops prevent the springs from relaxing, or when they are replaced by Sandows.

Abid said:

It is in post #16 in this thread.
Raghu's view is that it is not conservative enough. Martin in my view is giving rationale for why his calculation given gyroplane's behavior at touchdown is "equivalent" to an airplane limit landing load whose behavior is different.

Strong Tail?

So... Working toward more UK Section-T stuff for our kits. "Tail needs to support the weight of 3 men"..... Or something ridiculous like that.... No problem! We could put 4 on there if we had room!

www.rotaryforum.com

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Abid, Just to clarify personally I do not see the section t criteria as being very onerous- the Euroclones have all had major models comply, though I have no insight to the level of design changes they had to make. This is not to say that this should be consensus view here with the ASTM but it seems a simple and good start.

My Issue with the late Martin Hollman's calculation are that he simply, without explanation or calculation/analysis beyond, " gyroplanes land slower", suggested modifying the airplane formula by replacing wing loading by disc loading and removing the minimum sink rate. It may be a sufficient criterial in practice but seems a little arbitrary.

In the end the max decent rate absorbed by a landing gear that a regulation specifies is based on experience and judgement. The following may help inform this.

Disc loadings of our Bensen clones broadlly range from 1 to 2 lbs/sqft. The vertical decent rate is approximately given by 27*sqrt(DiscLoading) ft/sec, so the vertical decent range is 27 ft/sec (1620 ft/min) to 38 ft/sec (2280 ft/min) for our gyros . Absorbing this will definitely add a weight penalty and would be onerous as a minimum standard.

Let us next consider minimum decent rate (no flare or ground effect). The min decent rate in a glide is about 11.6 *Sqrt(DiscLoading) . Again in our range of disc loading that would be a decent rate of 11.6 ft/sec to 16.24 ft/sec. In other words designing to this rate would allow a gyro to be landed without a flare at the min decent rate speed. Still perhaps a little unrealistic as it does not account for ground effect and a flare.

Section T picks 8 ft/sec so that would mean they are assuming that min decent rate calculated above is halved to take into account ground effect and minimal flare. Hollman's calculation assumes a further halving of that to ~4 ft/sec (IOW 4 times less than minimum decent rate). Again in a average landing the decent rate would be below this but is it conservative enough as a minimum standard ? Well this is were the judgement and field experience comes in.

A few more thoughts on why Hollmann's criteria is not rational and a proposal for a more rational criteria. Hollman's proposed criteria of decent rate for calculating landing is :



How did he arrive at this ? Well, he used the fixed wing formula in FAR 23 by replacing wing loading with disc loading. FAR 23 however says that if the formula results in a sink rate less that 7 fps then it should be ignored and 7fps must be used as the minimum. Disc loading of gyros are very low and so if you use the above formula you will always get sink rates below 7fps. Because of this Hollmann proposes ignoring the minimum (7 fps) criteria in FAR 23.

There are two issues with this. First, ignoring the minimum seems arbitrary with no analytical justification. Second, and more fundamentally it is based on fixed wing thinking. To understand this you have to look at the factor :



How was this arrived at? In a fixed wing the minimum decent rate is proportional to the square root of the span loading. If we assume the aspect ratio of wings are relatively constant across wing loadings (wings are geometrically similar), then the decent rate is proportional to the fourth root of the wing loading.

Does this apply to rotary craft ? Not really. In gyros the decent rate is proportional to the square root of DiscLoading and not fourth root as in fixed wings ( and Hollmann's criteria). For example, vertical decent rate is roughly equal to

and minimum decent rate in a glide is around



This is why any criteria for decent rates in gyros for calculating landing loads should be based on the square root of disc loading and not the fourth root of disc loading like Hollmann suggested. Going with the fourth root is fixed wing thinking.

So what could a potential criteria look like ? I would go with something like



The drop height (in feet) for this criteria would be


This will result in very similar drop heights to Section T for heavily loaded gyros but give lower heights for lightly loaded gyros to take advantage of the lower sink rates.

Finally, the exact constant chosen in the formula can be different ( based on field experience and judgement) but it makes sense to base it on the square root of disc loading and not fourth root.

raghu said:

Abid, Just to clarify personally I do not see the section t criteria as being very onerous- the Euroclones have all had major models comply, though I have no insight to the level of design changes they had to make. This is not to say that this should be consensus view here with the ASTM but it seems a simple and good start.

My Issue with the late Martin Hollman's calculation are that he simply, without explanation or calculation/analysis beyond, " gyroplanes land slower", suggested modifying the airplane formula by replacing wing loading by disc loading and removing the minimum sink rate. It may be a sufficient criterial in practice but seems a little arbitrary.

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Hi Raghu
I agree with you that BCAR Sec T criteria is hard to pass on undercarriage. I know most production gyroplanes will pass it and actually go well beyond its requirements. I know I can drop test AR-1 from 12 inches and it would easily laugh off the drop test. My point is the principle of maintaining minimal requirements in industry consensus standard.
I would ask why disc loading and the fact there isn’t a stall at landing not good presumptions for the change. I assume you have an idea why tfat would not be wise.

Edit: just saw you message about your thoughts about why you think that. I’ll have to look at it on a laptop. Phones don’t do justice to serious reading.

Abid said:

Hi Raghu
I agree with you that BCAR Sec T criteria is hard to pass on undercarriage. I know most production gyroplanes will pass it and actually go well beyond its requirements. I know I can drop test AR-1 from 12 inches and it would easily laugh off the drop test. My point is the principle of maintaining minimal requirements in industry consensus standard.
I would ask why disc loading and the fact there isn’t a stall at landing not good presumptions for the change. I assume you have an idea why tfat would not be wise.

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see my post 38 ...it goes into details of what a gyro landing criteria should look like.