Low G and Rrpm decay rate

Abid

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I get dizzy easily. I'd barf if I rode around on a rotor-only gyro -- presumably sitting on a giant hub bar in the middle. Yecch.

There's no free lunch when it comes to trim drag. If you want a stable airfoil, you need to apply that download to the trailing edge. You can use an HS boomed out back (the normal approach) or you can make the download using trailing-edge reflex. In the latter case you're simply attaching your downloaded H-stab to the aft end of the wing (either fixed or rotorblade; the same rules apply; a rotorblade is simply a small flying wing).

Some designers try to cheat on this rule. My closest gyro-flyin' buddy died in the crash of a sailplane that was intentionally designed with a non-downloaded HS. The wing had drooped trailing edges to add even more nose-down moment. To keep the nose up, the designer put the CG way aft at 40% or more of wing chord. But this arrangement only works in a narrow range of airspeeds; either faster or slower, the wing's pitching moment varies as the square of airspeed, while the weight at the CG obviously doesn't vary at all. Bill might have been OK if anyone had told him about this aggressive setup -- but no one did. Sad.

We in Gyro-land had a rotor blade "design" foisted upon us some years ago with the same sort of drooped trailing edge airfoil -- an unworkable design.

Less radically, some experimental gyro rotor blades have been laid out with little or no reflex, in the hope that the blade's mechanical stiffness alone would keep the blade from "tucking." Yes, it would be nice to get rid of the trim drag caused by reflex. Rotorblades are pretty limber, though, and such a design amounts to yanking on the dragon's tail. Bensen put a little extra reflex into his metal blades at the tip, probably as a precaution against this sort of instability.

Fara, I agree that a farther-aft placement of the HS has real benefits. With a longer lever arm, the download in pounds (hence trim drag) can be less for a given number of foot-pounds of nose-up torque. Even better, the HS's role in dynamic stability (damping) increases as the square of the tailboom length.

OTOH, I disagree vehemently with Magni's decisions to (a) use no HS download and (b) place the HS so low that it catches no propwash. We need that propwash on the HS in these slow aircraft; HS lift varies as the square of airspeed. Immersion also makes the HS respond in proportion to throttle setting -- a desirable arrangement if you have HTL.

Finally, an HS centered in the propwash helps counteract torque roll. You can have a far-back, wash-centered HS as long as you have the rotor clearance, but the structure gets a bit unwieldy on a pusher.

I like the HS right at around 70% radius of the prop arc. Torque can be handled in other ways. But who knows I may change my mind with some experimentation. There are some other advantages to having the HS right at center in that it makes the vertical stab more effective at the cost of some weight in construction. I agree about a little reflex on your blade airfoil. It should definitely help even if its a little inefficient.
 

Tyger

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"The Magni prop thrustline is purposefully slightly high so that the horizontal stabilizer (HS) lift required to balance this HTL nose-down static moment is a DOWN-LIFT of the HS. With the other cumulative aerodynamic forces on the airframe being in the nose-down direction, airspeed static stability can only be (inherently and passively) accomplished by a down-lifting HS! A truly centerline thrust (CLT) does not require any lift of the HS to balance the prop thrustline. A low prop thrustline with power applied requires the HS to provide an UP-LIFT – a negative airspeed static stability condition! Any prop thrustline is CLT when it is not producing thrust; but nose-down airframe aerodynamic moments will be destabilizing for any prop thrustline, if not properly balanced by a DOWN-lifting HS balancing against these other aerodynamic moments. The Magni high propeller thrustline, requiring a DOWN-lifting HS to be consistent with the airspeed stability requirement, is easily “balanced” by the very effective down-loaded horizontal stabilizer. It is true, in general, that the higher the propeller thrustline, the more work (and perhaps drag) the HS (and rotor) is required to do to statically balance that high thrustline moment – the Magni prop thrustline and very efficient HS on a long moment arm has minimal effect on the performance of the Magni gyro, as evidenced by its superb performance on just 100 HP." – Greg Gremminger
 

Doug Riley

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My friend Greg Gremminger has done some very useful work on this topic, including some in-flight stability tests that involved calculated risk to himself.

I disagree with his reasoning supporting HTL, however. Here's the deal as I see it:

An object in space (such as a flying gyro) is in equilibrium when (among other things) the sum of all moments (torques) on it is zero. If you have precise CLT (hard to achieve all the time, with different pilots, fuel burn and so on), then there is no moment about the pitch axis caused by prop thrust. If the gyro's center of drag is also right at the CG, then the rotor thrustline will pass right through the CG, simply because there are no moments tending to shift it anywhere else.

Now let's disturb the equilibrium. In a sudden G-disturbance, the rotor thrustline will still pass thorough the CG; only the magnitude of the rotor thrust will change (the gyro will rise or descend). The nose will neither rise nor fall. IOW the aircraft will exhibit neutral pitch stability with respect to G load.

This picture assumes no HS. Now add an HS and rig it with zero down-load. Repeat the G fluctuation above, and what happens? Nothing different, until you look into what CAUSED the G fluctuation. Likely it's either a forward control input from the pilot or a downdraft from Mother Nature.

In the forward-push case, the control input has swung the rotor thrustline AFT of the CG. The tail will rise, the nose will drop, and this rotation of the airframe will tend to amplify the reduction in disk AOA caused by the initial control input. If the HS is effective, the nose drop won't go far before the HS begins to "bite" with negative AOA. That will snub off the frame rotation.

This is how the Magni reportedly behaves. It works, but IMHO it's... untidy. Any amount of nose drop amplifies the stick-forward control input. The pilot experiences this amplification as control lag.

If OTOH the HS is set up with negative incidence and/or negative camber (download), the rotor thrustline in equilibrium will pass BEHIND the CG, not straight through it. It has to, in order to counter-balance the tail-down moment of the HS. In equilibrium flight, the rotor thrust will be pulling UP on the tail and the HS will balance this effect by pushing DOWN on the tail. In a G reduction, the rotor thrust will pull up less, and the HS will push down the same as before. The net result will be that the nose will tend to rise in low G. This amounts to a built-in tendency for the aircraft to limit low G flight automatically. Again IMHO, this is tidier and reduces perceived control lag -- which is a trigger for over-controlling.

Where I disagree with Greg is in the "need" for HTL in order to have a down-loaded HS. You can, and should, apply HS download even to a perfectly CLT machine. This simply swings the rotor thrustline aft of the CG. Which is what you want.

With LTL, the rotor thrustline will end up behind the CG even if there's no HS at all. There's no need for HS download (nor up-load either; the LTL itself provides the equivalent of that). But the goal here is not HS download for its own sake. Instead, the goal is rotor thrustline behind CG. Either HS download or LTL will do that, but...

The LTL approach does have a limitation. It only works when the power is up. Close the throttle and you're back to neutral pitch stability, not the positive pitch stability you have when the rotor thrustline is behind the CG.

And this last point jibes with my experience in training in the tandem Dominator. When you perform an idling landing approach, the aircraft's "feel" goes from very solid to kinda rubbery, more like a Bensen. The control lag increases. My students would occasionally over-control in this flight mode. Of course, they couldn't PPO, since the aircraft was LTL and the power was off. But they had more trouble controlling airspeed, especially in rough air. Adding to this effect was the short lever arm that the HS had, and the fact that the HS no longer had propwash energizing it; only dirty air tumbling off the engine and the rest of the gyro ahead of it.

Fara, yes, the fastest propwash is located about 2/3 of the prop's radius from the center. The reason for this is that the propwash has a smaller diameter than the prop itself. The "necking down" of the wash aft of the prop varies with the ratio of slipstream to freestream; it's often most severe when the gyro is sitting still and less so at high flight speeds.

IMHO, the HS should have enough span so that its outer portion is outside the propwash -- but its inner span is immersed in the wash. Best of both worlds, so to speak.
 

Tyger

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One more quotation from GG, which speaks a bit to why the Magni horizontal stabilizer is on a long, low tail boom:

"The largest dynamic damping element on the Magni gyros is the very long tail to the HS. This very large HS does not depend on the fan blowing on it to achieve either static or dynamic stability margins. Not saying embedded tail feathers are not good, but they are mostly good when the fan is blowing hard on them. Otherwise, when the fan is not blowing, there is no “accelerated” air and the disturbed air from the airframe might even hinder or complicate the proper stabilizing moments that do remain from free air flow. I think Magni (and Jukka [Tervamäki]) had purposefully chosen to locate their HS below the prop in free air so that the stability margins and control responses are not so much a function of propwash strength.

A long tail is certainly very good for static stability - the long moment arm multiplies the static effect of the HS and reduces the rotor download penalty from a downloaded HS. BUT, for dynamic damping, the long tail boom provides an additional multiplier - a total SQUARE function multiplier for dynamic damping. The first multiplier from tail moment arm is the leverage arm that everyone clearly understands - well most everyone! The second multiplier for dynamic damping is the vertical rate of movement of the HS during a pitch action. When the tail is longer, the HS goes up and down faster. The up and down movement - during pitching action - produces an AOA change on the HS that produces an increasing force on the HS in the direction opposite to the HS movement - and a pitch moment opposite to the pitch rotation. This force is again multiplied by the moment arm of the HS. The force on the raising or lowering HS is "phased" with the velocity of vertical movement of the HS - and with the pitch rotation velocity of the airframe. This is the dynamic damping action of the HS. This dynamic damping pitch moment is the result of the vertical movement of the HS - not of the actual AOA of the HS. This dynamic damping function is a completely different element than just the static effect of the AOA of the HS. And, the dynamic effect is twice multiplied by the long tail moment arm - not just once multiplied as the static AOA moment of the tail is."
 

XXavier

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Just one comment: in my experience, the HS of the Magni 24 works perfectly well. The machine is always very stable in pitch...
 

Doug Riley

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No quarrel on the issue of dynamic stability. The damping power of the HS (that is, its contribution to dynamic stability) is indeed a square function of the distance between the HS's aerodynamic center and the aircraft's CM. IOW, its lever arm.

The relationship between static and dynamic stability is interesting. Dynamic instability is a tendency to oscillate more and more severely once disturbed. It's caused by positive static stability coupled with inadequate damping. So, no static stability, no problem with dynamic instability -- with static instability, you can die of PPO without any preceding oscillations (as in that Japanese video). Static stability must come first.

The acid test of static stability in a HTL gyro that has a HS is a low-airspeed, high-throttle pushover maneuver. The question is whether, with slow airflow (no prop blast), the HS still has enough down-load to prevent a nose drop. It's quite dangerous to experiment with this flight regime by just going up there and seeing what happens. There are ways to pre-test for PPO tendencies without using yourself as a crash dummy.

A wholly separate issue with H-stabs is the location of the airframe's center of pressure. Low-mounted "bathtub" fuselages, radiators, big wheels and (worst of all) floats can result in a low airframe center of pressure (or center of drag, if you prefer). This sets you up for a "drag-over." Unlike PPO, which is triggered by prop thrust, a drag-over can happen even with the engine off, at a high glide speed. To prevent drag-over, the HS must be able to generate adequate down-lift without the help of propwash. This argues for locating at least SOME of the HS in "clean" freestream air -- and as far aft as possible. Or, instead, you can mount your wheels, bathtub and such so that there is no net drag-over moment. Of course, you can't manage that with floats.
 

Jean Claude

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The miserable efficiency of a gyroplane is the result of its rotor going 500 mph while the rest of it goes 50 mph.
Compounding the inefficiency of a gyroplane is the method of powering the rotor.
Rather than by a gear train with an efficiency of 90%+ as is the case of a helicopter, the gyroplane rotor is powered by a windmill whose wind is generated by a propeller with an efficiency of no better than 70%.
The poor efficiency of the propeller is due to the slow flight and the small diameter.
The slow flight is due to the large parasitic drag ie a cosmetic or absent fairing.
The small diameter is due to the tail boom.

Below is my calculated comparison with the same the rotor.
The first table is a kind of Magni M16 pusher (parasitic drag S.Cd= 0.7 and propeller of 1.7 meter)
The second table is a tractor with a front propeller of 2.1 m diameter and an fairing of S.Cd = 0,25 permitted by the absence of rear walls stalled

Sans titre.png

So, you can see a consumption of 15.3 liters instead of 25.3 liters per 100 miles at the best forward speed.

Scd = 0.25 is not utopian because this aircraft side by side 100 hp has a S.Cd of 0.27 (wings deduced)

Sans titre.png
 
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Tyger

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The poor efficiency of the propeller is due to the slow flight and the small diameter.
The slow flight is due to the large parasitic drag ie a cosmetic or absent fairing.
The small diameter is due to the tail boom.

Below is my calculated comparison with the same the rotor.
The first table is a kind of Magni M16 pusher (parasitic drag S.Cd= 0.7 and propeller of 1.7 meter)
The second table is a tractor with a front propeller of 2.1 m diameter and an fairing of S.Cd = 0,25 permitted by the absence of rear walls stalled

View attachment 1153554

So, you can see a consumption of 15.3 liters instead of 25.3 liters per 100 miles at the best forward speed.

Scd = 0.25 is not utopian because this aircraft side by side 100 hp has a S.Cd of 0.27 (wings deduced)
Just empirically, I generally use about 20 liters to go 100 miles in my Magni. Your mileage may vary. :)
 

Doug Riley

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20 mpg or so. That's pretty decent for a small gyro.

The totally unfaired jobs, such as my little Air Command 447 and 447 Gyrobee, get more like 12-14 mpg. Of course, part of that miserable efficiency derives from their 2-stroke engines. A fair amount of fuel exits the exhaust pipe unburned. But lean them out and you risk seizure.
 

Tyger

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Of course, that's with just me flying... at closer to MGW it's rather less. Probably the numbers that JC is using assume that.
 

Aerofoam

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20 mpg or so. That's pretty decent for a small gyro.

The totally unfaired jobs, such as my little Air Command 447 and 447 Gyrobee, get more like 12-14 mpg. Of course, part of that miserable efficiency derives from their 2-stroke engines. A fair amount of fuel exits the exhaust pipe unburned. But lean them out and you risk seizure.

I was shocked when I found out most Rotax 447, 503, 582s burn about 5 gallons/hour, not very efficient, but I would say
it's not because they are 2stk., it's because they are a dated production design originally for snow mobiles and power was
the main consideration, not efficiency. I would bet the overall design has not been changed in over 40 years.
We use 2stk. engines on our UAVs that are excellent and very simple. They are all CNC machined and originally developed for
max. performance for large competition RC aircraft. a 100cc engine is between $800 and $1200, it only weighs about 9lbs. and produces 10+ HP.
I can fly a 65lb. aircraft for 36 hours at 55mph on 2.25 gallons of unleaded fuel with 50 to 1 -2stk. oil.
These engines run thousands of hours with little more than spark plug changes.
I wish they made a 50HP version, it would only weigh 45lbs.
A friend of mine makes a relatively new brand and he has been threatening to design some 400 to 600cc versions.
All of these engines are boxer twins.
 

martin-av8r

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I was shocked when I found out most Rotax 447, 503, 582s burn about 5 gallons/hour, not very efficient, but I would say
it's not because they are 2stk., it's because they are a dated production design originally for snow mobiles and power was
the main consideration, not efficiency. I would bet the overall design has not been changed in over 40 years.
We use 2stk. engines on our UAVs that are excellent and very simple. They are all CNC machined and originally developed for
max. performance for large competition RC aircraft. a 100cc engine is between $800 and $1200, it only weighs about 9lbs. and produces 10+ HP.
I can fly a 65lb. aircraft for 36 hours at 55mph on 2.25 gallons of unleaded fuel with 50 to 1 -2stk. oil.
These engines run thousands of hours with little more than spark plug changes.
I wish they made a 50HP version, it would only weigh 45lbs.
A friend of mine makes a relatively new brand and he has been threatening to design some 400 to 600cc versions.
All of these engines are boxer twins.
Hi Aerofoam

i am interested in your specification ?

if you take the average or best BSFC for just 5 hp for 36 hours = ?

 

Doug Riley

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Aerofoam -- Your vintage estimate for the Rotax 2-stroke aircraft engines is right on. The 447, at any rate, used parts from about a 1982 sled engine. I've bought certain items from the local Ski-Doo dealer here in Vermont.

My understanding about the excessive fuel consumption of these engines is that the limiting factor is heat. They're carbed intentionally to run rich (i.e. they are in part fuel-cooled), lest the pistons melt. I found that I could jet the 447 down one size (.165 to .162) without seizing the engine, running an EGT reading of around 1300 deg. in the winter, but this sort of "experiment" is courting engine failure.
 

Jean Claude

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Of course, that's with just me flying... at closer to MGW it's rather less. Probably the numbers that JC is using assume that.
As you can read in the tables, my quantifications were established for 4500 N, that is 459 kg in flight or 1000lbs
 

Tyger

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Well, my machine is 600 lbs, I'm 200, and full fuel with other fluids is perhaps another 125, so that's getting close to 1,000 lbs...
 

Aerofoam

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Hi Aerofoam

i am interested in your specification ?

if you take the average or best BSFC for just 5 hp for 36 hours = ?


I don't know what the actual HP is at cruise, I would estimate less than 3, probably around 1.5 to 2ish.
I used the larger 10hp motor because at the time, it was the only 2 cyl. boxer I could find. I initially used the 50cc single, but the accelerometers in the auto pilot
were registering 4 lateral Gs at idle, so I bumped up the motor for vibration issues. It allowed the use of a larger prop.
I do some alchemy with the props and carburetor to get the high cruise efficiency and don't want to give up my secrets.
The end result is an engine that can only deliver about 85% of it's rated HP, but that is more than double what the aircraft needs, so
it can climb out at absurd angles if needed. The sweet spot is in the low end. I originally built sailplanes and am an efficiency nut, so the plane
is very efficient to begin with. It actually cruises at about 25% throttle, almost a high idle.....
I understate the economy a little to make sure it is not exaggerated. An engineer from a small turbine company ran some calculations for me when we
did an SBIR submission and said I neglected to calculate the weigh reduction from burning off the fuel on a long flight and that it would add a couple hours to our numbers.
 

martin-av8r

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I don't know what the actual HP is at cruise, I would estimate less than 3, probably around 1.5 to 2ish.
I used the larger 10hp motor because at the time, it was the only 2 cyl. boxer I could find. I initially used the 50cc single, but the accelerometers in the auto pilot
were registering 4 lateral Gs at idle, so I bumped up the motor for vibration issues. It allowed the use of a larger prop.
I do some alchemy with the props and carburetor to get the high cruise efficiency and don't want to give up my secrets.
The end result is an engine that can only deliver about 85% of it's rated HP, but that is more than double what the aircraft needs, so
it can climb out at absurd angles if needed. The sweet spot is in the low end. I originally built sailplanes and am an efficiency nut, so the plane
is very efficient to begin with. It actually cruises at about 25% throttle, almost a high idle.....
I understate the economy a little to make sure it is not exaggerated. An engineer from a small turbine company ran some calculations for me when we
did an SBIR submission and said I neglected to calculate the weigh reduction from burning off the fuel on a long flight and that it would add a couple hours to our numbers.
Thank you for your interesting information

As Doug said with the old rotax snowmobile 2 stroke engine tech you have to run very rich of peak EGT to cool the piston

you have less time to cool the piston with 1 power stroke per 360 deg or rpm vs with 4 stroke 1 power stroke per 720 deg or 2 rpm

also you waste more fuel in the exhaust scavenge process

with the rotax ETEC direct fuel injection and RAVE exhaust valve they can burn less fuel and oil

i think BRP closed the ETEC outboard production and they retain it for the snowmobile engine only

they have it in the link in my first post the BSFC of the rotax 582 https://en.wikipedia.org/wiki/Brake-specific_fuel_consumption
48641989Rotax 582gasoline, 2-strokeAviation, Ultralight, Eurofly Fire Fox0.699425[1]19.3%
 
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Jean Claude

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I piloted gliders C800, Bijave, C 310, airplanes Piper J3 , PA 28, Jodel D117, DR 220, Cessna 150, C
Total Flight Time
About 500 h (FW + ultra light)
Well, my machine is 600 lbs, I'm 200, and full fuel with other fluids is perhaps another 125, so that's getting close to 1,000 lbs...
600 lbs + 200 lbs + 2 h of fuel = 860 lbs
Entering a diameter of 8.5 m and a total weight of 860 lbs, my spreadsheet gives 20.7 liters per 100 miles.
 
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Tyger

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It is interesting that you chose hours of fuel in the weight estimate, when part of what you are trying to determine is rate of burn...
I always top off when flying cross country: 19 gallons (72 liters). 19x6=114 lbs. I tossed in another 11 lbs. for oil, coolant, chocks, toolkit, etc., so I am sticking to my 925-lb. estimate. 🙂
I am thus still doing a bit better than your spreadsheet would indicate, I think.
 
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