Shameless update from Dale Kramer

On Friday, March 18, 2016 at 9:36:51 PM UTC-7, DaleKramer wrote:
On Friday, March 18, 2016 at 11:33:01 PM UTC-4, 2G wrote:
It is high because it IS high. It is 5-6 times higher than an R22.

Ok, so let us concentrate again on the rotor disk loading.
If you had evaluated the design as an engineer, then I would not have to had to assume that you were asking about rotor disk loading versus propeller disk loading. An engineering evaluation would have understood that there are two flight modes that use different disk loading calculations and the question would have been more specific.

Following that, you seem to have categorically determined that it has a very high rotor disk loading without specifying a class. When you start defining the class, you cite vehicles in 2 classes and now finally you are for some reason comparing my design only to a helicopter. It is obviously NOT a helicopter! It is not even in the Osprey tiltrotor class. The closest conventionally categorized class it could be put in is the tiltwing class and in that class it has a low rotor disk loading.

I believe if anyone should be criticized here it is not me.

Somehow we have rubbed each other the wrong way, for that I am sorry.

Dale - It's a pretty clever design. Thanks for sharing - gutsy move.

I expect the main reason to care about disk loading is to work out how many RPM at what propeller lift coefficient you need to produce enough total mass flow to hover. Presumably with the main engine running along with six electric motors you are within the operating parameters of the engines/props you have fitted and the thing can actually hover. It certainly appears to be a more highly loaded hovering design that a traditional single rotor helicopter in the same weight class, but I don't necessarily see that as particularly a big deal for what it's trying to do. I also expect that going to a higher disk loading than a typical conventional single rotor design will have some impact on efficiency and therefore endurance, but since you are not spending much time in vertical mode it's not a big factor for this design.. The bigger considerations here are the overall layout for prop tip clearance and commercially available brushless motor designs and the fact that you need multiple, displaced sources of thrust to control the thing in hover.

I'd have some questions about stability and control in hover mode. First, there is a fair amount of weight above the center of thrust for the electric motors - this includes the pilot and particularly the gas motor which is on a pretty long moment arm. This is a little like balancing a broomstick on the palm of your hand. You will need to counter any static or dynamic pitching moment with differential thrust on the electric motors, which could be problematic particularly in a low-speed transition between hover and forward flight when you have no aerodynamic elevator authority. Presumably the gas motor is pulling pretty hard which is stabilizing as long as you are vertical, but it doesn't provide any restoring pitch moment if you are at some intermediate pitch attitude, but not yet flying like an airplane. You'd need to use differential thrust on the electrics to keep the nose from tipping over, all while providing enough total thrust to hold hover.

A second potential issue is how to counteract the torque of the gas motor and prop, which see to be substantially larger than the electric motors and props. Since hexacopter yaw is controlled by adjusting the speed of the three clockwise turning versus three counter-clockwise turning props you'd have to have enough available angular momentum delta in three electric motors which are not on the centerline to counteract the angular momentum delta of the big gas motor which is on the centerline. I'm not sure how much being off centerline will affect the overall yawing moment.

Ideally, you'd like to be able to handle an electric motor bearing failure at an inopportune time in transition without losing control of the aircraft.. Some model hexacopter controllers deliberately gyrate in yaw to hold attitude with a single engine out. This probably wouldn't be a pleasant experience for a pilot onboard so you might need to consider how (or whether) you want to deal with that scenario.

Glad you're building a model first - lots of interesting challenges to work out.

I wouldn't worry too much about addressing Tom's criticism(s). He is oftentimes challenged understanding or conceding any points not made by him - though it certainly appears in this case that he's mistaken the bottom of the airplane (with the nose wheel stalk) for the top in your drawing of the aircraft with the pilot seat in hover orientation.

Andy

And oh by the way 2g, you questioned mr kramers academic credentials, just remember the wrights were'nt college educated, no degrees and they still beat the pants off of Langley, the college golden boy with all the "knowledge" and all the cash. Innovation and smarts are where you find them, sheepskin non mandatory or for that matter even desired. Most of the greatest innovations have come out of garages not universities.

On Sunday, March 20, 2016 at 4:41:42 AM UTC-7, Andy Blackburn wrote:

A second potential issue is how to counteract the torque of the gas motor and prop, which see to be substantially larger than the electric motors and props.

Re-read the Kickstarter description. Offset angle on the electrics should help. Presumably you'll get additional torque when accelerating to transition. You'll need to handle the variation presumably by accelerating the electric motors spinning the opposite direction from he main gas motor. I'm guessing you might need some limits on how much you can accelerate so you don't run out of countering torque.

On Sunday, March 20, 2016 at 8:48:34 AM UTC-4, Andy Blackburn wrote:
I'm guessing you might need some limits on how much you can accelerate so you don't run out of countering torque.

Don't forget full span aileron authority coming on board as you accelerate. I once landed my glider with 150 lbs of water in one wing and zero in the other, did not have wing drop till around running speed, just ended up with worn down urethane tip skid

On Sunday, March 20, 2016 at 7:41:42 AM UTC-4, Andy Blackburn wrote:
Dale - It's a pretty clever design. Thanks for sharing - gutsy move. ......

Thanks!

I am sure that a rotor disk loading of 18 lbs/ft^2 and thrust efficiency of about 5 lbs/hp is well within any theoretically feasible region for rotor design. At least it better be. I am basing my hover ability on motors and propellers that already exist. In fact on our Joby JM1 motors with 36x20 props at 4400 rpm we are getting about 110 lbs of static thrust at about 14 kw input to the motor controllers. That is about 5.9 lbs thrust/input hp (not motor shaft power!). This setup propels the eLazair in level flight at over 60 mph (the eLazair is pretty high drag at these speeds).
So, with about 500 lbs of static thrust from a WOT Rotax and 660 lbs from WOT electrics, the thrust to weight will be about 1.3/1. The electrics should only need about 1/2 power for hover.
The unknown is what amount of a zoom pullup versus a slow pullup and rotation thru a deep stall will be required. Rather than speculate on that I am just going to test it with model and full scale testing.
The transition is a very complex mixing of aerodynamic lift, aerodynamic drag, aerodynamic moments, thrust, thrust moments, mass and moments of inertia (probably other things too . I am sure I could spend a lot of time trying to model that mathematically but I choose not to at this point.

I think the broomstick analogy is not real good one for visualization because in my case the correction needed to bring the object into balance is not a sideways movement of the hand but simply an application of thrust moment about the center of gravity. During transition, the WOT signal sent to the multirotor controller should automatically result in WOT on the lower tail motors and 0 throttle on the upper tail motors and likely 1/2 power on the wing motors. Also there is still the elevator pitching moment that can be increased by design (at the risk of making high speed horizontal flight twitchy unless I use a separate, thrust vectoring horizontal plane or fly by trim tabs in high speed which I choose not to do right now). 3D RC modeling has shown what large area, high deflection surfaces can do. To start out I use the KISS principle and add from there.

Yes the electric nacelle 'tilt' method of countering the Rotax torque is expected to work for gross torque cancelling and the 'fine' adjustments will still be from the multirotor control of the electric motor counter rotations.

I am hoping to be able to handle at least one electric motor failure and possibly 2, depending on which 2. Model testing will determine this.

I am not saying the mechanical dynamics behind the broomstick analogy is different, just the visualization of it is a little different.

I don't think the dynamics of the mass moment being above the electric drives is necessarily a problem. The two necessities for electronic stabilization are that the drive moments are sufficient and that the electric drive dynamics are faster than possible plant disturbances. So the rate of spin up of the electric motor / prop system will need to be significantly faster than the rate that the broomstick can tip over or otherwise be perturbed by aero effects. The need for fast dynamics on the electric drives, in fact, argues for high disk loading.

The amazing effectiveness of electronic broomstick stabilization is routinely demonstrated by the various two wheel inventions that people zoom around on these days.

On Sunday, March 20, 2016 at 2:16:28 PM UTC-4, Steve Koerner wrote:
I don't think the dynamics of the mass moment being above the electric drives is necessarily a problem. The two necessities for electronic stabilization are that the drive moments are sufficient and that the electric drive dynamics are faster than possible plant disturbances. So the rate of spin up of the electric motor / prop system will need to be significantly faster than the rate that the broomstick can tip over or otherwise be perturbed by aero effects. The need for fast dynamics on the electric drives, in fact, argues for high disk loading.

The amazing effectiveness of electronic broomstick stabilization is routinely demonstrated by the various two wheel inventions that people zoom around on these days.

Agreed, but I have my doubts that these were all created by mathematically analyzing all of the Newtonian physics involved before they came into existence.

There is just a lot more involved in the transitions of the vLazair.

Keep thinking of more that I would need to calculate, derivatives of time, as you describe above.

On Sunday, March 20, 2016 at 9:53:48 AM UTC-7, DaleKramer wrote:
I am not saying the mechanical dynamics behind the broomstick analogy is different, just the visualization of it is a little different.

You are correct - it was a crude analogy. The main point (which you clearly understand) is that whole thing in vertical flight is quite likely statically unstable and if it tips over beyond a certain angle of vertical it is likely to flop over nose down. Unless you have enough altitude when this happens to get flying speed and pull out...well it could be a problem. Ideally you want to keep it stable so you never get to that angle until you have flying speed, which entails tipping over with differential electric thrust enough to get flying speed through the deep stall where the wing and elevator can operate. Yes indeed adding big controls on the tail and big old Fowler flaps on the wing could help you get stable at a higher angle of attack and lower speed. Lots of the hybrid aircraft-helicopter designs I've seen resort to tilt-wing to facilitate the transition more easily but all of that adds weight and complexity.

A simple calculation would be (without benefit of the dimensions on your plans) 220 lbs of thrust from the bottom tail motors produces a nose-up moment of 880 lb-ft based on a 4-foot offset from the center of mass. If the center of mass is 5 feet in front of the lift of those two motors that are trying to hold the nose up and you have a TOW of say 500 lbs you'd end up with a nose-down moment of 2500 lb-ft which would overwhelm the ability of the lower motors to right the aircraft as you approach horizontal. Now calculating the nose-down moment for totally horizontal is not realistic as you'd be accelerating before you ever got to horizontal but a little trig would tell you roughly what kind of angle off vertical the voters have the juice to recover from. Obviously you also have the other motors pulling as well and the prop wash over the tail adds a bit of moment, but the motors on the vertical centerline mostly just reduce the effective mass feeding the nose-down moment for reasonably vertical orientations. They don't help you at all as you approach horizontal. I expect there is somewhere around 30-40 degrees off of vertical where things get interesting and you better have some forward velocity and altitude before you let the nose get that tipped over. Sounds like you have a computer program to figure it all out but my gut feel of is that the last half of the transition to forward flight could get pretty sporty - wing still stalled but the two bottom motors have run out of ability to add enough nose up moment. The reverse maneuver could be even more exciting - a zoom has been mentioned.

Of course with enough thrust almost anything is possible. :-)

Again, thanks for sharing. Interesting design. All in all I think I'd rather have one of these than those scaled-up quadcopter drones people are promoting for personal transportation. Yikes!

Andy

Andy, I see that you have excellent experience and obvious insight as to what hurdles I will have to overcome in order to transition at slow airspeeds..

Before I continue I just want to see if you agree with me that it is less critical to iron out these 'low' speed transitions IF #1 the transition to horizontal flight instructions are simply to accelerate vertically to, say 40 (or whatever testing dictates) knots, then slowly push over AND #2 the transition to hover instructions are, apply WOT electrics, do a zoom pull up to vertical from 100 knots and on the way up, when the airspeed is less than 40 knots apply WOT on the Rotax?

Sorry for run on there, I am just trying to present transitions where there is less need for calculations.

And, if we agree on that, the minutia of how slow of an airspeed before pushover and how little zoom altitude is needed can be determined in testing.

I think your simple calculation does not take into account enough variables, mainly airspeed before pushover.

I am hoping 1.3/1 thrust will be adequate but I have ideas to get more if needed.