Shameless update from Dale Kramer

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.

On Sunday, March 20, 2016 at 3:08:51 PM UTC-4, Andy Blackburn wrote:
On Sunday, March 20, 2016 at 8:33:24 AM UTC-7, DaleKramer wrote:
On Sunday, March 20, 2016 at 7:26:55 AM UTC-4, wrote:
Surely Dale knows this is not the forum to use for gaining serious investment capital, or for that matter "intelligent" aerodynamic advice.

That is for sure! Exactly zero of the backers so far have been from my soaring acquaintances.


+1, but I want a delivery position...if it works. Fingers crossed.

Andy


Wow, thanks!

On Sunday, March 20, 2016 at 11:52:47 AM UTC-7, Andy Blackburn wrote:
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

People have been known to climb aboard rockets. Rockets have the exact same requirement: you have to keep them pointed upward -- at least until you get airspeed for meaningful wing lift.

One could argue that relying upon motors to keep working is pretty routine for flying machines. A helicopter needs to have it's one motor keep working when it's taking off vertically to avoid a dire consequence.

Back to battery life... there must be enough for an aborted landing scenario. That means electrics on as you're screeching to a stop through deep stall deceleration, then maneuvering to the desired landing spot in vertical and letting down sufficiently gradually. If there is too much wind or if the landing site didn't end up in the right place or something is wrong at the site, there must be enough power reserve to blast back up vertically to flying speed and perhaps a horizontal landing elsewhere.

Agreed, right now I am sort of allocating 1 minute WOT electric time with about 3 minutes of reserve (even though hovering should require about 1/2 power in electrics). This is another fine tunable value when the full size vLazair is detail designed. I am hoping that I can make vertical landings that use around 30 seconds of electric time.

On Sunday, March 20, 2016 at 3:08:51 PM UTC-4, Andy Blackburn wrote:
+1, but I want a delivery position...if it works. Fingers crossed.

Andy

I will consider that a deposit on delivery position #1

On Sunday, March 20, 2016 at 2:22:53 PM UTC-7, DaleKramer wrote:
On Sunday, March 20, 2016 at 3:08:51 PM UTC-4, Andy Blackburn wrote:
+1, but I want a delivery position...if it works. Fingers crossed.

Andy

I will consider that a deposit on delivery position #1

#1? maybe I need a few brave should in front of me.

Yes - way back when I worked at NASA Ames on variable stability helicopters as part of grad school. They had the XV-15 tilt rotor there and well as the RSRA - talk about bad transitions to forward flight. It has a rotor that was an oval cross section with blowing slots on both leading and trailing edges. The idea was to stop the rotor in flight and swap the slots you were blowing out of on the blades that were facing aft (they were symmetric airfoils (fore and aft AND top/bottom) - so "aft" was more of a term of art as it depended entirely on the blowing setup. In forward flight it was an X-wing planform. I understand the transitions were "exciting".

You have an easy problem in comparison.

I agree with enough power and pitch authority you ought to be able to make the transition to forward flight (and back) without to much drama. The question I can't answer with simple math is exactly how much power you need. You've got a swept wing so it ought to be able to produce lift at pretty high AOA which is good. At some point the vertical component of the thrust vector will be insufficient to hold the aircraft in hover and as you continue to pitch over the wing better take up the slack or you will be in ballistic territory. If your total thrust to weight is 1.3:1 you will run out of thrust to hold hover at T*sin(Theta) = W, or Theta = arcsin(1/1.3) = arcsin(0.77) = 50.3 degrees nose up pitch. That might be a bit high, even for a highly swept wing. You'll need to get the elevator unstalled as well if you want to actually fly the thing through the transition with the stick. The hope is that the nose will want to pitch over anyway once you get started so you'd think you could sort of mush your way through the transition until the boundariy layer on the wing gets attached and the wing gets lifting - and hope that it doesn't get so draggy that it wants to mush and settle for very long.

My Dad was the project test pilot for a variation on this theme back in 1958:

https://www.youtube.com/watch?v=wDstVGAmI74

Like I said, with enough thrust you can do pretty much anything you want.

Andy

Andy, give up now on the hovercraft and RC drones, just launch one of those F-100D ZEL from you back yard.

On Sunday, March 20, 2016 at 4:30:23 PM UTC-7, Andy Blackburn wrote:
On Sunday, March 20, 2016 at 2:22:53 PM UTC-7, DaleKramer wrote:
On Sunday, March 20, 2016 at 3:08:51 PM UTC-4, Andy Blackburn wrote:
+1, but I want a delivery position...if it works. Fingers crossed.

Andy

I will consider that a deposit on delivery position #1

#1? maybe I need a few brave should in front of me.

Yes - way back when I worked at NASA Ames on variable stability helicopters as part of grad school. They had the XV-15 tilt rotor there and well as the RSRA - talk about bad transitions to forward flight. It has a rotor that was an oval cross section with blowing slots on both leading and trailing edges. The idea was to stop the rotor in flight and swap the slots you were blowing out of on the blades that were facing aft (they were symmetric airfoils (fore and aft AND top/bottom) - so "aft" was more of a term of art as it depended entirely on the blowing setup. In forward flight it was an X-wing planform. I understand the transitions were "exciting".

You have an easy problem in comparison.

I agree with enough power and pitch authority you ought to be able to make the transition to forward flight (and back) without to much drama. The question I can't answer with simple math is exactly how much power you need. You've got a swept wing so it ought to be able to produce lift at pretty high AOA which is good. At some point the vertical component of the thrust vector will be insufficient to hold the aircraft in hover and as you continue to pitch over the wing better take up the slack or you will be in ballistic territory. If your total thrust to weight is 1.3:1 you will run out of thrust to hold hover at T*sin(Theta) = W, or Theta = arcsin(1/1.3) = arcsin(0.77) = 50.3 degrees nose up pitch. That might be a bit high, even for a highly swept wing. You'll need to get the elevator unstalled as well if you want to actually fly the thing through the transition with the stick. The hope is that the nose will want to pitch over anyway once you get started so you'd think you could sort of mush your way through the transition until the boundariy layer on the wing gets attached and the wing gets lifting - and hope that it doesn't get so draggy that it wants to mush and settle for very long.

My Dad was the project test pilot for a variation on this theme back in 1958:

https://www.youtube.com/watch?v=wDstVGAmI74

Like I said, with enough thrust you can do pretty much anything you want.

Andy

Andy,

Sounds like the RSRA had a reaction rotor powered by air, but I don't see that in the specs I found? And are you saying that after the RSRA rotor was stopped, they continued to blow out the 'horizontal flight' trailing edges for some reason?

I was assuming that there may be some loss of altitude during the vLazair 'push' to forward flight. Part of the 'push' transition procedure could not only be 'only push when you get to a certain vertical speed' but also a minimum height above ground to make allowance for a slight height loss. Or worst case it might be 'climb straight to X altitude and then push over, it will dive some but you'll pull out of it In the early 3D flight RC model days I had a model with not much over 1/1 thrust. I have never been able to hover very long but I don't remember ever having much altitude loss or control problems coming out of a hover attempt. I know Mr Reynolds and a lot of other things likely skew that example but I can't wait to see what my 1/4 scale will do on these transitions (btw I am planning a tether system for 1/4 scale testing).

Fortunately I believe the stability through transitions will be augmented by the multirotor controller which is closed loop on heading and pitch until the throttle command is shut down to it when horizontal flight is achieved..

I also think it is possible that the aerodynamic controls are not needed at all below their un-stalled AOAs.

Wow, that video clip is awesome! I think of my electrics on the vLazair as a sort of re-chargable JATO system

Ah, maybe the blown rotors was just for changing rotor speed during transitions?

Andy,

Since we are doing napkin calcs (I have this on a spreadsheet somewhere): Mass (M) = 850 lbs, Average Unbalanced upward force for 0 to 40 knts (electric props still gives about 80% of static thrust at 40 knts and assume linear decay)(F) = (250 lbs * 0.9 * 32 ft/sec^2) = 7200 lbf, Acceleration upwards = F/M = 8.5 ft/sec^2, Distance to go from 0 to 40 knts (67.5 ft/sec) using v^2=u^2+2as is 270 ft and the time to get there from s=ut+.5at^2 is about 8 seconds.

I hope I did that right, I even have those equations in memory after nearly 40 years

So, at 1.3 thrust to weight and a vertical acceleration to 40 knts, this puts us 270 feet high in 8 seconds after liftoff (accounting for some thrust loss at 40 knts on electrics, Rotax variable pitch should pretty well hold its static thrust at 40 knts, aircraft drag at 0 AOA and 40 knts not considered). Wow, rocket calcs

Knowing this, my gut felt good about being able to go straight up and pushover fairly easily and quickly so I went on to other things. Should I reconsider, I think 50 or 60 knts may work as well at which point I will add some airframe drag estimates.