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Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Eugene, May 29, 2017.
Your wife has threatened each of us and said that's not to happen. We're all afraid of her too.
So, on retractable Skyboy point B will move up a little, speed will increase a little, angle of attack will get smaller and tail will move up.
Moment between point A and point B will stay the same because power setting did not changed. But horizontal tail physically will be positioned little bit higher, will get a little bit better airflow and at faster speed will need smaller angle of attack to control this moment generated by A and B.
I do understand that there is more to consider like wing pitching moment Mz, but I am ignoring it for now on purpose.
And altogether this aircraft in flight will look much more normal. Without crazy angles and total decalage of 10°
Here is another idea from this morning to do tuft test with much longer tufts and hope to see something different.
Might need to try that while gliding with the prop stopped. Each time a blade passes it swirls the flow some upstream.
Probably not a very safe idea.
It's pretty clear the flow is not laminar, which is exactly what would be expected on such a thick object.
Well, I will not be doing this experiment over lake Michigan or something like that.
Our longest RWY = 5100 feet. I can do extra long and high final. Turn engine off, trim for 90 MPH and do power off landing.
This should give us same velocity and same picture as level flight
During my test flight yesterday I was doing some thinking. Like I always do. And it hit me that all this time I was thinking about my airplane as A-B-C triangle. But in reality there is no mechanical connection between point A and point C.
There is connecting brace going from point A to point D. This is a problem area on all Skyboys and needs to be inspected periodically for loose connection. It is apparently high stress point. If you don’t pay attention and your connection at point D is getting loose, you will have unpleasant surprise in flight.
All my connections are solid, but I still get very small sequence jerking that reminds me of driving small car with big trailer. I was complaining about it all along, but unfortunately nobody ever listens to me. I do believe that tail boom is flexing inflight up and down under constant load changes in bumpy rough air conditions. That is why it was always so much more enjoyable to fly at night for me when air is stable. Airplane feels 100% solid and nobody’s constantly jerking on your tail.
I would like to try installing two cables brace bailable two strut connections on the wings. I believe very similar cables you can see on Boorabee.
That flexible tailboom is an interesting clue.
Maybe your flexible tailboom is bending down 5° in flight and that's why the decalage appears excessive on the ground. The decalage in flight might be less. Your tailboom was designed for a slow ultralight.
Why didn’t I think about it myself ???
Maybe not so smart after all !
Thank you !
Put 100 lbs on the stab and see if it deflects.
I bought a 5" used irrigation tube to test years ago. It deflected so much with my 160 pounds I decided to reject it.
If you have to trim so much it means the angle is not enough anyways.
The angle should be high enough that no trim is required while solo and only trim has to be added when carrying someone else too. Since solo is the lowest takeoff weight possible and should not require trim at all at minimum weight, only as weight increases.
I estimated the negative pitching force is about 300ft/lbs for my 400 pound design. So if his is 1200 pounds the force would be about 900 ft lbs or say 90 pounds. But then also need to add the high line thrust force.
This is how much flex boom has under pressure from only one finger. I tried it this morning.
If I push a little harder, main gear motorcycle JAWA shocks starting to flex.
I think to know if this is big factor or not, I need to install cables and try.
Maybe on my first test flight I will not need nose up trim at all.
Or, horizontal tail should be large enough and effective enough (NACA 0009) to do its job at somewhat reasonable 2-4°AOA
Even 1° or 2° is better then nothing. But most importantly cables will stop vertical motions of horizontal tail inflight.
With the thrust mounted so high that greatly increases the angle required.
You can find this out accurately by marking the trim tab with engine at full power and then mark it again with throttle pulled back and diving slightly to test it at the same airspeed without power.
Then on the ground you move the trim to the settings you marked, then measure the difference in angles of the elevator.
This will tell you exactly how much of your trim angle is caused by the engine thrust and allow you to adjust the tail accordingly.
I am hearing something different.
With the thrust mounted so high = that greatly increases -horizontal tail volume -required. Angle of incidence should be somewhat normal = 2°- 4°, for lowest drag possible.
I am learning that increasing angle only to get what you need = not very smart way to do things. And should not be used on 100 MPH aircraft. But for 50 HP = 60 MPH was probably OK thing to do.
So, I can use something like 120-150lb to do this test?
I don't know what the tail load from thrust is. Just guessing, it could be 100 pounds from thrust and 90 pounds from the pitching moment.
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