Trim Speed under power greater than at idle?

[adamjedgar]

Hi guys,

I have been reviewing my igc files and i have begun to see what i believe is a consitent pattern. With trimmers left alone my glider appears to produce faster ground speeds (via gps) when engine is at idle and i am descending than it does cruise speeds. I have compared a number of flight records and this has been a regular occurance. Anyone else experiencing this? Any thoughts on why?

My theory is that its due to the following

When descending at idle, thrust is essentially gravity...vertical

Flying straight and level, thrust is from engine and is horizontal rather than vertical

 

I feel that perhaps the pendulumn angle is different between the two and so is the angle of attack. Someone has suggested to me it may be due to the way in which lines are loaded when engine thrust is increased?

Any thoughts? Anyone else checked their igc files to see if they also experience this?

[AndyB]

It is almost certainly due to the angle of attack of the wing.

[GR002]

I have noticed this as well and but it down to angle of attack of wing. You are certainly further back under the wing on idle. One other thing I notice on cruise or level flight  I can vary my speed by 1 mph depending where I sit in the harness and I wonder if this is due to angle of propeller?

 

[AndyB]

23 minutes ago, GR002 said:

I have noticed this as well and but it down to angle of attack of wing. You are certainly further back under the wing on idle. One other thing I notice on cruise or level flight  I can vary my speed by 1 mph depending where I sit in the harness and I wonder if this is due to angle of propeller?

 

Yes, I used to have my Parajet on hole 5 (hang point) from hang test, but then I lost 10 kg. On hole 5 I always found landings awkward because I was leaning backwards and it felt difficult to get my feet down! Having lost weight I moved in a hole to number 4. I am really in between 'holes'. 

Anyway, the more to the rear hang-point gives me a more upright position and this increased speed noticeably. So you are probably correct regarding prop angle.

[adamjedgar]

Woohoo so my observation is not a lone ranger one.

Also, i have two paramotors. One of them is about 6kg lighter than the other. Both units are high hang point. I find that the lighter one seems to move around a lot in turbulence (and in general)...both the motor about the risers and wing itself.

I find this movement a pain to be honest, it seems to make the machine unstable and am looking for ways to reduce it. 

In addition to the paramotor movement, the wing doesnt feel as solid either. 

I am wondering if the problem is that the recline angle of the propellor is to great thus tending to unload the wing and cause motor to twist and move around more? 

What i dont understand is that even when i feel more upright in the lighter unit it remains unstable. 

This leads me to this question, typically do your paramotors have an angle between thrust line and the rear of your harness (mine appears to be a 90 degree angle). My suspician is that if rear of seat is a right angle to thrust line, when one is setting up hang point positions for flying so that one feels comfortable there is too much "upthrust" unloading the wing and also causing the power pack to move/twist around a lot on the risers.

Now i can already here some minds thinking "setup hang points more vertical" but my body is already more vertical on the unit in question than the heavier one which does not have any issues. When landing i also feel that the vertical angle is changing a lot more on the lighter unter as i flare(it leans back significantly more than the heavier Nirvana Rodeo) Essentially i am saying i feel this is a geometry problem with the frame design and harness angle. I also geel as if the angle between my legs and the seat board is "acute" (less than 90) on the lighter unit. My gut tells me that the vertical distance between the upper harness anchors and the fixed jbars is not far enough.

Edited August 17, 2017 by adamjedgar