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Eric Yeoman

First Lift SEVTEC s
« on: Nov 04, 2018, 2:38 am »
Hi. I know the info is on here somewhere but I can't find it.  ::) .
Regardless of engine used. Can anyone tell me the shaft revs on first Lift, on various models? Thanks.

John Robertson

Re: First Lift SEVTEC s
« Reply #1 on: Nov 04, 2018, 10:20 am »
I can't recall ever seeing that info anywhere.   First lift is a bit conceptual rather than practical (other than ensuring it occurs above engine idle speed!).   It's the point at which the cushion pressure equals the craft mass with zero skirt leakage.  Of more interest is full lift - the point at which when the craft reaches it's full hover-height.  On a single engine craft the balances between full lift and thrust is critical in ensuring the craft is drivable (many craft can't stop without losing lift making hull and skirt damage inevitable).  Although full lift can be calculated (see performance calculator) it still needs to be "tuned" to suit the available thrust (many iterations of drive ratios and blade pitch) to get the best compromise.

Here is how I do it:
1. Set lift drive ratio to give maximum fan tip speed at max. engine RPM
2. Set lift fan blade pitch to 25deg (the most efficient angle for lift).
3. Subtract lift power at max RPM from total engine power available and design thrust drive ratio and prop/fan pitch (start with the most efficient prop pitch) to absorb the available power.
4. Make sure lift at idle is less than design lift.
5. check you can achieve full lift (if not, increase blade count or fan diameter).
6. Make sure thrust is no more than 10-15% of the craft mass (the stop-ability check).
... the odds of being able to achieve all of those things first time is zero.  Normally, you need to reduce the engine max. power/rpm and adjust drive ratios several times to arrive at something approaching a reasonable compromise.   Then you need to build and test the craft in a variety of different conditions (smooth surfaces, leaky surfaces, heavy chop/waves, etc.) checking driveability, cushion performance (does it have enough spare flow capacity to handle leaky stuff?), etc.

Rules of thumb:
1. full lift is normally achieved at around 63m/sec lift fan tip speed (if it isn't then you need to adjust the fan diameter/blade count).
2. limit lift fan pitch to 25-30degrees (little performance change and not very efficient out side this range).
3. Max safe(ish) tip speed for lift fan is 125m/sec. - that equates to the max speed you can operate the engine which, in turn, limits the power available for thrust.

This entire process is both time and money intensive - for a single-engine craft the easiest option is to pay someone else to do it (in other words buy a set of plans for an proven craft and stick to them!).  It also explains why it isn't easy just to "add more power" as most folk seem to think (power added will result in too much thrust at low RPM without a costly re-design - and often includes deliberate "wasting" of some added power to get a drive-able craft).

An easier solution for a the home designer is to use separate lift and thrust engines.  You've still got to do the basic calculations but at least the system impacts are decoupled.  However, the downsides are poorer reliability and more complex control.
« Last Edit: Nov 04, 2018, 11:22 am by John Robertson »