Re: Resistance calculation

DELFTship forum Hydrostatics and stability Resistance calculation Re: Resistance calculation

Peter Edmonds

A somewhat belated contribution from a brand new user.

For planing hulls I have been using the Taylor Model Basin Series 62 monohedron – 12.5 deg deadrise, which I have in part set up as an easy to use Excel tool This handles the lookup from Excel tables for drag and trim, the interpolations for length/beam and displacement/length, and presents graphs of drag against speed. An alternative is the TMB Series 65, 25 deg deadrise, which I have yet to implement into Excel.

They are both published, but are a bit labour-intensive to do the lookups off the curves and then the interpolations; hence the part implementation into Excel to support my own practice.

The seaplane float case is interesting. I haven’t looked at the model, but if it has the conventional step I would consider the bottom forward of the step only for the planing surface. This will be how it is working in areas of interest – say above 10 – 15 knots.

I don’t think the standard TMB approach of 4 positions of LCG is going to work well. I would look to determining trim angles over speeds for all the positions, then match the trim angle to the anticipated aircraft pitch angle prior to take-off (around level flight angle?), then use the drag corresponding to your assessed trim angle.

Displacement will be an issue, as aerodynamic lift (from wings) will be quite significant as lift-off speed is approached. I would suggest taking displacement as full, 70% and 40% of full. You would then have to start guessing from known seaplane take-off experience as to weight on floats as against speed. However, I expect you could use assessed “weight on wheels” against speed for similar aircraft on runway take-off. I expect experienced pilots would have a feel for this.

In light of this, I would be interested as to how you solved your drag analysis.