Speed Trials

One of the topics that has been discussed on the RC-Hovercrafts bulletin board is how to define, record and demonstrate model hovercraft speed trials. I hesitate to say "prove" or "records", because without vast complication and independent observers, any system is going to be easily corruptible.

This page describes my view and results for my hovercraft. I'd be happy to add results for anyone else's hovercraft if they are emailed to me with the appropriate information.

Rules

Not really rules, because there is no enforcement, but these are what I would propose as rules for a hypothetical governing body of model hovercraft speed trials. As noted above, I'd be glad to add other people's results to this page if they are emailed to me, and I'll add them even if they don't comply with these 'rules', but I personally would not consider them comparable.

A revision history for the rules is available.

Eligibility

1. The hovercraft shall be a hovercraft within the meaning of the UK Hovercraft Act 1968 - A vehicle which is designed to be supported when in motion wholly or partly by air expelled from the vehicle to form a cushion of which the boundaries include the ground, water or other surfaces beneath the vehicle.
2. The hovercraft shall be remotely controlled. No adjustments or control should be made directly on the hovercraft from the start of the first run-up to the end of the second pass through the timed or measured distance.
3. The hovercraft shall be self contained and self propelled. No power, power supply or fuel shall be delivered to the hovercraft between the start of the first run-up and the end of the second pass through the timed or measured distance.

   Operation

4. Speed shall be determined by measuring the time for the hovercraft to cross a predetermined marked distance (the timed distance), or by measuring the distance the hovercraft travels in a known time (the measured distance). In the latter case, the distance traversed must fall entirely within a predefined marked distance, and the hovercraft must traverse the entire predefined distance on both passes.
5. The hovercraft shall start from stationary, travel through the timed or predefined distance, turn, and travel through the timed or predefined distance a second time travelling in the opposite direction. Both times/distances shall be recorded, and the final speed shall be the average of the two results.
6. The timed or measured distance shall be a minimum of 2m or the time to cross the distance shall be a minimum of 5 seconds.
7. The end of the second pass through the timed or measured distance shall be no more than 10 minutes after the start of the first pass through the timed or measured distance.

   Reporting

8. The following information should be reported:
   • Date and time of the start of the first run-up
   • The timed or measured distance for both passes
   • The measured time for both passes
   • Propulsion method, including
     • For electric powered hovercraft,
       • nominal voltage of propulsion power source
       • number of propulsion motors
       • whether brushed, brushless or both brushed and brushless propulsion motors.
     • For internal combustion powered hovercraft,
       • engine type and displacement
       • number of propulsion engines
     • For other powered craft
       • Comparable generic information on propulsion arrangements
   • The means of measuring the time (eg, manual stop-watch, light-beam break, counting video frames)
   • The means of measuring the distance (eg, tape-measure, surveyors wheel).
9. For other's interest, it would be helpful to report
   • Lift arrangements
   • Hovercraft dimensions
   • terrain information (eg, grass, tarmac, deep water)
10. It is preferred that the speed trial be recorded on video, and video of the two passes through the timed or predefined distance made available for free download from an internet site.

    Rewards

11. There are no prizes, trophies, or other tangible rewards for establishing the fastest speed.

For like-for-like comparison purposes, I think it's necessary to further restrict speed trials to hovercraft which do not derive either propulsion or directional control from reacting against the surface beneath the vehicle. That is, a marine-only hovercraft might have screws ('propellers') and rudders in the water. A land-only one could be propelled by a wheel pushing on the ground. Neither of these should be directly compared with the speeds here.

Also, the hovercraft should be self-contained and self-propelled. It should carry its energy source, and be entirely controlled remotely. The means of transmitting control doesn't matter, but it should not deliver power to the hovercraft. For example, a model might be 'controlled' by trailing wires which are connected to electric motors. This shouldn't be compared with the speeds here if the power supply is at the ends of the wires and power is applied at the controlling end. The rules also prohibit changing batteries or refueling between teh two runs. Also, re-trimming or making physical adjustments to the hovercraft between runs is not allowed.

Finally, I note is amazing how apparently complicated a set of rules gets, even when I try and keep it simple!

Results

I have tried out these rules to get a speed for my own hovercraft (actually mark 2C, which (at time of writing) is not written up on these web pages - it's the mark 2B with thrust reversers removed and rudders added. It steers by a combination of adjusting power to each propulsion fan and the rudders. Rudders have gyro control. Mixing of rudder and motor power is done in the transmitter.

complying with the above, I can report:

• Runs started at 2:47 pm on 2 June 2005
• The first run was a measured distance of 1.965m in 0.320 seconds
• The return run was a measured distance of 2.315m in 0.480 seconds
• Propulsion is by 8.4V, two brushed motors
• Times were by counting video frames (8 frames and 12 frames respectively)
• Distances were measured by tape measure to establish that 24 bricks were 2.415m, and using the white lines and counting brick spacings.
• Times were measured by counting frames from the video, being 25 fps.
• Lift is by a single brushed motor running from a 7.2V pack
• Hovercraft has a 600x330 mm deck area
• Runs were done on a brick-paved car park
• Video of the two runs (plus one other) can be seen at the RC-hovercrafts site video section.

My speed is therefore 6.14 m/s for the first pass, 4.82 m/s for the second pass, giving a single value of 5.48 m/s. That is 19.7 kph, or 12.3 mph.

I didn't get a measured distance of at least 2m because my first pass was too close to the camera and I didn't get a wide enough field of view. However, the regular pattern of brick spacing makes it quite easy to get good distance values. Obviously, I need to do it again to fully comply with the rules.

Other methods

I also examined getting speed values by measuring distances from single frames:

Every image is double because that's how the camera works - two interlaced images per frame. I've used the first of each image pair.

Frame rate is 25 per second.

The paviors have an effective width of 100.6mm (24 of them across 2415mm).

The hovercraft travels 2.5 bricks, 250mm, every 0.04 seconds, giving 6.25m/s, which is fairly close to the 6.14 m/s calculated by the method above. The 6.14 is probably more accurate, since it would be 2.45 bricks per frame, and actually my scaling is not that accurate (in fact, the lines do look slightly closer together than 2.5 bricks).

Conclusions

1. My proposed rules for hovercraft speed trials are workable.
2. My hovercraft has a speed (almost to these rules) of 5.48 m/s, just below 20 kph.
3. I need to do the trial again to comply exactly.
4. More results are needed for other hovercraft!

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