JG3

<< I'll pay more attention next time I'm changing altitude to see if there is a ground speed difference. >>

Be sure to do a 4-way GPS at each altitude to get a true comparison allowing for changes in wind.

I had heard many theories that the speed would be faster in the thinner air at higher altitude. I'm quite skeptical of theories without solid evidence, especially amateur theories that have been repeated for generations. So I go and do testing to find the evidence. I've done that in the past with props comparisons and VGs, with revealing evidence in each case. The evidence in this altitude comparison testing was surprising, but satisfying because I now know the real world results. I have no doubts about the consistent results. The real airspeed is not faster up higher but in fact 3-4 kts less. So no speed advantage unless more favourable wind effect.

On x-country flights I seldom fly a straight line direct course. I'm forever watching the ground, and if I spot something interesting off-course I head over to have a closer look. I don't fly my aircraft just to get up in the air or just to get point to point; I use it more as a 'high clearance' vehicle for exploring the countryside. Of course this is more effective at lower altitudes. My Savannah handles turbulence pretty well, mostly just lively 'whoop-de-do' which I call 'rock and roll'. Not violent enough to cause structural overload in this well-braced high wing machine. A low wing cantilever design flying faster and thus hitting the bumps more abruptly is a whole different case. Yes the rock and roll is tiring, but I don't suffer motion sickness so just carry on, slowing down if need be. When it gets too annoying I either go high or call it a day and land at the next convenient airfield or off-field if suitable. Shouldn't plan on  still being airborne that time of day anyhow..... Each to their own, but I've really enjoyed flying like this for 3000 hrs and seen so much interesting countryside and activities all over the place......

Windy.com is a real boon for chasing wind at different levels. I have found it very reliable and useful so many times. 

I do quite a few long x-country flights in my Savannah, usually below 3000 QNH because I enjoy the perspective and the action I can see on the ground from that height. But often it gets fairly bumpy at that altitude by midday, and sometimes really rough in later afternoon.  Going up to 6500 or 7500 ft usually finds smoother air, but I don’t enjoy it so much up there because the progress over the ground feels really slow and tedious, and the perspective flattens the terrain and I can’t sticky-beak the same detail on the ground.
 

Of course indicated airspeed goes way down and that is expected, but I had thought that TAS made up for that and maybe some extra speed due to thinner air, etc. But measurements have shown that isn’t the case at 6500 ft, and actually lost 3-4 kts running at the same RPM by going higher.
 

First results surprised me, so I repeated the tests five times on four different days, and the results repeated consistently. It’s very satisfying when such testing is so repeatable; shows that the procedure and controls are valid.

Testing 4-way GPS at 2000 and 6500 QNH, several times.
From many 4-way GPS runs, I know my ASI is spot on at 2000, so the change in altitude from that base to 6500 is 4500.
Calculated TAS = IAS + (IAS x 2% x altitude in thousands). In this case IAS is 80 kts and difference in altitude is 4500, so the correction in brackets would be 7.2, rounded to 7 kts.

With prop pitched to 5800 rpm WOT at 2000 ft, found the same WOT of 5800 rpm at 6500 ft.

Observations from the results:
- IAS reads 8–10 kts less than at 2000 ft. To be expected.
- Calculated TAS is 1 kt higher than actual speed measured. (Maybe the correction factor should be 1.8% instead of 2%.)
- Speed at 6500 ft measured 3–4 kts less than at 2000 ft, when running at the same RPM.
- Fuel burn was essentially the same at both altitudes for the same RPMs….

The results for actual airspeed are consistently 3-4 kts slower at 6500 as compared to 2000. So to fly at the same RPM at 6500 actually lose 3-4 kts…..

We know that the engine produces less power in thinner air with increasing altitude, so that would account for the reduction in speed when running at the same RPM.
But that reduction in power means that 5500 is no longer max continuous, so there’s still a good margin to safely run continuously at 5500 rpm when at 6500 ft.

Running at 5500 rpm at 6500 ft gives an actual speed of 86 kts at 19 L/hr fuel burn.
Running at 5300 rpm at 2000 ft also gives a speed of 86 kts but at 18L/hr fuel burn.
So, higher fuel burn for the same speed at 6500 ft.
Would that higher fuel burn for the same airspeed be due to the basic carburetors not being fully compensated for altitude and thus running richer??

Comparing fuel efficiencies (mileage):

Flying at 86 kts at 6500 ft and 19 L/hr = mileage of 4.5 nm/L.
Flying at 86 kts at 2000 ft and 18 L/hr = mileage of 4.8 nm/L.
That works out to 18 nm more range from a 60 L tank of fuel to fly same speed but lower.

Flying at 5500 rpm at 6500 ft gives 86 kts at 19 L/hr = 4.5 nm/L
Flying at 5200 rpm at 2000 ft gives 84 kts at 17 L/hr, = 4.9 nm/L.
That works out to 24 nm more range from a 60 L tank of fuel to fly slower and lower.
The difference of 2 kts in speed makes for a time saving of only 1.4 minutes per hour of flight time…. Is it worth it??
 

So it looks like no speed or fuel advantage to going high, in fact advantages to stay low.
 

I usually cruise between 1500–2500 ft AGL  at 5200 rpm, making 84 kts, so I guess I’ll continue doing the same unless seeking a more favourable wind, or of course a smoother ride…..

On 11/01/2022 at 9:31 PM, APenNameAndThatA said:

I noticed you had a testimonial about VG’s and a Foxbat. IIRC, the factory tried VG’s but ditched them because they messed up the stall characteristics. Thoughts? 

The results quoted in the testimonial by the Foxbat owner at https://www.stolspeed.com/a22-fox-bat are pretty much what I would predict for that wing. If the factory got bad stall effects then what did they do wrong in the installation to generate that effect? Most likely placing the VGs too far back on the airfoil.... Also it's typical for manufacturers to not want to recognize that such simple mods can improve their products.

On 11/01/2022 at 9:25 PM, APenNameAndThatA said:

You have gone to a huge amount of effort. What motivates you? 

Good question. I started out many years ago hoping to discover the very best prop. Was surprised to find initial testing showed all props tested were pretty much equal. Eventually tested 11 different props and same results. I'm annoyed by hearing claims that I can tell aren't based on credible testing. Then it became an obsession as more and more repeat testing proved results over and over again.  Fascinating results from an engineering aspect, but I'm sick of bolting and unbolting props! One more report to come from testing already done regarding the difference between pitching to 5800 or 5500 rpm WOT, but then I'm absolutely done with prop testing...... After all that, I'm still flying the same prop that I started with 11 years ago, and wouldn't change for anything.......

The real difference that I was wanting to test this time was the static thrust. Previous testing had found that static thrust gives no real indication of cruise or climb performance. But of course it does make a difference in short takeoff performance. That initial thrust makes a big difference in how quickly the aircraft gets rolling and up to flying speed. With the popularity these days of seeking impressive STOL performance that could be significant.

So I purchased a digital load cell and set up a ‘stump pull’ arrangement to give all props equal conditions.  The differences in ‘grunt’ are significant, as detailed in the full test results at https://www.stolspeed.com/nid/46

This time also testing a couple of new scimitar-shaped props from Meglin, being imported from the Ukraine by https://www.bushaero.com.au

11 hours ago, Rodr said:

Who can I call in regards to ign problems with a 912uls  ie large rev drop on mag under 3000rpm but all ok on higher revs 

what to look for??

thanks

That's completely normal. Mag checks should always be done at 3000rpm or higher.

Noticed this neat tank vent mod on Charles Mollison's Savannah. Those original dangling tubes from the tank vents are a problem sometimes. If one gets facing forward and the other to the rear and tanks are full, the rear facing tube can start pissing fuel and keep pissing. This neat cable clamp pins the tube down and keeps it facing forward. Just be VERY careful drilling that rivet hole, all too easy to drill into the tank....

Back in about 1997 I was flying my ultralight in the Northern Territory and accidentally triggered the PLB while in the air. It was a surplus RAAF 121.5 unit with no GPS, but had a test switch with no cover so was easy to bump on. I didn't realize it was transmitting for about half an hour until I noticed static on the VHF, switched to 121.5 and the signal blasted in, so switched off right away. An hour later landed at Brunette Downs Station. An hour later the station got a call asking if anyone there could have accidentally actuated a PLB. Now that's very impressive, seeing as how they first contacted locals in the area who could be the most useful if assistance was required, rather than sending the cavalry.

A couple of years later the big rescue chopper was hovering over my workshop at Maleny and landed in the paddock. They were looking for a PLB signal. This had taken them a couple of hours because first they had found a PLB triggered by a lame at Caloundra airfield and had parked the chopper thinking their job was done. But Canberra called them to go again because it seems that both PLBs had triggered at the same time. I my case it was that mongrel exposed switch on that PLB had bumped while removing it from the aircraft. This time I rigged a cover for the switch that had to be pushed aside before it could be triggered.

But the real lesson is that PLBs mostly work really well, given a chance, and the monitors in Canberra were right on the ball. 

I want to have one in current condition and registered, and stored right handy so I can snatch it easily if leaving the aircraft in a difficult situation.

Has anyone here tried this plastic bag over a gum tree to see how much water you can actually catch? Not much I'm guessing, probably not worth the effort......

Always carry at least 5 litres of water in a hydration pack that can survive impact, and of course a good EPIRB.

Trigger the EPIRB and shelter in the shade of the aircraft and stay there and move as little as possible.

Keep a small fire going, with a bundle of grass and green leaves handy that can be dumped on the fire to make smoke when a search aircraft is heard.

If the radio is still working call on 121.5 and 126.7 and area frequency.

In my experience, an EPIRB signal will get response overhead in just a few hours if not sooner, anywhere.

10 hours ago, skippydiesel said:

Great! At last someone responds with real/recent experience. Thank JG3.

 

Any chance of some photos?

Why did you choose this particular paint? 

What are you painting on to metal/composite/fabric/wood 

Oh! & I could do with some idea of your your methodology: ie spray gun type & nozzle, pressure settings, number of coats, temperature, mix by weight or volume, etc

First of all I'm not an expert painter by any standard. Painted several cars and several aircraft, with variable results but mostly very satisfactory.

Not trying to make 'show pony' standard, just good utility finish. Minimum weight and reasonable cost.

If you want porcelain gloss finish then use 2-pack, with the down-side of more weight, higher cost, and difficult to touch-up in future.

This is the paint that the local auto paint store carries, and I found it excellent.

Painting aluminium. Over etch primer.

Spray gun is an old, much-used Devilbis. Don't know the nozzle size.

At about 40psi.

I just fill the pot to about half then add some hardener, not measured, but not the full 4:1 ratio. Plenty hard enough with less chance of cracking.

Then add enamel thinners to suit. That's hard to explain but dip a stirrer and watch the paint flow off it. Steady stream slows to drip fairly soon after lifting out of the paint. If the steady stream persists then it needs more thinner; if it drips almost straight away then it's too thin....

Try it on the shed wall and set up to a good pattern spread and good paint flow.  It's not all as critical as many instructions imply, just experiment a bit and then start on out of the way areas to get the hang of it. Multi-coloured tests on the shed wall leave memories of previous paint jobs.

First one light coat, not necessarily just a mist coat, 10 minutes, then a good wet coat, 10 minutes, and then a final wet coat should be enough. No need for more than necessary.

Wishing you good luck with your efforts.

Just the lower portion of the cowling new paint. 

Just finished some painting with Protek Polyurethane 4:1.

Excellent results and easy to spray, good finish.

Costs $160 - $200 for 5 litre kit of paint and hardener depending upon colour.

5 hours ago, jackc said:

Thanks 🙂. Worth a shot, have a spare set of skins that came from the plane originally, need to check their integrity and can experiment with painting them and see what happens.

Not sure WHY they were originally taken off? 

I used Solaguard on old perished skins and the result was really good. Looked like new and now protected somewhat from UV.

On 30/09/2021 at 9:06 AM, skippydiesel said:

No idea Jack - I would guess, it would be desirable to know that the solvents in the paint didn't "eat" the fabric (chemical compatibility) and that the paint used, had sufficient plasticisers to flex (rather than crack) with the fabric.

The good quality exterior water based house paintss work well on Dacron fabric. Stick well and are very flexible. Clean the fabric first with spraying thinners. Sounds vicious but doesnt hurt the Dacron at all. 

I use the vertical compass already in my Garmin GPS when displaying the HSI page. Works really well and easy to read and not influenced by magnetic effects. Another benefit is that it shows actual track made good, compensating for drift. I use it nearly all the time.

Could I please get some more info from you JG?  Any recommended brand / type, mixing ratio etc.  I want to paint the plane hopefully this summer.  Probably white with blue trim.

I don't have details about brands and ratio of hardener. I just go to a good paint shop that supplies the automotive respray market, and tell them what I need to do, and they should know the best combo. They tend to be experienced professionals as distinct from those that supply the domestic market.

Automotive enamel with hardener added. Sticks well, covers well, light weight, good gloss with hardener, easy to touchup, low cost. Fairly impervious to fuel with hardener added.

1800hrs over 16 years. All recreational, no commercial training.

That's not static; it's 5800rpm wide open throttle straight and level, just as Rotax recommended and their charts are based on.

That puts me at 5200rpm at 60kts best climb, and 5100rpm for best cruise right at the top of the Rotax torque curve between 5000 and 5200rpm. Not pitched fine at all, just correct by Rotax recommendation.

I found static to be not a consistent base line for props comparison due to cavitation and whatever effects at no airspeed, straight and level far more consistent.

Fuel consumption at WOT straight and level is 26 litres/hr.

JG3 you have a few errors ! 

TECHNICAL DATA FROM THE ROTAX BOOK

max continuous performance : 69kW at 5500 RPM.

fuel consumption at max continuous performance : 25 litres per hour.

**** specific consumption at max continuous performance is 286g/kW/hour ****

premium ULP caloric value ~ 46 MJ/kg, so translates to 13.15MJ/kW/hour, efficiency about 3600/13150 = ~ 27%

probably +/- 7 %

depending on how you value the weight of the fuel, the calorific value of that specific fuel etc

Yes, I realize that Rotax publishes 25 litres/hr, but I do not get anywhere near that consumption in real life. I get 20 litres/hr at max continuous of 5500rpm when pitched at 5800rpm WOT. Tested and retested many times and calibrated and rechecked my fuel flow gauge many times.

Long ago I did fuel flow calibration using a calibrated container and stopwatch at various power settings and airspeeds and t/o weights, several times over. These results were very consistent and matched the gauge readings. Those results were very interesting and I'll post those them on another thread presently. Then several times on Xcountry trips I checked that calibration on three hours legs and refilling from a metered bowser. A couple of months ago I did a 30.5hr trip and carefully logged all fuel used. Flying all the time at my usual leisure cruise at 5100rpm with prop pitched at 5800rpm WOT, which gives a speed of 80-83kts at 2500 QNH carrying my usual load, and fuel flow indicating 17 litres/hr. Total fuel used was 520 litres so that gives an average for the whole trip of 17 litres/hr. Can't get much better check than that.....

It's the real life results that I seek in all my testing. I take testing and analysis very seriously. I've studied the factors involved and tried to make all comparisons as valid as possible.

As for the thermodynamic efficiency, I concede that my estimate may have been overly optimistic.... That efficiency for car use on the road with varying speeds is usually estimated at 25% so I upped it for continuous operation and good Rotax design.... Maybe too optimistic.... So that means that even more waste heat has to be dissipated...... 

I do need to step in here and try once again to clear up some misunderstandings that are being tossed around. 

First of all, if an aircraft is pitched to 5500rpm wide open throttle (WOT) straight and level, then 5500 is no longer max continuous, not at all, it's now wide open throttle full power and is limited to 5 minutes! I guess I need to try to explain that....

The max continuous specification for an engine is not governed by the revs but rather by the amount of power being generated. That max continuous power is limited by the ability of the internals of the engine to dissipate heat away from such items as exhaust valves and pistons. The thermal efficiency of a Rotax at constant rpm will be better than a car on the road, but still only about 35% for the carbureted version. That means that only 35% of the energy in the fuel being burned goes to driving the propeller while 65% of that energy needs to be dissipated as heat. About half goes out in the hot exhaust gas and the rest through air cooled cylinder walls and liquid cooled heads. But the critical part is getting that heat away from the internals exposed to the fire and out into the cooling medium. Imagine a poor little exhaust valve with all that super hot gas going by and only the little valve stem to carry away the heat and through a layer of oil into the valve guide. That contact can only carry away heat at a limited rate, and the recommended max continuous is based on that conducting ability. At the Rotax recommended max continuous of 5500 when pitched to 5800 WOT, I measured fuel consumption to be about 20 litres/hr. That's as much fuel burn and thus heat as Rotax recommends for continuous operation. When pitched to 5500 WOT and then flown at 5500 I measured the fuel consumption to be 26 litres/hr. That extra 6 litres/hr is a lot of extra heat to be dissipated, and why Rotax recommends only 5 minutes at that power rate.

The torque curve published by Rotax is based on WOT 5800rpm, and the top of the torque curve is between 5000 and 5200rpm. That's what they have determined by dyno testing to be the most efficient operating speed for this engine. If pitched to 5500 WOT then that curve will probably be very different. I measured the fuel burn at Rotax's recommended max continuous to be 20 litres/hr when pitched at 5800 WOT. When I pitched to 5500 WOT that 20 litre/hr showed to be at 5200rpm so 5200 should be the max continuous in that case. Fuel burn is a very dependable indication of power generated so I am very confident in these results.

No doubt Rotax's recommendations are conservative, but are you comfortable exceeding those recommendations in your engine??

Wandoan, Qld airstrip 17 Aug 21.

I sure wish that I could delete those old ads that have sold months ago. Surely it's not that hard to make it that clicking 'Complete' could delete the ad. I won't be adding more ads until I can delete when I want to.

It's easy enough to post the ads. Surely we should be able to delete when we wish to. Could be made possible under the 'Edit' function.

I have found the Classified section to be very useful for selling items and has potential for much more. Is there any way to delete those items that have long since been sold and now labelled 'Completed' or 'Expired'?? They clutter up the list and make scrolling tedious......

Please do let us know what works the best, if anything.....