Jul 21, 2009, 12:46 AM
Registered User
Joined Oct 2002
1,938 Posts
Quote:
 Originally Posted by David H In turn the again the drag is the same for the same Cl irelevant of mass. the distance travelled in the turn is directly proportional to the mass aand the energy loss is also proportional to the mass. So if we take the horizontal turn, the exit speed is the same for heavy V light model, but the heavy model takes longer because of the extra distance.
I dont understand. I would think in F5 one would attempt to fly the tightest pole-to-pole course trajectory. That requires smallish turns radius connecting the straight line segments. The smaller the radius, the higher the G-load, the heavier mass model generates higher centrifugal force vs the lighter model, thus requires higher Cl to maintain the turn trajectory, which requires increased AOA, resulting in increased drag, which scrubs more speed.. So wouldnt heavy model exit the turn at reduced speed all other things being equal?
 Jul 21, 2009, 01:44 AM Registered User Joined Aug 2004 1,163 Posts Ptxman, Your thinking of the radius of the turn being constant rather than the Cl. The model has an optimium Cl for the turn. ie at a given speed the lift is fixed. If the model is heavier it has to turn a larger radius. The drag in the turn is constant independant of the model mass. The energy to complete the turn is drag x distance. The distance in the turn is proportional to the model mass and so is the KE of the model, as a result the exit speed is the same heavy or light. In a constant radiusturn it is possible for the heavier model to come out faster than the light because the heavy model has more KE.
 Jul 21, 2009, 04:43 AM Registered User Belgium, Flemish Region, Oosterzele Joined Dec 2001 5,539 Posts In the old days, Rick Ruysinck tried to calculate the difference Light/Heavy. He concluded the weight did not pay a big role in the laps. The lighter model climbs higher but then has to fly longer distances and has a slightly worse gliding angle. The heavier model will however be handicapped in the thermal part (climb, glide and landing). Latest blog entry: That elusive thermal...
 Jul 21, 2009, 05:47 AM Needs to do 52 legs !! Verenigd Koninkrijk, Fareham Joined Aug 2008 7,237 Posts Wim, i wonder if Rick took into account that a heavy model (more or bigger NiMh cells) had more energy to burn. Where as now we are fixed to 1750W.min no matter how big the battery. In either case, your point of the handicap is very true.
Jul 21, 2009, 06:21 AM
Needs to do 52 legs !!
Verenigd Koninkrijk, Fareham
Joined Aug 2008
7,237 Posts
Quote:
 Originally Posted by David H Joe, F5B differs significantly from crosscountry gliding. In F5B the drag of the model in a straight line is not really influenced by the wing loading(ie Cd is much the same at the low Cl, both the Cd profile and Cdi are the same)) so the energy for the straight leg is just drag x distance. So yes a heavier model will lose less speed or use less height. But for the same energy input(watt.minutes) the heavier model will be moving slower or have less height proportional to the extra mass. So net energy gain from the heavier model is zero. But from a time perspective(and F5B is a race) the lighter model will reach the other end quicker because it will travelling faster when it enters the course. In turn the again the drag is the same for the same Cl irelevant of mass. the distance travelled in the turn is directly proportional to the mass aand the energy loss is also proportional to the mass. So if we take the horizontal turn, the exit speed is the same for heavy V light model, but the heavy model takes longer because of the extra distance. So looking at the fundumental physics rather than applying the physics of another discipline leads to a different conclusion. Cross country gliders whether model or fullsize are interested in L/D. F5B is really mainly about drag( actually it is energy management) A lighter model in windy and adverse conditions will require a greater change in the flight path than a heavier model to achieve the best result.
David, i agree on about half of this.

Sure F5B is not the same as cross crountry gliding. However the simularities between the two are more relevant then you give it credit for.

Personally i think the turn is a pritty insignificant phase of the F5B pattern. A lot of people think we really loose a lot of speed in the turn but the reality is not the case. When you fly with the Unilog speed sensor you will see what i mean.

Although the radius of the turn might be bigger for the heavyer model, this does not really translate into that much of a loss in energy. Given that the base can be in the apex of the turn and not at the start of the turn, the only real extra travelled distance will be that distance flown perpendicular to the course direction. Now put some wind component in the mix that is also not in line with the course and all of a sudden there is actually no wasted "energy".

To put this in to cross country gliding terms. If you carry water you will end up flying a larger circle in the thermal and thus going up slower due to being further away from the strong core. This is on top of the higher minimum rate of sink that comes with the higher wing loading. You can tighten up the circle if the thermal is really small but this will result in a higher rate of sink again.

Basically i see the thermal part of cross country gliding as the equivalant to the turns and climbs in F5B. The legs are similar to the inter thermal gliding. I will leave you to figure out what % of time each discipline spends in each phase.

Interrestingly, you say F5B is much more about drag. So lets compare the flying speeds of F5B and real cross country gliders. I see about 300Kph entry speed vs as low as 100Kph on exit. Lets consider the climb and turns to be 0Kph and we just compare the legs to the gliding. Real gliders in non wave competition on a good day fly 200-250Kph in real heavy sink vs 90Kph in lift that they don't thermal in. So yes there is a bigger speed difference in F5B but i wonder on avarage who is actually flying faster.

Keeping that in mind and the bit i wrote about different flap settings for different speeds. Now consider the large difference in speed reduction i measured on the S13 vs the B1L given virtually the same dirty fuselage, weight and power setup. It seems to be the higher camber section on the S13 wing is solely responsible for the worse performance in the high speed regime. Both wings have about the same thickness and the same high quality finish.

With respect to your last sentence. I would argue that in adverse weather conditions (heavy wind) it will be more efficient to add balast to the lighter model.
Jul 21, 2009, 11:09 AM
Registered User
Joined Oct 2002
1,938 Posts
Quote:
 Originally Posted by David H Ptxman, your thinking of the radius of the turn being constant rather than the Cl. The model has an optimium Cl for the turn. ie at a given speed the lift is fixed. If the model is heavier it has to turn a larger radius.
I can’t see how anyone can win a race by going a further distance assuming equal speed. An RC pilot does not have any control feedback on what CL is. But good pilots have good sense for course positioning inclusive of turns. I guess you could set up max elevator throw on a heavy model to constrain turn radius equivalent to its CL sweet spot, but that’s getting pretty technical. Maybe a simplistic example would help clarify what I’m saying, hopefully I did my math right:

Assume constant model speed = 150 km/hr (41.7 m/s), pole to pole distance = 150m. The F5B pilot turns on the pole judges ‘beep’ indicating he has ‘made’ the 150m distance. A 10m turn radius yields a 1-lap total closed course path = 363m (2x150m straight legs + 2x31m semi-circle end turns). On this trajectory he would complete 1 lap in 8.7 secs, 40 legs = 20 laps = 174 secs total duration. Now take the exact same (speed) model & increase the turn radius to 15m. That yields 394m closed lap distance, 9.5 secs/lap = 189 secs to do the same 40 legs. The difference is 15 secs extra time purely based on flightpath dictated by radius. If the heavy model was CL constrained by this 5m, it would have to be 8.7% faster to compensate, all other things being equal, no?

Caveats: there are many, but the big one: this assumes the F5B pilot is waiting for the beep to indicate commencement of each turn. Of course he could tighten the course trajectory by anticipating the pole/beep, entering the turn early, as long as the belly of the plane peaks just outside the 150m line as viewed by the course judge. This becomes more like F5D pylon racing which requires assistance of a good caller perpetually anticipating the far pylon-1 distance & generally the pilot is right by P2/P3 so has a good local sense for that turn. But from what I’ve seen, many F5B guys turn hard on the beeps to avoid a cut. So lighter is better!
 Jul 21, 2009, 04:17 PM Needs to do 52 legs !! Verenigd Koninkrijk, Fareham Joined Aug 2008 7,237 Posts Hi. Care to do the math for the beep at apex scenario? The needed increase in speed will be how much in that case? What % increase in mass are we talking about? What about the extra distance travalled vertically by the light model?
 Jul 21, 2009, 05:08 PM Registered User Calgary, AB, Canada Joined Oct 2002 1,938 Posts Using the same example, if you managed to time the turn exactly so the apex coincided with the pole distance beep, the 150m straight line segments are reduced by 20m (2x10m radius) = 130m. Total course distance is now 323m, time required = 155 secs @ same 150 kph. Using the same method on 15m turn radius you get 120m straight segments, 334m closed course distance & 160sec time respectively. In terms of the other questions… I dunno… I don’t fly this silly event! Just kidding. You know better than me what min/max F5B weights are out there as a function of setup & rules. In terms of power on climb-out & weight implications… way outside of my mental grasp. This example is more analogous to constant speed pylon flying. I’m just saying course trajectory has a noticeable cumulative impact on utilizing speed & energy. Big surprise, tight flying and high speed yields the best results. Ideally bring both to the gunfight! But I still cant see how an overly heavy model can be faster considering it requires higher lift (thus higher drag) to maintain it through 80-odd hard turns all other things being equal.
Jul 21, 2009, 05:36 PM
Needs to do 52 legs !!
Verenigd Koninkrijk, Fareham
Joined Aug 2008
7,237 Posts
Quote:
 But I still cant see how an overly heavy model can be faster considering it requires higher lift (thus higher drag) to maintain it through 80-odd hard turns all other things being equal.

Quote:
 Total course distance is now 323m, time required = 155 secs @ same 150 kph. Using the same method on 15m turn radius you get 120m straight segments, 334m closed course distance & 160sec time respectively.
Basically if flying perfectly, the 50% increase in turn radius only results in a 5 second loss. Where the extra weight (don't know how much weight corresponds to 50% increase in turn radius) will make a big impact on the position of the polar on the L/D graph. This means the best L/D will be at a much higher speed.

Now try and figure in the extra distance travalled vertically for the lighter model.
Jul 21, 2009, 09:04 PM
Registered User
Joined Aug 2004
1,163 Posts
Quote:
 I can’t see how anyone can win a race by going a further distance assuming equal speed
You can't. I never said the heavy model was better, but that the light model was.
Quote:
 Using the same example, if you managed to time the turn exactly so the apex coincided with the pole distance beep, the 150m straight line segments are reduced by 20m (2x10m radius) = 130m. Total course distance is now 323m, time required = 155 secs @ same 150 kph. Using the same method on 15m turn radius you get 120m straight segments, 334m closed course distance & 160sec time respectively.
This applies to laps 2 and 3 but not laps 1 and 4 where you still have to make base A. For a 1.5kg F5B model the turn radius is in the order 10-12m depending on the section. A 10% heavier model need a 10% bigger radius. Yes you can start the turn earlier but will you.
Quote:
 Although the radius of the turn might be bigger for the heavier model, this does not really translate into that much of a loss in energy.
I disagree that the turn energy is not significant, but the fact remains that the heavier model uses more. So again the light model is better.
The Cd is so high in the turn compared to the straight , especially induced drag,3-4x is my calaculation. so the energy lost in the turn is close to the same as lost in the straight.
Jul 21, 2009, 09:19 PM
Registered User
Joined Oct 2002
1,938 Posts
Quote:
 Originally Posted by jjmouris Basically if flying perfectly, the 50% increase in turn radius only results in a 5 second loss. Where the extra weight (don't know how much weight corresponds to 50% increase in turn radius) will make a big impact on the position of the polar on the L/D graph. This means the best L/D will be at a much higher speed.
You asked what difference flying perfect (flying beep at the apex) resulted in. Its simple geometry & I answered the question with simplistic assumptions, namely constant speed. Whether that 5 secs is actually realizable on an F5B course, or whether you can personaly actually achieve that level of perfection anticipating the beep on 80+ turns without a cut is really a moot point independant of airframe. Thats 100% flying skill.

But I would think 'any' seconds is worth pursuing be it flying style, drag reduction, airfoil selection, weight, motor, prop whatever. I also wouldnt underestimate the cumulative effect of even 'small' increases in Cd. Every turn you scrub speed results in less velocity entering the next segment. Lift required is a function of V2, so higher AOA is necessary to get higher Cl, which means higher Cd, more drag again. Its a vicious circle.

I'll leave the vertical engineering to you. Pylon guys prefer level altitude.

# Images

 Jul 21, 2009, 09:21 PM Tragic case Sydney Australia Joined Feb 2002 5,875 Posts Perhaps the heavier model may also have a small advantage in 6 legs, although since I never do enough of those its hard to be sure. Certainly the light model always seems to be going slow down that last leg. Number of legs done with NiMh batteries 1100 g pack was similar to those being done with Lipo 500 g pack. However the Lipo packs do those legs with less energy consumption (based on energy put back into the pack) than the NiMh even though the peak power from the Lipo packs is probably higher. So doesn't this bring us back to Dave H's original point that lighter is better because its more efficient not because you can necessarily do more legs.
Jul 21, 2009, 09:25 PM
Registered User