DJI S800 prop pitch

[MombasaFlash]

Does anyone know the pitch of the standard DJI S800 15" flat carbon props? Firstly, I would like to change them anyway and secondly, I also want to get some steeper pitched props to cope with higher altitudes, but I would prefer to know the 'starting point' pitch to avoid buying unnecessary quantities of ultimately useless props.

 

[redridinghood]

Does anyone know the pitch of the standard DJI S800 15" flat carbon props? Firstly, I would like to change them anyway and secondly, I also want to get some steeper pitched props to cope with higher altitudes, but I would prefer to know the 'starting point' pitch to avoid buying unnecessary quantities of ultimately useless props.

I tried and compared them to my APC 13x6,5, JXF 14x4 and 15x5. Amp wise the DJI props draw (5,5 amps at 50% stationary throttle) close to the 15x5 JXF and lift about the same. I have seen them being called 15x6,5. Not sure if this helps.

 

[gtranquilla]

But also note that greater prop pitch can also lead to a higher prop stall speed especially at higher altitudes!
So with the complete AUW, make sure to confirm your hover speed at the intended altitude is set up correctly so that your MR will still hover.

Does anyone know the pitch of the standard DJI S800 15" flat carbon props? Firstly, I would like to change them anyway and secondly, I also want to get some steeper pitched props to cope with higher altitudes, but I would prefer to know the 'starting point' pitch to avoid buying unnecessary quantities of ultimately useless props.

 

[MombasaFlash]

But also note that greater prop pitch can also lead to a higher prop stall speed especially at higher altitudes!
So with the complete AUW, make sure to confirm your hover speed at the intended altitude is set up correctly so that your MR will still hover.

Presumably you meant to write lower prop stall speed. Yes, but in order to push more of the thinner air for the same motor effort an increase in pitch is necessary. Finding the happy medium is the trick - assuming one can find a choice of props in the first place. Most things seem to point towards the Graupner 14x8's and Zoar 15x6's. There doesn't appear to be much else to choose from. I will probably go for the Graupners. Don't like wooden props.

 

[redridinghood]

The Graupners 14x8 have even less lift than my 14x4 JXF. Just an FYI. Maybe the new Tiger motors props are a good choice.

http://www.rctigermotor.com/show.php?contentid=177

 

[gtranquilla]

No... the stall speed of a wing or propeller is higher at higher altitudes. You need more airspeed to create lift at higher altitude. I should know as I used for Hang glide from various mountain peaks in BC, Canada. At 14,000 ASL.... you need a steep downslope and a very fast run to get into the air without stalling the hang-glider wings. If you cannot get to take off above the stall speed....... you die!!!

Presumably you meant to write lower prop stall speed. Yes, but in order to push more of the thinner air for the same motor effort an increase in pitch is necessary. Finding the happy medium is the trick - assuming one can find a choice of props in the first place. Most things seem to point towards the Graupner 14x8's and Zoar 15x6's. There doesn't appear to be much else to choose from. I will probably go for the Graupners. Don't like wooden props.

 

[MombasaFlash]

My bad.

Then does this not lend argument for increasing the prop pitch in order to move more air? If the standard S800 props are somewhere around 15x5 and it behaves well at 100m, I am wondering what sort of pitch increase is needed to cope with 2000-3000 metres. Or perhaps that is not sufficient an altitude change to warrant any prop changes.

 

[hjls3]

Mombasa - I happen to live just below 7000ASL. I fly around a fully loaded stock S800 Z15n 2x6s 5500 lipos. I can testify that I have it flying pretty good at that altitude. However, last weekend I went to 10kASL and can also testify that some changes are needed. It flew, but I felt it had plenty of room for improvement. Im just not quite sure which prop setup to try. I am not opposed to the wood - might give them a go. Perhaps we can trade notes.

edit - where I really noticed the difference was on the decent. I did up the gains and that helped but was for sure thinking....MORE PROP

 

[gtranquilla]

I think the Helicopter pilots who fly heli-skiers will have the best possible answers to these questions.
IMHO - Increasing prop diameter is better than increasing the pitch for higher altitude flights. But when the maximum diameter prop is installed and the maximum speed does not give you the lift you need, the only remaining option is to carefully increase the pitch..... but at the same time there is a tendency for the MR to drop suddenly unless your hover speed has been adjusted up for the intended altitude. Maybe the DJI controllers automatically adjust for that but I am not sure.

My bad.

Then does this not lend argument for increasing the prop pitch in order to move more air? If the standard S800 props are somewhere around 15x5 and it behaves well at 100m, I am wondering what sort of pitch increase is needed to cope with 2000-3000 metres. Or perhaps that is not sufficient an altitude change to warrant any prop changes.

 

[MombasaFlash]

With the S800 prop length limited to a maximum of 15" about the only thing one can change is pitch. Or is it .... ?

I asked APC. This is their response.

"At higher altitudes you will need more blade area to offset the effects of the thinner air. Higher pitch will also help but your battery life will go down. I would suggest the 13x4.7SF and the 16x4E (we just came out with this size, it will be on our website next month). Some folks use our 15x10E and EP on quad-copters, you can try this setup at higher altitude as well.

I don't know of a pitch vs. altitude chart. It should really be a diameter (or blade area) vs. altitude chart.
You are welcome to study our performance data:
http://www.apcprop.com/v/downloads/PERFILES_WEB/datalist.asp

The coefficients are dimensionless, just plug in the air density to get the relative thrust increase/decrease.

BTW, if you're interested in trying the 16x4E and EP ASAP, let me know."

It doesn't really help with the S800 - except for the 15x10 suggestion perhaps. But then again, that does not really mesh with what he was saying about blade area being more relevant than pitch.

So we are still in Guessitland. Interesting info on the APC site about maximum prop rpm's. The SF props have a surprisingly slow maximum. Way under what we run them at !


I have a suspicion that the Wookong senses performance (or altitude) and adjusts things so that the Tx stick is always around centre for hover.

 

[hjls3]

Mombasa at 10kASL I would hover at 50% throttle. I found the issue being when descending, the bird would act like not enough lift just under 1/2 throttle pretty unstable - as if the gains were way too low. - I dont want to be the first to try it - but I wonder if this is not an argument to look at 3 blade props - more prop area same pitch.

With the S800 prop length limited to a maximum of 15" about the only thing one can change is pitch. Or is it .... ?

I have a suspicion that the Wookong senses performance (or altitude) and adjusts things so that the Tx stick is always around centre for hover.

 

[olof]

Any decent with a MR will be unstable because you are flying through "dirty" air, turbulence from the props, arms and the main board. These things are not aero dynamic. And just flying through prop wash even in an airplane (with wings) will cause a lot of instability, rotor craft are much more suseptible.

If you travel laterally much faster than the decent then it will be smooth.

I think these instabilities are basically caused by bad piloting, and not understanding how an aircraft interacts with the environment.

Also not understanding staying ahead of the aircraft or power curve will lead to very erratic flight, and often a crash. When you fly full scale and you are actually in the aircraft you feel this very well, but it is hard to get the feel when flying a model, just by watching.

So I am just saying work on being a better pilot and staying ahead of the aircraft, will prevent a lot of bad outcomes.

 

[gtranquilla]

Good info sources you have provided Mombasa Flash....... BTW - In the movie "Vertical Limit" there is an excellent scene where mountaineers needing to be rescued climb to the "vertical limit" of what a heli was capable of reaching...... a real nail biter.... you can see the chopper pilot was really risking everything trying to gain lift in very thin air to rescue the climbers..... ofcourse that choper was much higher than your MR and choppers have the benefit of variable and collective pitch!

With the S800 prop length limited to a maximum of 15" about the only thing one can change is pitch. Or is it .... ?

I asked APC. This is their response.

"At higher altitudes you will need more blade area to offset the effects of the thinner air. Higher pitch will also help but your battery life will go down. I would suggest the 13x4.7SF and the 16x4E (we just came out with this size, it will be on our website next month). Some folks use our 15x10E and EP on quad-copters, you can try this setup at higher altitude as well.

I don't know of a pitch vs. altitude chart. It should really be a diameter (or blade area) vs. altitude chart.
You are welcome to study our performance data:
http://www.apcprop.com/v/downloads/PERFILES_WEB/datalist.asp

The coefficients are dimensionless, just plug in the air density to get the relative thrust increase/decrease.

BTW, if you're interested in trying the 16x4E and EP ASAP, let me know."

It doesn't really help with the S800 - except for the 15x10 suggestion perhaps. But then again, that does not really mesh with what he was saying about blade area being more relevant than pitch.

So we are still in Guessitland. Interesting info on the APC site about maximum prop rpm's. The SF props have a surprisingly slow maximum. Way under what we run them at !


I have a suspicion that the Wookong senses performance (or altitude) and adjusts things so that the Tx stick is always around centre for hover.

 

[hjls3]

Olof - the craft was noticeably different upon descent then at lower altitude. Was not dropping straight down, in fact always use a glide path upon decent. My observation was - at altitude it certainly felt different then at lower altitude and it just so happen that where i really noticed it was descending. But hey what do i know - maybe i am a bad pilot ps I also happen to fly "fullsize" quite a bit.

Any decent with a MR will be unstable because you are flying through "dirty" air, turbulence from the props, arms and the main board. These things are not aero dynamic. And just flying through prop wash even in an airplane (with wings) will cause a lot of instability, rotor craft are much more suseptible.

If you travel laterally much faster than the decent then it will be smooth.

I think these instabilities are basically caused by bad piloting, and not understanding how an aircraft interacts with the environment.

Also not understanding staying ahead of the aircraft or power curve will lead to very erratic flight, and often a crash. When you fly full scale and you are actually in the aircraft you feel this very well, but it is hard to get the feel when flying a model, just by watching.

So I am just saying work on being a better pilot and staying ahead of the aircraft, will prevent a lot of bad outcomes.

 

[DennyR]

It is true that you need more speed and more blade area. But not more pitch. To draw a comparison, if you reduce the head speed of a SR heli then the stability will drop off and you will see the characteristic wobble in the descent that is apparent in MR's (because lift is proportional to RPM only) If you can keep the rpm higher (less pitch) then stability will improve. large pitch is what you don't need. In an ideal world you would have idle-up constant rotor speed and VP props. In the meantime I would try 15x4 wooden Gemfans. Adapters and special instructions I have......

 

[MombasaFlash]

... but I wonder if this is not an argument to look at 3 blade props - more prop area same pitch.

.... if only someone would make them.

Any descent with a MR will be unstable because you are flying through "dirty" air ...

Always good to have a knowledgeable and objective contribution.

The DJI S800 has a bunch of minus points when compared to 'standard' designs - grossly overweight, wobbly booms, expensive proprietary parts - but I have to say that its descending hover flight is the smoothest I have come across yet in a multi-rotor. The last video I posted demonstrates this well at 01:03. I have taken the dihedral of the boom/motor layout to be the main contributor to this welcome attribute.

Otherwise yes, as olof points out, lateral travel on a descent, as opposed to straight down, is always going to be smoother. That 'frisbee' of spinning props creates a virtual wing to glide on - much like a single-rotor 'disc'.

 

[MombasaFlash]

It is true that you need more speed and more blade area. But not more pitch. To draw a comparison, if you reduce the head speed of a SR heli then the stability will drop off and you will see the characteristic wobble in the descent that is apparent in MR's (because lift is proportional to RPM only) If you can keep the rpm higher (less pitch) then stability will improve. large pitch is what you don't need. In an ideal world you would have idle-up constant rotor speed and VP props. In the meantime I would try 15x4 wooden Gemfans. Adapters and special instructions I have......

Would you take tke 15x4 Gemfans over the 14x8 Graupners? I have read elsewhere your positive comments regarding the Graupners' suitability on the S800.

 

[FerdinandK]

Just a short comment on the 50% throttle, I am sure you all are aware of that, just do not write it down. In GPS-Atti and Atti-mode of the WKM the Copter holds altitude at 50% throttle, descents below 50% throttle and climbs above 50% throttle (if possible). So what you want to know is, at what throttle - stick position in manual-mode the copter hovers. If you want to go to fly in thin air, the best way to do that, is an "over-powered" copter, so something that hovers already at 30% throttle, or change to props that allow that to happen (in manual mode if you use a WKM).

What I do not agree is, that "low pitch props" are needed for MR. On my copter I never go below 1:2 pitch:diameter ratio, but of course that is just my personal experience. If diameter is limited (like on the S800) the only thing you can do is to increase pitch to get more lift.

best regards

Ferdinand

 

[DennyR]

The 14x8 graupners were very quickly out performed by both the Xoars and the Gemfans, for windy conditions I always used the 14x4.7 APC's with blunted tips for safety. Slightly less endurance but improved stability due to pitch angle, blade area and rpm.

The most important consideration when using adapters to use other props. is that this mod. increases the motors center of gravity. This means that any imbalance will be magnified and turned into a torsional vibration around the arms. Props should be balanced in the vertical and the horizontal plain and the motors themselves should be tested with an iphone vibration app. Hand wound motors are sometimes not consistant and can generate their own torsionals within.

 

[DennyR]

Just a short comment on the 50% throttle, I am sure you all are aware of that, just do not write it down. In GPS-Atti and Atti-mode of the WKM the Copter holds altitude at 50% throttle, descents below 50% throttle and climbs above 50% throttle (if possible). So what you want to know is, at what throttle - stick position in manual-mode the copter hovers. If you want to go to fly in thin air, the best way to do that, is an "over-powered" copter, so something that hovers already at 30% throttle, or change to props that allow that to happen (in manual mode if you use a WKM).

What I do not agree is, that "low pitch props" are needed for MR. On my copter I never go below 1:2 pitch:diameter ratio, but of course that is just my personal experience. If diameter is limited (like on the S800) the only thing you can do is to increase pitch to get more lift.

best regards

Ferdinand

I am afraid that this theory is totally wrong. What is wrong is that when you are in the descent a MR reduces rpm in order to reduce lift and as that scenario increases in magnitude you eventually reach a point where a partial or even a complete stall of the blade occurs. (Blade stall, like a wing is a function of angle of attack and nothing else) The only time when it would be mechanically necessary to increase pitch would be if the motor Kv was too low and you could not turn the prop. fast enough to generate the required lift.
In aerodynamic terms the most efficient lift will occur when the available power is absorbed into the maximum thrust area that will produce a velocity close to the relative air speed, which in the hover is almost zero. (Cast in stone) Maximum lift efficiency is however always compromised by the fact that it does not produce the best stability hence less dia. than the optimal is needed to improve stability by using higher RPM and lower angle of attack. Especially in descending turns.

It is very easy to see this theory in action if you fly SR helicopters. Just switch you idle up rpm's to lower rates and you can see the stabilisation deteriorate as the speed gets lower.