Re: .010 TeeDee Tiny Speeder

balogh Tue Sep 19, 2023 6:15 am

Encouraged by this thread, I ordered some HQ 3x2 props that were delivered today.

Just bench-tested my TeeDee010 with a fuel of 20%castor+30%nitro+50% methanol mixture, and the following rpm-s were audio-tached:

1. 27600 rpm with the stock 3x1.25 prop
2. 22800 rpm with the HQ 3x2 prop

Assuming the same rpm ratio in the air, too, the theoretical speed increase ratio with the 3x2 prop is calculated as:

(22800 x 2)/(27600 x 1.25)/ = 1.32 that is the flight speed increase is expected by 32%...

This seems to contradict to the basic rules of aerodynamics, that the power to keep an object flying at the velocity "v" against its drag forces "F" (P= F x v) is proportional with the velocity on the 3rd power (Drag force F is further proportional with the square of velocity v^2, so P~v^3) ..so a 1.32 speed increase with the 3x2 HQ prop would require 1.32^3 = 2.3 times the COX power required at the flying velocity with the 3x1.25 stock prop, whereas the 010 Tee Dee generates at 22.8k only 79% (~15W) of the power it generates at 27.6k (~19W), if the performance curves at https://www.mh-aerotools.de/airfoils/cox_frameset.htm are correct:

.010 TeeDee Tiny Speeder - Page 2 Perfor10

So with only 79% engine shaft power available at lower rpm with the HQ 3x2 prop, in fact the flying velocity should be not more, but less, about the cube root of 79% = 92% of the velocity with the stock 3x1.25 prop..

Of course these are highly theoretical numbers, because the efficiency of the 3x1.25 stock prop in converting the engine shaft power into aerodynamic power (the product of plane velocity and drag force, P=F x v) at 27.6k speed maybe different than the similar efficiency of the HQ 3x2 prop at 22.8k...
I will check in the field soon to what extent this velocity increase (or decrease?) with the use of the HQ 3x2 prop becomes visible...

balogh Tue Sep 19, 2023 6:44 am

Deleted for double posting

rdw777 Tue Sep 19, 2023 8:30 am

Andras, I found a good article when I was looking to how more accurately measure pitch when carving my own props from wood…. I checked the pitch of some known props and found the method presented to be accurate…..I think overall a good article on the mechanics of propellers…

http://www.stefanv.com/rcstuff/qf200203.html

balogh Tue Sep 19, 2023 8:49 am

Thanks a lot Robert, a good, but detailed explanation of what I was also trying to present in a condensed form about how the COX engine and 2 different props will interact and what the consequence of the larger pitch of a prop with unchanged diameter may be.

If the engine were capable of rotating the larger pitched prop at unchanged rpm, we could reasonably expect a higher airplane velocity...but because our model engines with wide open carburetors have a shaft power reducing when the engine rpm reduces ( in its section before the peak power is reached, see the chart in the above post), the flight velocity increase proportional with the pitch increase is not realistic.

At the end it is the conservation of energy: the energy produced by the engine is consumed - mainly, but to an extent depending on the efficiency of the prop - by the drag forces acting on the plane along the flight path.

But because the power provided by the engine at lower rpm will also be reduced, no matter what pitch the propeller has, the energy provided by engine will convert also into less friction-consumed energy i.e. the speed of the plane cannot be more if the energy provided by the engine is less. That would contradict the energy conservation.

Anyways, these are theoretical statements that give only approximations, due mainly to the propeller efficiency which is not constant at all speeds or pitches.

gkamysz Tue Sep 19, 2023 11:27 am

Don't confuse shaft power with propulsive power. The propeller converts shaft power to thrust power. We can safely assume a low pitch/diameter ratio prop is inefficient at high advance ratio, so thrust for the engine output power is low at high speeds with the 3x1.25. If a prop with more pitch is used, it should make more thrust at speed. The diameter should also be reduced to allow the engine to run at peak HP at flight RPM. The hard part is knowing when drag and thrust will be equal and sizing the prop accordingly.

Determining inflight thrust and drag isn't easy. I have a clean flying wing (800mm) on electric. I thought it was fairly low drag. I was flying it on 2S with 5x5 prop and it was pretty fast. I bought some APC 5x7.5 to test a theory that these smaller electric were solely limited by pitch speed. This prop made no difference in perceived top speed, even though input power should have been higher. I switched to a 3S battery and now run a 4.1x4.1 and it's faster, but not quite as fast as I had hoped. I've never measured any power or RPM only estimated in calculations. I've concluded the wing thickness is the limiting factor with the associated drag. I have GPS on it, but 1hz isn't very useful. I think I'll install an FPV flight controller for full data. My next step? Either far more power (problematic with the glass fuselage pod for battery space) or a much thinner 600mm wing with an all new design.

.010 TeeDee Tiny Speeder - Page 2 Img_3510

balogh Tue Sep 19, 2023 1:01 pm

Thanks, Greg,

from the point of view of energy conservation, there can be no confusion if we speak about the shaft power or propulsive power ..given the propeller losses, the useful, propulsive power will always be less than the shaft power from which the propulsive power derives...

Shaft power = Propulsive power+ Propeller losses

so if the shaft power reduces from 19W to 15W as in case of my TeeDee10 when swapping the props and thus reducing the rpm and output (see the power curve of the TeeDee010), then the propulsive power will also drop...resulting in lower plane velocity, unless the efficiency of the 3x2 prop is so much superior to the 3x1.25 (i.e. the propeller losses are so much less with the 3x2 prop) , that the higher prop efficiency will offset the drop in the shaft power..

In your test you prevented the motor output loss by changing the battery from 2S to 3S...unfortunately such power ramp-up with a COX 010 is not possible, so lower rpm (caused by increased prop pitch), and lower output walk hand in hand along the power curve.

rdw777 Tue Sep 19, 2023 1:16 pm

Working with props is interesting to me….It’s one of the big three!
Lift, Thrust , and drag….I liked the idea of trying to best match the prop to the task ….I like the “zip” of my Pageboy so I’d like to prop that up some more, pun intended Very Happy

I’ve been using a cut down Cox grey to 4 x 2….. I’ve made a 4 x 3 and 3.875 x 3 for starters….Haven’t flown them yet but the air behind static tests seems promising ….I like the idea to be able to change one small parameter at a time and compare results….

.010 TeeDee Tiny Speeder - Page 2 33d99f10

gkamysz Tue Sep 19, 2023 2:43 pm

balogh wrote:....unless the efficiency of the 3x2 prop is so much superior to the 3x1.25 (i.e. the propeller losses are so much less with the 3x2 prop) , that the higher prop efficiency will offset the drop in the shaft power..

There is no question, above a certain speed, efficiency of the 3x2 is better than the 3x1.25. Whether the airframe reaches a higher airspeed is the question. I think this low drag design could fly faster, even with 70% power available. Trimming diameter would probably allow even more speed. It's all dependent on the drag of the airframe. If the model was flying at 70% of pitch speed with the 3x1.25 I wouldn't expect more speed with more pitch. If it's currently flying 100% or more of static pitch speed, there is more airspeed available. You don't know how much until you try.

I have never flown a model using a propeller with a P/D of less than 0.6. 1.25/3 is 0.42 and should be relegated to free flight climb or very draggy airframes. Which is probably fine for most designs for these engines. For this airframe, it's a poor choice for maximum speed. I expect Tobaiies to sort it out like he did with the TD .020 speeder.

When more power isn't available, drag reduction is almost always possible. Such as; aileron servo fairing, fillets, engine fairing, gap sealing, etc....

It's been a while since I've looked at Selig's site, and it was great to find data for such a small prop. https://m-selig.ae.illinois.edu/props/volume-2/propDB-volume-2.html#APC Here we can look at eta, CT for APC 4.2x2 and 4.2x4. You see that thrust drops to nothing at a much lower advance ratio for the lower pitch prop.

gkamysz Tue Sep 19, 2023 2:46 pm

rdw777 wrote:Working with props is interesting to me….It’s one of the big three!….

Absolutely! I learned the same in the early electric days. You did homework to see what was going to work, but you still tried a few. You're going about it the right way, with limited selection of commercial props.

balogh Tue Sep 19, 2023 2:58 pm

Thanks Greg, maybe I am underestimating the propeller loss of the 3x1.25 and then indeed the 3x2 will yield higher plane speed even if the shaft power is less.

In any case I will field test the difference between the speeds with 3x1.25 vs 3x2 on my Li'l Spee-Dee and revert.

Good discussion, I have never dug myself so deep into relationships between propulsion and drag and pitch/dia issues before, thanks for the clarifications!

1/2A Nut Tue Sep 19, 2023 4:10 pm

Speed manipulation with everything being a factor.

To gain 2S max speed same motor / batts - could mod the 5x5.75 prop to keep the needed pitch.

Yes thinner airfoil always, thinner airfoiled wing tip plates help and nix the rounded tops
to a straight sharp exit. The lip of the wing at the saddle could be shaped to meet the
front lip of the wing saddle.

Stock Green 3x2 drone prop has been caught on video pulling a TD .010 4.7oz plane near straight up
from idle using a throttle sleeve. Breezy day for a SIG airfoiled balsa sheet wing.
Lean Nano design better with 3P best so far 4P prop 130 mph pass.

3x2E Ok prop for this design;

Lil Speedy TD .010 / Test Flight 3x2E Green Drone T style prop.
Best pass cross wind 32,792 rpm / 62 mph / per prop calc.
4.7oz RTF solid pull better loops and faster turns great prop with a throttle.
Best brief peak down wind pass: 33,941rpm / 64 mph - per 2in pitch .
Bench static: 26,647 rpm 3x2 prop / 2.6oz thrust / 51 mph pitch speed.

The HQ T3x2 blade shape profile and weight loads more yet still provides if with a suitable plane design / weight.

.010 TeeDee Tiny Speeder - Page 2 Vlcsna12

gkamysz Fri Sep 22, 2023 7:41 am

1/2A Nut wrote:Speed manipulation with everything being a factor.

To gain 2S max speed same motor / batts - could mod the 5x5.75 prop to keep the needed pitch.

Yes thinner airfoil always, thinner airfoiled wing tip plates help and nix the rounded tops
to a straight sharp exit. The lip of the wing at the saddle could be shaped to meet the
front lip of the wing saddle.

My wing is power/drag limited. Reducing prop diameter certainly won't increase speed. 3S gave 50% more power and the 4.2x4 about 130MPH pitch speed, but I doubt it's over 115. Last time out was after a bad launch(crash). The motor just didn't run right so testing an APC 4x4.5 didn't prove much.

The wing is 5/8" (16mm) thick for the full 31.5" (800mm) span. The old Speed 400 racers 25 years ago didn't even have wings this thick.This 90's era model is a Dart by Voster from Germany. The tip fins are 1.3mm thick and per the kit. It could use a nice fillet at the fuselage, but that will be in a new design. Flying wings are tricky things in terms of design.

At some speed, for a given airframe, the only way to get more speed will be with more power. At this point, prop choice should already be highly optimized and it will be necessary to adjust diameter and pitch to get more speed. There is a big difference between the low hanging fruit of speed when you start at a relatively low speed, to where drag and power is the main factor.

Any how, keep having fun and fast is always fun.

1/2A Nut Sat Sep 23, 2023 1:30 am

For me the wings are fun to rip an acro -yet are dirty birds creating stability with reflexed trailing edges.

If the E motor is pushed the iron saturates, copper lags, mags heat up and the prop spins less.
Unloading some amps / thrust for better straight away speeds is an option with "fixed" power.
Such is the case with many small IC flying projects tending to be set on a coveted engine selection.
Choose a TD .010 and the challenge for speed absolutely begins.

I have found this cut keeps thrust respectable. The APC material is easy to taper back the trailing edge.
Leaving the leading edge untouched speeds up the mod and retains some inherent dynamic balance.

.010 TeeDee Tiny Speeder - Page 2 Lp050710

APC had adapted this speed solution at some point too, for me about 8 yrs. ago.

.010 TeeDee Tiny Speeder - Page 2 Apc_sp10

TD .010 outstanding results- prop mod to proper proportions to propulate propformance Tether Car Track

2.1oz Nano will go straight up at 80 mph +

.010 TeeDee Tiny Speeder - Page 2 -19h2610

Sponsored content