Grass Operations

[Waconut]

Hi guys,
My club has a well groomed 800ft grass strip. I have had no problems flying my Avanti at the field but last weekend I went to maiden my F-86 and could not get enough speed to rotate on takeoff. I discovered that it sucked up a lot of grass clippings clippings loading the fan and stator. I am going to add some screen over the cheater hole on the bottom of the fuse to hopefully eliminate the problem. I am concerned I will have the same issue with my F-4 and F-18. Has anyone added screen to the cheater hole and did you notice any issues with performance?
Thanks!!
Anthony

[xviper]

I fly at 2 different fields. One is a GeoTex surface and the other is grass. I have planes that I won't even consider taking to the grass field, based on past experience. Being able to take off from grass depends on the amount of thrust the model has and the size of tires it has. If you have enough thrust, you can often times get away with smaller tires.
My F-86 should arrive in a few days. It's an ARF plus. I have and inrunner system from a recently deceased plane. I know for a fact that the stock F-86 has extreme difficulty getting off the grass. There were two of them at my old field and both were pretty much lost causes. It wasn't due to grass being sucked into the cheater so much as the EDF itself simply couldn't put out enough grunt. I believe with the inrunner system, the F-86 should be able work off the grass field, but only time will confirm that.
I had the first version Blue Angels F-18 and could NOT get off the grass. However, I think the new version with the inrunner could do it easily. I also have the Eflite F-4. I seriously doubt it has enough punch to get up enough speed to take off from grass. Those tires are just too small.
As for putting a screen over the cheater hole, I believe it will reduce airflow significantly. By how much, I can't say for sure but I used to experiment with different kinds of bug meshes on a sports car to protect the radiator. Different application, I know, but the principles are similar. On the car, the mesh reduced air intake by as much as 15%. That's with airflow being forced in from the front. In an underbelly cheater, air is being sucked in and not really being forced, so I predict the reduction could be more. In a model jet, cheater intakes are more for when the plane hasn't built up enough airspeed. Once up to speed, the cheater is not so important.

[Wrthog]

On point

[Mizer67]

Stick an FMS 2100kv pro motor in the F-86 and grass ops. should be possible regardless of conditions, it has about 20% or more installed thrust than the FW fan (1857kv 12-bld).

If you can't get there with 90mms, I would put the FW 1835kv HET and 12-bld fan on 6S, for 10% more thrust vs. the 1750kv 9-bld.

I would not cover the cheater, it typically reduces the static thrust which you need to get to V(r).

[timmybeetle]

Hi

See my post,

https://www.hobbysquawk.com/forum/rc-airplanes/rc-military-jets/225884-official-freewing-t-33-shooting-star-80mm-edf-thread?p=260743#post260743

I use mesh over cheater and fly from grass no issues with take off, only thing I did was my CG is 5-10mm more back.

grts Hans

[Waconut]

A grass strip is my only option. The Avanti and 80mm A-10 operate fine from the grass but they do not have cheater holes.
I will have to see if I can get everyone who mows the runway to start in the center and blow the clippings to the sides.

I will add screens over the cheater holes and see if I have any issues with getting speed on takeoff. I will post the results here after I try to fly Sunday.

[Alpha]

Waconut Let us all know how your results fare, Anthony. For a model like the F-86, if the screen mesh idea doesn't work out for you, a more extreme mod would be to aspirate the fan using a cheater door on the upper surface of the fuselage instead of under it. Look to what we did with the Freewing 90mm F-104's air brakes, which also served a dual function as spring-loaded suck-in cheater doors. Springs would hold them flush to the outer fuselage wall when not needed, and yet when the fan required it, the suction would collapse the door inward.

[Bgelok]

I fly off of a 450’ grass runway. My avanti takes off no problems with or without flaps. The FW F4 gets airborne with take off flaps in about 200’ give or take.

[Hugh Wiedman]

Hi guys,
My club has a well groomed 800ft grass strip. I have had no problems flying my Avanti at the field but last weekend I went to maiden my F-86 and could not get enough speed to rotate on takeoff. I discovered that it sucked up a lot of grass clippings clippings loading the fan and stator. I am going to add some screen over the cheater hole on the bottom of the fuse to hopefully eliminate the problem. I am concerned I will have the same issue with my F-4 and F-18. Has anyone added screen to the cheater hole and did you notice any issues with performance?
Thanks!!
Anthony

Waconut , I fly almost exclusively off of grass, albeit our grass is Bermuda and cut to about 1/2" to 3/4" but grows incredibly fast this time of year. Our runway is just under 800 feet, but I have never needed it. The jets I have that did not need any modification are as follows: 2 FW 90mm F-4's with stock outrunner and flying a RT 6250, the F-18 (repainted and 3D cockpit that added over 1 pound of extra weight) with the stock outrunner from the original grey version flying with a RT 6250, Avanti with stock inrunner and flying the RT 6250, the Stinger 90 with the stock outrunner flying a RT 6250.

The only jet's I had a slight problem with are the F-16 with the upgraded inrunner flying an HRB 6000 and the SU-30 stock inrunners with the HRB 6000, but that was when the grass had dew on it in the morning. After drying out, they get off in just under 200 feet (F-16) and just under 300 feet (SU-30). I changed the nose wheel on both to give them a higher AOA and because I wanted to try a much heavier battery, the Hobbystar 8000. Went to a Hangar 9 Pro-lite wheel on both, the F-16 fit a 2" (1 3/4" was stock) and the SU-30 fit the same brand but a 2 1/4" (1 3/4" was stock). After changing those nose wheels, the F-16 got off in less than 100 feet (even with the much heavier battery) and the SU-30 in just over 130 feet (also with the heavy battery). But if I fly it on the regular HRB, the stock nose wheel on dry grass would have been sufficient.

Your going to get grass in the intake, just make sure to clean it out after each flight. IMO, a screen over any intake may get clogged and reduce the air flow even further than blowing a few blades of grass through the EDF.

These jets on grass require substantial up elevator at the beginning of the take-off run to alleviate pressure on the nose wheel which almost acts like a brake pushing down into the grass. After it picks up speed and just before V1, reduce elevator to a "take-off normal" amount (whatever that is) so it lifts off level instead of in a high alpha rise that could cause a stall. This in itself will cut your take-off run possibly in half! I learned this from all the "master" turbine jet pilots at our club who told me that this is the only way they take off, even with a high thrust turbine. They all noticed that I didn't use any up elevator until very late in the take-off run and then showed me the different. Give it a try, but make sure you're not holding full up elevator just as the jet lifts off or you might end up in an extremely high angle and stall.

[mshagg]

The F-86 performed well off of grass in my experience. You can upsize the wheel slightly on the nose, there is a dubro which fits the bill. You can even experiment with longer struts to increase the AoA. CG with the tail sit method and use half flaps, the plane can take off with a shockingly low ground speed.

If the lawn is full of clippings or wet, all jets will suffer the same fate.

The bigger the better when it comes to grass - freewing's 90mm jets are awesome for grass ops.

[Bgelok]

The bigger the better when it comes to grass - freewing's 90mm jets are awesome for grass ops.

Agreed, just wish we could get more than 3 mins flight time.

[Waconut]

We always fly in the morning at our field with the sun at our back and of course there is always dew on the grass. When the F-86 sucked up the wet grass clippings it really packed in on the outer portion of the blades.
I will get it figured out.

[Hugh Wiedman]

We always fly in the morning at our field with the sun at our back and of course there is always dew on the grass. When the F-86 sucked up the wet grass clippings it really packed in on the outer portion of the blades.
I will get it figured out.

That happens to us as well, especially since our summertime humidity is around 80%, even in the morning. I like to get to the field early, so take at least 1 prop plane on "jet day" to fly on the wet grass. The Avanti also is not bothered much by wet grass and it's always a good time to work with my 3D planes. I once tried my F-16 at 8:30 in the morning (which has it's primary air intake on the underside of the frame just in front of the EDF) and of course it never got off and looked like the Miss Budweiser Hydroplane cruising down the runway. It was actually quite hilarious to watch.

[The43rdHammer]

Waconut Let us all know how your results fare, Anthony. For a model like the F-86, if the screen mesh idea doesn't work out for you, a more extreme mod would be to aspirate the fan using a cheater door on the upper surface of the fuselage instead of under it. Look to what we did with the Freewing 90mm F-104's air brakes, which also served a dual function as spring-loaded suck-in cheater doors. Springs would hold them flush to the outer fuselage wall when not needed, and yet when the fan required it, the suction would collapse the door inward.

This is what we have been doing here on our grass fields in the UK, which tends to have different grasses to North America and the ground tends to be softer. I've modded models for 3 different members of our club so far by adding an angled slot cut in the top of the model just in-front of the fan. All have been complete successes.

If Freewing are listening to this thread, please stop putting intakes on the bottom of models, not only does it suck in loads of debris but the airflow actually physically sucks the model to the ground.

Andy

[Radar-Guy]

That's why I prefer the 10s 1400kV 4500m Ah Lipo (980g battery weight) combos for the 90mm jets. TORA about 350 ft. No mesh on the F-18. Sorry for the poor quality (Mobius "Hatcam")

Frank

[Mizer67]

Some planes are worse than others with the "Hoover" effect on grass.

My Mig-21 is an especially good runway vacuum. The Mirage on the same runway has few if any issues.

The F-22 must have great intakes as the two small cheater holes on the belly further in front of the fan also have no issues in sucking up debris.

[The43rdHammer]

I've had no issues with the Mirage, amazing plane... but the F-35B ... another story.

I should add that I don't own one and never will, but I have test flown and maidened 4 for other people now (all successfully in the end). In some cases I have more minutes on the airframe than the actual owners do 🤣

Andy

[Waconut]

I pulled the EDF unit out of my F-86 this morning. A lot of grass in there.

[The43rdHammer]

There are out of my SU-35 (yes, I still have one) 😁

[Waconut]

I epoxied aluminum screen over the cheater hole. As you can see there is a large opening at the front landing gear and I can see where the wheel slung grass clippings up in there as well.